Idle issue- Huge drop in RPM from Park to drive??

[1985gmcsierra]

Vehicle is a 1985 gmc sierra 1/2 ton pickup.
Carb number is 17085248- no after market mods other than a new float and seat- also reset float height.

I have rebuilt the carb and also followed the steps in the book.

The carburetor will idle high in park- at say 1200.
when put into drive, the vehicle rpm's will drop to 400-500 max.
the idle mixture screws have little to now effect on the idle.
When backing out the idle mixture screws the engine will eventually stall out if they are backed out all the way.

timing is good. no vacuum leaks on distributor, pcv valve, base gasket, etc.

When driving, the vehicle is fine.. especially going fast.
at a stop in gear or just backing up or moving around slowly, the rpms drop so low it almost stalls, and sometimes does.
It appears that the idle circuit is having some sort of issue.

I adjusted the idle speed screw by the throttle cable to compensate for the rpm- and keep it from stalling out in gear.  no other modifications really had any effect on bringing up the idle speed on the idle circuit?

I read through most all of your forum responses and didn't come across anything that was similar.
Thanks, Jason

[Cliff Ruggles]

Did you remove the idle tubes and clean in and under them?

Did you install one of our kits, or over the counter?.....Cliff

[1985gmcsierra]

I believe I installed one of your kits.  I can't remember to be honest. I did buy your book and plan to order my next kit from you as well.

I did clean under the idle tubes (originally, one of the idle tubes had fallen out which caused a hesitation upon acceleration).

I didn't take the idle mixture screws out but cleaned all openings and blew them out with compressed air...

I'm not too sure what to be looking for but will tear it down again.

Could the power piston cause any issues like this?

Thanks for the quick response!

[429bbf]

jason I've never seen an idle tube fall out. the only thing it could do is go down . I've always had to give them a pretty good pull to get them out of the fuel bowl.the only thing I've seen fall out are the main well bleed tubes.did you enlarge your holes under the idle screws?fwiw

[Cliff Ruggles]

Most likely confusing the main bleed tubes in the airhorn with the idle tubes.  Idle tubes don't "fall out" they are pressed into the casting. 

I'd pull the idle tubes and clean in and under them, that should remedy the idle issues, if everything else is in good working order and correctly adjusted.....Cliff