400 sb uneven feel from 2800-3000 rpm - Hot Rod Forum : Hotrodders Bulletin Board
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Old 11-13-2003, 07:02 PM
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400 sb uneven feel from 2800-3000 rpm

I have a 77 chevy 3/4 ton 4x4 with 35s, 4.10s, th 350 with stock conv, and a 400 sb with 9.8-1 hyperutes, 993 heads with 194 ints, performer cam and manifold, stock exh manifolds into 2 1/2" pipes with glasspacks, stock hei and 74 pontiac 455 qjet. The carb and hei were set up to Clifford Ruggles recomendations (Absolute genius). This setup really runs. I have never had a sb powered rig pull throughout the rpm range like this one does. My problem is that as i am accelerating I get an uneven feel in the engine at 2800 rpm that goes away at 3000. There is no power loss, but it drives me nuts. The hei has 12 deg initial, 20 mech all in by 2700. crane adj vac adv has 12 deg all in by 10". Total with vac hooked up is 45 deg all in by 2700. Is this curve suitable with a rig this heavy? Also, would headers be beneficial to this setup? Thanks, im a first timer.

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Old 11-13-2003, 10:25 PM
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Does it detonate under load? If not then the timing is fine. Try a little more if you want.

Headers will improve on what you already have.
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Old 11-13-2003, 11:13 PM
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Was the engine properly balanced when you had it built? Out of balance vibrations usually come in at lower rpms like where you are having problems.

Its also possible that you are feeling the transition of the primary metering rods pulling up a tad late. Could be moderately lean at 2700 and then by 3000 the rods are totally up and richer. A plunger spring change may help.

Mark
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Old 11-14-2003, 08:07 AM
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That compression ratio and cam combination is goring to make a lot of cylinder pressure. If you wait to hear detonation or spark knock, you may be noticing parts of your pistons flying out of your exhaust pipes. I wouldn't run more than 34-36 total with that set up and don't use less than 93 octane fuel.
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Old 11-14-2003, 09:37 AM
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Thanks for the replies fellas! I have changed power piston springs a number of times experimenting. The problem doesnt seem to ever stop with that. The qjet set up now is the most responsive set up I came up with throughout alot of jet/spring/rod combos. If my engine was out of balance, wouldn't the problem be constant no matter what fuel or towing I was doing? Do you think That I am using too much timing now? Also, is this excess cyl pressure a bad thing?
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Old 11-14-2003, 10:15 AM
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Excessive cylinder pressure and/or too much timing equals cupped valves, broken ring lands and rings, damaged head gaskets, and other fun things!
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