Miguel

My motor seems to be breaking up past 3000 rpms in neutral and running, by breaking up I mean rpms are going down between 50-200 rpms. It doesen't backfire but makes like a slight air puffing noise through the exhaust when rpms pass 3000. Checked fuel pressure at 7 psi gradually goes up to 12 at full boost from a procharger making 7.5 psi. Checked timing and when I lowered it to 7 degrees it made white smoke come from tail pipes and still broke up in same rpm range, going to 20 degrees initial stopped the smoke but same reaction. Temperature is at 150, oil pressure is 42, volts 14, boost at 1000 rpm idle is 2 psi. Filled the tank with gas and carb cleaner, no difference, Spark plugs look tan color and everything looks good. I have a 383 with a intercooled procharger, Dean Nickerson 750 cfm holley carb, Boost referenced fuel pressure regulator, 130 gph mallory electric fuel pump, 1/2 fuel lines, 4" Open exhaust, full MSD ignition distributor,coil,6M2 and a bunch of other mods. What could it be, because I'm stumped? Any help will be greatly appreciated. Thanks.

Cord

Often a "Pffffft Pffffft" sound from the exhaust is a burned exhaust valve. What you are hearing is the compression leaking past the valve. If the electrical system checks out, then I'd start with a compression test.

blown formula

7 # boost and a TOTAL of 12# pressure ....man, your in trouble....base pressure should be 7.5# to 8.0# PLUS 1# for each pound of boost......let's see 7.5 + 7#=14.5-15# total pressure. Maybe the boost is getting high enough and the pressuire is low enough, it is pushing the fuel BACK into the pump....leaning out the motor???? It sounds like a good possiblity here......
Boost the pressure out of the regulator and be sure the boost compensation is working on the regulator.....VERIFY your boost source point as the blow box...NOT the intake. Good luck.

Miguel

I don't think its the fuel pressure but hope its not a burnt vavle, if it is can I still run the motor if I dont go over 3000 pms or do I have to fix it right away, also what is a burnt vale, is it bent or something? And might It be that the roller rockers are misadjusted, and not letting the valve close? Thanks for the help

Cord

Well, if we assume that there is 20 degrees of advance built into your module, and you set 20 degrees of initial advance, you'd have 40 degrees of total advance. Now we could get a pool going on how long your engine will live under boost with that much advance, but I think that most people would agree that you wouldn't finish your first hard run.

Now if only the valve is burned, then I suppose that you could procrastinate the repair. Your motor won't make full power and you'd be open to a catastophic failure if it was damaged by over advancing the ignition. It's amazing how big of a piston chunk can actually pass through a valve, but if it gets caught, that piston is going smack it and that's when things get ugly.

Now, lets be clear, I'm only suggesting the burned valve based off your statement of the puffing sound from the exhaust. A burned valve wouldn't get you any white vapor out the exhaust. But a blown head gasket would give you the vapor. That would also be consistent with over boosted/timed/lean application. Do you see where I'm going?

A burned valve is actually a catch-all phrase. The valve could be warped so that it no longer seals on the seat or there could actually be material eroded away. In either instance, a leakdown test will tell you if the engine is still healthy. From there we can try to determine if it's a fuel or ignition problem.

Miguel

Did a compression test on all cylinders and there all where they supposed to be and match good. Also increased fuel pressure to 9 at idle and found no difference. Someone told me it could be the distributor pick up. Still stumped

Cord

Put a timiming light on the engine (better yet a scope) and try running the revs up. See if you can spot a skip in the ignition. If you want you can start swaping parts. Do it one thing at a time. Cap, rotor, coil, pickup, module, module harness, etc...