Ran out of gas...now rough idle...vacuum leak?

[Mbeef61]

I was out today testing my brakes and I was an idiot and ran out of gas. No biggy, pulled over and called AAA. They brought two gallons.

Truck has been running great lately. Recently swapped hei distributor, plugs, wires. And did timing at 12* advanced. Running great. Until it ran out of gas.

Put in 2 gallons and the thing won’t idle. Barely holds 500 rpm if it will idle at all without dying. But it runs great under throttle. No hesitation or stumbling. Just won’t idle.

Things I’ve done:
Tested fuel pressure before filter 7 psi
Changed fuel filter thinking maybe it picked up some crap
Listened to carb and it is spraying fuel
Checked carb intake screens they are clear

Only thing I have found is I have no vacuum at idle. Plugged power brakes and pcv valve. Plugged vacuum gauge into vacuum advance for hei and get 0 when rough idle at 500 rpm. Give it throttle it jumps to 30.

Trying to figure what running out of fuel would cause this? Any ideas?

[p0508]

Id say you sucked something up in the idle circuit of the carb.

[Mbeef61]

Originally Posted by p0508

Id say you sucked something up in the idle circuit of the carb.

Any idea how to look into that?

Also it’s a 71 c10, 350, hei with Holley 670 street avenger.

[Mbeef61]

I was even considering if it somehow ruined a intake gasket or flange gasket. It’s just weird it’s only effecting idle vacuum, vacuum boosts under throttle which makes me think it’s a small leak?

[p0508]

Any engine idling rough at 500rpm isnt gonna have much vacuum. Did it backfire when it ran out of gas, were you WOT and it ran out?

[p0508]

You can try the old rag trick. Info on how to do it is on the web. But essentially rev your engine up to 2500 take a wetted rag or put a glove on however you can smother the carb while trying to hold that 15 - 2500 pull the rag or whatever you use just before it dies. Obviously you want to bunch the rag up and not just toss a rag over it......

Also if you suspect a vacuum leak, take a can of carb and choke cleaner, spray it around all your vacuum sources and intake gasket, if the engine revs up you found the leak.

[Mbeef61]

Originally Posted by p0508

You can try the old rag trick. Info on how to do it is on the web. But essentially rev your engine up to 2500 take a wetted rag or put a glove on however you can smother the carb while trying to hold that 15 - 2500 pull the rag or whatever you use just before it dies. Obviously you want to bunch the rag up and not just toss a rag over it......

Also if you suspect a vacuum leak, take a can of carb and choke cleaner, spray it around all your vacuum sources and intake gasket, if the engine revs up you found the leak.

Thanks I’ll give that a shot tomorrow. I think my neighbors have had enough starting and revving for the day haha.

I have heard of the rag trick before, but what is it accomplishing? I never quite understood that

[p0508]

Basically hit the carb with a huge shot of vacuum. Like sticking a super powerful vacuum cleaner on the back if it.

By blocking the carb you are creating a huge momentary vacuum pull hopefully sucking out any debris

[Mbeef61]

Well I said the neighbors can deal with it and tried both.

Wet rag didn’t help. Carb cleaner didn’t do anything around the base or vacuum lines but it did help down the throat of the carb

[Mbeef61]

Another thing I am noticing while messing with it is that the choke plate on the carb seems stuck open. Usually I can flip the throttle and it pops closed. It is stuck like this

[p0508]

Open the throttle then move the choke.

[Mbeef61]

Originally Posted by p0508

Open the throttle then move the choke.

When it’s off and I move throttle to move choke plate it’s just floppy and returns to open. When running I move the throttle to close it and it revs up but it returns to open

[p0508]

Yeah on a Holley when the engine is off you have to open the throttle to move the choke plate. It's working as it should

[Mbeef61]

Choke is getting 14 volt while running

[Mbeef61]

Yeah I cooled engine off and hit the throttle and the plate closed. I guess that’s not the issue

[Mbeef61]

The things that stand out to me are it idles rough and stalls. But runs fine under acceleration. And it had no vacuum today at rough idle of 500rpm but with throttle jumps to 30, but when I tested it doing my timing it was 17-20 while at 900 rpm idle.

