Need 383 help please!!

[Gary_gtx67]

Guys I'm banging my head here trying to figure out what the heck is going on. I have a 383 bored .30 with the Keith black flat tops, 906 heads pretty much stock, comp cam with .525 lift. What's going on is the motor starts misfiring after 5100rpms but cam has a power range to 5700. What am I missing? About done with this 383 and about to just build my 440....

 

[daredevil]

Weak coil, bad valve springs maybe. Or even a fuel issue. Will need more info.

 

[moparphilip383]

I have had the same problem. What I found is it maybe a weak spark from the ECU. Are you running the orange bax or gold or silver. I bought mine (orange box) from Summit racing, after 4300 it began breaking up. The only reason I new it was spark was that my shift light began missing along with the engine. Long story short I switched to the ignition box by mopar good for 10,000 rpm ( never been past 6,500) and she now pulls with no break up.

If not that check timing and octane.

 

[moparbob498]

I agree with above...need more info....but off the top of my head...I also say coil or fuel or valve spring ...hi-speed miss....good luck

 

[Gary_gtx67]

I'm running the orange box, new 3/8 fuel line and sending unit and a carter mech fuel pump. I'm not sure on the. Valve springs..... When it shifts to the next gear it will continue to misfire for a few moments then it clears and pulls hard again..

 

[Gary_gtx67]

Oh and coil is a super stock coil

 

[Gary_gtx67]

It will also do it without a load.

 

[RustyRatRod]

I'm not sure on the Valve springs.....

You built an engine and don't know what valve springs it has?

 

[court9155]

Probably ignition but man I would pull a set of springs down and check into them........the valve train is the most commonly overlooked part in the typical shadetree build.

Dan

 

[RustyRatRod]

Even the 383HP suffered the same factory maladies as the rest. Poor machining. TALL deck heights compared to spec. LOW compression ratios compared to spec. I mean don't get me wrong, they were made outta good stuff, but attention to detail was lacking. Still though, the 383HP made more power. That is the most important difference. I bet through blueprinting the entire engine to spec and degreeing the cam and heating up the advance curve you could unlock another 50 plus HP. Just my guess.

 

[Gary_gtx67]

These heads my dads. The ones I had built for me I took off due to a bad head gasket and tried these for shots and giggles. The ones I had work are a 516 head hardened seats 207,174 valves. And comp springs @200lbs

 

[Gary_gtx67]

Starting to think the 906 heads were a bad idea. I know it dropped my compression from 165 per cylinder to 125

 

[RustyRatRod]

Starting to think the 906 heads were a bad idea. I know it dropped my compression from 165 per cylinder to 125

Had you made a piston change, they could have been a good idea......

 

[67 B-body]

Electrical:
How old is the dist?
Do you have any play side to side of the reluctor? Is your air gap set correctly between the pickup and reluctor?
Is there a good chassis ground for the ECU?
How many volts are at the coil + terminal when the engine is operating?

Mechanically:
Do you have a fuel PSI gauge to confirm adequate pressure / volume?
Did you measure the fuel pump pushrod before installing it to confirm its length "not worn down?" Todays off the shelf oil can kill those things!

 

[Gary_gtx67]

All of the ignition is new from Mopar performance. I havnt checked air gap. What does it need to be at? When I built the motor I put a new fuel pimp
Rod in. Idk what the pressure is but I will look into that. Maybe run a gauge and line into the car?. The Ecu is mounted directly to the fender well. I will check the volts to the coil later just to be sure.

 

[Gary_gtx67]

The pistons are not a stock flat top. They are the kieth black hyper utectic with valve reliefs.

 

[RustyRatRod]

The pistons are not a stock flat top. They are the kieth black hyper utectic with valve reliefs.

What cam is in it? Seems like you oughtta have more than 125 even with 906s unless you gotta fairly large cam. Nevermind. I see it. The comp 525

 

[RustyRatRod]

Which one is it? This one?

http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=703&sb=2

If that's the cam you have, that ain't "pretty much stock". That thing has 253* duration @ .050". There's your cylinder pressure leak. Probably don't explain the miss, but you can certainly have a more matched cam in it. I bet it needs valve springs.

 

[67 coronet]

you said it does it with no load.how is the float level,could it be running the carb low at high rpms?since it goes a bit after your shift i wouldnt think it was electric since that should recover almost instantly with rpm change.

 

[Budnicks]

All of the ignition is new from Mopar performance. I havnt checked air gap. What does it need to be at? When I built the motor I put a new fuel pimp
Rod in. Idk what the pressure is but I will look into that. Maybe run a gauge and line into the car?. The Ecu is mounted directly to the fender well. I will check the volts to the coil later just to be sure.

There is nothing wrong with a 383ci except it's smaller cubic inch displacement, compared to the 440ci... You need to learn "how to tune" & diagnose problems with either set up... Get a MP Chrome for street/strip or Super Gold "race only box" for at the track, keep the Orange box for a back up or spare for on the street, if that's your preferences, You absolutely need to have the properly set up valve springs & push rod lengths & valve-train geometry, to match your camshaft... It maybe even a Carburetor tuning &/or fuel or delivery problem, some or most mechanical fuel pumps are just crap for over 5000rpm, I suggest an electric pump with a fuel regulator preferably a bypass style, with a good cleanable fuel filter before the pump & before the carburetor, wire all the fuel pump system with a relay to keep voltage constant & from spiking... I would also check make sure the wiring is all good & you have adequate grounds everywhere {run a 8ga.-10ga. ground wire directly from the block to the ECU}, also the 906 open chamber heads are 10cc+ or so larger combustion chamber, dropping the static compression, if you don't have the proper piston & head gaskets combination, to compensate for the larger 88cc-90cc combustion chambers... What do your plugs look like ??, what does the dist. cap look like ??, is there any black or orangish crusty stuff in there ??, you could have a cracked cap or something simple like bad wires, fouled/bad plugs, rotor, cap, reluctor air gap should be 0.008"-0.010" {make sure to use a non-magnetic bronze or brass type feeler gauge}, bad pick up or coil, etc... how long since you changed the plugs, wires, rotor & cap ?? Good luck & happy Moparing...