Throttle bodies

Hi, so yesterday I thought I'd balance the idle air control valves as I've not been happy with how it's running at idle. Is a pain to start when cold, fires then dies until it finally stays running. Also the idle screw is turned all the way out to get a reasonable idle which I thought was strange. The throttle response is snatchy too. Started adjusting them, I couldn't get #1 anywhere near #2 vacuum reading, I think the nearest I could get it was around 5inHg. Also every adjuster had been wound out so they weren't clicking against the indexed points, definitely not right. So I reckoned something must be blocked, or maybe a vacuum leak but I've not found a course yet. Stripped the idle air control valves down, can see the return spring being a right bastard to reassemble but one problem at a time! All seemed cleanish though, valves moving freely. Will chuck them all in the sonic cleaner tank at work on Monday. My concern is the special coating I've read about on here, supposedly being quite important. Well on mine, I have noticed some of the coating has chipped off in places. I just can't see it making a whole lot of difference myself, what do people think? It's definitely lean at idle too, a quick crack of the throttle causes a lean bog. If it was carbed I'd want to turn the air screw in a 1/4 turn. The ends of some of the vacuum pipes have perished at the ends which could be causing some of the symptoms if they're not fully sealing, will change them all anyway. Will clean the injectors as well, I'll see if I can borrow a mates injector cleaner, be able to see what the flow rate is like then too. I'm hoping to ride it to Assen to watch the GP next year so I want to get it right rather than having problems in a different country!

Cheers

Cameron

Hi Cam.

The coating on butterfly edges will flake off at some point, manual states it's an important feature for sealing & never use brake cleaner,  I've cleaned dozens of Honda throttle bodies, some butterflies are coated, some aren't, always used paper towel soaked in brake cleaner, never affected the coating, your's look filthy & defo in need of a good fettle, personally I wouldn't be concerned about that coating, it's coming away anyway, in your case it's probly better not being there at all, Ultrasonic clean might fetch it all off, at least you'll have a clean assembly to work with. 

What air filter are you using? it's not doing a great job, could be contamination from old engine oil mist up thru the crankcase breather into airbox, ideally this ought to be filtered, but it's not in a stock airbox.

Master throttle stop screw is factory set, ensure all 4 butterflies fully close & aren't being held open in anyway, butterflies need to be fully closed during a cold start crank, if they're held open in any way, you'll have issues with a cold start, you should not be operating throttle on a cold start if bikes set up correctly, this is what starter valves are for, they allow an amount air to bypass butterflies, make sure all vac pipes are in good order or you'll get nowhere.

Ensure all starter valves can be wound fully in & out or you'll get nowhere, idle adjuster screw accessed through the right side frame rail affects all 4 starter valves at once, use it as per idle adjust on a carb'd bike (set to approx 1,200rpm on a hot motor).

Once all the above is good, confirm you've no air leaks with brake cleaner squirts to all 4 throttle body- cyl head mount rubbers, spray vac pipes etc.

Run motor up to temp, then adjust starter valves, sequence is shown in manual, many don't understand how to read vac gauges, if you've any doubts, there's a few vids on Youtube that might help.

You've not stated which year bike you're dealing with, early 5th Gen uses a bar mounted/cable for fast idle, late 5th Gen use the crappy wax unit that is problematic, ensure this is not being held open by a sticking wax unit or a tight cable if you've an earlier 800.

Hi Gaz, thanks for the reply. Air filter is a k&n type filter rather than a paper one, can't remember off the top of my head. You're right though, underside of the butterflies are far worse too! That said, it did 45k miles before I bought it, I managed a meager 600 miles since I started using it a year ago! It was a near scraper when I bought it, I made it usable last year but I plan on doing a proper restoration going forward.

I presume you're talking about the throttle stop with a locknut that seems to be factory set? Mine has some paint marker on it that's not been broken so happy that's ok. Butterflies do seem to fully close.

