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Old 08-23-2012, 12:53 PM
oldchevy1956 oldchevy1956 is offline
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383 low vacuum and idle issues

My problem is that my engine easily dies when in gear and stumbles / misses from time to time at cruising speed. They two issues may be related but I am not sure why. I am not new at engines and tuning as I have done this for several years but I am somewhat at a loss for my current problem. I have a newly built 383 with slightly worked over heads with a roller and somewhat aggressive cam with a 2800 rpm stall converter in a 700r4 trans. All is fresh with this engine and I am running headers with dual exhaust, a Mallory dual point distributor and Edlebrock 600cfm carb. My dwell is set at 31. My engine vacuum is low (9) at idle (due to the cam I am sure) and 15 at 2500 rpm. I have set the base timing (with distributor vacuum unplugged) at between 5 and 7 at 750rpm and the mechanical advance will move it to 25 at 3000 rpm and then the vacuum advance will take it to a total of 35 at 3000 rpm. I have been running full vacuum off the carb to the distributor. It has good and loopy idle at around 750 rpm but with low vacuum at 9. When I put in gear the engine rpm drops slightly and then I loose engine vacuum (goes to 5) which in turn causes the distributor vacuum to drop below what will advance the engine (takes about 15sec to loose distributor vacuum) and then the idle becomes very rough and slow (advance is back to 5) and then will die within about 10 secs. To fix this I have to have the rpms up real high around 1500 (to keep vacuum up to 12) and now we have both vacuum and some mechanical advance on and when I put in gear I am now running to much advance causing around 1350 rpm which is way to fast. There seems to be no middle ground between idle and in gear in terms of advance and rpm. Too low of rpm I get no advance when in gear and it dies and to keep the advance from losing vacuum I am running to much rpm for idle and in gear. I also mentioned that the engine seems to miss from time to time at cruising speed and I donít have a good explanation for this. I do need to add that the engine at WOT will run crazy fast with no hesitation, engine detonation or spark knock. So if I could just run at WOT all the time and not have to stop at stop lights and such I would be fine!! Any ideas how to deal with my idle Ė advance issue, as well as, my cruising speed issue???
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