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Old 06-18-2004, 02:39 AM
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396 won't rev

Hi Guys,
Have been lookin thru previous posts but can't find the answer.
My fresh built 396 runs fine to about 5000rpm then goes all hairy and misses and farts.If the throttle is released it carries on running rough all the way back too idle.
It seems to be the same on the road as it is being revved at standstill.
The motor has 9.5:1 comp,Edellbrook Street tunnel ram,Two 650 vac sec Holleys(one is blocked off with a plate underneath - although the egine did the same when running both carbs),Hydraulic cam (cam report has 0.29258 peak cam lift and 0.45813 peak valve lift - does this make sense to anyone?) valve springs had 130lbs on the seat? (I think) comp cams roller tip rockers.MSD billet dissy and 6AL with 7000rpm pill plus blaster 3 coil.The motor was ballanced and had some minor porting on the oval port heads(iron).Timing was set to about 38 degrees total.Runs Holley Blue puimp and reg set to 6lbs - Fuel pressure does not drop off at all when running rough.Plugs seem to be o.k colour.

Help please!!!! I thought 396's were meant to rev????

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Old 06-18-2004, 03:21 AM
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What kind of air filter set up do you have?? Sounds like you'r starving for air, going rich and loading up. Run it down the road (somewhere where you can pull off the road and pull the plugs) Run it to where it acts up for a few seconds, keep the throttle where it is and turn off the ignition (you want a snap shot of what is going on at that instant, if you let it run back to idle and pull the plugs you will only see what it's doing then), pull the plugs and look at the color. If sooty black your rich ( not enough air or too much fuel, make a pass without the air filters ). If they are white ( too lean, not enough gas, bigger jet size).. Good luck...

Last edited by Pony; 06-18-2004 at 03:28 AM.
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Old 06-18-2004, 04:05 AM
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yeah, thanks for that - have been trialing without air filter - pulled plugs and they are pretty right - jet sizes/ 1 or 2 carbs have made absoutely no difference to this problem
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Old 06-18-2004, 04:24 PM
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Sounds like you got the intake end pretty right. How about the exhaust end. A restrictive exhaust system will stop your revs when it can't get rid of the exhaust fast enough?
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Old 06-18-2004, 08:22 PM
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No - that is pretty good too - headers ito 2 1/2 system with one 2 1/2 chambered Forzaflow muffler which has 2 inlets and 2 outlets - problem still there with open headers!
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Old 06-18-2004, 08:30 PM
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May be the heads? What are the casting #s not many oval ports will flow past 5000 rpm or so. And Check your fuel pressure.
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Old 06-19-2004, 02:02 AM
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O.K. - but surely no matter what the heads are the engine would still run above 5000rpm without going all hairy - bearing in mind also that once this happens it stays that way for a couple of seconds even as the revs are falling back to idle.
The fuel pressure is solid as a rock - I have a isolated fuel pressure gauge on the dash.
Thanks for your reply - but I'm still scratching my head????
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Old 06-19-2004, 08:13 PM
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High end drop

Doc here

Check with your Cam provider, and let them know what you are experiencing.

It sure sounds like "Valve Float" to me.

Doc
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Old 06-22-2004, 01:24 AM
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Thanks for that Doc - the more I think about it and the more guys I talk to - the more I think that could be the problem.How much effect would the lash of the rocker gear on the hydraulic lifters make to this problem?
Cheers,
Dom.
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Old 06-22-2004, 01:53 AM
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LASH

[color=orange]Doc here

Not sure How much effect it would have, The guy's with the software Could tell ya' Better than I can...

Let them run the numbers and see what they think.... I would think duration would be more the offender at high RPM's

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Last edited by docvette; 06-22-2004 at 04:17 AM.
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Old 06-22-2004, 03:22 AM
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i had a simular problem once on my 87camaro , cept it turned out to be the fuel pump it would bogg down at like 4500-5000
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