Handy tips on how to fix those XR600R carb issues

Read on for carburation tips, hit the blog for XR updates, or if you're rebuilding click here for the rebuild tips page

An introduction...

There's a lot to love about XR600Rs, but people do seem to have a few issues with carburation. So, in an effort to share the love (all care taken, no responsibility accepted etc., etc.) and hopefully make some XR600R owner's life happier, here's a rundown of "what we've learned so far".


The information here relates to single-carb XR600Rs equipped with the Keihin PD carburettor (basically 1988 through to 2000). It's set up in a series of stages to work through to get your XR600R running right, and will be added to progressively so check back for updates. To avoid confusion read the glossary of terms (right) before you get the spanners out.


The most common issues are hard starting, a high or hanging idle, and a 'sticky' throttle. We'll deal with all those, but first...

Rich vs lean: petrol engines like an air/fuel ratio around 14.7:1 (14.7 parts air to one part fuel). If the carb is mixing in too much air or too little fuel you're said to have a lean condition. Too much fuel, or too little air, and you're running rich. If you're too lean you need bigger jets (more fuel), too rich and you need smaller jets (less fuel).


Idle screw: that's the big black knob at the top of the carb. Turning the knob clockwise raises the slide (and airflow) to increase the idle speed, anticlockwise backs it down. Honda calls it the throttle stop screw but everyone else calls it the idle screw, so that's how it's referred to here.


Pilot screw: that's the little knurled brass slot-head screw at the front (engine side) of the carb just ahead of the float bowl. Some people refer to this as the slow speed screw or air screw (that's just wrong), but Honda calls it the pilot screw...and it's easier to stick with that when you start talking about making adjustments. Turning it in (clockwise) makes the mixture leaner. Turning it out (anti-clockwise) makes the mixture richer.


Slow jet: is what Honda calls the smaller jet at the front (engine side) of the carb ahead of the main jet. Together with the pilot screw it makes up the carburettor's slow-speed circuit. Some people call this jet the pilot jet (see where the confusion comes in) but we're sticking with slow jet. Jets generally carry a number stamped on them – numerically bigger is richer, numerically small is leaner.


Slow-speed circuit: the pilot screw and the slow jet make up the slow-speed circuit. This basically controls the air/fuel mixture at start and idle through to somewhere between quarter and half throttle.


Main jet: this is the guy that's doing all the work when you're WFO (or, for the politically correct, at full throttle). Together with the needle, needle jet and the slide, it meters the air/fuel ratio from half to full throttle. This is generally the first thing to play with once you've got the slow circuit sorted out – numerically bigger is richer, numerically small is leaner.


Slide, needle, needle jet and needle clip: the slide controls air flow and the others meter the fuel. Unless you're a race mechanic the only thing you'll play with is the position of the needle clip on the needle. This affects the air/fuel ratio around quarter to three-quarter throttle. Moving the clip up on the needle (towards the blunt end) makes the mixture leaner, moving it down makes it richer.


ACV: that's what lives under the little round plate behind the choke lever. Honda calls it the air cut-off valve, but some people like to call it the air control valve. ACV works either way, so that's what it's referred to here.


Hanging idle vs high hot idle: chances are you'll experience both, so let's differentiate between a high (or fast) hot idle and a hanging idle. A high hot idle is when your bike idles (too) fast when it's up to operating temperature (like after a ride). A hanging idle is when your bike idles high when you back off the throttle (or come to a stop) but returns back to 'normal' idle speed after you blip the throttle. Here's a teaser: if your bike revs really high when it's hot your slow speed circuit is too lean.


Hard starting: you probably already know what hard starting means, so it's better to define what you want. One kick hot or cold is the ideal to aim for...and it's not unobtainable.

A glossary of terms

Do the maintenance

Assuming your bike is not worn out and you're getting good spark there are a few things to do before you start playing with the carburettor.


First off, clean that air filter properly (good to have two so you can oil one and let it drain off overnight – you're aiming for consistency). Make sure your fuel is good (fresh, with no water or dirt), open the petcock and float bowl drain screw to make sure you're getting decent fuel flow (clogged petcock, filter or even a blocked tank vent cap), and check for blockages in the overflow and breather tubes.


