Do I have a vacuum problem?
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Senior User
Joined: Oct 2002
Posts: 106
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From: Bucks County, PA
Do I have a vacuum problem?
My newly rebuilt 460 has a Comp Cams 268H cam. It seems to be missing at idle and continuing through all RPMs. Hooking a vacuum gage to the intake at idle, the needle rapidly bounces between 16 and 21. It continues to bounce throughout all RPMs. Is this bouncing normal with this cam?
It has an Edelbrock 750CFM with factory jetting and adjustments. #1 and #6 spark plugs were black on insulators. The other plugs seemed light tan. Replaced those 2 plugs and ran on the road for 20 minutes. They were light tan after that. Have MSD distributor and wires and Accel Super Coil. Timing 10 degrees BTDC.
Engine does not seem to have the pickup I expect for a 450 HP low RPM engine. Also, there is a vibration that might be associated with a cylinder or two not firing properly.
Any suggestions?
It has an Edelbrock 750CFM with factory jetting and adjustments. #1 and #6 spark plugs were black on insulators. The other plugs seemed light tan. Replaced those 2 plugs and ran on the road for 20 minutes. They were light tan after that. Have MSD distributor and wires and Accel Super Coil. Timing 10 degrees BTDC.
Engine does not seem to have the pickup I expect for a 450 HP low RPM engine. Also, there is a vibration that might be associated with a cylinder or two not firing properly.
Any suggestions?
Postmaster
Joined: Aug 2001
Posts: 2,882
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From: Nevada
Sure sounds like either a vacuum or intake manifold leak problem.
You can spray carb cleaner or propane along the edges of the intake manifold and carb base while the engine is running and the rpms will increase if there's a leak. If that doesn't turn anything up, I'd do a compression check on it to see if its leaking between cylinders.
You can spray carb cleaner or propane along the edges of the intake manifold and carb base while the engine is running and the rpms will increase if there's a leak. If that doesn't turn anything up, I'd do a compression check on it to see if its leaking between cylinders.
Thread Starter
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Senior User
Joined: Oct 2002
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From: Bucks County, PA
Thank you, Georgedavila,
I will check this tonight using carb cleaner. The new cam seems good. Should I get a real loping idle sound with it? Should the loping sound continue throughout the RPM range? Thank you for your help.
I will check this tonight using carb cleaner. The new cam seems good. Should I get a real loping idle sound with it? Should the loping sound continue throughout the RPM range? Thank you for your help.
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Senior User
Joined: Oct 2002
Posts: 106
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From: Bucks County, PA
What can cause misfiring? How can I check for misfiring? The engine has a serious "vibration." I want to fix that, and I wonder if it could be a misfiring. Sure sounds like it might be, but not sure.
It's a 1971 block, and harmonic damper has straight spacer. I think it is an internal balanced flexplate. Could I have an externally balanced flexplate--would one fit, or are bolt holes different internal to external? Thank you.
It's a 1971 block, and harmonic damper has straight spacer. I think it is an internal balanced flexplate. Could I have an externally balanced flexplate--would one fit, or are bolt holes different internal to external? Thank you.
Postmaster
Joined: Aug 2001
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From: Nevada
An easy way to check for misfiring in the ignition system, distributor, wires, plugs, is to pull and replace spark plug wires one at a time while the engine is running. If that cylinder is firing correctly, there should be a noticeable drop in rpm. Caution! use insulated, long-nose plyers to handle sparl plug wires when current is running through them or you can get a nasty shock.
I've found checking the ignition system won't always find a vacuum leak.
I've found checking the ignition system won't always find a vacuum leak.
Senior User
Joined: Dec 2003
Posts: 109
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From: central illinois
could also be cam timing, misadjusted valves[depending on heads], stuck open valve, or a valve not opening. is the valvetrain adjustable? did you mill block or heads? what heads are on engine?
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Thread Starter
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Senior User
Joined: Oct 2002
Posts: 106
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From: Bucks County, PA
I'll check using your method, Georgedavila. Also, Cam timing should be correct—Zero degrees, new Cloyes chainset. Valves were adjusted and readjusted after cam breakin. Harland Sharp adjustable roller rockers. Neither block or heads were milled. Heads are D0VE C heads were cleaned, new valves and springs. I’ll sure check and double check everything til it’s right. Just want to start with the most obvious and easiest to fix first. Thanks for your input!
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