350 engine idle shake HELP!

[Awann99]

I’ve had the truck sitting for about 3 weeks while I redid the door rubber and now when I start it, the engine shakes vigorously. It ran perfect before I let it sit. I’ve checked and rechecked the wires and they are all going to the same spot. I’ve moved the distributor clockwise and counterclockwise to see if the timing was off but it stays rough no mater how far I turn it each way. I put my hand over the top of the carb and it seemed to slow the shaking. I think it may be a vacc leak. How would I check for a vacuum leak? It did this once before and I took off the rotor cap and filed all the buildup off of the points about a 2 months ago. That helped but I don’t see how so much buildup could occur that quick if that is the problem again. .
350 abc crate engine. 3 years old. Edelbrock performer intake, HEI, and stock q-jet.

[truckster]

Carbon arcing in the distributor cap or arcing wires are the two things that come to mind first. Have you tried running it at night to see if there are sparks jumping anywhere?

[Wrenchbender Ret]

There is a vacuum connection in the intake manifold behind the carb. If one of the hoses that connect to this outlet fitting comes off the fitting or the other end, it will cause a shake in the engine as this connector goes into a runner that feeds 2 cylinders. It usually connects to the vacuum reservoir ball & the auto trans modulator.
George

[rockyrivermark]

Check all your vacuum lines.
Get some carb cleaner with nozzle.
Spray around base of carb and vac line fittings and lines.
If the engine Boggs down that means the vacuum leak sucked the carb cleaner in the leak point.

[KQQL IT]

Dirt in the idle circuit will make them shake too.

[Awann99]

Thanks guys. It started raining so I will try these tomorrow

[Awann99]

Here’s what all the plugs looked like. Maybe time for new plugs?

[Corts60]

Throw a set of plugs in it and check for vacuum leaks as mentioned above. The dead give away that it is a vacuum leak is the fact you put your hand over the carb (choked it) and it started running better.

[Awann99]

I ordered a set of R44TS spark plugs along with a new carb stud kit and gasket. The passenger side carb stub was loose. I attempted to tighten it but it happened to be stripped. So that might be where the vacuum leak is.

[Steeveedee]

Originally Posted by Awann99

I ordered a set of R44TS spark plugs along with a new carb stud kit and gasket. The passenger side carb stub was loose. I attempted to tighten it but it happened to be stripped. So that might be where the vacuum leak is.

When you start it next time, spritz that area with carb cleaner and see if the idle smooths out.

Though that's not the kind of thing that happens a/b- working OK and then not working OK. I'd more suspect dirty fuel.

[rockyrivermark]

Originally Posted by Awann99

Here’s what all the plugs looked like. Maybe time for new plugs?

Well yeah. Always perform a general tune up first it’s its due.

[Greasey Harley]

My truck idles weird sometimes , when I get "iffy" gas. I recommend topping off with some good premium (Non ethanol if available) and see if it clears up.
Could be a lot of things, but fresh gas will eliminate one variable.

BTW
Timing NEVER changes while a vehicle sets not running, no matter how long it sets. If it "It ran perfect before I let it sit", the problem isn't the timing. The only way the timing changes if something is moved.

[Awann99]

Alright fellas. I coated the carb to manifold gasket in carb cleaner and started it up. It ideled just fine for about 10 second and then started tuning horrible. So I’m pretty sure I found the vacuum leak. New gasket and bolts will be here soon. Thanks guys.

[Steeveedee]

Always nice to pinpoint the problem! What year is your truck? Some years have a crossover for the heat riser that can give you grief. You will know when you get the carburetor off and there's a bunch of built up carbon in the manifold.

[Awann99]

Originally Posted by Steeveedee

Always nice to pinpoint the problem! What year is your truck? Some years have a crossover for the heat riser that can give you grief. You will know when you get the carburetor off and there's a bunch of built up carbon in the manifold.

The truck is a 1969 but is has new crate engine in it and different exhaust manifolds. I’m just using a generic air cleaner until I can find a correct snorkel one.

[rockyrivermark]

Originally Posted by Awann99

Alright fellas. I coated the carb to manifold gasket in carb cleaner and started it up. It ideled just fine for about 10 second and then started tuning horrible. So I’m pretty sure I found the vacuum leak. New gasket and bolts will be here soon. Thanks guys.

I didn’t specify but spray the carb cleaner while the truck is running to find the vacuum leak
Also sometimes people will warp the carb mating surface by overtighening the mounting bolts.
Can be filed level again if needed.