Once you've looked for blockages, look for leaks. Air leaks at the head and inlet manifold, carb and manifold, or even carb and air filter hose, will screw with your jetting. So will leaky gaskets on the top of the carb or the ACV. With the engine running spray some starting fluid (ether) or WD40 around the joints – if rpm changes you've found an air leak.


Throttle cables should be lubed, properly adjusted and checked for any binding (you should hear the slide snap shut practically immediately when you let go of the throttle). While you're playing with cables, ensure the free play on the manual decompressor is set up correctly – nudging a valve open isn't going to help anything.


The auto decompressor system seldom has issues. What does cause problems are valve clearances that are too tight. They close up over time, so make sure the clearances (particularly on the exhaust valves) are right.


There's an old saying that 99% of carburettor problems are electrical, but the XR600R's ignition system is largely bulletproof (although pre-'88 models apparently had some stator issues). Timing is pretty much set and forget, so if that's okay it's really just down to checking that you've got the right plug and that the cap, lead and coil are all okay (that said, a faulty kill switch can give you all sorts of intermittent hard-to-diagnose problems).


The key to fault-finding and tuning is isolating issues and controlling variables. If  there's one area where that's particularly true, it's the nut behind the 'bars, so....

Standardise your starting procedure

The trick to starting any big single is to find TDC on the compression stroke (where you encounter resistance), edge the piston just over TDC, and then kick. The auto decompressor on XR600Rs makes the process easy, but it can also mask how far past TDC you've gone – go too far and you're not getting the full benefit of the swing.


Factory XR600R race mechanics of the day apparently recommended 'the Honda dance' for cold starts:  kill switch to off, choke fully on, manual compression release pulled in, wide-open throttle, kick through five times in succession, get off the manual compression release, bring it just past compression, ignition on, no throttle....and give it a good kick. Drop the choke to halfway soon after she fires, then fully off as she warms up.


Once the bike's up to temperature a single kick (from just over TDC) with no choke and no throttle should do the trick.


Bottom line, hot or cold, is no throttle ever. Okay, it might be permissable to whack on a bit of throttle just the get the little bugger started before you start tuning, but it's a variable you want to eliminate. Using different techniques and throttle openings to start the bike isn't going to help you get the jetting right.


And here's a teaser: if you can start from cold with no choke your slow speed circuit is too rich, if you start fine when hot but poorly when cold your slow-speed circuit is too lean.


Now, assuming you've done the maintenance and can actually get the bike running, the first step towards a sweet-running XR600R is to...

Set the idle up right

All pretty simple, but something that gives plenty of clues  on slow-speed jetting. The fist step, before you even start the bike, is to grab a pen and notebook and check the position of the pilot screw. Counting the number of turns, gently screw it in until it gently seats...then put it back where you found it. The convention is to measure pilot screw position in the number of turns out from fully-seated (in), so note that.


Okay, fire her up, let her warm up, and set the bike upright and level.


Second step is to adjust the idle screw and get a good idle speed. Turning the knob clockwise raises the slide (and airflow) to increase the idle speed, anticlockwise backs it down. Honda specifies an idle speed of 1300rpm (plus or minus 100rpm) but that's probably a little on the high side. It's all a bit academic unless you're using a tacho, but adjusting by ear generally works out fine (too high or too low should be obvious, you're aiming for a consistently smooth idle where you can still tell it's a big thumper).


Now, here's the important bit, it's time to adjust the pilot screw. On an XR600R turning the screw in (clockwise) makes the mixture leaner, turning it out makes the mixture richer. Gently turn the screw in until the idle starts to drop or miss a little, and note that position. Now turn the screw back out until the idle starts to drop or miss, and note that position. Roughly half-way between those two positions you should find a sweet spot that gives you the smoothest and highest idle. Note of that position.


The 'standard' setting is 2.5 turns out, but anything from 1.5 to 3 turns is okay. If the best setting is under one turn you need a leaner slow jet, if it's over three turns out you need a richer slow jet. Swap out the slow jet accordingly and run through the whole procedure again.


If you're in the the 1.5 to 3 turns range you should be pretty good but (and here's where it gets tricky) if the best setting has raised the idle speed too much and you've had to back the idle screw out, the next time you try a cold start it might not leap into life until you wind the idle screw in...and you've just 'adjusted' yourself into a high hot idle situation.


