I have a 70 Vette with a RHS built 383 magnum series SBC. I have 270 duration comp cam, holley street dom. intake and a 750vs holley carb, my ignition is a mallory unalite and msd blaster 2 coil. The car runs great at part & medium hard throtle but under full throtle the car stumbles and surges badly in the upper rpm band. I have tried 3 different carbs and all seem to do the same thing although my edlebrock 750 cfm carb off my camaro did better than the holleys but the factory breather will not fit it. Any thoughts on this? The car runs wicked fast when its right but I cannot seem to find this problem.

Help!

 

If the same thing happens with different carbs, you can rule that out, most likely... How old is the distributor? I went through a very similar thing a few years ago with missing and poor top end performance, and found that my distributor needed to be rebuilt/replaced. Dropped in a new one and the problems were over. Do you have another distributor you could swap in to check this?

 

I have a similar problem, runs great until you stomp it and hold it there, then it surges a bit. I suspect I have a fouled spark plug or spark plug wires that may be cross-firing. As far as the distributor goes, I had a road runner that did the same thing, the distributor shaft would get sloppy and wobble, throwing off the points adjustment, then the car would sputter and miss a lot.

 

Thanks for the replys. The distributor is only a few years old and was purchased when I bought the motor from RHS. The car sits allot but does get started monthly but I dont get to drive it a great deal as I have 4 old muscle cars and a full time job / life.
The motor has maybe 2500 miles on it. Plugs were replaced but it still does it. This problem has been reoccuring since I put the motor in the car. It comes and goes but seems to be here to stay this time. The car pulls hard then stumbles, then pulls, then stumbles, surges under full throttle like its trying to make power but cant. It doesnt back fire or miss it just surges.
I guess I could replace the distributor but like I said its only got a few miles on it. It could be timing related I guess. I set the timing by ear. I advanced it until the car spark knocked and then back it down until it just stops knocking. The cars starts easy enough and runs cool on the temp. guage. The engine compartment seems to hold allot of heat inside and the motor is quite hot to the touch but the water temp. stays around 180. I have a holley high volume mechanical fuel pump but I am unsure of the fuel preasure. The engine has dart 2 sportsman heads w/ 2.05 valves and was a long block crate motor built in the mid 90's by RHS as I said. I paid 3K for the motor back in the 90's and when it runs right its unreal but this problem is kicking my butt.????

 

BTT I need help!

 

This is like a Sherlock Holmes mystery.

First off, DON'T ASSUME that what you have installed on your engine is working properly. THAT'S A FATAL MISTAKE.

I would first THOROUGHLY investigate the ignition system. That means plugs, plug wires, coil, cap, rotor, module, ground connection, on and on.

When I had a similar problem, turned out to be SLIGHTLY TOO LONG SCREWS HOLDING THE COIL TO THE CAP ON THE HEI that caused arching.

Attention to detail, and I mean ATTENTION TO DETAIL, is your best friend here.

Hope this helps.

Jake

 

Check to make sure all your plug wires are not laying on headers or exhaust manifolds,... check plugs to see condition (black ) and are they gapped correctly?

 

I would also be suspicous of the condition of your rubber fuel lines. I have heard of cases where the lines were actually getting sucked shut.

 

Wow. Sucked shut.. I never thought of that. Before the fuel pump or after? Or both? Thanks

 

make sure fuel is getting thru..a part plugged filter,trash in tank,to small fuel lines..rule out fuel starvation problems.Can you put a timming light on it and take it up in RPMs,by hand? Does the light bounce all over the place or pretty much stay the same on the same mark? You might have to find aplace that still ahs the old "sun" tune up machine and plug it into that to check for firing problems.I think with lazy lift (valve)springs, the problem wouldnt change with changes of carbs.Oh,have you an extra coil to sub in place ,for a test?

 

Before.

 

Where is the power for your distributor coming from?

 

The unilite distrib. hooked up to the wires that were feeding the other distributor as I recall. Its been a few years since I installed it though. Whats your line of thought? Please explain.

 

I don't know much about the Unilight,but aftermarket units need a solid 12vlt supply. The stock red/white is a resistance wire that drops voltage to around 6vlts. That made my MSD do really strange intermitant things, alot like you describe...was just a thought...

 

Wow thats a good thought! I checked and the directions call for the loom resistance wire to be used. I checked my voltage and I have 3 volts to the coil & dist. with the motor running or with the key just turned to the on position. I would say that the load of the coil & dist. drops the actual voltage somewhat but 3 volts is what I read with my fluke meter. Here is a link to my dist. wireing diagram. My car is wired per figure 2 drawing. Good thought though 68RATVT. You had me excited for a minute there. Too easy I guess.
http://go.mrgasket.com/pdf/Unilite_Wiring.pdf

 

I took the time to wire the distributor per figures 1 as well. No change in performance though, the problem is still there. The people at Mallory suggest I replace the MSD coil with one of their own but couldnt tell me why other than "past experiences". I also saw where you must not use a solid core plug wire like copper with the unilite, but my wires are Moroso glass core so Im guessing they should be ok but I may try another set of wires. I hate just changing parts without a known defect though. Another frustrating afternoon.

 

Have you checked the fuel filter?

 

Yes the fuel filter is good. I have 2. a see thru russell just before the carb and a large fram cartridge type just after the fuel pump.

 

You say it doesnt miss or break up , so it just feels like it falls on its face or surge like it's not getting enough gas,need to check fuel pressure to rule out fuel pump,diaphram in fuel pump could be bad from sittin,so check fuel psi.

If it was poping and breaking up I'd look into dist.but if not ,then fuel

 

Looking at the diagram,it says to use the resistance loom wire with 12v,you are reading 3v. If the voltage is low then current is high,bad according to Mallory. What kind of module are you using?Find a clean 12v source....