1971 rebuild won't idle down

boxofparts · Aug 15, 2015

Hello, my name is Justin. I am currently finishing a 1971 roadster and am having issues getting the bike idle down after being revved. The bike starts and idles great but when I hold the throttle for 2-3 seconds the bike revs for 8-12 seconds. Originally, this was only occurring when the bike was warm but after rebuilding the carb it started to happen even with the bike cold. It is worth mentioning that the bike has less than a 100 miles on it since its rebuild.

My first thought was that I had an air leak but I have had no luck finding one. I have tried spraying the motor down with both carb cleaner and starter fluid when running but I didn't have any change in RPMs. Next, I tried rebuilding the carb (single VM34) but it made the issue worse. I believe I may have made a mistake reassembling the floats which I will double check tomorrow. Any suggestions on what to try next?

oldmikew · Aug 15, 2015

make sure you do not have a sticking throttle cable or cables

boxofparts · Aug 15, 2015

oldmikew said:
make sure you do not have a sticking throttle cable or cables

Cable was checked and replaced when I took the carb off to rebuild it. I have also checked and made sure the slide closes all the way after pulling the throttle.

Rohan · Aug 15, 2015

If the auto advance unit is sticking, and staying fully advanced, then it might do this.

Best way to check this is with a strobe light, and the little plug in the primary case that lets you see the degree scale,
and the timing mark on the alternator rotor.
Weak or worn little springs or lack of lube can be the culprits.
Hopethishelps.

boxofparts · Aug 15, 2015

Rohan said:
If the auto advance unit is sticking, and staying fully advanced, then it might do this.

Best way to check this is with a strobe light, and the little plug in the primary case that lets you see the degree scale,
and the timing mark on the alternator rotor.
Weak or worn little springs or lack of lube can be the culprits.
Hopethishelps.

My bike is currently fitted with a Boyer ignition which I believe replaced the mechanical advancing unit.

Snorton74 · Aug 15, 2015

boxofparts said:
oldmikew said:

make sure you do not have a sticking throttle cable or cables

Click to expand...

Cable was checked and replaced when I took the carb off to rebuild it. I have also checked and made sure the slide closes all the way after pulling the throttle.

A little cable lube goes a long way to reducing friction with the housing. Mine wasn't returning cleanly until I lubed it.

Rohan · Aug 15, 2015

boxofparts said:
My bike is currently fitted with a Boyer ignition which I believe replaced the mechanical advancing unit.

Have you strobed it to verify that the ignition is indeed retarding at lower rpms ?

Throttle cable sounds a strong possibility, but after that still be worth checking.
You can usually feel if the throttle cable isn't dropping the slides fully, doesn't feel right.

concours · Aug 15, 2015

Sounds like an inoperative idle circuit, idle speed screw turned in enough begin running on the needle. Enrichener plunger seal in good shape?

boxofparts · Aug 16, 2015

Rohan said:
boxofparts said:

My bike is currently fitted with a Boyer ignition which I believe replaced the mechanical advancing unit.

Click to expand...

Have you strobed it to verify that the ignition is indeed retarding at lower rpms ?

When I timed the bike a few weeks ago it was but I didn't have this issue then so I will double check.

concours said:
Sounds like an inoperative idle circuit, idle speed screw turned in enough begin running on the needle. Enrichener plunger seal in good shape?

I went back and looked through the Mikuni's manual. Is the enrichener plunger part of the choke starter system?

L.A.B. · Aug 16, 2015

boxofparts said:
I went back and looked through the Mikuni's manual. Is the enrichener plunger part of the choke starter system?

It is. If the rubber seal has degraded or detached itself from the plunger or the plunger isn't fully seating in the "choke off" position then it could cause a high idle.

boxofparts · Aug 16, 2015

concours said:
Sounds like an inoperative idle circuit, idle speed screw turned in enough begin running on the needle. Enrichener plunger seal in good shape?

I rebuilt the carb again this morning paying a little bit of extra attention to the idle circuit and the problem seems to be gone. My only concern now is how it got to be so dirty that the bike didn't run correctly in such a short amount of riding (maybe 30 miles). Could my fiberglass tank be causing this even with me running a fuel filter? Then again maybe it was the seal on my plunger and I am overthinking this.