 XR600Rs do tend to idle a bit higher when hot, but only a bit. The idle screw generally sits around a turn or so off the stop. If you can't find a 'happy medium' hot/cold idle speed it's time to go back and adjust the pilot screw/slow jet (try richer) and run through the whole process again.


If you've got a hanging idle, or a sticky throttle that wasn't resolved with proper cable maintenance, running through the above process might cure the problem, but there can be other complications (which we'll get to later).


If you've had a good result by this stage you can stop reading and start riding, but if you've still got starting and idle issues, there are a couple of things that might be behind them...

Probable cause...

Two things that can cause havoc with your slow-speed jetting are a faulty ACV and pilot screw issues. Both are best addressed with the carb off the bike but, if you're careful, you can work with the carb in situ.


Let's look at the pilot screw first...specifically the O-ring.


The perversity of O-rings is that they act shy and inaccessible when you want to check their wellbeing, but make crazy escape bids when they're not the center of attention.


Previous-owners may well have 'mislaid' the pilot screw O-ring (and/or the metal washer that sits behind it), so check that everything is there and that it's assembled in the right order (from bottom – pilot screw, spring, metal washer, O-ring). After decades of adjustment the O-ring may well have lodged itself in the carb body and be flattened, scuffed or replace as necessary.


The tapered upper part of  the pilot screw interfaces with a tiny hole in the carb body (ahead of the slide) and acts as a jet. Prising out an old O-ring (use thin-gauge wire with a very short  90-degree hook) can leave bits of decayed rubber that will block the opening. Any obstruction here will give you starting issues, so make sure it's clean and flowing free.


You can't really check this without the carb off the bike, but an easy intermediate 'solution' is to flush out the pilot screw hole using an aerosol with an extension tube you can slide up into the pilot screw opening – a can of electronics cleaner is ideal because you can really go to town and it'll evapourate (eventually, wait a bit) without leaving any residue.


While you're there, it doesn't hurt to pull the main and slow jets and give the housings a squirt through (open the throttle to give the needle jet some space when you do the main).


Don't underestimate the probability of a blocked pilot screw hole – it can surface on a fresh, clean rebuilt carb purely on the grounds that it's the smallest hole any random bit of dirt can block.


Which brings us to the ACV.


The ACV's job is to richen the mixture under deceleration to prevent 'backfiring' or 'popping' (more technically called afterburn because it occurs when unburnt lean fuel/air mix ignites in the exhaust system). Theoretically the ACV has no effect until the throttle snaps shut, but it's an integral part of the slow speed circuit if there's anything wrong (like a hole in the diaphragm or blocked passages) it can put your jetting out.

Time may have caused the rubber diaphragm in the ACV to develop a leak, or allowed the various passages and orifices to become clogged. Check the condition of the spring, diaphragm and O-ring (install with the flat side towards the carb) and ensure that all the orifices and passages are clear.


A can of electronics cleaner works well here too. The 'nipple' on the carb side of the diaphragm acts on a spring-loaded button valve (behind the little brass ring) so check that this isn't sticking and give it a bit of a flush out. Likewise, flush through the hole behind the O-ring (and associated openings on the cover) and the two passages hidden deep at the base of the ACV housing behind the diaphragm.


If the diaphragm has any kind of leak, the good news is that you've found the problem and dropping in a replacement is a simple fix that should cure things. The bad news is that you'll be out some cash and that the new diaphragm will fail one day and eventually need replacing.


The alternative is to eliminate or defeat the ACV altogether. There's a good news/bad news side to this too...realistically it's just easier to drop in a new one if it's damaged. That said, you'll find details of how to defeat the ACV if you keep scrolling. But first...

Stay afloat...

If  you haven't already checked you float bowl it now!


Honda specifies a float bowl level of 14.5mm. It's measured  from the base of the carb body to the flat edge of the float in line with the main jet holder as the float tang touches (not depresses) the plunger on the float valve.


Minor variations from spec shouldn't make too much difference, but if the float is too high (under 14.5mm) you get too much fuel in the bowl and symptoms of jetting rich. Too low  (over 14.5mm) and you don't get enough fuel and can run lean. Either way, it's a variable that you want to eliminate when chasing 'perfect' jetting.


