I am running the following combo:
454 +".030 with big valves on mildly ported 049 heads
Crane 272 cam with recommended lifter, stock stamped steel rockers
9.25:1 CR
Q-Jet - 3 different ones
T-400 with 3:55's and a Stall of 2200-2400.
MSD billet distributor triggering a Crane HI-6 into a Crane coil with Accel 300 wires and R44t plugs.

The engine starts and idles well pulling a steady 16-17" of vacuum. Timing is at 10 degrees basic and about 35 total.

The engine will take off and at about 2000 rpm's (this used to be 3500rpm's) it just stops - literally quits, like either the spark went away or you are out of gas. Let off the gas and in about 3-4 seconds it comes back and runs ok at a lower rpm. Now if you exceed 2000 rpm's in drive or any gear you hit the wall and it quits. So you go home at 1500 rpm's - slowly. I know I am answering my own question of it is spark or fuel problem - but where???

I have changed the following and the problem still is there just at different rpm's.

- Fuel pump, rod and "S" hoses.
- Several differenct Q-Jets even one from Lars that he ran went through and tuned up.
- Spark plugs
- I even changed the Crane HI-6 coil from an old style to a new one.
- Tried timing at 6, 8, 10, 12 degrees and no change
- Vacuum advance using ported and manifold

Nothing changes the basic problem of dieing out at somewhere betwen 2000-3500 rpm's.

I need some of the expert minds out there to suggest ideas or a plan to troubleshoot this problem. Thanks for your assistance.

 

I ain't no expert but I went to university with some guys who might have been!

Got the same problem.

I have had the QJET rebuilt twice, new fuel hoses, gas tank, pump, PERTRONIX Ignitorm, new coil, new plugs, new leads and timed it to perfection. I have got it to the point where it runs OK but still "stutters" in the 2500-3500 range. I am halfway through changing the ignition switch on the steering colum right now. Symptoms were that it would fire on start but die when I wen to run. Occasionally it took a while to start because it would not run.

Check your positive voltage at your coil. I honestly have no idea what it would be for mine but a friends BB 74 had 10.2 volts on it when it was running. (Mine is 4.5) .Check out this other thread where another CF guy had a similar experience with voltage.
http://<a href="http://forums.corvet...?id=645216</a>

My theory is this. Under no load the engine draws less current. Once you start to give it a little it starts to put load on th engine. More gas and more compression changes the density of the dielectric between the spark plugs ie the gas and air is more resistive as more gets in the cylinder. Hence the spark plugs need more spark to break through the spark plug gap and ignite the fuel. The spark is supplied by the coil. A lower voltage at the coil means less discharge at the spark plug. The voltage at the coil comes from two places. At start it comes via the starter motor and puts 12 volts directly on the coil. During run it comes directly from the ignition switch "RUN" position via a resistive stainless steel wire which will drop around 5-6 volts. This puts 6 or so volts on the coil to help protect the points. If you have a PERTRONIX fitted you can bypass this wire. You can also bypass it if you have points but you will burn out your points faster. Points are only $7 a throw so not a real big expense.

So here is what you do as a check. You have to hotwire the car to a good 12V source. Go from the positive terminal on the alternator to the positive terminal on the coil. Use the ignition key to start the engine. Drive the car. It will need the wire to be pulled off to stop the engine as you have bypasssed the ignition switch. Make sure it is a decent piece of multi core copper so that there is minimal resistance on the wire. To check the resitance of the wire put a multimeter on it and measure the reistance. It will change under load but you can use this as a comparison between the different wire you have available. Did the engine improve when hotwired?

Over time the ignition switch can also wear. This will increase the resistance of the switch. The only thing between the battery and coil is the switch and the resistive wire. So if the hotwire technique worked then it may be time to replace the ignition switch ($27 or so). What happens is if the resistance is high due to a worn switch that the distributor will work fine under no load. When you load up the engine you increase the current through the switch, this give a voltage drop (V=I*R) and then there is less voltage available at the coil thus less spark.

If the hotwire check improved the engine then I would replace the switch (which I am halfway through now).

Another check is the vertical slop on your distributor shaft. Do this check. take of the cap of your dizzie. Pull up on the rotor. Does the shaft move up and down a lot? If it does then you need to put shims (aka washers) on the end of the dizzie shaft. It is not a difficult job. The most time consuming thing is setting up TDC and timing aftwerards. I knew jack about Vettes until a year ago but managed to shim mine and get the engine running in less then two hours.

I would also go to tech tips under this forum and search on LARS papers. I think he talks about how to put in a dizzie and commmon problems also.

If none of that works then you need to capture the symptoms. What I mean is you need to know what the engine was doing when it died. You will need a friend to monitor gauges and record. Hook up a fuel pressure gauge and make sure that WHILE you are driving that fuel pressure is good. CAUTION - fuel pressure in the cockpit is a very bad thing! I taped the fuel gauge to my windshield like another CF member did.

You could also hook up a multimeter to the coil and monitor its voltage under load. Hook up wires to the coil and run them into the car onto the multimeter.

Do multiple runs, capture RPM, SPEED, gear, voltage and fuel pressure. The more info the more likely you can home in on the problem. If you know someone with a DYNO it would be handy to do it there. Try the car at a steady speed ie no hard accleration, does it still do it?

Good luck.
:cheers:


[Modified by stingry, 4:04 PM 9/10/2003]


[Modified by stingry, 7:34 PM 9/10/2003]

 

Could be ignition related. I have had two HEI ignition modules fail. They run fine until the distibutor body reaches engine temp then they would miss until the unit was replaced.

Not sure what module the MSD unit runs, but it could be a posibility.

 

Update on what I wrote before. Found partial break in the stainless wire going from ignition switch to distributor. This would explain voltage drop. At switch end of wire it had 12.06 volt sin the run position.

THis wire is stainless and braided in white. The wire from the starter motor is red and should only have the battery voltage when the ignition switch is in the start position.

The wiring is 33 years old so I guess this stuff happens! Tug all the wires on you firewall and look around with a strong light, you may have a break also.

Good Luck

:cheers:

 

Many thanks for the time you spent writing out the posts. That will keep me busy for some time. Best wishes.