I guess that’s pointing towards vacuum leak? I kind of hear a hissing but the air cleaner is off. So it’s hard to tell. All vacuum lines are new not cracked. All are plugged. Intake manifold maybe? A good way to diagnose that?

I just don’t understand how running out of gas caused it?

[dmjlambert]

Do you have a fuel filter inside that carburetor inlet? It looks like the style that holds one. Perhaps it's clogged, and at higher-than-idle the fuel flow forces the spring that holds it in place to allow filter bypass.

[Mbeef61]

Originally Posted by dmjlambert

Do you have a fuel filter inside that carburetor inlet? It looks like the style that holds one. Perhaps it's clogged, and at higher-than-idle the fuel flow forces the spring that holds it in place to allow filter bypass.

That was one of the first thing I checked. Changed fuel filter and checked those two inlet screens. All clear. Wondering if something got passed and clogged something in the idle circuit of the metering block.

[ga-stepside]

Back in the day we constantly had issues with idling with Holley’s. The trick was to drill out the idle bleeds on top inside of the carb, I think to .062 but don’t quote me on that. The other trick was to remove the idle adjustment screws and use a carb cleaner with the straw and squirt cleaner into the idle screw holes, make sure you face is not over the carb or you will get a face full (don’t ask). If you try spraying the carb cleaner into the bleed holes inside the carb top it will only clog the idle circuit more.

[Mbeef61]

Originally Posted by ga-stepside

Back in the day we constantly had issues with idling with Holley’s. The trick was to drill out the idle bleeds on top inside of the carb, I think to .062 but don’t quote me on that. The other trick was to remove the idle adjustment screws and use a carb cleaner with the straw and squirt cleaner into the idle screw holes, make sure you face is not over the carb or you will get a face full (don’t ask). If you try spraying the carb cleaner into the bleed holes inside the carb top it will only clog the idle circuit more.

So remove the idle screws on the sides of the metering block and spray? And it comes out the air bleeds? I guess that would be an indicator it’s clogged it nothing comes out

[ga-stepside]

Originally Posted by Mbeef61

So remove the idle screws on the sides of the metering block and spray? And it comes out the air bleeds? I guess that would be an indicator it’s clogged it nothing comes out

Exactly.

[toolboxchev]

Vac Leak, Carb problem, Other problem.

For a vac leak spray some carb clean around the base of the carb and see if there is an decrease in idle. Hard to do with the fan blowing air by. Try the base of the intake manifold gasket also, both sides and front and back.

[Mbeef61]

Originally Posted by ga-stepside

Exactly.

Does it need to be running?

[Mbeef61]

Originally Posted by toolboxchev

Vac Leak, Carb problem, Other problem.

For a vac leak spray some carb clean around the base of the carb and see if there is an decrease in idle. Hard to do with the fan blowing air by. Try the base of the intake manifold gasket also, both sides and front and back.

I am going to try that. I did the carb. No changes in idle around base or vacuum ports. I’m going to try the manifold in a little bit here. Hard to get to the back due to motor being in rear position but I’ll spray and pray haha.

Man a sniper efi and fuel cell are looking enticing...

[HO455]

My feeling is that you sucked some junk up (or the fuel pump diaphragm was disintegrating due to no fuel) and some if it got past the filter into the idle passages. The idle orifices usually are in the .015 to .005 sizes so it doesn't take much to affect them.
Pull the float bowls off the carb and look for dirt inside. Before reinstalling them, try removing the idle mixture screws (Remember to count the turns as you take them out) and spray some carb cleaner in. You may be able to flush the dirt out. Clean the bowls, let dry and reassemble.
If this fails you may have to take the carb apart and spray and blow all the passages out.
Have you inspected the tank for debris inside?

Good luck neighbor