Intermittent severe missing or hesitation

[dmjlambert]

My 1969 CST/10 has a 350 engine and TH400 transmission. This is my daily driver and I estimate that I put about 3000 to 4000 miles per year on it. When I got the truck in 2014, I had a local shop rebuild my engine and the engine builder replaced the heads with 1990s crate engine heads and installed a GM HEI donated by a friend. I installed a newly remanufactured quadrajet carburetor and new water pump and fuel pump, and new distributor cap, rotor, and AC Delco plug wires. The coil looked like it was in good shape and I installed the new rotor bushing with spring and rubber washer properly that goes under the coil. I installed a fresh M&H Electric Fabricators supply wire to replace the resistor wire, attached at the the firewall connector to make sure it has a good 12V supply even though the truck was running fine with the resistor wire. Spark plugs are gapped at .045".

3 or 4 months ago the engine started burning rich and spitting very black water drops out of the tailpipe upon startup. A local mechanic had a look at it and found the choke pull-off inoperative and replaced it, and adjusted the carburetor idle mix and idle and got it running well again. Also he noticed the vacuum advance on the HEI was not working, and was physically bent and we figured it probably did not work since installed. He replaced that and adjusted timing. The engine was pinging so I further adjusted the timing and ended up at 7 BTDC where it seems to work the best. The new vacuum advance provides about 20 degrees of advance. I have had it connected to ported vacuum and manifold vacuum, and it works well in either place. When I change the vacuum source, I adjust the idle to 600 RPM with transmission in drive. Currently I have it connected to ported vacuum.

Overall the truck runs very well. It starts easy, and drives down the road just fine. Nice and smooth. In the past few weeks a problem came up. Intermittently, probably once every other day, at random, the engine has an episode of severe missing or hesitation which lasts about 5 seconds. The engine almost dies, but then it comes back to life. I do not take any action to bring it back to life. I thought at first it was during acceleration, but then yesterday it happened when just driving along at 40 MPH.

I'm wondering if it is a gas supply issue such as the float sticking up and finally falling when the bowl is empty, or the HEI module or pickup going bad. I suppose it may also be a problem with the ignition switch. Is there anything else that should be suspect? Are there any tips to figuring out what is going on? The problem only happens when going down the road, and it is completely random as far as I can tell. I don't have a tach installed. How can I tell if I am running out of gas in the bowl temporarily or losing spark temporarily when going down the road? Which condition is most likely based on the story I tell? I'm kind of thinking about the pickup coil because the vacuum advance was recently fixed. How often are those things intermittent? Should I do some sort of test on it while installed?

I don't have experience with removing and re-installing distributors, but I suppose I can give it a shot.

[dmjlambert]

I forgot to mention, I replaced the fuel filter the other day, thinking that could be the problem, and the hesitation or missing episodes were not cured by that new filter. In 2015 the truck got a new fuel tank and sender, replacement rubber fuel lines between tank outlet pipe and frame and between frame and fuel pump, and the fuel line along the frame and from the sender through the cab floor flushed out. It has a mechanical fuel pump.

[Richard2112]

Could your carb be overheating? Fuel lines run close to/in contact with radiator/heater hoses? I've found that my Q-jet didn't like to sit directly atop my manifold and runs better with a thick gasket. Fuel, especially with ethanol added,will boil at a fairly low temp.

[dmjlambert]

Thanks, I will add that to my suspect list. I do have the thick gasket installed that the new Quadrajet came with, and no new changes in the steel fuel line position. I will have a look to make sure I didn't miss any problems in that area.

[Barnfind46]

The fuel here in DFW causes vapor lock issues at about 190 to 200 degrees on non fuel injected cars and trucks .My truck has had a problem with the temps starting to rise. I installed an electric fuel pump and made a new fuel line that did not hug the engine block. I also put a heat sleeve on the line and a carb spacer. So far so good. We will see what happens when summer gets here. A fuel return line can also help although I have not done that yet. With fuel at 10% ethanol and talk of 15% in the future this may be a real problem for vintage cars and trucks.Hope this helps.

