Chevy 350 High rpm break up/power loss. Help me figure this one out - Hot Rod Forum : Hotrodders Bulletin Board
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Old 09-14-2009, 11:27 AM
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Chevy 350 High rpm break up/power loss. Help me figure this one out

hey guy im running a chevy 350 with summit brand vortec heads a Comp XE274 cam all comp cam valve train( pushrods, Spring , retainers , locks) 1.6 ratio full roller rockers, flat top pistons, a holley 670cfm street avenger, and a summit stage one intake... o and a shorty headers to 2 1/2" y pipe to 2 1/2" cat back with no cat and ultra flo ss muffler.

My problem is when i run my car it loses power at around 5300rpm and sometimes it will start to break up at around 5600-5900rpm. The cams RPM range is 2000-6000 and the intake is idle-6000. I tries multiple carbs from 625 demon and a edelbrock 650 and still the same problem. i had a 3 " cat back on it for a while and was no different. the heads are ported a little and the intake is gasket matched to the heads.

Any ideas????

Thanks alot

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Old 09-14-2009, 11:39 AM
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Distributor is breaking up...HEI?
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Old 09-14-2009, 11:57 AM
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Quote:
Originally Posted by BBCMudbogger
Distributor is breaking up...HEI?

well i bought a brand new HEI distributor and this happend so i tried my old one and the same thing happend. I tried timing every where from 30degrees Total to 38 degrees total. Right now its about 36-37 total and about 11-12 degrees intial. its got medium springs in the mechanical advance and the rest is vacuum. sometimes it does it worse than others. its got good plugs in it with a real close heat range they are light brown.

It gets just a little bit of oil on every plug out past the threads. it doesnt smoke though and idles fine.

Im pretty confused with it. im pulling the motor out this winter to take a look at it and see inside but it was a crate motor years ago and the motor has VERY low miles on it. The inside of this engine was like brand new. even the lifters were like brand spankin new. but it was like a 190 hp replacment truck engine so i had to amp it up just a little

the car ran a 13.9 with a 3.73 posi and a stock torque convertor and i know it has more in the upper rpms.
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Old 09-14-2009, 12:02 PM
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Somebody have an msd setup you can try? How about your valve settings...is the cam torn up?
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Old 09-14-2009, 01:00 PM
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Quote:
Originally Posted by BBCMudbogger
Somebody have an msd setup you can try? How about your valve settings...is the cam torn up?

The valves are set at 1/2 past 0 lash. the cam only has 5,000 - 6,000 miles on it. It always kind of had this problem since it was together. I dont think anyone i know has a msd around ill have to check. I runs real goo up to 5400 and it just doesnt pull near as hard as say 4800rpm. Then it breaks up. It kinda sounds like detonation but kinda doesnt. I showed a few people the sound when it breaks p and everyone said they never heard nothing like it. This setup should be a grreat combo to 6,000rpm and I should be able to take it even a little higher if i wanted to.
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Old 09-14-2009, 02:17 PM
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What fuel pump? fuel pressure? it`s either running out of gas or running out of spark.
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Old 09-14-2009, 03:12 PM
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Quote:
Originally Posted by DoubleVision
What fuel pump? fuel pressure? it`s either running out of gas or running out of spark.


Well it has a stock pump on it but i tried an edelbrock pump and it didnt seem to help.

something else i noticed is when i run use to run 87 octane in it, it would deisel over when i turned it off. So i started putting 93 in it and it pretty much cured it except if I just finish a race and then turn it off, it still deisels over a little. So is there some way it could be maybe having pre ignition or something from a hot spot in the chambers when its at 5k or is that just a crazy idea??

just an idea.
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Old 09-15-2009, 09:09 PM
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Check the voltage to the hei must be above 12.Also what module is in the distributiors? stock or aftermarket what brand and coil? Could also be valve float but if it is should be getting worse happening at lower and lower rpm's.
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Old 09-16-2009, 08:46 AM
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man you have the same exact problem as my 81 trans am, with a 355 afr heads and a big crane cam, originally my timing was way off cause the hold down for the distributor wasnt working right so my timing would just go everywhere after i set it, it would start to spudder around 4500 though and chug it would make sounds like pops but they sounds more like bass then pops, then i noticed i had a oil leak from the lifter valley to the intake ports on the heads, puta double thick gasket on now thats better,
as soon as i set my timing right the edlebrock fuel pump broke went through 2 so far and it takes like a week to send one back in and get it back so i finally bought a expensiver holley got it yesterday and gonna put it on hopefully thats my problem, but when i set my timing right itll pull to 5000 so i know that helped
oh yeah my car dieseled to when i shut it off till i changed the intake gasket to the double thick one, i hear diesling can be caused by the intake gasket not sealing on the inside could be caused by porting i read to if you port it to big for the gasket to seal? so i read anyways im no expert but going through the same problem on my car for months, my car will spin the tires for in every gear to if i keep the rpms low enough for it not to hit the brick wall so it runs real good till those rpms

if my holley fuel pump dont cure it im gonna get a full msd setup distributor 6al box and wires and a blaster coil then if that dont work i drive my car off a cliff and put it on youtube for you guys to watch just kidding

my car however in reverse will rev to 7000 rpms+ idk why it will do that in reverse but not the forward gears, cause its geared lower of course but what does that tell me?? nothing really does the shorter gear take less fuel? less spark?? i never checked my current going to the hei im starting to think that might be a good thing to check out
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Old 09-16-2009, 10:01 AM
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Sounds to me like 93 octane isn't working for you. Dieseling is usually caused by the valves getting red hot. 93 is yesterday's regular. Have you tried 102 or 108 race gas? Octane booster will leave a redish colored residue on the valve faces and head chambers and won't help for too long. You need to cure the dieseling problem first before you start burning valves.
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Old 09-16-2009, 11:55 AM
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Do you know what your timing curve is? How much total?.....nevermind
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Old 05-05-2010, 03:55 PM
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O yeah just thought id add i figured out the problem. My spring height was not set right causing the valves to float. Machince shop fixed it up.
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Old 05-06-2010, 12:08 AM
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Oops!

Good to hear you got it going.
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