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Old 05-05-2013, 07:28 PM
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383 misfire/cutout/bog at high rpm

I have a fresh built sbc 383 that cuts out at 5k rpm like clockwork under load. I can stand on it and it runs very hard and strong right up to exactly 5k every time and falls on its face. feels almost like its hitting a rev limiter. im running a holley 600 carb with float level set correctly, edelbrock rpm dual plane manifold, 9.9:1 comp, lunati voodoo 60103 cam, 041 casting 1.94 heads and roller rockers. .035 plug gap. msd 6al box with a new hei dist, and a 65k volt coil. advertised to be good to 7500 rpm. this engine was a 355 until this winter when I built it as a 383. it ran all the same components except a more aggressive cam before and I installed the new distributor mentioned above as the old one was clearly worn out upon inspection, the msd was run on the old engine also. I could run the 355 to 6500 no problem and did so right before tearing it down, ruling out valve float. initial timing is 16 degrees, total falling right at about 36 degrees, set with vac advance disconnected. total is all in long before 5k rpm. rev chip installed in msd is a 6700 so its not that either. im at a loss here, it runs great right up to exactly 5k everytime, and cuts out/bogs/misfires, whatever you wanna call it, it falls on its face. feels to me like its loosing ignition but who knows. never did this before. is it possible the msd doesn't like this new dist/coil combo for some reason? would installing the ign module back into the dist and forgetting the msd be worth trying?
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Old 05-05-2013, 08:11 PM
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I would remove the dizzy and MSD parts, install a boneyard HEI and take it down the road.
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Old 05-05-2013, 10:45 PM
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I doubt that cam would make great power over 5k but should rev ,,,
check fuel delivery rate,ignition system.bone stock HEIs dont work all that well at high RPM without a better coil.600 cfm is a little small for top end charge or engines over 400 hp,,,
check exhaust system for flow restriction...valve springs,,,look inside cap for cross fire? check plugs and gap?
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Old 05-06-2013, 03:57 PM
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Swapped out msd box and put stock ign module back in, no change. Right at 5k it sounds like its detonating, something like a popcorn sound. Even though i thought i ruled out valve float, would floating valves make that kind of racket under wot? It doesnt work into it, it slams into a detonating sound instantly at 5k like i hit a wall.
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Old 05-06-2013, 04:07 PM
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what is the maximum timing your engine reaches?what kind of fuel? hows the cooling system? what heads?,,,,,
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Old 05-06-2013, 04:19 PM
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I guess a good quick question would be how do i verify if the valves are floating? If it revs out fine in neutral would this mean that the valves are not floating?
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Old 05-06-2013, 05:50 PM
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It has 041the casting heads, 1.94 valves,64cc chamber, which my research has led me to believe they are old vette fuelie heads. They were gone through and checked over before this build, new seals installed but the rest of the work was still good from previous build. Valve job, bronze guides, hardend seats etc. 91 octane. Total timing is set at 36 degrees with no vac advance, if i hook it up and rev engine in driveway, it will go up to something like 58, but no load part throttle like that, it pulls manifold vac and thus advances timing. I confirmed vac advance wasnt the issue by disconnecting and takin it for a drive. Same result. Cooling system is adequet and operates correctly. Running aluminum radiator with elec fans, no overheating to speak of. Runs a pretty consistant 180 to 190:
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Old 05-06-2013, 06:17 PM
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flat valve springs ,the engine will just roll over and make no power in upper RPMs.check for fouled plug(spit on all the header pipes.
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Old 05-06-2013, 07:48 PM
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By spit on the headers to check for fouled plugs, i assume that means your wanting me to look for a cold tube, indicating a weak cylinder, i have a temp gun to use, ill check into that tommorow, but it runs so sweet everywhere else i doubt i will see anything significant. By flat springs do you mean my springs are possibly inadequet/worn out and need to replace them with a set recommended for my cam? An added note, i free reved it in the driveway and it does the same thing, only comes in slightly later like about 5500. Ive gotta guess its making it to 5500 as opposed to 5000 because it free revs much faster than it would under load.
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Old 05-06-2013, 11:26 PM
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I'm not a cylinder head expert, but are these 327 "fuelie" heads? If so, with the 1.94 intakes, I'm guessing you get great performance to about 4500 RPM, a little fading, and hit the wall at 5000 RPM. The guess is that you are flow limited. At 5500 you may be floating the valves as a result of improper springs, and see that when you just spin it up in the driveway. Does the engine behave differently in different gears? Like a slightly better peak in 1st than second, second than third, etc? If so, I think flow is the issue. If I'm right, porting may help, but I'm certainly no expert on that. I'd be looking for a different set of heads, or at a minimum rework to accommodate 2.02 valves. And, always use the right springs for the cam.

PatM
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Old 05-07-2013, 06:02 AM
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Its possible those could be 327 fuelie heads but im not for certain. It doesnt really fade, just hits a wall right at 5k. Same results no matter the gear, doesnt get worse in higher gears. These same heads were on this engine as a 355, and would rev out 6500+ no problem. I assumed my springs were adequet as i went with what i consider a less agressive cam (almost same lift, much less duration) but maybe the ramp of the lobes is more aggresive and these springs were right on the verge for the last cam and just cant handle this one.
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