[Coley]

...couple of quick things, this does initially sound like a fuel problem.

That said, are you sure your distributor was re-installed correctly?...and not one tooth out when being re-inserted.
7° is a bit low for a timing set point...even for stock, which is normally 8°....and most guys can set theirs up around 10-12° no problem without pinging showing up.

Back to the fuel, are you familiar at all with the Quadrajet carbs?
It might be worth while to take a saturday morning and remove it from the truck (15-20 minutes) and take it into your workshop and take a quick peek inside it by removing the top air horn.
This is not too difficult and does not take too long to do...simply hand tools, 5-10 minutes with some basic ability and care and you can inspect your primary float chamber to see if everything in there is ok.
This also gives you a chance to thoroughly clean the car with carb cleaner and compressed air.

....just a few thoughts and some input here.

Regards,
Coley

[68 P.O.S.]

Get your timing straightened out first before moving on to the carb. Your initial timing should be between 12-14 degrees with vac advance disconnected, the mechanical advance in the HEI should provide 20-22 degrees, and your vac advance should be limited to 10-12 degrees. Also, the springs for the mech advance should be changed to provide full advance all in between 2800-3000 RPM. Total timing should be around 34-36 degrees. How much vacuum do you have at idle?

[RichardJ]

None of those suggestions address the intermittent problem.

Your comment,
>> I'm kind of thinking about the pickup coil because the vacuum advance was recently fixed.<< could have merit.

The pickup doesn't usually go bad, but the two wires to it are constantly flexing with the operation of the vacuum advance. I would connect a hand vacuum pump to the vacuum advance and with the engine idling, operate the vacuum advance through its full range, multiple times. It wouldn't hurt to have the timing light connected and check that the advance is operating smoothly and returning to base timing as it should.

[davepl]

Way out thought: rubber line on the suction side being collapsed which cuts off fuel flow. There should only be a couple of inches of rubber fuel between the tank and the pump.

[dmjlambert]

Thanks everybody for your comments.

Originally Posted by RichardJ

None of those suggestions address the intermittent problem.

Your comment,
>> I'm kind of thinking about the pickup coil because the vacuum advance was recently fixed.<< could have merit.

The pickup doesn't usually go bad, but the two wires to it are constantly flexing with the operation of the vacuum advance. I would connect a hand vacuum pump to the vacuum advance and with the engine idling, operate the vacuum advance through its full range, multiple times. It wouldn't hurt to have the timing light connected and check that the advance is operating smoothly and returning to base timing as it should.

This is my top suspect. This is a good troubleshooting idea. I connected my timing light and went through about 100 cycles of letting a manifold vacuum port suck the distributor advance, and removing the tubing from the port and blocking the port with my finger. The vacuum advance worked great and showed with the timing light. I was thinking at some point it might have a tantrum but it did not. I did this at idle, and I repeated with my Lisle Throttle Pedal Depressor holding at a higher speed.

What I have here is an intermittent tantrum that lasts a few seconds, and so far recovers without the engine completely stalling. It is annoying.

[Coley]

I get the 'annoying' part of this for sure....that being the 'intermittent' nature of a problem.
Sometimes the best thing that can happen is a full shut-down/quit failure which allows you to find the culprit in its needed, clearly 'dead' form.
...keep going you'll find it.

All Good
Coley

[dmjlambert]

Today I was just driving along a road at 45 MPH, and it had a tantrum. It ran poorly and just struggled and almost died for about 10 seconds. Then it went back to running excellent for the rest of my trip. Where exactly is the corner of the Bermuda Triangle today? Maybe I'm running through it.
Before my trip I connected an analog voltmeter to fuse panel at the unfused ignition terminal, so I can monitor my voltage just for the heck of it. It ran steady at about 15 volts the whole time, including during the tantrum. I'm still poking around trying things. Coley, I will probably take your suggesting of having a close look at the carburetor, one of the next steps.

