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Discussion Starter · #1 ·
i just put a crate 350/330hp motor w/ gm hei distributor into my 70 chevelle. i've had some problems with timing, carburation, and advance so i took it to a mechanic to take a look at it. he adjusted the timing, and carburation and told me that my vacuum advance was kicking in too early. it tends to die at stop signs or when i first put it in gear. i know they make an adjustable vacuum advance, would this be the way to fix this problem or should i go with new springs and weights? i also noticed that after he changed the timing, it had less power. i assume that he had to compensate for the early advance by retarding the timing. the spec sheet says to set the timing at 10btdc but i'm not sure if that can be trusted. the camshaft specs have been crossed out and new numbers written in. anyone know how to tell what the timing is just with the lift, duration and centerline? the mechanic also told me i could just disconnect the vacuum advance, i tried that and noticed a big decrease in power. thanks.

Rich
 

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This does not sound right. Vacuum advances "kick in" at idle and then decrease the advance as the manifold vacuum decreases with throttle opening. The exception to this would be if the advance were hooked to the vacuum port on the carb.Then there is a "kick in" point that is determeined by the carb machining. Maybe you should try manifold vacuum and see how that works. Be aware that the advance degrees for your vacuum advance can may be stamped on the flat metal bracket for the can.You may have too much vacuum advance (like 20 degrees; look for a 12 degree unit). Speed shops do have adjustable vacuum advancers for HEI.
 

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Discussion Starter · #3 ·
that's strange, right now i have no vacuum from the carb at idle. so basically vacuum advance is not doing anything for me. i'll try hooking it up to the manifold. i'll let you know if that helps. thanks.

Rich
 

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Discussion Starter · #4 ·
that didn't work. it still dies when i put it in gear. i'm not getting much vacuum from the manifold either. could this be because of the timing? i know that before i brought it in, i had quite a bit of vacuum and when i hooked up the vacuum advance at idle, it did advance the timing. any thoughts? thanks for everything.

Rich
 

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Hey Rich,
Do you have a vacuum gauge? If so try setting the timming like so; set idle speed about 750-800rpm. Then advance timming until you have about 15 inches of vacuum. I have a 300/350 target engine in my 65 SS which is basiclly the same engine except for the heads. My engine idles about 850rpm in neautral and shows 15 inches of vacuum with inital timming set at 12 degrees btdc. In gear against the brakes it idles about 750rpm (with B&M hole shot converter) and holds about 12 inches of vacuum. Your problems sounds like the timming is late (retarded) or you have a vacuum leak. Also on my AFB, I had trouble getting the engine to to idle under load when I first put it back together because one of the idle jets had picked up a piece of junk during the engine swap. Check that also. I have two of these engines both with Edelbrock performer manifolds and 625 AFBs and they idle very reliabily around 800-900 rpm. They both pull good idle vacuum as well. Please post what you find out. Good luck. mgg

[This message has been edited by Manny Gomez (edited 07-17-99).]
 
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