Thread: there is definetly a vacuum leak

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Originally posted by S19243H
ps but the motorwill be idling at 1500

it want be if you idle it back down. you cant get a vac. reading at 1500 rpm's, i think you might be trying to set the rpm's with the dist. cant do that. move the dist up, idle the carb. down untill you have max. vac at 900 rpm's. the timing will take care of itself if you do that.

Ok, I came in a little late here so let me toss in some stuff learned by experience . . . otherwise known as, "the hard way."

How big a cam do you have?
Using advertised duration, a 284-292 degree cam will idle 600 rpm or so and vacuum will read 10-12"

A cam with advertised duration of 260-266 degrees will idle 550-600 rpm at 18-19" vacuum.

Stock cams idle the same as the mild cam listed above.

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Where are you plugging your vacuum gauge?
If into the ported - or timed as some call it - vacuum source idle at 600 rpm will show zero and 900 rpm will show 12".

Plugged into full time vacuum - or manifold vacuum as it's usually called, you will get 18-19" of vacuum at a 600 rpm idle assuming a stock or very mild cam.

Incidentally, radical cams when spun up to 1200-1500 rpm or so will read 18-19" on a well sealed engine.

Ported vacuum is usually tapped at the carb above the throttle blades.
Usually being the key word, some Holleys have a full-time vacuum source above the throttle blades so you need to be careful here.

Manifold vacuum is anywhere in the intake manifold and below the throttle blades.

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Are you timing the engine with the vacuum advance line disconnected?
If so, that's correct.
If not, you will not have sufficient advance to have a good running engine as well as give away a lot of horsepower.
Pull the vacuum line at the distributor and plug it - golf tees work great for this.

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Do you have an MSD ignition - or similar - module?
And are you using a dial-back timing light?

I ran into a problem using a new dial-back timing light.
It showed 20 degrees off toward the retard side if I remember right.
Re-checking timing with the old non dial-back light - same brands on both by the way - showed the timing to be at 8 degrees initial like it's supposed to be.

You have to be careful here.
Some guys use a dial-back light on MSD equipped engines and get the same reading as a non dial-back and others report the same problem I had.

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Are you running an Edelbrock or Carter carb?
And is the engine running very rough on the bottom end of the rpm scale - and won't hardly idle?
Pull the metering rods and look for a bent one.
I had a bent metering rod in a brand new 500 cfm Carter on a dual quad setup and when it stuck, I thought I had a heck of an ignition problem.
The engine ran strong when revved up and with a bit of throttle applied, but at light throttle settings it wouldn't hardly run.
The bend had locked the metering rod in the up position which made the engine overly rich.

You can check these by rolling them on a known to be flat surface - piece of thick glass works well - with the 90 degree bend hanging over the surface plate and if a bend is found, straighten it by hand.

C9x we found the problem look under the thread faulty vacuum gauge for sale a lot of good info in youre post thank you