Hello. I have a one year old Edelbrock 4 barrel 750 cfm carb on the 350 in my 69 El Camino. I tired adjusting the carb and found that the idle mixture screws make little or no change in the rpm's and in fact when screwed all the way in don't kill the engine. I called Edelbrock and they are thinking vacuum leak. The thing idles fine and runs smooth. Why am I then adjusting the carb in the first place you ask. Well, it is dieseling when I shut it off. Also, after it is warmed up and I drive it and then shut it off for maybe 15 minutes it starts back up fine, but if I drive it and then shut it off for maybe 45 minutes I have to crank it for awhile before it starts. To me it acts like it isn't getting fuel although pumping the gas when starting it does not seem to make a difference. Sorry for being so long winded here. Thanks for any input you can give.

My .02, 750 on a 350 is alot of CFM unless you've got that sbc breathing real well. I'd check fgor a vaccuum leak around the base of the carb as a starting point. The idle crews would have much to do with RPMs but with gas/air fuel mixture, helps erradicate hestitation amoung other things.
The dieseling leads me to think your too far advanced timing wise which would also come into play concerning your idle mixture screws also.
My question is, why did you make a adjustments to an out of the box Edelbrock? The most anyone I know had to do was lean up the choke a notch.
Gets some starter fluid and start spraying away, find that vacuum leak. You're running it 45 minutes and it's not starting also says, timing.
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Emil
GCi
http://groups.msn.com/randomrodders
Drive it!
46 Ford Sedan
37 Packard Super 8

i agree w/ Emil, 750 is a lot on a 350. I'm thinking the idle speed is too fast, not letting the throttle plates close all the way. If it's not closed, air goes by the throttle plates and not thru the idle circuit. That's why your mixture screws are inneffective. Drop the idle speed down and adust them again. Unless you have a lot of cam, this should work ok.

bjb
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There's a world of difference between "good" and "good enough".

You may have to play with the timing alittle to keep it running but you need to close the throttle plates before you can use the idle screws. Start with them turned out 4 or 5 turns. Than turn them in a quater of a turn at a time. Rpm may increase as you turn them in. Adjust timing and throttle plate accordingly. You should end up with a 20 rpm drop per screw(LEAN DROP METHOD). Once this is set, what you use for vaccum (manifold or port) advance should not effect the carb operation. Takes alittle time.
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Mike J
Unchartered DSRA Member
If it ain't broke, fix it anyway...

All fine answers in adjusting the carb. Also agree about 750 being maybe a bit much for a 350. You may be getting a bit of heat soak after the engine is fully warmed up. Seems to be a fairly common problem. Might want to look into using a phenolic type base gasket for the carb. Tech at Edelbrock recommended that to me. Have used one type or another on various applications with good results so far. Have also heard of some carbs coming with the float level set too high. Also make sure the vacuum advance line is hooked up to the right port in front of the carb. Good luck and keep us posted.
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Ken

"Also make sure the vacuum advance line is hooked up to the right port in front of the carb.

Is that "right" port as in correct or "right" port as in left?

The drivers side port is the one you want.
Use the passenger side port and the engine will overheat in traffic on hot days.
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C9

Please explain your theroy on why drivers side port is the one you want.
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"Big Al" Liebmann

Chief Legal Council & Charter Member: Dipstick Street Rod Association
Technical Advisor - Barris/Kind Chrysler City Coup'e Kustom

Illegitimate Son of Ed Croozer & Mr. Willys ....

The problem is that the idle plates are not shut off..usually meaning too high of idle, also explains the run-on after shutting off you referred to. Could be a vacuum leak tho....you can start with both. While the 750 is not the perfect carb unless you are turning a burning 7000 rpm all the time, the idle circuit and jetting can be made to work even tho the throttle response may be spongy!

I would like to chime in with my two cents on full time vacuum if I could Al. The answer is it depends really? Now I am talking street engines.......not racing!

Usually, if you are running a stock or close to stock engine with low compression and mild camshaft, the timed vacuum source is what you should be using. When accelerating, the additional timing that occurs as the engine RPMs increase will create some additional power and the throttle response will show marked improvement. Low compression engines also should tolerate the additional timing with little or no pinging and the fuel economy will also be improved slightly.

On the other hand if you have a higher compression (over 9 to 1) engine with more of a camshaft you'll be much better off using manifold vacuum. The additional timing will give a big cammed engine a much more stable idle yet the additional timing backs off once you get into the throttle, minimizing the chances of detonation or ping occuring at higher rpms with higher peak cylinder pressures. The tip-in response is better too with full manifold. The engine will like the increase in timing as well in the cooling department. I like a full 20 degrees of advance at idle for most of my engines. This usually translates to 10 -12ish static and limited to 8-12 vacuum. If you start with alot of static, well poor starting when hot is the result as we all know.

Just my thoughts....how about you Al?

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Steve Jack
Marketing Technologies Limited,LLC
ConceptOne Brackets and Pulleys

I just switched back to the port vacuum ( passenger side port).Vacuum increases as you open the throttle plate. Gives a little rougher idle and I'm checking the mileage difference. Also pings less so far. May run alittle warmer. Still playing with it. Exhaust pipes are running a little cleaner. Right now I can just switch the vacuum hose to either port and it will run decent.
I ported the intake this winter and my vacuum increased at idle. Now I need to change the spring as I believe C9 suggested a couple of month ago. Also read Carter Carburetors by David Emanuel. A good book to have if you want to play with the Edelbrock or Carter carbs.
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Mike J
Unchartered DSRA Member
If it ain't broke, fix it anyway...

"Please explain your theroy on why drivers side port is the one you want."

Drivers side is full time vacuum.
GM engines need a lot of advance at idle to forestall overheating.

Passenger side is ported vacuum.
Meaning that, as more air flows through the carb, the more vacuum advance you get.
When you do that, you're adding advance when you don't want it.

Using full time vacuum allows timing to back off - or retard - when the throttle is depressed.

Refer to any Motors Manual for notations on the vacuum level that vacuum advance starts as well as information on when and how much centrifugal (mechanical) advance starts.

The two advance methods are complementary and not additive.
They take their cues from engine load and engine rpm.

We can't go far wrong in following what the factory does as far as timing goes for hot rods run on the street.
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C9