Engine cuts out after driving 20 min, ideas?

[Osker]

OK friends, ….71 C10 big block, not a daily driver but I want to get it there.
Issue: Driving back from work, it is about a 20 minute drive, it was running fine on the interstate got up to about 75mph right before I got off, driving through town at 25mph after about two miles of that she shut down, just died while rolling. Sounded and felt like she ran out of gas, but I had ¾ of a tank. I pulled her off to the side of the street. I started her up and it runs for about 10 seconds at a normal idle, nothing missing or lumpy then she dies again, I did that 3 times but every time she would die out.

Outside temperature was 75, engine temp was normal, oil pressure was normal, battery is charged. She cranks fine every time.

A few weeks ago, the same thing almost happened on the interstate, but she kept running after the stumble and a slow down and I made it through town with no more problems and back to the house. I had driven her since then mainly around town and no stumbling or cutting out. Then yesterday she went flatline on me.

I’ll check it out tomorrow after work when I have some time. I’m thinking fuel pump or the in line fuel filter in the carb inlet? I’m hoping after sitting for a day she will start and run home where I can really dig into the issue.
Any recommendations I am all ears!

History:
Fuel pump (40 gph) replaced in OCT 2020 with a NAPA MF001 because old one was leaking from the housing.

After I replaced the fuel pump the Rochester M4MC started flooding out and she wouldn’t run. Taking the carb off I found that the well plugs had been epoxied, but the epoxy had come off and was left on the gasket after the carb was lifted off. I think the new fuel pump was flooding the carb out. Looking in the manifold there was a big pool of fuel just sitting in there. I changed the oil not knowing how much went down. Plugs were fouled and smelled like gas. I put in new plugs and wires. Instead of rebuilding the carb I purchased a rebuilt one from the Carburetor Exchange in El Monte CA. With minor adjustment to the fast idle she ran great, but I still need to raise it a bit as she doesn’t always start right away. I’ve been cheating with starting fluid. One shot of that and she hits right away.

Spec’s
Non-original 396 cu in engine block code 3916323
TH400
MSD ignition, Mallory, new cap in OCT 2020
New plugs and wires in OCT 2020
Spark Plugs = AC Delco R45XLS @ .035 gap
Stock intake

Thanks in advance. I’ll keep you posted with what happens or doesn’t.

[Stocker]

Originally Posted by Osker

I’m thinking fuel pump or the in line fuel filter in the carb inlet?

Had a problem with my K20, would run okay under light load that doesn't require much throttle. Going uphill it acted like it was running out of gas, stumble & die. Had to sit several minutes before it would fire. Finally got it home & replaced the fuel pump even though it seemed okay and was fairly new. Still conked out going uphill. The carb rebuild wasn't all that old but I pulled out the paper filter from the carb inlet to check it. It was plugged, could not blow air through it. I put in a new bronze filter and the problem was solved. Good luck with yours, I hope it's something as simple as that little filter. PS -- get the sintered bronze one, not the paper one.

[franken]

Just a guess but maybe take the gas cap off when it happens.
I think 71 had the vacuum canister to vent the gas tank into the intake rather than into the air. If so the gas cap would have originally been unvented.

[Happy_dan]

Do you have a fuel pressure gauge on there? Just guessing of course, but if it was flooding the carb after changing, maybe too high fuel pressure?

[MARKDTN]

When it doesn't start, are you still getting spark?

[Davidf]

Check fuel flow. My truck acted similar it turn out to be a bad fuel pump. Initially, I would guess heat related failure of an ignition part, but for the running 10 secs after restarting makes me think fuel starvation.

[Steeveedee]

Originally Posted by Davidf

Check fuel flow. My truck acted similar it turn out to be a bad fuel pump. Initially, I would guess heat related failure of an ignition part, but for the running 10 secs after restarting makes me think fuel starvation.

I also suspect ignition. Once the system is all heated up, the running for 10 seconds just brings it back to the "bad" temp.

I'd be for checking the coil and condenser, personally. If the engine is flooding and not starving at 75 MPH, the fuel filter is probably OK

[Sheepdip]

Also check and make sure your distributor if H.E.I. has a full 12 volts going to it.

[Davidf]

Originally Posted by Steeveedee

I also suspect ignition. Once the system is all heated up, the running for 10 seconds just brings it back to the "bad" temp.

