K20 runs rough

[onemean85]

i have a k20 with a 350. has edelbrock intake, edelbrock carb, and headers. put the motor in the truck and it ran perfect. was driving to town the other day and it started running rough. erratic idling. sometimes high, sometimes it wont idle at all. backfires on deceleration. but seems to run good accelerating hard. im thinking an air leak somewhere? tried cap and rotor. also has HEI distributor. tried messing with timing alittle but nothing changed it. anyone have any input? thanks

[rich weyand]

Check the step-up pistons on the carb.  They get dirty and stick, giving you the wrong mixture in either cruise or power circuit, depending on whether they stick up or down.  They gum up.  When you kick in the secondaries, those are usually set rich enough that if the pistons are stuck it won't matter.

Remove air cleaner.  Loosen two torx screws on the step-up covers, and rotate the covers out of the way.  Pull out the pistons and metering rods.  Clean the pistons and rods.  Use cotton swabs to clean out the cylinders.  Reinstall.  Do not over-tighten torx screws.

Also, Edelbrock carbs can be heat sensitive.  If it only misbehaves when it is hot out and the engine is hot, then it could be that you need an insulating spacer between the Edelbrock and the manifold.

[mechanic58]

This is the best tool for working on those Edelbrock carbs with:

[thefarmboy21]

You sure the timing didn't change? I'd check it with a light.

[rich weyand]

You sure the timing didn't change? I'd check it with a light.

Yeah, could also be the vac advance canister is shot, or the hose is split, or the hose is not hooked up.

[onemean85]

timing didnt change i checked that. also tried moving around a bit. vac hose is hooked up to distributor

[rich weyand]

Can you get a timing light on it and see what the timing is with the vacuum connected versus disconnected?  No need to plug the hole, just look at the timing with the vacuum disconnected, then plug it in and make sure it moves.  Should be 12-16 disconnected, and 15 degrees more connected.

Did you clean the step-up pistons and verify they were moving freely?