So on mine I have to give it some throttle otherwise it won't stay running, definitely not right I know. The the fast idle with the "choke" lever won't come up until it has been running for a few seconds, before that it'll barely stay running with the throttle closed. And it is an early 99 I belive, pre wax model.

I'll get the new vacuum hoses ordered, injectors and throttle cleaned etc and go from there I think. After a bit of research, I do understand the starter valve adjustment, I can't even get them equal at the moment though.

Another point to mention, I previously thought someone had fitted too short a choke cable, fully closed it held the starter valves open. I actually had to cut a bit of the plastic choke cam off on the bars to allow it to fully close! I'll change it at some point if need be. Always thought this was odd though. 

Not saying this is definitely your issue but on my 6th gen, a couple of starter valves were seized open (in the cold fast idle position) and balancing was really hard. Have a look at the starter valves as you pull the "choke" lever open and closed and make sure they are all freely moving, especially the "unadjustable" one. I had use penetrating oil and some persuasion but got mine moveing freely, and then balancing was normal. Your comment about the idle screw being wound right out suggests the starter valves are all set unnaturally far open, and that is probably also why the choke cable seems wrong (but is probably normal).

That was one of my thoughts but all the plungers are moving freely. I've got them all removed for cleaning at the moment but I don't think I'll achieve anything from that. Nice to know it's clean though lol.

12 hours ago, Cameron

Another point to mention, I previously thought someone had fitted too short a choke cable, fully closed it held the starter valves open. I actually had to cut a bit of the plastic choke cam off on the bars to allow it to fully close! I'll change it at some point if need be. Always thought this was odd though. 

Yeah sort vac hoses, they do rot over time, agree with Terry, starter valves seem to be all screwed up, they're supposed to be coated & run dry, if oiled they might gum up especially using a K&N that filters poorly, they appear to catch a load of dust, in reality they're basically 💩 & let a ton of crap thru into the motor, even when oiled, proven by the filth inside your throttle bodies, I keep advising people about this rubbish, but they've been around that long they're a good seller regardless of the shit filtering & poor fit, a lot bikers fall for the hype, (Sprint or BMC) are a way better quality filter, aint cheap tho.

A while back on another forum I use, we did our own filter/dust trapping tests on the big filter names K&N / PIPERCROSS/ SPRINT / BMC.

Paper filters came top, Sprint & Bmc came second place, even (dry) filter better than K&N oiled, Pipercross or Pipercrap as I call em are just total 💩 fall to bits & go thru your motor,  K&N came out the worst for filtration.... Nuff said!

Yeah UK 98/99's should have a bar mounted fast idle lever affair, it gets called a choke but it's not actually a choke as such.

I've noticed Honda weren't very generous with cable length, even with the low stock clip-ons, when you start fitting bar riser blocks or a conventional bar setup, you quickly run out of cable length.

Eg) I've fitted conventional bars that sit as high as the fairing allows, fast idle cable is only just long enough, stock throttle cables were too short to run the stock route, looked a dogs dinner routed any other way,  my research led me to ST1300 throttle cables which are basically same cables albeit a few inches longer, expensive new, bagged a good pair from an eBay breakers for £15 inc throttle tube, return cable has too much slack, so not a direct transplant, nowt ever is on a VFR ffs, so I McGuyvered it at throttle body end with a cable tie to take out 10mm of excess slack, i'll make an extra split lock nut as a spacer next time I get tank up.

My point is, cables can easily cause you some shit if routed wrongly, too short, kinked, splitting etc, just ensure your's are aren't contributing to your gremlins, I hate a cable slack at throttle, but a tiny bit is necessary so butterflies fully close.

Online Manual clearly shows all loom & cable routing if you wanna compare how yours are laid out.

While air box is out it's a good time to remove "flapper" & "Pair valve" crap, if not done already.

Let us know how you get on 👍

Interesting point about the filter, I don't think it is a k&n but similar. I'll check at some point.

Someone has fitted heli bars at some point, I wonder if the choke cable has been changed at some point for a longer one but with too short an inner as such, could be the reason for that. Pretty sure everything is routed as it should be, I'm a bit ocd with stuff like that!