davepl I have added the fuel line to the suspect list, but I think it is a long shot. I have pretty new 3/8 rubber hose in the stock locations and it is in good shape and feels firm. Both lengths of it are before the mechanical fuel pump. They are stock length of about 8 to 12 inches long to make the distance from the frame to the section of metal tubing that goes up into the cab, and from the frame to the mechanical fuel pump inlet. The sections of hose should have pretty much no serious suction going on with them, just gravity fed fuel going into the low-pressure fuel pump. It is not a performance engine, just a plain old stock setup and I'm driving like a granny. Also, the fuel cap is vented and I never hear or feel a suction or pressure release when removing the cap to refuel.

Thank you everybody for your comments.

[Grumpy old man]

Take it back to the mechanic who has been fixing it all along , He knows more about it than anyone who hasn't seen or touched it .

[dmjlambert]

Yes, wouldn't that be nice. I am the chief mechanic of this truck now and most familiar with it. I moved since last year, and probably didn't have a mechanic who knew it better than me at my old home anyway. I've come to a Y in the road and will try either the fuel path or distributor path first, and see which fixes it. If neither fixes it, I will be surprised.

[Mike C]

I'd look at putting a Skip White HEI in it for $69. That gives you all new components and should eliminate the ignition except for plugs and wires at that point.

Does it backfire through the carb? Mine did years ago with an intermittent issue that turned out to be a valve guide that had failed and randomly the valve would hang on the seat and it would spit back out the carb. Next cycle it would seat fine and might be weeks before it would do it again. Only found it when I had the valve cover off and I was leaning on the motor. Pushed on the rocker/valve and it moved over! WTF?

[dmjlambert]

I don't think I have any backfiring going on. I don't know if I would go with a Skip White HEI or get an AC Delco one from Rock Auto. I'm thinking about it. After giving the vacuum advance a work out and not revealing any problems, I may start with diving into the carburetor first.

[HO455]

Do you have a tach installed? If so try disconnecting it to see if the problem disappears. You might try monitoring the voltage at the distributor when driving. With the ground wire attached to the engine block you will be able to detect having a ground issue as well as a 12+ issue. You also might retest the vacuum advance with a vacuum pump instead of using the engine. This will ensure you have tested the advance through its full range. Are you running an aftermarket air cleaner? If so try a different one to see if it is related to the problem.

[OBS454]

Since you are running an HEI is sounds like a bad ground or the dielectric grease on the bottom of the HEI module has degraded or the module itself is getting hot and starting to go out. Like someone else said, be sure to closely examine those wires coming from the pickup coil.

[toolboxchev]

Yup that module will call this type of issue.

[dmjlambert]

I did what HO455 suggested and ran wires directly to the power input terminal of the distributor, and to the metal distributor body, and attached those to a voltmeter in the cab so I can monitor power at the distributor while driving. I do receive steady normal voltage to the distributor even during these tantrums, and this confirms no ground or power problem. So, it is either inside the distributor or inside the carburetor. I'm going to start with the carburetor and leave the distributor alone for a while, and resist the urge to just pull both and refresh both at the same time, so I can know for sure what fixed it. I will consider it a good time for carburetor tune up anyway, even if I put the carburetor back on it and problem is still there. Then I will move on to the distributor.

I don't have a tach or vacuum pump. I may get a vacuum pump, I'm sure it will come in handy. I have a stock air cleaner. I will let you guys know what I find. I appreciate the tips and comments.

[dmjlambert]

Here is the conclusion about my random engine episodes of severe missing or hesitation which lasts about 5 to 10 seconds, engine almost dies, but then it comes back to life. I pulled the carburetor and sent it over to National Carburetor. I had purchased it with the lifetime warranty, so I thought I would take them up on that. They worked it over and sent it back to me. I put it back on the truck and made minor adjustments to the choke and fast idle speed. It has now been back on the truck for a couple weeks, it runs well, and I have been driving the truck every day. The problem first reported in this thread appears to be cured. I would like to thank everybody for your input, ideas, and encouragement.