I was chasing an intermittent random cutout under WOT after installing my new engine.

After replacing plugs, dizzy cap, leads, HEI module and ign coil I found problem was an after market theft alarm.
The ignition cutout relays would go open circuit at elevated rpm under full load due to some vibration in the firewall where it was mounted.

The other thing I found was the terminals in the new dizzy lead connector were not seated properly... Make sure the terminals are locked correctly in the plastic connector and make solid connection when plugged into dizzy.

Not likely to be your fault though if issue occurs all the time... but if anyone has an intermittent issue..... worth checking.

Good luck with finding that gremlin!

 

had an old camaro with a q-jet that did the same thing. at rpm you should have way more that 3 psi at the carb. check the fuel pump, might need replacing. also, i had to rebuild my carb as there was an internal leak.

 

Update;......umm, c377 I got distracted by your avatar. What were we talking about? Oh ya, gremlins

Realized my intake has a dual pattern and borrowed a 600 Holley from a friend, just off his running truck. Guess what? Same thing.

New plugs today. The plugs that came out are real dark. Indication of running rich? Ran smoother with the new set but did not correct the cutting out.

Going back out to run a direct test lead from battery to HEI. Eliminate everything in between.

I understand that 3psi might be low so I put and inline electric rated at 5-9 PSI with yet the same results.

Cap and coil are the only components not tested.

Thinking back on my rebuild. I re-installed the cam that was in it when I bought it. Had a broken piston at the time so I never wound it up. Supposed to be a Erson Viking 101. Can't find specs anywhere. I put it back in the same timing holes on the drive gear (sprocket?). If the cam timing was off could it be causing this? Not sure how far either way those mounting holes are, if the engine would even run if installed incorrectly. And without specs I don't know how to check the timing.

Chasing my tail here......Ohhhhh avatar.....

 

Update #2 for the day:

Reconsidered the fuel pressure and what it meant.
By-passed the mechanical altogether, Electric direct from the hardline into a clear plastic filter and to the Holley.

The electric pump spit and pumped air bubbles into the filter. Never completely filling the filter. Car runs with about 1/2 the filter filled with fuel. Test drive gave me the same results.

So I'm thinking that the 3 lbs I saw at the first test may have just been air pressure and the carb is starving for fuel.

Time to drop the tank.

Not too bad, 45 minutes to having the pickup assembly out. Sock flowing well but starting to open at the seam so off it came. Eliminating anything in the tank.

Wouldn't thing that I have a leak in the steel line from the tank to the fuel pump. Never any fuel under the car or gas smell at all.

Had about 1/4 tank of fuel when the pump was putting out air. And I know this is close because I put my hand down in the tank when I had the pickup unit out.

So, air in the filter and erratic flow going in. Back to starving for fuel? If so where else could the air be coming from?

Bummer Saturday.

 

yeah, she's kinda cute isn't she.

are you running a return line? if the return line's not used, or blocked or the vent's not working properly you can't get the fuel out of the tank to the carb properly. check your vent line to make sure it's right. maybe the supply line has a blockage?

 

Return is not used neither is the vapor line. But I removed the fuel cap and, well, you know.

 

hook that return line back up and see what that does for you...

 

Try to eliminate fuel delivery.
Try some tests.
So far by changing carbs you either have fuel delivery probs or ignition/timing/valve train probs.
If you suspect pumping air:

If the tank is still out of car, temporarily hook up up electric pump and power it and pump gas into a 5 gal bucket.

If the tank is in the car then just get a 5 ft length of gas hose and pump into bucket. You can then even hook up the see thru filter and check it's operation. Used to use those filters years ago at times and just because they have air in them doesn't mean they are pumping air. It was common for them to initially run that way. Check the exit stream of gas.

I shoot for around 3-4 psi on qjets, nothing wrong with that. It's the most flow at the least pressure that you want.
As above, use the return line. Could be overcoming and flooding out at higher rpm only, but unlikely.
You can even run a few feet of clear gas line where you can see it at high rpm.

The only way to see if the secondaries are working properly or not is on a dyno or prop the hood up in back enough to see and drive with the air cleaner off to view operation. If you are used to them, you should be able to hear them.

A note to your secondaries.
If you just blip the throttle, they won't open.

If you, rev it high enough and long enough in neutral, you can make the air valve start to open about 3/8" measuring from the front, but certainly not all the way.
Can do it to any qjet set properly, but not recommended.
I think that's where the confusion lies.

