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Old November 14th, 2015, 04:17 PM   #1
bob706
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Rpm real slow to come back down after giving it some throttle

Just got finished doing a valve shim adjustment and lo and behold the bike started up! With my luck I thought it would explode . Got it warmed up and adjusted the rpm to a little over a grand. When I goosed the throttle to about 4 or 5k it took several seconds for it to come back down and it stopped at about 2k. I proceeded to chase my idle but would hang high after blipping it or plunge to nothing. My main needles were filthy and the whole carb was cleaned. While it was apart I added 1 washer per side for a total of 3 per side. Also have not synced the carbs yet. I plan on going back to 2 washers and syncing but can there be another reason for the rpm to hang up? Thanks for any advice
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Old November 14th, 2015, 04:50 PM   #2
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Vacuum leak? Carburetors not thoroughly clean and/or adjusted? How did you clean them? Any other modifications?

If your gonna sync the carburetors I recommend the bench synchronization method, as the vacuum method has it faults, and it's not a true sync.

My write-up on the subject.


You can sync with the carbs in hand. Back the idle adjustment off to close that carb then match the second carb to it with the sync screw. You can then turn the idle adjustment up while using a thin feeler gauge (maybe .002") between the throttle plate and the throttle bore, adjust to just fit, and then check the second one and adjust to match. If you have good feel for this it will run fine once installed.

It would do folks well to take a minute to review and understand what a sync tool does, how it works. And take another minute to look at the carb linkage and sync adjustments available to fix irregularities that might occur. It can help take some of the mystery out of this as well as stop unnecessary carb sync screwups.

Meaning, that the carbs were right once, the butterflies were matched and it performed well, but then changed. Undecided And the problem somehow occurred in the butterfly direct shafts, or the sync screw turned or wore that pad that it rests on, or the carburetor pairing/ bridging brackets twisted in relation to each other, and enough so that the butterflies no longer operated in sync, such that performance noticeably suffered. And a fix will be accomplished by changing the butterfly position, the only thing the sync adjustment does. Hmmm. Really? Undecided

Meanwhile, the tool measures vacuum at each individual runner. That's all it does, how it works. And vacuum is effected by a huge list of things, butterfly position being only one of them, and the only one addressed with that tool while turning those sync screws. The rest of that list includes, valve adjustment, jetting, float level, compression differences between cylinders caused by wear as well as factory CC differences in the head, intake runner, as well as flow by port variations, etc. Variations in cams, wear or factory tolerance. Then there's the potential big one, pilot jets and pilot screw mixture settings. All of those effect the vacuum and will be read by the sync tool, accurately identifyng the difference that exists, and with the only adjustment being used to "correct" the problem, move the butterfly relationship, one to the next.

It's that clear understanding that has me of the opinion and long time practice to not touch the sync screws on a set of carbs that were right once and were never dismantled from the rack. And jet cleaning as well as float needle changing doesn't require that they be dismantled. And further, if I DID dismantle a set of carbs, I just matched the butterflies on the bench at assembly, using a feeler gauge, and then never touched them again, never gave them another thought.

That method, properly executed, will have THAT portion of the complete equation, balanced flow to each cylinder in a multi-cylinder/ carb application due to butterfly position, satisfied entirely adequately. And in fact, it has worked for me every time I've done it on every bike (as well as individual runner/ butterfly V-8 intakes, all eight) that I've ever owned or any that I've fixed for others. And the times I fixed for others was usually after an attempt by others to sync the carbs, chasing a problem, or sometimes not even chasing a problem, but one they created wth a sync tool, chasing a problem from that list, that was never a butterfly position change problem. Those linkages are so direct and simple that they don't know how to screw themselves up. Not enough to care about.

I'm sure this will start a lot of controversy so I won't argue it, just offering it for those who understand the whole picture that I outlined and might make good use of it. It is what I've done, on every engine I ever worked on, including blue printed racing engines as well as regular old, high mileage street stuff. I've had at least four, four cylinder bikes with over 100,000 miles that ran quite well, the whole time, and never had a carb sync performed. When they did finally get a total disassembly, this is how I set them up, on the bench. Over the years I've straightened out at least a half dozen messed up ones, probably more, and on the bench, after fixing the original problem which was fouled pilot jets.

I only posted this because it seems like its coming up fairly often, especially with problems from a sync gone bad. One I would suggest was never a sync problem to begin with. A big or sudden change in performance is NEVER a sync problem. NEVER. Remember that. Cool If there is a problem, and you connect the gauges? YES, there will definitely be an imbalance indicated. But because that hole has a problem that is from the rest of the list. A bad plug, a plugged pilot jet, a screwed up float, a bad valve or setting, etc. Every one of them effecting the vacuum in the intake but NOT from a sudden move of a butterfly position, a sync adjustment.
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Old November 14th, 2015, 05:56 PM   #3
Yow Ling
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Too lean, look for a leak. maybe richen up your idle circuit
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Old November 14th, 2015, 09:16 PM   #4
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Quote:
Originally Posted by Yow Ling View Post
Too lean, look for a leak. maybe richen up your idle circuit
I don't believe it's lean. I went from 2 to 3 washers under each needle. I did this because i installed a k&n filter. Also sprayed wd40 around all vacuum lines and boots with no results.
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Old November 14th, 2015, 09:34 PM   #5
Yow Ling
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if you get a butane or propane torch and feed some gas into the airboxwhile it is running give it a rev and see if it settles quicker , this will confirm if you have a lean condition.
Im not familiar with CV carbs , on regular flat or round slide carbs the needle doesnt do much at idle so any effective adjustment is normally done with the airscrews, which could possibly be sealed on some US engines
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Old November 15th, 2015, 07:10 AM   #6
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MOTM - Jun '18, Oct '16
What did you set the idle mixture screws at? 2.5 turns out is a good starting point.

If the carbs were dirty, the idle mixture circuit may be partially clogged unless you cleaned it very thoroughly.

Shimming the needles has minimal effect on the idle mixture. 3 washers is a lot.

As noted, what you are seeing is typical of a lean condition/vacuum leak.
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