You'll also want to make sure the float doesn't have any leaks and that there's no dirt or deposits stopping the float valve from clean or replace as necessary.


If you're checking float levels with the carb on the bike (and you don't have a float gauge) cutting a piece of stiff card into a 14.5mm 'L' shape should let you line things up and get a consistent measurement in a tricky position.


Another handy hint is to check the float valve seal (and float position) by attaching some clean fuel line and gently blowing through it until you feel back pressure as the float valve seats.

…and clear your passages

Checking for blockages in the breather and overflow tubes was mentioned briefly in the ‘do the maintenance’ section, but the subject of airflow through the PD’s various passages is worth expanding on a bit.


The overflow’s purpose is to (hopefully) stop your engine filling with fuel if the float valve sticks open or if you lay the bike down. The two breather outlets on the carb body are there to vent the float bowl. Vent, in this case, means to let air both in and out…so if there’s a blockage you could be getting a partial vacuum in the float bowl with consequent jetting issues. Checking that they’re all clear is a simple matter of seeing if you can suck or blow through the hoses (you’ll have to separate the breather/vent hoses on the bike’s right-hand side).


Checking airflow through the three little brass openings along the bottom of the carb inlet isn’t quite as easy, but if you’ve come this far and you’ve still got problems this is probably where you’ll find them.


The opening in the centre of the carb leads into the main/needle jet tower…presumably to aid in fuel atomisation. The left-hand (highest) opening leads to the brass button valve in the ACV while the middle opening feeds into the ACV way down hidden at the back of the housing.


All three passages are small diameter and can become clogged relatively easily with correspondingly negative results.


Don’t try sticking bits of wire down them, grab a can of electronics cleaner (with extension tube) and give them a squirt through – depress the button valve and squirt from there to check the top left-hand side passage, go from the inlet end for the others. If you’re not getting decent flow it’s time to give the carb a good soak (acetone or white spirits) and blow through the passages with compressed air.

Some notes on jetting

Stock 'Aussie spec' jetting is a #62 slow jet and a #152 main with the needle clip set in the third (middle) groove and the pilot screw at 2.5 turns out (although some manuals show different sizes for different years and,  presumably, different markets).


A size up from stock is generally considered a good move, and a #155 main and #68 pilot is a common recommendation for 'uncorked' XR600Rs. Don't just slap in the biggest jets you can find though – optimum jetting varies according to weather, altitude, engine condition and modifications.


Starting issues aside, the XR600R is pretty tolerant towards jetting. It'll still run with jets a size or two so off (just not terribly well) so don't be shy about changing jets – it's not a two-stroke so you don't have to worry about seizing the engine unless you go way too lean.


But...swap out jets incrementally (i.e. go up or down one jet size at a time), and only make one change at a time (i.e. don't swap filters or pipes and drop in a new main and pilot jet, until you've seen how the bike runs after just one change).


If you're new to the game there are some excellent articles about jetting techniques online – just be aware that they might be referring to carbs with a different set-up (like ones with an air screw instead of a pilot screw where going rich/lean is the other way around).


The best advice you'll ever get on jetting is to keep a diary. Note down your jet changes, mileage, date, weather conditions, any pertinent mechanical factors (new air cleaner, new pipe, different octane fuel etc., etc.) and add a few comments on how the bike feels after riding.


Tracking changes and monitoring results will show if you're heading in the right direction...or where you stuffed things up. The emphasis should be to keep that jetting diary too – besides being invaluable while you're actually juggling jets, it'll get you back on track after a break and help eliminate guesswork in the future.


Anyway, once you've solved any slow-speed circuit issues, and got your main jet right (bear in mind there's a bit of a 'trickle down' effect every time you change the main), you're going to want to fiddle with the position of the needle clip to give you the best throttle response and mid-range performance.


Technically speaking, it's a combination of needle clip position, needle taper, needle jet and slide profile that controls the air/fuel mixture, but unless you've reached 'guru level' the only thing you really want to mess with is the needle clip.


That's fine, because adjusting clip position affects things from somewhere between 1/8 and 1/4 throttle through to around half throttle where its influence starts to taper off and the main jet begins to take over. In short, just where you probably ride most off-road and at that 'part throttle' opening where you sometimes feel a flat spot.


