engine rev,s with throttle closed

[derrickjeff]

Hi guy,s.
just finishedthe latest build of mine.Ihad a 2 into 1 inlet manifold made,and fitted a mikuni vm 36 carb.It run,s great on tick over,but when you rev it and let the throttle off the rev,s stay high and only drop down again after a couple of minutes.I have tried moving the needle up and down,and adjusting the mixture screw.

 

[fredintoon]

Hi derrick,
next thing I'd check is the throttle cable.
Take off the air cleaner so you can see up the carb's entry, then rev up and let go of the twistgrip.
The carb slide should slam shut right away but from what you say, yours don't.

 

[TwoManyXS1Bs]

Higher than normal manifold vacuum from the 2-into-1 may be causing the slide to hang near the bottom. A small angled pickpoint can be used to pull the bottom of the slide back to help it settle. If that works, may want to consider:
- Check for burr or interference near bottom of slide travel.
- Light chamfer of lower edge of slide, on intake manifold side.
- Polishing slide's intake manifold side.
- Stronger return spring.
- Excess drag in throttle cable.


Watch rotor timing marks with timing light. See if it's holding a bit of advance during this high idle, then retards a bit after it settles.

During the high idle, momentarily put your finger over the
- Main airjet port, note any change.
- Idle airjet port, note any change.

 

[Brew]

cable is getting hung up or you are sucking air from somewhere. remove the air cleaners so you can watch the slides

 

[LuckyQXS650]

I have a similar set up,mine was the trhrottle cable,switched from the "one into two" junction with one side taken out to a dedicated single throttle cable,made a night and day difference in response as well.

 

[Gibson]

Check advance mechanism, oil it and check springs. If advance is sticking it will cause the engine to hang and not return to idle.

 

[jonpals]

How about a vacuum leak? Try spraying starter-fluid on the intake manifold at idle, if the rpms rise you have a vacuum leak,.

 

[derrickjeff]

thanks for the help,the manifold had a hole in it,so it was sucking air.the throttle slide was sticking,so fitted a stronger spring,and re jetted the carb.now running a lot better.