Fun with vacuum? Yeah, right.
Thread Starter
Burning Brakes
Joined: Oct 2001
Posts: 1,185
Likes: 0
From: New Wilmington PA
Fun with vacuum? Yeah, right.
Really I just need you guys to tell me if I'm smoking crack or not. I've had an unusual high idle problem ever since I got the car but have been busy with other problems until now.
Facts:
PCV attached to fitting on back of carb. As I understand it the PCV valve will give a sort of metered vacuum leak so on a dual plane intake this is the best place to attach it. (The air will be distributed among the cylinders instead of being routed to just a couple.)
Rest of vacuum system attached to hole just behind carb.
Oddity:
Pinching off the PCV valve results in major RPM drop at idle.
Pinching off other vacuum hose results in somewhat smaller RPM drop.
Questions:
Is this a sign of a vacuum leak somewhere in the system?
Is pinching off PCV supposed to have such an impact on idle RPM?
Any help would be great. Thanks!
Facts:
PCV attached to fitting on back of carb. As I understand it the PCV valve will give a sort of metered vacuum leak so on a dual plane intake this is the best place to attach it. (The air will be distributed among the cylinders instead of being routed to just a couple.)
Rest of vacuum system attached to hole just behind carb.
Oddity:
Pinching off the PCV valve results in major RPM drop at idle.
Pinching off other vacuum hose results in somewhat smaller RPM drop.
Questions:
Is this a sign of a vacuum leak somewhere in the system?
Is pinching off PCV supposed to have such an impact on idle RPM?
Any help would be great. Thanks!
Race Director
Joined: Sep 1999
Posts: 11,358
Likes: 379
From: Plano TX
Re: Fun with vacuum? Yeah, right. (Cookwithvette)
Pinching the PCV will drop the rpm. Guess it depends on what you mean by "major rpm drop". 100 rpm or so is normal. If it's more, try replacing the PCV valve. You can get them at any auto parts store.
Safety Car
Joined: Apr 2002
Posts: 3,551
Likes: 0
From: No link, No dupe
Re: Fun with vacuum? Yeah, right. (Cookwithvette)
If you think you have a vacuum leak, there is an old trick. Take a can of starting fluid, and spray in various places where you see vacuum lines. If at any time idle changes, then you may have located a vacuum leak. Just be sure the spray stays off of the exhaust manifold for obvious reasons, and be sure none enters the carburetor.
Agree with other comment about the PCV valve. It sounds normal.
Agree with other comment about the PCV valve. It sounds normal.
Thread Starter
Burning Brakes
Joined: Oct 2001
Posts: 1,185
Likes: 0
From: New Wilmington PA
Re: Fun with vacuum? Yeah, right. (zwede)
Pinching off the PCV valve would take the engine from around 900 RPM to sputtering and nearly dieing. I can't pinch and see my tach at the same time but I think if I continued to hold it the engine would stall.
I'm thinking I have a leak in my main vacuum system and the symptoms are being exaggerated by the PCV system.
Hmm.... I wonder what would happen if I pinched both hoses simultaneously?
Here is a strange nugget of info. When I first got the car I pulled one of the plugs to see what sort of condition they were in. YUCK! Absolutely caked with carbon. The other day I finally got around to changing the plugs. (this is with a few tanks through the new carb) Well.... best to see.
I'm no expert but I'd guess it's running a little lean? Is it possible for all the carbon to have been burned off?
[Modified by Cookwithvette, 2:56 AM 6/2/2002]
I'm thinking I have a leak in my main vacuum system and the symptoms are being exaggerated by the PCV system.
Hmm.... I wonder what would happen if I pinched both hoses simultaneously?
Here is a strange nugget of info. When I first got the car I pulled one of the plugs to see what sort of condition they were in. YUCK! Absolutely caked with carbon. The other day I finally got around to changing the plugs. (this is with a few tanks through the new carb) Well.... best to see.
I'm no expert but I'd guess it's running a little lean? Is it possible for all the carbon to have been burned off?
[Modified by Cookwithvette, 2:56 AM 6/2/2002]
