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Discussion Starter #1 (Edited)
Gentlemen,

Following on from this original thread:

https://www.hondatwins.net/forums/56-fuel-supply-carburation/56...setting-2.html

and a more recent one looking for help on pilot jet assembly, I suspect there's something still not right with the whole pilot jet/idle circuitry. I was wondering whether you might give me pointers as to where to go next ?

General:

The bike has a standard K5 engine, with the original air-box and exhaust. I rebuilt the engine to sort low compression issues a couple of years ago. After numerous teething problems (down to my own inexperience and stupidity), it has been fairly usable after the initial period and has put up perhaps about 1000 miles with no major problems since. The carbs were stripped and cleaned a couple of times (by me, with a little home ultrasound cleaner) at this stage as part of the initial troubleshooting. The tappets, timing, cam-chain tension have been checked recently and are within spec.

The problem/symptoms:

- The bike starts and runs reasonably well. It will pull strongly above 4k and rev out cleanly.
- There is, however, a noticeable flat spot when you open the throttle - either quickly or slowly - from 2.5k.
- Once you are through this, the bike pulls and revs cleanly.

Other symptoms:

- Opening one throttle at a time confirms the right hand cylinder is slower to rev, and prone to stuttering, the left is fine.
- When trying to adjust the mixture screw setting as per the recommended procedure, I notice that whilst the left works as you would expect - the right hand one has negligible effect until it is almost fully in, and then the revs rise progressively over the last 1/2 turn until it is fully home.

Where Im at at the moment - rightly or wrongly - is assuming that the low-speed flat spot and the mixture screw setting difficulties are connected. I suspect that there's an outside chance the type of mixture screws I have might also be the wrong profile for the right hand carb, based on the fact they dont seem to do anything until they almost seal the circuit in its entirety.

At a bit of a loss as to where to go next. Any pointers greatly appreciated.

Thanks, Alan
 

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How do the spark plugs compare, soot-wise? The flat spot and pilot screw symptoms say rich on that side, but it could also be that the idle is set higher (out of sync), and the needle jet is contributing more fuel, so the pilot setting is set to less.

The flat spot may be to a too-full bowl, or another path making the mixture a bit rich, like a loose jet. Basically, when you open the throttle, a CV carb is temporarily richer, as the air velocity over the needle/main jet is higher until after the piston lifts, causing more fuel to pull through the jets until the velocity drops to 'normal'. If the carbs are out of sync, then the left one will pass through this rich zone more than the right, and make for an imbalance. So, verify your idle and throttle sync are very close, before chasing mixture issues.

You may also have a weak spark plug or spark on that side, and the inrush of air is blowing the spark out. Once the sync and mixture is checked, this would be next on my list.
 

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Discussion Starter #3 (Edited)
How do the spark plugs compare, soot-wise? The flat spot and pilot screw symptoms say rich on that side, but it could also be that the idle is set higher (out of sync), and the needle jet is contributing more fuel, so the pilot setting is set to less.

The flat spot may be to a too-full bowl, or another path making the mixture a bit rich, like a loose jet. Basically, when you open the throttle, a CV carb is temporarily richer, as the air velocity over the needle/main jet is higher until after the piston lifts, causing more fuel to pull through the jets until the velocity drops to 'normal'. If the carbs are out of sync, then the left one will pass through this rich zone more than the right, and make for an imbalance. So, verify your idle and throttle sync are very close, before chasing mixture issues.

You may also have a weak spark plug or spark on that side, and the inrush of air is blowing the spark out. Once the sync and mixture is checked, this would be next on my list.
Thanks WintrSol. I always find your explanations educational and enlightening.

Had only time for a short run, so a lot of the plug colour is indicative of that - but there definitely seems to be an imbalance, with things running richer on the right:



That ties up with the smell, smelt like it has been running with the choke permanently on.

From here - logical next step would be to remove the carb, check the float height, pull the jets (and squirt some more cleaner through them as I have the carb off anyway) - and then check, reinstall and tighten all of the jets ?



edit: One other thought strikes me: if the piston were slower to rise on this RH carb than the other, that too might give rise to that transient over-fuelling situation you describe. I should perhaps give the carb a through cleaning throughout, and do the 'straw test' procedure to cover that off as well.
 

