350 Chevy running rough, lots of time and effort spent

[thewrai6th]

I have a 69 GMC with a 350 (85 Blazer block?)/3898993 heads/Holley 600 carb, fuel pump and intake.

It has been running great for years.

The truck seems to start and idle fine. As soon as I take it down the street, it starts running really rough. Idle drops and it almost dies. If I shut it off for a few minutes, it's fine again until I drive it down the street. No choke, manual or electric, never need it before. Choke is wide open.

This seems to have happened when I ran the tank low on gas. So I've replaced the fuel filter and found alot of rust and crud in the old one. I took out the FSU and blew it out with compressed air.

I have new plugs r43ts ACDelco, .035 gap. New wires, cap and rotor. Fresh coolant and oil. Timing is set at 8 BTDC. I can't find any exhaust or intake leaks. I was thinking maybe the wires were overheating, but I don't think so. I checked each wire with a timing light and they are all sparking.

I'm running out of ideas and I'm about to do a leakdown test. Seems like a fuel problem to me though. Any suggestions?

[rsavage]

Did you pull the carb apart to see how much gunk and rust made it into it? Could be a bunch of crap moving around in there.

[cdowns]

have you check cleaned and gapped the points?

[martindc1]

What are the odds that your fuel line has a pinhole in it?

[thewrai6th]

Thanks for the replies.

I have not taken the carb apart. I guess that should be my next step. Any suggestions as to what to take apart and clean? It's a 4160 Holley.

The cap and rotor is new.

There are no holes or leaks in the fuel lines.

[cdowns]

cap and rotor dont mean much if you have points and havent maintained them

on the other hand if you have an HEI dizzy, your plugs are gapped wrong they should b at >045 another most common problem with hei conversions is failure to have it wired properly with a 12v supply. if you use the original wiring it get lo voltage and causes the module to overheat and cause a problem like your describing

i'd check all that stuff b4 touching the carb

[Shyguy]

Another thing to check, the in tank filter may be clogging.

Danny

[junkyardjohn]

IF YOUR FUEL FILTER WAS FULL OF RUST & CRUD... & THERES NO FILTER BETWEEN YOUR TANK & FUEL PUMP... I WOULD BE SUSPECT OF THE FUEL PUMP DIAFRAM. JOHN (I THINK I WOULD ALSO PULL MY GAS TANK & CLEAN OUT THE SOURCE OF THE RUST & CRUD)

[thewrai6th]

Old points distributor. No filter in the tank or before the pump.

[thewrai6th]

Took it out this morning. Got about 2 miles without any issues. Engine runs strong. On the way back to my house, it started shaking and running rough just as I got home.

I'm at a loss here. If it were fuel delivery, then it wouldn't run fine when it's cold, would it? I'm starting to think something is heating up and causing this, but what?

No smoke from the engine and the head gaskets were just done about a month back.

I might just post a video here in a few.

[QKENUF4U]

kinda sounds like a vacumn leak at the base gasket on the carb..... have ya checked it once hot ? maybe the modual in the HEI getting warm ? def. worth your time to pull the bowls and check the carb for crap....

[mstrav1973]

anyone cosidered vapor lock? maybe weak fuel pump

Matt

[D-Rat]

Were the #7 and 8 plugs fouled at all? I had this and replaced the vacuum modulator on the back of the transmission because I was getting trans fluid up the vacuum line. Seems to have helped.

Also ensure your wires are separated, particularly #5 and #7.

[dodo]

Try checking the fuel filter right after it dies, mine would suck up rust and crap then after it sat it (the crap) would go back down the fuel line. When i started it again it would slowly accumulate again and die. Got so P*ss&d off I cut some windshield washer hose out and siphoned some gas into a jerry can and ran fuel line into the jerry can on the seat beside me and drove home like that.

[trusty72]

sounds like the coil maybe. my timing light has a long lead so i can drive it and have the light in the cab to troubleshoot.

[thewrai6th]

Originally Posted by trusty72

sounds like the coil maybe. my timing light has a long lead so i can drive it and have the light in the cab to troubleshoot.

Great replies. Thanks for all the help. I feel like I know almost every part on this simple engine, it's frustrating when I can't diagnose what's going wrong.

The coil and alternator are ancient. The volt gauge seems to read the same as it always has and no issues with the headlights or battery, so the alternator at least works. Coils are cheap, might go pick one up tomorrow.

Originally Posted by dodo

Try checking the fuel filter right after it dies, mine would suck up rust and crap then after it sat it (the crap) would go back down the fuel line. When i started it again it would slowly accumulate again and die. Got so P*ss&d off I cut some windshield washer hose out and siphoned some gas into a jerry can and ran fuel line into the jerry can on the seat beside me and drove home like that.

OK, this sounds almost exactly like how my truck runs, but I keep the idle high enough that it doesn't completely die. If I shut the car off for even two minutes it runs fine again until I take it down the street. Which brings me back to the crap in the fuel filter and this starting when I almost ran it out of gas.

The tank is full now. I need to run the gas down a bit before I can clean out the tank. I will probably take the air compressor to the entire fuel line and clean out the carb tomorrow. Also might borrow a friends HEI, to swap out and see.

I did untwist and reroute a couple of the new plug wires I put on, didn't seem to make a difference, but I will check that 5 and 7 are not touching. These are brand new plugs, they might have 5 miles on them. I pulled a couple and they just looked brand new, might as well check them all.

I was looking up vapor lock, as Matt suggested. I wonder if I have an exhaust leak in the collector that is heating up the fuel pump. I put my hand down there and didn't feel any exhaust leak though. Just a thought.

[thewrai6th]

Alright, I think I fixed the problem. Unfortunately, 3 things changed at once and I'm not sure which one did it, but I have a good idea:

#3, Had to pay a bill downtown, so I drove it on the freeway for about 5 miles. Possibly losensing up some gunk and clearing out the carb, but it ran like crap the whole way until I stopped and idled for a minute, then shut it off. I had already cleaned out the lines and replaced the fuel filter, so this is unlikely the fix.

#2, I reseated the spark plug wires on the cap, after shutting it off. They were all on there pretty well.

#1 probability, I had a manual choke cable that rusted and siezed. I rolled it up and stuck it behind the dash one day, not realizing that the terminals on the back of the instrument cluster are live. My temp gauge started flipping out after the freeway run and I reached my hand under there and felt the choke cable rubbing on the terminals. Started it back up. Now my temp sensor doesn't work, but the truck drives great. Back to normal/full power.

My guess is the really old (possibly original) alternator couldn't produce enough spark with the temp sensor grounded out to do anything but idle. Still, the headlights were working bright, so it's hard to say for certain.

Thanks again to everyone who helped me out with this! Much appreciated!

Time to get back to sanding the new cab