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Old 08-08-2003, 08:55 PM
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392 hemi problems

My current hotrod project is a '38 Chevy coupe with a '58 Chrylser 392. The engine is equipped with an Isky 280 degree/ .440 lift cam, Holley 650cfm manual choke carb, 11 to 1 forged pistons, stock bore and stroke. My problem is this. At idle it only has 7 inches of vacuum(idle speed is approx 850 rpm with engine warm) If I check vacuum at 1500 rpm it increases to close to 16 inches. Needless to say it idles like a Top Fueler on nitro. I have checked every possible source of vaccum leak, intake gaskets, carb gasket, all vacuum ports on carb, disconnected vacuum advance, compression averages 155 lbs, 160 highest, 150 lowest, which should rule out sticky or burnt valves, timing is set at factory 8 degrees btc. I know the cam will affect vacuum some, but this is a rather mild cam, and cannot account for this low a vacuum.

Any suggestions?

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Old 10-22-2003, 10:05 PM
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I would check valve timing. I had a hemi timing gear that was marked one tooth off. Harmonic balancers can slip. Check cam and timing mark against true tdc. Put some advance in it and see if it helps.Good luck
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Old 10-23-2003, 05:37 AM
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Try this. Suggestions from the badasscars web site.

You really have to time an engine of this sort solely by the total advance. Most engines like a total advance of 34 to 38 degrees BTDC.

Leave the vacume advance hooked up and take your engine up to about 3,500 RPM and set the timing to around 36 degrees.

An engine with the timing too advanced will have a high and erratic vacuum signal, might have a snappy throttle response but not pull very well under a load, will have pre-ignition (detonation) problems and it will idle rough, like it has a bigger cam than it actually does.

This sounds like this might be your problem providing all valve timing is correct.

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Old 10-23-2003, 08:06 AM
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I run a 354 hemi with an old Honest Charlie 3/4 cam, whatever that is. Once you modify an engine significantly, factory timing specs don't mean much. I learned a timing trick on this board that I now use on my hemi and it works great.

First, adjust the idle and splice a vacuum gauge in the vacuum advance hose. Leave the vacuum advance hooked up. Now adjust your initial timing to maximize vacuum at idle. May need to go back and forth between idle speed and distributor advance 'til everything is optimal. Once vacuum is maximized, retard the distributor to back down vacuum reading a couple of inches and it should be set perfectly. Remove the gauge from the vacuum advance hose. If the engine pings, back down the advance a couple more degrees. Works for me.
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