 

Well, I had to walk away from this for a couple of weeks before I burn't the vett to the ground.

I have swapped, replaced and bypassed all fuel and electrical related componets on the engine.

Consulted with experts here and local racers.

Read Lars' Q-jet papers.

Scoped the ignition.

Read an interesting thread running here now on a big block with the same symptoms. Turned out to be the condensor. Ha! that was the first thing I tried.

My options are narrowing to include arson or Craigs list.

Any new ideas out there?

Blain.

 

Thousands of forum users and not one more idea? Thats disappointing.

 

Man... these kind of problems are real frustrating. I just reread all the replys and nowhere is your timing at 3000 rpm mentioned. What is it? It should be between about 32-36 degrees at 3000 with the vacuum line pulled and plugged. As well you might try to run the engine in a dark garage at a higher rpm, just to see if there is any electrical energy escaping in the form of sparks to ground from any part of the distributor or plug wires.
I'm not sure about the discharge from the carb, but increasing rpm,fouled plugs,bogging or cutting out at rpm.....they point me to look at a plugged exhaust system. If everything looks OK externally, hook up a vacuum gauge to the carb and see if it builds pressure as the rpm rises......Hope these ideas help

 

Do you know for certain that pushing your accel pedal to the floor is fully opening the secondary throttle plates. (Opening them by hand with the throttle linkage is NOT the same thing.) Also, on the Q-jet, you have a secondary lock-out link that needs to be released after the choke has completely pulled off. Is it working properly? {Of course, that wouldn't explain the same problem with the Holley...}

 

Sounds like it couldn't happen but.....
When I bought my 68 bubba had tried to fix a broken accelerator pedal mount at the firewall. He forgot how the adjustable arm went back on and he put it backwards. This meant that the accelerator arm on the q jet went about 1/2 throttle at full pedal. I turned the arm around and ,wahlah, had full response from carb. Could even burn tire with auto transmission.

#2 A weak coil on an HEI will sometimes overload at high rpm. This produces a weak spark at wot/advance timing. I believe it is called over saturated.

Keep at it.
Dave

 

Got those ideas flowing again...

I'll address each point;

I don't know the exact timing at 3000 because I don't have a dial timing light or degree tape on the balancer. I estimate it to be high 20's maybe 30

I have run in a dark shop looking for spark leaks-nothing.

Have not checked vaccuum at various RPM's only at idle. Can Hooker sidepipes with glasspack inserts plug? Keep in mind that this does not just ease off like it is choking, it runs up to 4300 hard then shuts off. Not a hesitation, just like you turn the key off and on. I remember the old days as a dumb kid we would turn off the key while coasting in gear then turn the key back on to blow the mufflers out. I don't get the same backfire in the exhaust.

Watched the secondaries open. Even tightened the windup spring to hold it closed longer. And yes when I put the Holley on it did the same thing.

I'm sitting at my desk trying to visualize the throttle arm and if it has full throw. Pretty sure it does but I'll check it out tonight.

Now for the HEI. I replaced the coil, fired it up and I got 6 grand on the tach the first time to the floor. Not wanting to abuse it anymore and thinking that the trouble was corrected I removed the Holley and gave it back, along with the electric fuel pump. Guess what? So back on with the Holley and fuel pump. Still have the problem. I have gone back to the HEI in an attempt to identify what could have been wiggled, moved, changed etc. Can't get it pass 4300 again.

I know this seems like a distributor problem. Feels like one. But I have fire even when shutting down. Watched the timing light while putting my foot in it. Shows spark continuous.

Blain

 

 

It may rev to 6 grand without a load on it in the driveway !

 

-make sure vent line isnt block
-at that rpm fuel demand is high your mechanical f/p may not keep up with the supply.
-i know because ive been down this road recently

 

I connected an electric fuel pump directly to the carb drawing out of a 5 gal can. Think that eliminates the return and vent line?

Blain

 

If it cuts out at 4300 every time, then I would look at the coil/module. Years ago the GM in cap epoxy coils were known as built in rev limiters. If you have another coil/module, try it. Make sure you have a good ground at the manifold or add a wire.

I think the genuine GM modules are the only ones with the dwell extension capability affecting the coil at higher rpms.

 

I bet the cat converter is plugged up. Is it real quiet?.......thanks , bryan.