Pair valve I was thinking about, they're always so untidy! I had a xj900 a while ago and the amount of plumbing on that for the pair valve was ridiculous! The flapper valve I was probably going to leave, I was under the impression it gave a slight low rpm boost, bit like a power valve I suppose but on the inlet. Does it not do an awful lot then?

Been reading more into the coating on the throttle bodies, apparently they're used in all sorts of honda bikes and cars and seem to be more critical than I expected. I wonder if that could be the cause of the irregular vacuum readings and needing to have the idle speed screw wound fully out? Has anyone used any particular product to recoat them? Apparently molykote 321 is the stuff to use so I've read? Has to be worth a go I reckon.

Cameron, 11/9/2025 at 12:52 PM

Interesting point about the filter, I don't think it is a k&n but similar. I'll check at some point.

Someone has fitted heli bars at some point, I wonder if the choke cable has been changed at some point for a longer one but with too short an inner as such, could be the reason for that. Pretty sure everything is routed as it should be, I'm a bit ocd with stuff like that!

Pair valve I was thinking about, they're always so untidy! I had a xj900 a while ago and the amount of plumbing on that for the pair valve was ridiculous! The flapper valve I was probably going to leave, I was under the impression it gave a slight low rpm boost, bit like a power valve I suppose but on the inlet. Does it not do an awful lot then?

You should see a Gen 1 Fazer 1000's "Pair valve setup, proper dogs dinner/octupus pipework mishmash, nightmare to get at plugs, mine's long since been junked & blanked off.

Genuine K&N's have their emblem cast into the silicone section, no emblem, its a Chinesium copy.

If you're lucky, you've got a sprint filter, strongly doubt it by how much filth's got passed the filter.

If you can get on with the Helibars, keep em, they're silly money, scrutinise the welds with a magnifying glass as they're know to crack if bikes been over scuffed clutch cover fins, bike's defo been over on right side.

With Helibars you might need longer cables, stock cables weren't any use when I fitted a conventional bar setup, never had Heli-bars, but they do look pretty tall.

Re: (Pair valve system) all of it can be junked, blanked off, cap off reed valves stubs or fit new CNC covers, I've a contact that machines em up & sells em on eBay, cheap too.

Flapper does naff all, bike will run better without it.

They all run lumpy & a bit woolly below 3,000rpm even when everything's correctly setup, uneven firing order etc,  it'll run better with decibel killer removed if using a sports slip-on can, it's an over square motor, loves to be revved & happier at higher rpm, Iridium plugs if funds allow, check gaps, plugs get their earth from cyl head thread, use only a tiny smear of anti-seize on plugs, Halfrauds usually sell em discounted in a sale (NGK CR9 EHIX-9)

Leave snorkel in situ, it'll run better.

Check rubber throttle body mounting rubbers are still supple enough to actually form an air tight seal when clamped up, could be dragging air in?

Tourmax are my go too brand, same spec as OEM at a fraction of the price.

So if the starter valves are all moving freely but the starter valves are all slackened off (closed valves, no tension on the adjusting nut) then you must have a significant source of air from somewhere else. Maybe it is obvious but is there slack in the throttle cable so this is not holding the throttles open? I also had a brain-fart once and got a vacuum hose trapped in the fixed throttle stop under the throttle body (underside, right front) that stopped the butterflies fully closing after I replaced a thermostat.

Otherwise, I'd be looking for some significant vacuum leaks in the hoses. Or maybe the rubber boots on the inlets have got cracks?

I recall that you should be able to seal off the throttle bore with your hand and have no air passing through the butterfly, as all should be entering via the starter valve. 

Wasn't a k&n filter, it's a paper hiflo filter! A lot of dirt has got passed though, not necessarily past that filter mind but over the years. dB killer will have to stay, I borrowed it to use of my aprilia on a track day once, rode the honda and it was deafening! Not a huge fan of it tbf, right at the bottom of the list that though lol. Someone's had the snorkel off, I might get one actually. Again, down the bottom of the list lol. The manifold rubbers seem ok, bit hard and the clamps are rusty. Don't think they were leaking though. 