In a nutshell moving the clip down raises the needle and makes the jetting richer, moving it up (closer to the top) lowers the needle and makes the jetting leaner. Try one notch at a time and pay close attention to the results after a test ride.


Sounds good, but the big problem is that the needle and clip on a PD Keihin are a pain to get to. You need to remove the throttle cables, pop the top off the carb, unscrew the linkage arm, withdraw the throttle spindle, pull the slide and linkage out of the carb, and reach deep down to undo the linkage plate that holds the needle and clip in the slide. Which is why some people say the PD is "hard to tune".


You can do it with the carb on the bike – loosen the clamps at the manifold and airbox so you can rotate the carb back and forth to give you sufficient clearance at various steps – but working with the cross-head screws is tricky (especially when you come to button it all up again). So...

Make your life easier with Allen heads

Butchered Phillips (cross-head) screws are pretty much to be expected on old bikes, and if someone's been playing with the jetting in the past you can pretty much guarantee the cross-heads on the carby are well past their use-by date.


Allen heads are the answer, but most commercial Allen head kits only cover the easy-to-reach bits. So here's a list of everything you need to swap to Allen heads on an XR600R Keihin PD carby.


M3x6mm button head x2: choke plate

M3x8mm x2: needle clip retainer

M4 x10mm button head + split washer x1: choke lever retainer

M4x12mm button head + split washer x2: ACV plate

M4x12mm + split washer x2: top cover

M4x12mm + split washer x1: throttle spindle

M4x16mm + split washers x3: float bowl

M5x12mm button head + split washer x1: throttle cable retainer plate


The button-heads are needed to clear the choke lever and cable assembly...and to give you smooth airflow over the choke plate (see the section below, and remember to Loctite these two). Keihin uses a M4x13mm cross-head with a 7mm shank on the throttle spindle but it's a bit of an 'orphan' size and, since it's as much about clamping force as lateral play, an M4x12mm Allen head  works fine.


Replacing the float bowl screws turns swapping jets from a painstaking operation to a piece of cake, and while the Allen heads on the needle clip retainer are a bit tricky to reach with a standard size Allen key they're a damn sight easier to whip in and out than butchered cross-heads.


Watch out though. You get a lot more leverage on an Allen key than a small screwdriver and you can strip the threads on the carb body if you overtighten things.


And while we're talking Allen heads, if the cross-heads on your twistgrip throttle body body have corroded themselves into place (as steel bolts tend to do in aluminium housing) M6x15mm is the replacement size.

The hanging idle issue...

Jetting right usually solves high hot idle issues, but a persistent hanging idle can be down to other causes.


A hanging idle is when your bike idles high after you close the throttle but returns to 'normal' idle speed if you give it a quick blip. It might not happen all the time, but chances are it'll surface just as you back off for a really tight technical section on a rocky trail, or as you pull up opposite a bunch of grumpy OAPs at a bus stop – not good either way.


Assuming your jetting is right, the ACV isn't faulty, and you're not pulling in air from somewhere unexpected (perfect gasket seals and a light grease on the  throttle and choke shafts entries), what's basically happening is that the slide is failing to close fully when you back off...then, when you blip the throttle, it settles back into its proper place. In essence something is causing it to stick.


Discounting problems with cables for a moment, that something is a combination of engine inlet vacuum, age and wear.


The XR600R's big piston pulls in a lot of air at higher rpm, and when you suddenly go from WFO to a fully-closed throttle it's still trying to suck much the same amount of air through a much smaller opening. That creates a 'vacuum' that pulls the slide against the barrel of the carb body...and that's where problems can start.


Honda fits a pretty fierce return spring, and doubles up with a push/pull cable arrangement, to make sure this isn't an issue when the bike is new. Eventually, however, you get some wear in the carb body and a few deposits on the slide that can add up to binding.


So what to do? Double-checking the cables should be the first stop before you tear the carb apart. Besides checking for smooth operation and any binding caused by routing or fraying, it's worth pulling apart the twistgrip to make sure it's not gummed up and that free-play is as per spec.


