pvc. to much vacuum?

[rdc767]

Strange question. I'm trouble shooting a high idle. I have a edelbrock 750 on my 351. The idle creeps up after start from 900 to about 1300 once the engine is fully warmed up.

Its a manual choked carb. The choke cam is off the the throttle arm and is not affecting the butterfly's.

I have the low idle screw all the way out and is off the throttle stop as low as it can get 900 rpm.

The primary butteryflys to my look completely shut. I've taken off the carb off to verify this. The throttle linkage isn't binding.

I've check for leaks around the base of the carb with starter fluid. No rpm change.

I've set the idle mixture to get max rpms.

Timing is set to just below detonation.

Here is the strange part. I think the pvc port of the front of the carb is draw too much air. I have it plumbed to a air breather on the valve cover. Once I take off the hose and cover it with my finger the engine dies. If I screwed in the idle screw and crack open the butteryflys the engine idle fines and is adjustable. I am able to set 700 and it stays there no creep.

I have a 3/8" hose on the pvc vacuum port. This is the same size as the breather nipple. If I was able to go smaller or restrict the air Thur the pvc breather I would be able to use the idle screw to set the rpm.

Edelbrock tech is closed this late at night. Just wondering if you think this might be the right trouble shooting track.

Ron

[Ronbo]

Does that cap actually have a PCV valve in it? One thing you could do is replace it with a chevy valve with a breather grommet. The valve has a floating metal plug that gets sucked upwards to block flow under high manifold vacuum. (what you hear rattle when you shake them)

[Rick Parker]

You have created a huge air leak in you intake system. Plug the hose, or better yet the tube at the base of the carburetor. Set your idle accordingly. Attach the hose to a shielded PCV valve in the other cover. The hose from the breather would go to the base of the air cleaner assembly. You are forcing the engine to run extreemly lean and can possibly damage the internals such as a piston or valve.

[CHANMADD]

Put a PCV valve in the line and it will be fine. Like Rick says you will damage the motor the way it is.
John.

[rdc767]

Thanks
Screwed that up.

Ron