erratic idle

[Mr Hand]

I had primary throttle shaft bushings installed into my baseplate but  still have an unsteady idle. Some propane down around the RH side of the carb down by the secondary throttle shaft hole smoothed it right out.

Obviously a vacuum leak thru the shaft hole but is it common to leak there? Can bushings be installed to fix this?

[Mr Hand]

Hello? This thing on?

[71vette]

I had a very similar situation with the idle after I installed new primary shaft bushings.  I sprayed carb cleaner at the secondary throttle shafts and sure enough, the idle smoothed out.  If you read through the history of posts, you will find that the rear throttle plates should seat themselves which requires some play in the secondary shafts.  If you put your hand over the airhorn when it is running, does it idle better?  If it does, you may have a vacuum leak (other than by the secondary shafts) or you may need more idle fuel. 

Because I wasn't using the original carb and had taken a core and rebuilt it to Cliff's specs, I had to open up the idle down channel restriction as well as the idle mixture screw openings using one of Cliff's option settings.  It idles the best it ever has.  (454 with mild cam)

I didn't see any other details about your vehicle so if you can provide more information on make, engine and the work you've done on it, I'm sure there will be others on the forum that can provide some excellent suggestions.

[Cliff Ruggles]

We can install 3/8" bushings for the secondary throttle shaft.

Before doing that, I would remove the throttle body and check to make sure the secondary throttle plates are fully seated when closed, shaft not bent, twisted, etc....Cliff

[Mr Hand]

just rebuilt the carb using one of cliff's kit' and although the idle improved and seems a bit rich at idle (oww my eyes burn!) the engine falls flat on its face under anything past 1/4 throttle........driving away from a stop under anything more than softly on the gas pedal results in hesitation, stumble, bog and backfire......backfired so bad it blew out the muffler! !@#@%*@!

In park it tachs up with no problems this only happens under medium engine load.

Distributor rebuilt and recalibrated by suntuned aka Bob Davis so safe to assume it's not ignition. Timing set to 12 degrees BTDC and jumps up when the vacuum advance is hooked back up......using manifold vacuum.

Low compression Pontiac 400......stock cam

[Cliff Ruggles]

What is the carburetor number?

Is the engine completely stock and the one for the application?.....Cliff

[Mr Hand]

17058274

yes

[Cliff Ruggles]

Rebuilding the carburetor with good parts and correct settings should have it working fine for what you are doing.  Were the idle tubes removed and cleaned during the rebuild?  Did you adjust the APT back to it's original setting.   Power piston hanger has the metering rods exactly even.

The backfiring situation to a point where it blew up a muffer is most likely ignition related.  The engine quits firing the plugs, fuel builds up in the exhaust system, then it starts firing again and ignites the fuel.

Do you have control of the idle fuel with the mixture screws?

Is the accellerator pump delivering a good shot of fuel when the throttle is moved quickly?

Was manifold vacuum used to the advance before the carb and distributor were rebuilt?.....Cliff

[Mr Hand]

 Were the idle tubes removed and cleaned during the rebuild?

no but they were cleaned with solvent & brake clean and appeared to be clean when looking through them with magnifying glass.


The backfiring situation to a point where it blew up a muffer is most likely ignition related. 

agreed although this is the third Bob Davis HEI I've had him do and all prior units have been perfect. Not sayin this one is ok but.........


 Did you adjust the APT back to it's original setting.   Power piston hanger has the metering rods exactly even.

no wasn't sure how to


Do you have control of the idle fuel with the mixture screws?

yes


Is the accellerator pump delivering a good shot of fuel when the throttle is moved quickly?

yes


Was manifold vacuum used to the advance before the carb and distributor were rebuilt?.

yes but the car was never driven after the initial rebuild

[Cliff Ruggles]

Idle tubes can not be cleaned with compressed air and any sort of carb cleaner of solvent.  They "varnish" inside, and the only way to effectively clean in and under them is to remove them. 

It is possible to "rod" them out with a piece of .030" spring or music wire, but we highly recomend removing them, to effectively clean in and under them.

We also recomend installing a removable set screw over the APT adjustment, to fine tune the part throttle A/F ratio without having to take the carburetor apart.

The items mentioned above will help make sure the rebuild is done correctly.

Some of the symptoms mentioned still don't sound carburetor related, at least the backfiring and blowing out the muffler thing.......Cliff