Handy hint is to initially leave the bottom part of the twistgrip housing off so you can see what's happening with the cables while you use the screw adjusters to get the right play (after making major adjustments at the carb body). If you've rounded off the cross-head screws trying to get them out, now is the time to replace them with Allen heads – M6x15mm.


Okay, if that hanging idle is still there it's time to tear the carby down.


The slide itself is hard chromed so it shouldn't (theoretically) be worn much, but you'll probably see some dirty deposits on it and, if you look closely, some vague outlines of where it's been spending most of its time.


Despite expectations of 'precision tolerances' don't be surprised if finger pressure from the inlet side reveals a bit of play. That's not necessarily an issue...but obvious ridging or grooves on the slide or the barrel of the carb body means replacement time.


A simple clean and polish generally goes a long way towards restoring original functionality, but don't use anything abrasive that will remove too much metal. Autosol (or similar) chrome polish applied by hand will clean up the slide, and a few minutes on the slide barrel inside the carb with a Dremel-type tool and a small buff should polish out any imperfections.


Make sure you get rid of any traces of polish residue. You should end up with a carby barrel that's smooth to the touch and a slide that...err...slides freely and easily. A good dousing with silicon spray doesn't hurt either.


If everything else is good, polishing should solve your problems. That said, some people advocate a couple of more invasive solutions that you might like to know about...and possibly avoid.


The first is to lightly round (fine grit wet'n'dry, finger pressure) the leading edge at the base of the slide and the edges at the top and bottom of the carb bore where the slide bears against it. The edges on the carb body are pretty sharp, but Honda puts a slight bevel on the base of the slide so this shouldn't really be an issue. Probably best to pass on this 'solution'.


The second solution is the hole-in-the-slide mod which basically involves drilling a 2mm hole 23mm down from the top of the front (engine side) of the slide.


The theory is that this reduces the 'vacuum' that causes the slide to bind against the barrel of the carb body as you throttle off. It's also claimed to improve response on the bottom-end.


Reduced vacuum-induced stiction seems a likely outcome, but any improvement in low-end response is probably down to the fact that you're jetting richer to compensate for the added air the hole delivers at 'closed' throttle and then getting a little bump of extra fuel in the mix as the slide rises and the hole is covered up.


Does it work? Well, an early trial of the drilled-slide modification on a carb without a polished barrel (see the Camel build blog for background) showed improvement, but the hanging idle problem wasn't completely eliminated until a refurbished carb with a polished slide and barrel (but no hole in the slide) was slotted in.


Looking for a definitive answer a 'full combo' (fully refurbished, polished barrel, drilled slide) carb was prepared and trialled back-to-back against the 'no hole'  carb with a polished barrel. Neither carb displayed a hanging idle, so it's a pretty fair call to say that polishing the barrel is the best bet to solving any hanging idle issue that correct jetting doesn't eliminate


Running a drilled slide back to back with an undrilled slide didn't show up a noticeable improvement in bottom end response either. Given that it's hard to move back from (unless you have a spare slide), and given that it adds another 'unknown' into the jetting equation, it's probably a mod that's better to pass on unless all else fails.


There are some changes you can make to your carb that will show real benefits though, namely...

Lighten up that throttle

If you do much road riding, or keep it pinned for long stretches over fire trials or in the desert, the stiff return spring Honda fits makes holding the throttle open a bit of a pain.


Lightening the load is as simple as pulling out the throttle linkage spindle and cutting a 1mm groove at the spot where the return spring seats against the spindle 'quadrant'.


There's plenty of meat in the casting to allow this, and the belt-and-braces push/pull throttle cable arrangement ensures there's no fear of the throttle sticking wide open.


The result is a noticeably lighter throttle that's easier on your wrist at sustained speeds. It's much less tiring and, as an added bonus, the reduced spring resistance gives you better feel and control when you're pottering around pretending you can ride trials.

Don't get all choked up

Choke plate failure is a known issue with XR600Rs, and looking at the stock unit it's not hard to see why some people report the spring and flap leaping into the cylinder, bouncing around causing scarring and, in the worst case scenario, bending valves.


Replacing the stock unit with a 'billet' aftermarket unit is a pretty common modification. 'Billet' is a bit of hype, you can make one from 1mm aluminium sheet – just remember to put some vents in it, the flap on the stocker is there to stop the carb blowing off in the event of a backfire and the holes in the aftermarket units serve the same purpose.


