Engine runs fine then dies at idle

[Guy Dudebro]

I have a 305 SBC. I had to take out the distributor to replace the intake manifold because I snapped off a bolt in the thermostat housing gasket. ANYWAY ... most everything has been taken apart and put together. I had help setting the timing from a guy who knows his stuff.

So set off on my first few maiden voyages. Things would run fine on short trips. But if I drove more than a few miles the engine would sputter and then dies when it came to idling. I'd have to turn the engine and floor the pedal to get to come back up.

I drained the gas tank because the gas was about 6 months old. I replaced the fuel filter (from pump to carb) and that seemed to help a bit. But I have the same issue. It sounds like its running out of gas even though I have some in the tank.

Next step? Replace the fuel pump. Its only about a year old but the guy I bought it from put it on and he might have bought a cheapo.

Any other trouble shoot ideas would be appreciated.

[Davidf]

I suspect the carb is leaking fuel past the needle valve and flooding the engine. The reason it runs initially is the extra fuel is like choking a cold engine. Run the engine, shut it off and look (down throat of carb) to see if fuel continues to dribble into the engine. If so, carb rebuild time.

[Guy Dudebro]

So does that mean excess fuel is sitting in the mainfold? Is there a way to flush it out, adjust the carb and start fresh?

[TxGoats]

I had the same problem with my '68 GTO, after numerous top notch mechanics looked at it , I started doing my own research. To cut a long story short it ended up being a intake gasket leak, I had swapped carbs too but it didn't help. With the engine running, Get some ether or carb cleaner and CAREFULLY spray around the edges of the intake gasket and under the carb. You will find the leak when the rpm's go up. It could start leaking air after it warms up. My .02
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[Userone]

rebuild the carb. I let my truck set about that long and there was so much crap in the carb from the evaporated gas it looked like gravel in it.

[Guy Dudebro]

I noticed a small leak at the back of the engine which may be due to an intake manifold leak. Which may explain the idling issue? When you found the leak, did you replace the gasket on the mainfold? Or did you patch the leak?

[davepl]

Originally Posted by Userone

rebuild the carb. I let my truck set about that long and there was so much crap in the carb from the evaporated gas it looked like gravel in it.

That's a can of worms for people that haven't done it before. You gotta start somewhere, but I think the first time you should have a "backup" carb.

I do agree in principle though: I bet the needle/seat and other passages are gummed up, maybe hanging the needle open, etc.

To what extent do you think you could you "fix" all of that by spraying carb cleaner into the right places rather than disassembling it?

[davepl]

Originally Posted by TxGoats

I had the same problem with my '68 GTO, after numerous top notch mechanics looked at it , I started doing my own research. To cut a long story short it ended up being a intake gasket leak, I had swapped carbs too but it didn't help. With the engine running, Get some ether or carb cleaner and CAREFULLY spray around the edges of the intake gasket and under the carb. You will find the leak when the rpm's go up. It could start leaking air after it warms up. My .02
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Anyone have a good way to spot an intake leak --under-- the valley side? I'm suspecting one on my own car. I've got an original oval port intake on a ZZ502 "roval" port heads and it might be a mismatch. The shop drilled holes in the butterflies, all of that, but I don't think it was the right approach.

Short of just swapping intakes back, I'd like a way to -know- if its leaking or not in the areas I can't reach.

Ideas?

[Guy Dudebro]

I had the carb laying in the garage for awhile and, while I sprayed carb cleaner, maybe I didn't get it good enough. Will start with that, look for a manifold leak and if neither of those work I'll huddle in the garage with my knees to my chest and weep.

[TxGoats]

Originally Posted by Guy Dudebro

I had the carb laying in the garage for awhile and, while I sprayed carb cleaner, maybe I didn't get it good enough. Will start with that, look for a manifold leak and if neither of those work I'll huddle in the garage with my knees to my chest and weep.

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[TxGoats]

Originally Posted by TxGoats

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Sure sounds like a intake gasket leak to me, mine drove me nuts, ran good until it warmed up then wouldn't idle unless I was about 1500 rpm's after I replaced it, I had to reset timing, and idle, I had it kinda tuned to deal with it.
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[bigbadchev72]

I just had something like that happening after my last engine swap, ended up being cheap fuel hose from the frame to the fuel pump that was sucking itself closed and choking the motor out of fuel, it would die and fire right back up after a few cranks 3-5min later rinse and repeat

[hugger6933]

When I first read the title I thought vacuum leak then after that I would check hose then the carb needle and seat. Pull out the dipstick, does it smell like gas? That would be a definite indicator or a needle and seat problem. It seems to me to be running too long of a time to be the N/S though, so start with the carb cleaner test then look the hose over those two thing are as close to free as you can get. Jim

[Churchill]

also might be the fuel getting too hot. the original intakes have small valley pan things underneath where the carb sits, on the underside of the intake.

[Guy Dudebro]

I took off the carb and thoroughly cleaned it. Then put back on and cleaned some more. I had some vacuum lines misarranged and got that corrected. Took the truck out on Sunday and it ran great for about 4 miles. Then a spitting sound came out of the exhaust and it dies on me at the next stoplight. A few teenager pushed me into the nearest 7-11. After that, it wouldn't even turnover.
Raining today. Planned to see if I can detect and intake leak tomorrow. Maybe a small leak is messing with the air/fuel mixture and causing it to stall out?

[Davidf]

It takes a pretty large vacuum leak to cause dying at idle. Big enough that it should be obvious. I still think you are flooding the engine after it warms up. About 4 miles and the engine is good and warm and cannot handle the extra fuel from a stuck or leaking needle valve. Right after it dies, pull a plug and see if it is wet with excess fuel.

[Userone]

sounds like your float is out of whack

[Guy Dudebro]

So I replaced the battery (it wasn't starting) and the voltage regulator along with the alternator. Truck starts up and idles great. But when I get going faster than around 35 mph I hear a backfire from the driver side exhaust ... kind of a pop, pop like a low volume gun shot. And my exhaust is coated with black soot. And it doesn't shift gears in relation to the RPMs. You think it still might be a carb issue?

[Davidf]

Originally Posted by Guy Dudebro

So I replaced the battery (it wasn't starting) and the voltage regulator along with the alternator. Truck starts up and idles great. But when I get going faster than around 35 mph I hear a backfire from the driver side exhaust ... kind of a pop, pop like a low volume gun shot. And my exhaust is coated with black soot. And it doesn't shift gears in relation to the RPMs. You think it still might be a carb issue?

Too much fuel! Your carb needs attention. Soot is from two things: fuel or oil (could be both). Fuel is dry and powdery. Oil is, well, oily.

[larley]

With the engine running,spray starting fluid at potential vacuum leaks. You will hear the difference, if leak is there.
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[Userone]

black soot=running rich sounds like it's flooding I say float problem.

Recent experience: I gave up on my float problem because it was pissing me off (and I didn't know what I was doing adjusting it) and luckily my uncle had a holley on his shelf in the shop. I bolted it on and runs like a top now.

I'd recommend new plugs after you get the carb situated because they are more than likely fouled out