Breaking up above 4500 RPM
 

Aright here's the deal,:blah:

I've got an '86 Iroc that I've put a 355 in with a fresh bottom end, flat top pistons with valve reliefs. Toppin it are a set of AFR Eliminator Street 180cc heads a performer intake and a rebuilt quadrajet from a early 80's 454 truck motor. The cam is a CompCams XE268H. For the ignition I have a GM module, accel weights and springs, MSD cap and rotor with an accel hei supercoil. Wires are accel 300 8.8's and AC plugs. When I hit about 4500 under 3/4--> full throttle it starts breaking up on me. Car runs strong up until this point. I have the timing set at a base 10 degrees BTDC. The only thing that is less than a year old on this setup is the bottom end and the coil. The bottom end is about 3 years old, still only has about 7k on it and the coil I'm not sure, was in the car when I got it 4 years ago.


Any Ideas?


Thanks

Re: Breaking up above 4500 RPM
 

could be the control modual... try upgrading it to an accel/pertronix. do u kno wat the total timing advance is??? should be around 34-36 total. the control moduals cause missfires in the above 3000 range... since it might be faulty it could decrease the charging of the coil resulting in miss fire/ miss/ and hesitation aka breaking up on u... still the GM's/standard are cr@p and i would put in a newer one like accel/pertronix bc they have better electronics and help for longer charging of the coil and are rated to higher RPMs. i have an accel on mine.

wat about ur fuel pressure in the 4500 and up range? id say spark/timing/or fuel is your problem.

Re: Breaking up above 4500 RPM
 

I would check the fuel pressure with a gauge at WOT.

Re: Breaking up above 4500 RPM
 

I believe my total is about 34* I'm not really sure what exactly my fuel pressure is, haven't checked it. I am running a mechanical pump nothin fancy. Whats a good pump to use?

Re: Breaking up above 4500 RPM
 

What engine came in the car originally?

Re: Breaking up above 4500 RPM
 

Originally it had a Smog 305 4bbl. with a 700r4. Thats long gone...

Re: Breaking up above 4500 RPM
 

:iagree: :iagree: I'm thinking fuel delivery problem.
Alot of the time getting a fuel gauge between the carb and F/P is difficult. You can tell if it is fuel related or ignition by holding it in 1st gear and running the rpm up using less than half throttle. An ignition problem will disrupt the engine above a certain rpm regardless of throttle.
If it is smooth above 4500 at part throttle
I would replace (not check) the filter in the carb. I think it's $1.49 ,and report back.
Other possible causes:
mechanical fuel pump clogged.
drawing fuel thru a turned off/dead pump in the tank.
ignition module (especially if it is from 4 cyl).
coil that pulls more amps than module can supply.
insufficient valve spring pressure for cam.

Re: Breaking up above 4500 RPM
 

The filter in the carb is gone I put an inline filter in between the pump and carb. There is no pump in the tank either I know.

Re: Breaking up above 4500 RPM
 

Aright I took it out and eased it upto five grand and she didn't break up once, so I guess that settles it that I have a fuel supply issue. As far as High flow fuel pumps who's using what? Whats a good pump thats gonna work well for me? Holley, Edelbrock..etc...


Thanks