Big engine problem

[Peter1962]

Hi,
I recently bought a 1970 Corvette LT1, the Corvette had stand still for three years, in the beginning it run great, then after a week that I had bought it, it began to shake and backfired. We changed the plugs, plugcable, Ignition, airfilter, we cleaned the gasfilter and the Q-jet Carburator was also cleaned.
When the corvette stands still the engine runs great. When I pull up till 2500 rpm it still runs great but when I pull further up, above 3000 rpm, nothing happens, it looks like the engine is dead, but when I put my foot of the gas, it will run again. It looks like it has something to do with the secondaires.
Who can help me?

Peter

 

[AKRAY4PLAY]

double check your points in the distributor. i'll bet that is your problem, if so look to upgrading to a module or replacing the distributor with an HEI style for reliablility and worry free driving. Brian

 

[JohnZ]

LT-1's came with a Holley on a high-rise aluminum intake - someone has changed the intake and carb if it has a Q-Jet on it (which doesn't have a power valve - they're only on Holleys). You either have a restriction in the fuel feed system or an internal carburetor problem; check the float levels and the adjustment on the secondary air valve opening spring.

 

[cwerve74]

I second the re-check the points. Be sure you have replaced the capacitor. A weak spark can cause the symtoms you describe. If the spark is weak the plugs will look black & sooty. I always like to do the easy fix first. If thats not it press on with the fuel deliver as a problem.

 

[Ol Blue]

Peter
I had a problem that sounded similar to yours. I put a new coil on and the problem went away.
Ol Blue

 

[Jack]

Did all 70 LT-1 come with Transistorized Ignition (TI) ... or was it an option? I have no experience with the TI but I understand it was forerunner to later HEI and like HEI had no points but unlike HEI it had separate amplifier box and separate coil.
JACK:gap

 

[tomtom72]

Hi Peter, I would recheck the point gap and swap out the coil if the point gap is correct as was said by the guys here. Then the hard stuff comes next, the fuel pump on a LT-1 has specific tests to pass: 1.)preassure 2.) leak down test 3.) Volume over time test. Assuming you changed the filter, the pump must pass all three tests. If it passes, then I would look at the Carb. You should get a licensed reprint of the factory shop and assembly manuals because they will be a great aid in servicing your car. The generic manuals, Chiltons, Clymers, etc., are okay; but nothing beats the GM manuals. Also, as a point of originality the 70 & 71 LT-1s came with Transistorized Ignition(TI), 780cfm Holley dual feed single pump Vacume secondary Holley number 4555 or 4489; further they all had aluminium Hi-rise intake manifolds,P/N 3972110. Also all LT-1s had flat tappet mechanical cam and lifters( requiring valve lash adjustment periodically).When and if you have to check the fuel pump you will have to figure out which pump you have so you will use the correct benchmarks for the test. An LT-1 pump has only a fuel feed line and no return line, P/N 40709. Hope you have good luck with the car! tt72

 

[mike weyman]

my nickel says coil or fuel pump, maybe fuel filter,mike