Besides being preventative maintenance, another upside to replacing the stock choke plate is improved air flow. The stock unit is a pretty chunky bit of kit with lots of lumps and bumps. A flat plate unit installed with button-head Allen screws (look at that little smiley face!) smooths things out considerably. So...improved reliability plus free horsepower, what's not to like about this modification.


There are, however, a couple of traps.


The screws holding the stock choke plate are punched to hold them in place – which makes them hard to remove without shearing, and dubious for re-use. Using a hammer and punch on replacements invites a bent shaft, so semi-permanent Loctite (or similar) is the best option.


Chances are that the screws will come straight out without a problem, but if you do shear the heads off don't panic. If you're lucky there'll be enough meat left to get them out with a pair of pliers. If you're unlucky they'll sheer off flush and you'll have to remove the shaft and drill them out.


The chances of drilling into a sheared M3 screw and keeping the threads intact are pretty slim, but if you're very careful you can drill the old screw out and tap the hole for an M4 which will work just fine.


Removing the shaft can create its own problems though. The half-round end meshes with a tiny coiled spring nestled in the carb body (that's what closes the choke). That spring can get dislodged so withdraw and replace the shaft carefully – rotate it gently, don't force it or you'll bend the tangs.


If the spring isn't rotating the shaft to its normal closed position (check where it sits in relation to the choke lever) you're going to have to find a drift (or, realistically, a nail with a flat head) that'll fit through the shaft opening, and punch out the brass plate in the carb body.


A few light taps will reveal the spring and allow you to install the shaft so it meshes with the tang inside the spring while the outside tang sits in the recess in the carb body (see photo). Support the carb body and the outside end of the shaft, check that the spring is in position and causing the shaft to rotate properly, tap the brass cap plate back firmly into place, lightly peen it in position, and you're ready to go.


Well, not quite. Even if you didn't experience any problems with sheared screws take the time to trial fit the new choke plate before you get the Loctite out.


The aftermarket  plate shown here was a 'precision fit' on the shaft (much tighter mounting screw tolerances than the stocker) but at full choke it was binding against the carb body leaving an air gap. Make sure your choke plate is fully choking air flow (hit it with a file as necessary) and not contributing to any cold starting problems.


Don't be put off by thoughts of sheared screws (might never happen) this is a worthwhile modification and, if everything goes smoothly, it's an easy piece of preventative maintenance that could add some horsepower.

Defeat that ACV

Eliminating the ACV is another modification that falls into the preventative maintenance category. Despite what some people will tell you it doesn't necessarily  improve performance.


Instructions to defeat the ACV were included in Honda race kits back as far as first 750/4s (hence the performance myth). The purpose then, as now, was to get rid of a potential failure point and make race-tuning a bit easier.


The ACV 's role is to richen the mix when you snap the throttle off. Doing so reduces the afterburn 'popping' you get with a lean mixture and allows bikes to be jetted leaner from the factory (for whatever purpose).


On the PD Keihin it does that using inlet vacuum (from a hole in the left-hand side of the carb body ahead of the slide) to move the diaphragm and close the button valve to cut air flow (hence Honda's nomenclature) and decrease atomisation in the slow speed circuit.


This all goes wrong when the diaphragm develops a hole (which, given the workout it gets in normal operation, is almost inevitable over time). With the diaphragm breached fuel can be sucked up directly into the carb body at low speeds and idle making you run rich.


On the PD the solution is relatively simple – just block the vacuum inlet.


Honda's 750/4 race kit instruction was to block the hole between the carb body and the ACV cover plate with a ball bearing...but cutting a rubber plug is a more practical (and probably better, if everything seals right) solution.


You need to leave the diaphragm and spring in place – the diaphragm (holed on not) to seal the ACV plate, and the spring to keep the button valve depressed. Bolt it all back up together and the job is done.


So, is it worthwhile? The downside is that you'll get more pops from the exhaust on the over-run (hey, some people like that!). The upside is that you've removed the niggling question of whether the ACV is making your slow-speed jetting rich, avoided potential future failure, and saved yourself a few bucks on a replacement diaphragm.