High Idle (1 Viewer)

[Catching Neutral]

My 40 started idleing high yesterday. This was after a cold start. Pushed the choke in and it didn’t return to the normal 750-800rpms. I haven’t been able to put the timing gun on it to see how high the RPMs are. I’m guessing 1250ish. I checked the choke mechanism to make sure it wasn’t sticking a bit even with choke all the way in. I lubricated the choke mechanism on the carb. It appears to be returning to closed position. Also checked all vacuum hoses. All connected and intact.
I plan on adjusting the carb as soon as I have some spare time, possibly Tuesday.
Anything else I should be considering in the diagnosis?

 

[duncanrm]

A vacuum/intake manifold gasket leak. Visual inspection of hoses isnt enough.

 

[1MAC]

Put a vacuum gauge on it or spray carb cleaner on the hoses/connections until you find the leak.

 

[cbmontgo]

Double check that linkage.

 

[Catching Neutral]

@cbmontgo …linkage is good. Timing gun showed 1400 RPMs. Carb cleaner under the carb heat shield yielded an drop in RPM. Now to find time to get the air cleaner off for a closer look. Looking like it could be an intake manifold leak. This engine was rebuilt/refreshed about 12 years/30000 miles ago.
Isnt a bad intake manifold gasket reason enough to ship it to Mosley Motors???

 

[GA Architect]

Isnt a bad intake manifold gasket reason enough to ship it to Mosley Motors???

OH heck ya it is! As a 350 mile view, I suspect you are due for one of then-thair HiPo units Mr Mosley sells.

 

[bhsdriller]

I've always had the experience that the vacuum/intake leaks will cause it to run poorly and not idle as good compared to the high idle that you're seeing

 

[Catching Neutral]

@bhsdriller Its running great.…cant really notice any difference there. Still pulling 15-16 on the vacuum gauge which is right where it was after my last valves/ carb/ timing adjustment.

 

[OhioFJ]

I recently had something similar happen. The paper gasket btwn the carb and base had become deformed and was holding the secondary butterfly open. Maybe worth checking, but it sounds like you also have a gasket leak to some degree.

High idle / Secondary butterfly question - Solved!

Greetings all - I'm having trouble fighting high idle (~900). Everything was running well - I had tuned via lean drop to an idle of 650 with a vacuum of 19-20. Ran this way for about a year. Seemingly overnight the idle went to 1200 and vacuum of ~15. Tore into everything and found a leaking...

forum.ih8mud.com

 

[Catching Neutral]

@OhioFJ ….thank you for sharing that…I think I have to consider anything until I am able to break into it and get some more precise info

 

[graham5david]

I would look for vacuum leaks.

 

[Skreddy]

It is a 100% guarantee that a carb base gasket can’t leak on a sniper…. For when you get the engine back from Mosley. Just something to ponder.

 

[Catching Neutral]

Air cleaner housing off. I have sprayed almost an entire can of carb cleaner around the entire intake manifold, base of carb and carb itself. I am getting nothing. No decrease in RPM anywhere. I have checked the choke linkage 4 times. Unless I am missing something ( which is entirely possible) linkage is ok

 

[OhioFJ]

Air cleaner housing off. I have sprayed almost an entire can of carb cleaner around the entire intake manifold, base of carb and carb itself. I am getting nothing. No decrease in RPM anywhere. I have checked the choke linkage 4 times. Unless I am missing something ( which is entirely possible) linkage is ok

Can you peek down the carb barrels while the engine is running? Both butterfly valves closed?

 

[Catching Neutral]

Running..

 

[cbmontgo]

A long shot, but maybe the idle speed screw got bumped somehow. Give it a couple of half turns and see what happens. I’d take it exactly back to where it was if it doesn’t solve the issue.

 

[Catching Neutral]

@cbmontgo I started from scratch adjusting the carb….got summoned to the table for Thanksgiving dinner right in the middle. Now I have to watch the Egg Bowl. Good luck to your Rebels. Busy tomorrow so I hope to get back into it Saturday morning.

 

[OhioFJ]

To me it looks like the primary butterfly is open at idle, which I do not think is correct. Hopefully someone more experienced can chime in.

At idle I believe the fuel is drawn through a passage below the primary butterfly, adjusted by the idle speed screw.

Does pulling up on the pedal decrease the idle? I know you checked the linkage.

 

[OhioFJ]

To me it looks like the primary butterfly is open at idle, which I do not think is correct. Hopefully someone more experienced can chime in.

At idle I believe the fuel is drawn through a passage below the primary butterfly, adjusted by the idle speed screw.

Does pulling up on the pedal decrease the idle? I know you checked the linkage.

You know I think I’m actually seeing the Venturi.. not sure if my theory is correct as I can’t see the butterfly. Maybe get a headlight and look down there.

 

[Catching Neutral]

@OhioFJ …I’ll report back when I’m able to get back under the hood.