69 coupe 427 390hp 4 speed. Bought the car in April this year. Relatively fresh motor, cam is a mystery. Car always idled rough and never had the power that I thought it should have. Vacuum at idle with a new Edalbrock Q-jet and the correct 69 carb base gasket setup (thanks Lars) and new Fel-Pro intake gaskets is about 7". Put a degree tape on and found that the best operating condition was at 40 deg with a total of about 65 deg- yikes! Have no response from the idle mixture screws- both all the way in and it will not stall. Checked the power valve at idle and it appears to be seated (thanks Lars) don't know what to do next. If valve timing is off that would explain the low vacuum but what is up with the idle mixture screws? Returned the timing to 20 deg with a total of 45 deg and the performance is gone. I put the lightest spring in the centrifical advance I could find and am running a single point distributor. Any advice will be considered. Thanks.

 

Ok, let's do a basic check....

With idle vacuum SO low, you have one of two issues:
A radical camshaft (which, at 7" of vacuum would be INSANELY radical), or
A monster vacuum leak....

My best guess, without physically looking at the car, would be that you have a vacuum leak. An EASY way to check is the following:

With the car running at idle, put your hand over the primary side of the carb, blocking flow to the engine... IF the engine RPMs suddenly go up, you've got a BIG vacuum leak... The fact that you turn the idle-screws in, tends to support this theory...

On the timing side, remember that you should divide the timing marks by 1/2, to get a true indication... So you're *really* at 20 degrees with 32-degrees max... Not TOO bad, not out of the range of reasonable engines... I would ignore this, and move back to vacuum issues...

Let's talk about carbs, for a moment. IF the carb is not a brand-new unit (not rebuilt), I would ask to borrow a spread-bore carb from a buddy, and try it out.... Q-Jets are notorious for running WONDERFUL when they're alive, and being a miserable experience when they finally "die." Borrow a carb, and see if you CAN dial it in....? If yes, then problem is mostly solved.... Hell, I would probably swap carbs anyway, just to see what happens!!!

I hope all this helps.... Keep us posted!

 

Sounds to me like the damener ring has slipped, giving you a false reading on the timing. That's for the timing issue. There are many things that affect performance but if you can't kill the engine with the idle mixture screws, the carb is a good place to start.

 

time to put a dagree wheel on it and find out

 

Do a TDC verification. It's very common for the balancer rings to slip, and with the timing settings you're getting it sounds like that may be the case. If you need a copy of my paper on how to check TDC, drop me an e-mail.
V8FastCars@msn.com

 

[QUOTE=breathial]Ok, let's do a basic check....

With idle vacuum SO low, you have one of two issues:
A radical camshaft (which, at 7" of vacuum would be INSANELY radical), or
A monster vacuum leak....

Carb is brand new. I'm sure I don't have a vacuum leak. I'm thinking the cam is not timed correctly and the ignition timing is compensating for it.
I wonder how effective the idle mixture screws are at 7" vac.
I don't follow your thought on taking the timing indication and cutting it in half. Isn't the degree tape an extension of the timing marks already present? Why cut it in half?