The butterflies were definitely shut, did make sure of that. I should have covered the throttle bodies before I stripped it apart to see if that affected the running, if it stalled I'd know it was definitely allowing a significant amount of air through. 

Update for today, the ultrasonic cleaner got them very clean, so clean it took the rest of the coating off the bores! I'll have to try and recoat it with something, I think I'll try some of this molykote stuff and see what sort of finish I get. I'm reluctant to buy new throttle bodies as they're all old now and there's no guarantee the replacement will be much better than they were,  before I stripped it all off anyway![image: 20251110_204426.thumb.jpg.22eb19c9f0d7f7f89d6f1a2a96f179a0.jpg]

Plenty of throttle bodies don't have any kind of coating on the trailing edge, seems to be a Honda thing, it's bound to come off at some point, seems the coating is Honda's attempt to minimise butterfly/bore wear, never known anyone replace it after it's come adrift, personally I'd leave butterflies clean & fuel test em, see if they'll hold fuel which will confirm if stop screw needs a tweak, don't be afraid to move it if necessary, it's only a throttle stop screw, butterflies only allow air thru not fuel.

Bare in mind everything that moves wears, now coatings missing, factory set throttle stop won't likely be set as it once was, stop screw may need a readjust to compensate.

Just to follow up on this thread, I sprayed some of that molykote on the throttle body bores and butterflies, was just enough to stop any daylight getting through, whether it makes a blind bit of difference, who knows? Vacuum hoses all replaced, I'm pretty sure that was the problem tbf. I was planning on leaving flapper in place but I broke part of the solenoid getting a hose back off to reroute it so it's all been removed and port blanked off at the throttle body. It's a lot tidier and the air box is much easier to fit now, so that's a definite bonus! Will order some pair valve blanks soon too.

I did spend many hours trying to work out why it wouldn't run properly and backfiring after I put it back together, had 3 and 4 injectors round the wrong way! Balanced the idle air control valves, looking forward to riding it now, feels much smoother straight away.

Interesting read. I am removing my 2000 throttle bodies to check operation and clean the injectors as the plugs looked dry after trying to start the engine. I have tested the pressure and flow of the fuel pump and it looked good though the test was suppose to be with the engine runnning. I agree the throttle butterfly coating will come off on all the older bikes sooner or later. The 2000 is the same but slightly less than yours.

For those that don't know it is recommended to change out the 3 rubber parts per injector (seals and o-rings) if removed. OEM are available.

If needed I have found that 1999 to 2001 Honda CR-V injectors have the same part number 06164-PCA-000. A set ofHonda OEM injectors will setyou back around $300 cad. The part number can also be found on Amazon and assume they are  cheap chinese knock offs or other aftermarket injectors for $100 a set.

I have two 1998's and a 2000. The 2000 is the only one not running at the moment. I really need to figure out which one to keep. At this point I think it will be a 98. Lots of options.

Hi. If you made just a simple passage in ultrasonic washing machine, the special coat have no damages (i did it some time ago and i had not any kind of problem).

I just suggest to reassembly all throttle body and balance it without try to reach a specified value of vacuum for all 4 cylinders. It's enough that you hear the engine running "round and smooth". The vacuum values are affected by many other factors (as valve clearance, for example). If all other components are not in perfect conditions it's almost impossible reach the vacuum value (theorically) indicated. Work on screws commanding the valvle util you see on vacuometer the all 4 cilinder working at same level. Any other work are unuseful and you risk to go crazy. If you have some difficult to reassembly the thottle body, somewhere in this forum you can download the Service Manual, where everything are explained. 

A set of new aftermarket injectors (made in Japan) with all new rubber o-ring/cushion/seal are cheaper than buying a set of o-rings/cushion/seals seperately.

Thats crazy!