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I would not remove the carb to check the float level, but instead, raise the float with a float gauge and open the petcock. If fuel flows, the level is clearly wrong; if it just moistens the float valve, it's good.
Yes, a sticking piston could enrich the mixture, especially during transient operation. The straw test allows you to roughly compare both sides, to see that similar pressures raise the pistons similar amounts, and at about the same rate. It could just be the piston cover gasket rubbing on it, delaying the rise just enough.
 

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I'll second the straw test as well. The barrel in the carb where the piston travels has to be spotless and smooth. It doesn't take much for it to bind up the piston.
 

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Discussion Starter #6
Thanks guys.

I would not remove the carb to check the float level, but instead, raise the float with a float gauge and open the petcock. If fuel flows, the level is clearly wrong; if it just moistens the float valve, it's good.
Yes, a sticking piston could enrich the mixture, especially during transient operation. The straw test allows you to roughly compare both sides, to see that similar pressures raise the pistons similar amounts, and at about the same rate. It could just be the piston cover gasket rubbing on it, delaying the rise just enough.
Ill check the float levels that way - a lot easier :)

I have a sneaking suspicion the only time I attempted the straw test, both pistons rose, but not at the same rate. If the float levels check out ok, Ill pull the carbs and see what I can find.
 

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Discussion Starter #7
So, with a little time free this afternoon, I removed the float bowls to see what the respective fuel levels were like in the right and left carbs:



The one at the top is off the right hand cylinder, which is the one with the stutter c.2.5k, and the plug showing signs of running rich.

Not a great photo, and not a very accurate test either, but the fuel level in the RHS bowl is not higher than the other - and if anything is even a little lower.

From here, I think it's reasonable to assume that the signs of rich running is not down to the float level on the right.

Based on this, it seems to me the more likely suspects at this stage are bad spark/poor burning on the right - or the piston being slow to raise.

Qn: Is this a fair assumption - and if so, what's the best next step ?
 

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The mixture screws are only in effect in the idle circuit. As soon as you crack the throttle, they are not in play anymore. At 2500 rpm the main jets take over and are the main contributor to the fuel mixture. At around 4000 rpm, the slide will come in play and will lift to open the main jet.

Have you checked or synchronized the carbs? This should be verified first. There are two things to check during synchronization; 1) make sure the idle screws are producing the same level of vacuum on your gages and 2) make sure that the vacuum rises the same on each side when you crack the throttle. Any imbalance in the above will greatly impact the performance.

It looks like you have one good running cylinder and the other one is playing catch-up, but it is not winning!

Attached is a diagram to help understand the workings of these carbs. View attachment 450 carb.pdf
 

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Discussion Starter #9
Yeah it definitely feels like it's playing catch up as the revs rise.

I've studied that carb diagram ad-nauseum. Interesting - but probably raises more questions than answers at the moment.

And there's no vacuum gauge facility with the 450 alas.. That would much too easy .. :)
 

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Yeah, the 500's have them built into the carbs. I've seen guys drill and tap into the carb manifold. Easier said than done.
 

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Yeah it definitely feels like it's playing catch up as the revs rise.

I've studied that carb diagram ad-nauseum. Interesting - but probably raises more questions than answers at the moment.

And there's no vacuum gauge facility with the 450 alas.. That would much too easy .. :)
I added vacuum ports to a spare set of carb-to-cylinder boots; others have drilled into the intake ports to add them. It is fairly easy to swap the boots out, once you have the vacuum matched, as long as you lock the cables down well enough. Idle vacuum sync is less important than open throttle sync, IMO, since a little stumble at idle is no big deal.
 

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Discussion Starter #12 (Edited)
Well it's been a long and (mostly) productive day with the 450.

Started out with a bit of reflection over a cup of morning coffee. Wanted to try to determine for sure if the issues were with the ignition, fuelling, or perhaps something in the mechanicals peculiar to the right cylinder.
Decided to swop the carbs left to right first - to determine if it was the carb or one of the other two areas,

Biggest pain (as usual) was draining the tank to remove the balancer pipe and be able to lift the tank off. After that, it was fairly straightforward to remove the carbs and linkages, and bolt them in to the opposite sides




Next half hour was spent juggling with the two now separate carb linkages, to get it to start and then fiddling with the throttle cable tension and idle screws to stop the thing revving sky high and bring things back to normal. Kind of awkward when all of the adjustment screws are stuck in between the carbs when you swop them over - but hey ho, must needs :)