L.A.B. said:
It is. If the rubber seal has degraded or detached itself from the plunger or the plunger isn't fully seating in the "choke off" position then it could cause a high idle.

I did not see any issues but after being cleaned and reassembled the bike returns to idle almost immediately.

DogT · Aug 16, 2015

I hope you're not using ethanol in a fiberglass tank. Some people have luck doing that and others make a gummy mess. I use avgas to be on the safe side.

MikeM · Aug 16, 2015

DogT said:
I hope you're not using ethanol in a fiberglass tank. Some people have luck doing that and others make a gummy mess. I use avgas to be on the safe side.

Some of us have no choice. There is no place to get non ethanol. I suppose race gas or av gas but its certainly not a convenient option. I drain my tank and dry it out after every ride. So far, its not been an issue.
I do have a steel tank building up in my change bucket. I am about 1/2 way there.

acadian · Aug 17, 2015

quick way to test if it is a slide return issue is to duplicate the problem with the air cleaner off, then you can quickly check to see if the slides are indeed returning. If not, could be that the cables are binding when you fit the tank. The cable junction block can be a problem area, especially if the cable housings are not fully seated in the junction block. Sometimes the single cable (from the twist grip) will pull out from the junction block seat and get hung up on the lip (hard to visualize but if you look at the setup it will make sense). I solved the problem by ensuring the cables are properly seated in the block, and wrapping the whole thing with duct tape.

dynodave · Aug 17, 2015

Been gone for a week so first view of this thread.
Boyer to me.... automatically means too much advance at low speed/RPM, since they call for timing (sic) at 5000 rpm .
advanced low speed timing makes higher vacuum @ idle,
more vacuum makes you close the slide to make the engine throttle down the RPM.
more vacuum makes the slide stick (suction) in the body.
Very old problem and symptoms. Known for easily 20 years or more.
Cure has traditionally been a stronger spring or adding a amal spring inside the MIK spring.
Dragging cable only compounds the problem.
Sorry no first hand experience on MY bikes as I don't care to use single MIK on my bikes.
And I'm not claiming Mikuni is a bad carb just that an old analog boyer is a huge compromise for an ignition due to it's crappy advance curve.
Yet you could have an entirely different problem....?
IMO...As DogT suggests ethanol dissolving the polyester resin or the epoxy fuel proofer/liner is a strong second possibility.

boxofparts · Aug 17, 2015

The problem is back and only occurring after the motor has been running for 20+ minutes.

dynodave said:
Boyer to me.... automatically means too much advance at low speed/RPM, since they call for timing (sic) at 5000 rpm .
advanced low speed timing makes higher vacuum @ idle,
more vacuum makes you close the slide to make the engine throttle down the RPM.
more vacuum makes the slide stick (suction) in the body.
Cure has traditionally been a stronger spring or adding a amal spring inside the MIK spring.

Thank you, I will try that.

Rohan · Aug 18, 2015

If it only occurs after 20+ minutes, it sounds like its a fuel feed problem - or maybe heat related ?
Checked that the little breather hole in the fuel cap is clear and unobstructed.

Checked that you have good fuel flow to the carb. ??
Maybe pull some choke on, and see if it improves it, or splutters.
If it helps, the fuel level is low, and needs checking for obstructions or blockages.
Fibreglass in the fuel ??

dynodave · Aug 18, 2015

If it only happens when warm???
1. If you run thick oil
2. Boyer analog, they all have rapid advance off idle.... even more the lower it goes 500-700rpm.
3 They almost always speed up as the oil warms up it is a regenerative thing, ,,,,,,more speed more advance, more advance more speed etc.
If this is the boyer analog (rita too!) advance curve effect it can almost instantly be confirmed by substituting the old AA system which will behave much better.
Most of the time a boyer analog (& rita), if it can idle cold will speed up rather high when warm. Normally I have to hand hold the throttle until it warms up a bit or it will stall.
While not desirable, I consider it normal for this set of hardware.

1971 rebuild won't idle down

boxofparts

oldmikew

boxofparts

Rohan

boxofparts

Snorton74

Rohan

concours

boxofparts

L.A.B.

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boxofparts

DogT

MikeM

acadian

dynodave

boxofparts

Rohan

dynodave

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