I'd be for checking the coil and condenser, personally. If the engine is flooding and not starving at 75 MPH, the fuel filter is probably OK

Don't disagree with this logic at all. I was just saying that I had a bad fuel pump that behaved strangely. All seemed fine for many miles on the highway, then all of a sudden, severe engine misfire and ultimate shut down. Wait a bit and it would restart. Sometimes for only a few seconds/minute, then sometimes for several miles. Limped her home, replaced the pump and all good.

[Warrens69GMC]

When that happened with mine it was a plugged fuel filter.

[truckster]

One possibility that hasn't been mentioned is the fuel sock in the tank.

[Osker]

Thanks everybody for the replies. I got it started, stumbled a little at first and I got a intake backfire. Second try the idle evened out and she seemed OK. I let it run for a few minutes and revved it with no issues. I got it back to the house but I didn't get on it while driving. Fuel filter when pulled out looked good nothing clogged up.

I have not had the flooding issues since I replaced the carb in October.
I didn't put a regulator on. I thought about it but I figured if it ran OK with a new pump and carb there would be no need. So there's no regulator on it.

I'm getting spark, she's running.

I think the fuel pump is good but I won't rule it out. Filter checks out but I like the idea of the sintered bronze suggestion.

I didn't think about ignition but maybe I need to after the suggestions and the backfire. I'll look into it. I had replaced the coil back in October, it's not HEI, just the Mallory unilite dizzy with the infrared shutter gizmo. It's old but it checked out OK when I went through it in October. It might not be as good a shape as I think it's in.

Fuel sock in tank, good point but I'm getting plenty of fuel but it still could use a inspection.

Thanks everyone, I appreciate the recommendations, I'll work on it over the next few days and see how it turns out.

[1970 Custom GMC]

If you have not already, Compression test the engine first! It helps a lot to rule out valve and ring problems first! 8)

On a side note, I put a new mechanical fuel pump on my 1970 and the secondary bowl on my Holley 600 started to flood.

I ended up having to regulate the pressure to keep it stable.

[i82much]

make sure your brake booster and check valve are working properly if you have power brakes.

[mikeshome]

I would try removing fuel cap when it happens.

[CC69Rat]

Mallory Unilite distributor ?

[slikside]

Originally Posted by truckster

One possibility that hasn't been mentioned is the fuel sock in the tank.

When my sock disintegrated, the first clue was flooding in the carb. That lead to checking the filter, and it was packed with whisker looking debris.

[jjzepplin]

Subbed for info.

[bowtie72]

Vapor lock? Is your fuel line from pump to carb steel? If you have a temp gun take it with you next time you drive it and when it dies check the fuel line temp.

[68 P.O.S.]

Do you have any rubber lines going to the fuel pump? Mine has rubber lines at the fuel pump. When on the freeway one of the lines would kink and shut off the fuel flow. Reclocking the line fixed it.

[Osker]

Originally Posted by CC69Rat

Mallory Unilite distributor ?

CC69Rat.... This is what it looks like.

[Osker]

I did the compression test cold and I came up with 178 to 184 on all cylinders.
I drove it, ran fine, got it up to 60 and punched it a few times and nothing crazy happened. I'll keep going through some more of the suggestions.

[CC69Rat]

The module in the distributor is going bad. (Circled in blue) common problem with these distributors. Summit carries a replacement for it.

See where it says "Unilite" on the tag (red) ?

99% sure that's your problem. I would also look at the coul to see if its touching the intake on the bottom edge, it may be getting hot and cutting out. Or, the ballast resistor may be getting hot after 20 min and cutting out, causing further drop in voltage to the coil and explaining the cutting in / out or loss of spark completely.

After it cools off for a few min, does it start back up ?

[Osker]

CC69 Rat,
I didn't wait long enough to see how long it took to restart after a cool down.
Given that she restarted with no big issues a day or so later, I would think it would restart after a cool down on the same day, I just didn't wait around long enough, I had to get going, luckily I was walking distance from the house.

I agree with you, the more I think about it, the more I think the distro setup is going out. I went through the system test on the module last fall and the readings were in line with the accepted or stated voltage limits so I assumed it was serviceable. But I did read posts online about the unilite module reliability issues so the doubt has been in the back of my mind when this happens. It's been a few months and anything can happen since then. I replaced the coil in the fall but I will double check that I didn't bottom it out in the bracket. Thanks for the tips and advice. I'll check out the replacement module and new ballast resistor.

[geezer#99]

Considering the lack of vac advance with that Mallory and tons of room, you should consider a swap to an hei.