Vacuum leak?

[68350]

Ok so before i go tearing into my engine and see what the intake manifold gaskets look like, i wanted to reach out for advice.

I was out for a drive here a few months ago. Had ran perfectly fine this year until this drive. About half way through the drive, once i slowed down for a town, i noticed my idle on my 383 was way higher than normal (usually around 750, probably is around 1500 now). I thought maybe the carb was just sticky from sitting so i continued on with the drive (about 100 miles round trip). At the point of hearing/feeling the faster engine idle, it started to sound like it almost had a miss. But seemed to run better if you got on the throttle. Everything to me points towards a vacuum leak that randomly started while driving down the highway. So tonight i pulled the quick fuel carb and replaced the carb to manifold gasket thinking that was the problem. I also checked the vacuum port caps on the carb, as well as the power brake booster vacuum line. Everything looked good to me. Once i had the new carb to intake gasket installed, its actually idling even higher now. When revved up the rpms come down pretty slow as well. So im thinking now that its probably the intake manifold gaskets.

Anyone have any other advice for checking things before i go through the work of pulling the intake? I did make sure all the bolts on the intake were still torqued, had one that required maybe half a turn.

[geezer#99]

Check the weights in the distributor aren’t hanging up.
All your symptoms point that direction.

[68350]

Ok i will do that. That would be nice if that was the problem lol

[Steeveedee]

Check the throttle body where the shaft goes through, on the primaries. I've had the same experience when the throttle body was worn, but on other carburetors. Check both sides, because sometimes the leak is on the passenger's side. A little carb cleaner or maybe drip oil on the shaft, your choice.

[68350]

Well the distributor weights seem to be functioning correctly. I don’t see any issues with them. I will check the shafts next

[68350]

Another note to add, this motor is about 6-7 years old. It maybe has 4000 miles on it. And it was a brand new motor when i purchased it, as was the carb

[phat69]

Just get a can of brake\carb cleaner and spray it where ever you think there is a problem.

[Steeveedee]

Also check to see if your throttle linkage moves freely enough. Do you have a manual choke? Maybe the fast idle cam i out of whack?

[68350]

Throttle linkage seems to be move freely snd comes all the way forward without binding up. I will take a look at the choke tonight. Thats one thing I didn’t check last night, is to make sure the choke if functioning properly when warmed up

[rpmerf]

I would remove every vacuum line and cap it off. Could be the vacuum advance, brake booster, or a cracked line. Inspect any caps currently on there that they haven't cracked.

Spray anywhere there CAN be a leak with carb cleaner to see if that changes the idle.

When you replaced the gasket, did you use the same gasket? Did you compare it with the base of the carb to make sure it covered all the vacuum ports?

Floor mats on the gas pedal?

[Wrenchbender Ret]

I'm not familiar with a Quick Fuel carb but on Q-jets sometimes the secondary throttles don't shut tight. Like the guys said, spray some carb cleaner down the throats. I don't think it sounds like an intake manifold leak. Remove the PCV from the valve cover & see if you can feel a vacuum at the valve cover. That would indicate a leak in the intake gasket.
George

[68350]

So update. I sprayed carb cleaner all around the intake and base of carb. Checked the vacuum lines again. No change. I sprayed some down the secondaries and it idled down for a second. So im assuming this points to a carb issue?

[68350]

Also the carb is an 850 double pumper design with vacuum secondaries. Its a Quick Fuel Technology Slayer carb.

[Wrenchbender Ret]

With the engine not running, open the secondary throttles & spray carb cleaner down the throat. Let it set a few minutes & start it up. I think you have some trash along the edge of the butterflies.
George

[68350]

Ok so i found the issue. Had the carb off and cleaned it. Reinstalled with no change. Starting manually plugging vacuum hoses and as soon as i pinched the pcv valve hose it idled down and ran normal. This would indicate intake gaskets if i thought this through, correct? Instead of creating crankcase pressure the intake is leaking out air? Or does it simply mean the valve is bad. I pulled the valve out of the valve cover and plugged it with my hand and it has a massive suck to it when plugged

[Steeveedee]

Could be that the guts of the valve are bad. Though they usually stick shut. The PCV system does flow a lot of air, though. Look inside and see what you have.