Once the bike was ticking over properly, a quick check revealed that the problems had in fact moved over with the carb. Stuttering when the throttle on that side was blipped, and the air-screw doing nothing until it was all the way in. At that point I decided to pull the problem carb, strip and clean everything on it , and then put both carbs back in their original positions as the electrics and mechanicals seemed to be working ok on either side with the better carb. There was some light black staining on the inside of the problem carb, possibly contributing to a bit of stickiness on the throttle plate - especially just as it closed over the two little air-holes coming up from the idle circuit. Anyway - plenty of carb cleaner and half the wife's jar of cotton-buds later, it was deemed fit for service. Only other thing of note was that one of the screws holding on the carb top had rung. No way to retap it, so I just cut a similar threaded bolt to length, and used a nut underneath. Might have been causing a bit of a drop in the vacuum under the piston, or perhaps allowed a little twist causing the piston to stick a little - or alternatively mightn't have been an issue at all - hard to say for sure.

All back together (well almost) :



The old original paper filters could probably do with a refresh at this stage, but one thing at a time..

Couple of test spins and some fiddling with settings later .. and.. the stuttering seems to have gone. No hesitation between 2500-3000 revs, and you can blip either side individually with no noticeable difference between the two :) Don't seem to have made any progress with getting the airscrew to function normally on the right as per before, but since the bike is running better, Im gonna park that one for now.

I guess you could call that some kind of a result. Only problem is that with both sides running reasonably well now, Id like to get stuck in and really get the whole thing running sweet as a nut. I'm beginning to think these old Hondas are a never-ending challenge :cool:
 

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Discussion Starter #14 (Edited)
Ah, you've caught on.
:p:p

Bill - or indeed anyone else who knows the answer - could I ask a favour:

I've been trying to find the measurements for your original credit-card fuel gauge, but although there seem to be countless references to it on here, I haven't been able to find one with the actual dimensions anywhere:

- could someone point me in the right direction please ?
 

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I've made a few just by using the picture of one in my fsm as a guide. I think the first one I made sat on the rim and then I realized it should measure off the gasket surface. Did you check that the throttle plate is attached so it closes squarely? Did you try backing off the adjustment screw so it closes a bit? If it's open too much the vacuum signal may be weak. I suppose if the left side is adjusted strong, it's doing the work and right side adjustments won't have much affect until they overcome the left.
 

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Float gauge

Just cut the plastic or metal to the legs span to the sides of the carb, and the step marked '20' holds the float up. The deeper part has to clear the main jet:
floattool20.jpg
 

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Discussion Starter #17 (Edited)
Thanks guys. I get the gist of the template :)

I've made a few just by using the picture of one in my fsm as a guide. I think the first one I made sat on the rim and then I realized it should measure off the gasket surface. Did you check that the throttle plate is attached so it closes squarely? Did you try backing off the adjustment screw so it closes a bit? If it's open too much the vacuum signal may be weak. I suppose if the left side is adjusted strong, it's doing the work and right side adjustments won't have much affect until they overcome the left.
Yes visually it looks as if the throttle plate is centred on the bore and closes properly - but based on what's happening at the moment, it may well be that it's not closing properly, and allowing more fuel in through the little transfer ports
 

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What number is stamped into the jet? If the wrong jet is in, or it isn't seated properly, the mixture will be too rich, and possibly cause the pilot screw problem. The pilot air jet could be partially blocked, too, making for almost all fuel being drawn in.
 

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:p:p

Bill - or indeed anyone else who knows the answer - could I ask a favour:

I've been trying to find the measurements for your original credit-card fuel gauge, but although there seem to be countless references to it on here, I haven't been able to find one with the actual dimensions anywhere:

- could someone point me in the right direction please ?
Her ya' go........
 

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Discussion Starter #20 (Edited)
Her ya' go........
Thanks Bill & WintrSol.

Still trying to get my head round how the idle circuit works in those CV carbs.

I think the sequence is as follows:

(i) When the butterfly / throttle plate is fully closed, the cylinder ticks over on the fuel supplied through the mixture screw circuit alone - and the air fed solely through the single brass tipped hole you can see at the six o'clock position at the rear of the carb inlet

then ..

(ii) When the throttle plate is opened a fraction, allowing some air to start flowing through the main Venturi, in order to cope with the extra airflow the idle circuit is then supplemented by fuel coming through the two little holes in the base of the Venturi, previously hidden behind the throttle plate.

(iii) As the throttle plate is progressively opened, the piston lifts the main needle off its stop, and additional fuel is then also added to the main Venturi airstream through the main jet.


Is this correct, is that how they're supposed to work ?
 
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