Hi all, I'm quite new to posting on this message board. In any rate, my '86 based 350 motor has had a very strange problem that prevents me from tuning, idling, and enjoying the ride everytime I go out for a hot lap. I guess I'll start with describing the problem, then go into how the motor is built.

This motor has an (what I initially thought) erradic timing condition. I cannot for the life of me get the motor to idle without the rpm slowly raising and leveling off at around 2K rpms. It will idle at ~750 for a short amount of time, then it goes into its little fit, and the rpms float on up. Sometimes it idle perfectly for almost a minut, then very sightly increases rpms until it ultimately surges on up and staying at 2K. In traffic, if it does this, I'll hold the brake and engauge the clutch to load the motor to return it to idle. It cams like a freak for a little, makes me smile, sounds killer, then the demons come back out to ruin all the fun. I thought it was my distributor at first, but when I installed the new one MSD sent me, it still did it. I cannot tell if it a timing problem or carb problem. Timing at idle is a conservative 8-10* base, and around 15-20 when self-fast idled. Very hard to describe, and equally hard to diagnose.

I've tried different carbs, new distributor, any amount of base timings, and the heaviest dist weight springs MSD sent. Even when it does idle, if I so much as drop a hair on the throttle, the rpms soar. Almost acts like a vacuum leak, but I do not have any. I've even tried disconnecting the power brake booster as a possible vacuum leak, and even replaced the intake mani gaskets! I'm going bald from all this stress. Not to mention my tires too cause this is really a PITA to drive. I'm leaving posi marks all over town I'm so pissed.

Here's the run down on what I'm dealing with....Like I said before, the block is an 86 1-piece rms truck block, .040 over flat tops, scat crank and 6" rods, trick flow 23* heads, Comp XE284 grind hydraulic tappet cam kit, vic jr intake, and a Holley 750 dp carb. Timing is handled by an MSD 8570 pro billet dist, 6al ignition, and a blaster ss coil. Simple as that.

Amazingly enough though, I cannot tame this very simple set up. If anyone has any suggestions, questions, comments, words of hope, or pep-talks, please feel free before I go insane, lol. Any help at all is GREATLY appreciated. Thanks for reading this and for your time...matt

 

It may sound overly simple but, check for a vacuum leak. Be aware of the places that could cause problems. Is the car fuel injected or carbuerated? Good luck

 

check the intake end gaskets.

 

The motor is carb'd and I'm pretty sure there are no vacuum leaks. In regards to 1981zzz's suggestion, could the end gaskets of the intake manifold affect anything is they do leak? Crankcase pressure is separate from a motor's vacuum pull isn't it? Thanks for the replies guys...matt

 

Yes the end gasket if it is leaking will make a difrence there is a way to check for vac leaks you take a propane torch and turn it on start the car and run the propane torch all around the carb and intake if the motor reves up or down were you hold the propane thats were your vac leak would be

 

Thanks for the tip zzz. That's a good idea of checking for leaks. I'll try it out tomorrow. Thanks again for the help....matt

 

Does the dist have a vacuum advance on it? Where is the vacuum coming from? You should be connected to the side of the carb, "ported" vacuum.
Also, have you checked the power valve to see when it is coming in, or if it is blown? If it is adding fuel, it could creep your idle up slightly...

 

Thanks for the reply Tex. It's funny you mentioned the power valve b/c I just pulled the carb to check it out. Before I removed the car, I checked the vacuum at the front base of my 750dp, and it read 5.5" at ~800rpms. After pulling the power valve, it looked as if it had seen better days. The diaphragm looks kinda suspect. I reallt doubt this power valve is the optimal one for my motor with the radical cam and 5.5" of vacuum. I'll throw in the right pv and some new gaskets and see what happens. Oh, the distributor is not vacuum advance. It's the msd with advance stop bushings and the works. Thanks again for the help. Fire away with any more tips if I'm missing something...matt

 

Be aware that some Holley DP have a PV in the back, also. If you don't want to pull it down, you can go to Holleys site, look up the list number, and it will tell you the jets and power valve sizes..