68sting

My carb is puking gas out the vent sometimes. I have checked my float level numerous times, replaced the needle and seat and add a new fuel pump and filter. It does this off and on. It seems most likely to do this after I have been sitting at a stop light for a while and then get to second gear as I'm pulling away.

Here's a video

jimvette999

What is the engine temp when this happens? Fuel might be boiling. If not that, perhaps some reversion/pulses from intake manifold.

Timsride

Check Your fuel pressure it might be too high. carbs only use about 5 to 7 psi if its 9 or more it might be holding needle off seat then the only place the gas can goe is out the vent.When Your car is moving it is using up some fuel in the bowl. So I would think it would happen more during idle. Just a thought.

68sting

Hard to tell actually temp with the factory gauge but its not running hot. When I shoot my temp gun here are some of the numbers I get. Heat does seem to play a role though. It doesn't happen until things warm up a little.

Carb 90-110 degrees
intake below the carb 140
radiator 170-180

What causes reversion/pulses from and intake?


I did consider fuel pressure and switched from a Holley blue pump to a mechanical pump. I doubt both pumps would be bad.


Here's a little more info on the car.

509BB
750 holly vacuum secondary
9 hg of vacuum at an idle @ 6600ft elevation
I didn't have this problem at 1200ft elevation
cam specs
http://www.compcams.com/Cam_Specs/Ca...?csid=445&sb=1


When I re jetted for this altitude I brought it to a shop and ran it on there dyno for a tune up. Air/fuel mixture was good and of course it didn't do it during that session.

68sting

bump

gerry72

It's hard to see in your video but if the fuel level is high (leaking needle and seat, sunk floats, or just set too high) in the bowls you would have fuel dripping out of the boosters. It looks to be spashing, or maybe shooting drops out of the vent. I can't tell if anything is coming out of the boosters.

Going by the conditions it occurs, I would guess at fuel percolation. But it seems odd that it occurs only in a very narrow operating range. I would have to ask if you are running the carb without an insulating gasket? When this condition occurs, can you dump some cold water on the pump, line, and carb to see if it goes away?

68sting

I'm not sure if its coming out of the boosters or not. I let it idle in the garage tonight for well over 30min. I played with revving it up and just couldn't get it to do anything. I then took it out for a drive and got maybe a half a block and could feel it cutting out. I then got out on to the main road by my house and it flooded out and died. I get it back home and let it idle in the garage revving it up and no issues. Very frustrating.

Earlier in the night I made a quick trip to Costco. Maybe 5 minutes away so no time to really warm up. It ran fine but I checked the carb when I got there and the accelerator pump discharge nozzle and boosters were wet. No flooding problems though.

I would also like to note that while idling in the garage it smells extremely rich. I did change my power valve to a 4.5 earlier this week after checking my vacuum. It was 9 hg in neutral and 5 hg with the clutch slightly engaged to give it a load.

Thanks for the help so far. I'm at my wits end with this B.O.B (bucket of bolts).

1971 LS-5

My big block with a Holley was doing the same thing. Lars steered me in the right direction by telling me I needed a heat shield and a fuel return line from the carb. Since the heat shield was easiest, I did that first. After I test drive on a hot day I can report no problems. One more successful test run and I am going to post a hearty thanks to Lars.

68sting

Do you have a picture of this heat shield? Thanks

Dantana

this is may be a silly thought, but do you have an extra carb or know someone that does? I'd be a little curious if someone elses 750 Holley did that same thing on your engine, especially since you mentioned that you rejetted at higher altitude and you had no problems prior.

Timsride

If Your power valve is that close to what Your vacum is gear ey time You touch the gas to drive Your power valve will open. It was meant for bigger vacum drops. The rating for power valves can be out by .5 so it could be opening every time You put it in gear. If the heat shield was a problem it would happen all the time when it was hot not just on the road, it would happen in the garage too. When You have the bowls off try to clean Your air bleeds out with compressed air. Put a new fuel filter and see if it runs better. It may just need to run a bit cleaner to get vacum up and everything else may fall into place. Try to make changes one at a time so You don't end up with another problem. Thats My opinion.

chevymans 77

Its hard to tell from the video but it looks like fuel droplets coming from the vent, this would make me think that the "whistle" vent slosh tube has fallen out of place.

this may also support the fact that its happening when you take off.

Fuel may be sloshing up and into the vent and into the carb throat and into the engine.

the power valve fuel is passed through the main metering system once the fuel starts flowing through the boosters. the only way for it to make your engine run rich will be once your running on the man circuit or the PV is blown and since you've changed it I doubt its blown.

Do you have a fuel pressure gauge on the fuel log at the carb? If so whats the fuel pressure?

I guess if there is a small leak in the fuel line before the pump it could draw air into the system. If this was to happen it may blow small fuel droplets out the vent, I've never seen this happen but it may be possible.

Just trying to think of possibilities.

Neal

68sting

I don't know anybody around here that has a carb I could borrow. I have considered just buying one to eliminate the carb though.



What size power valve would you recommend I get? Where are the air bleeds located? I will blow them out.

I was thinking the same thing but didn't know for sure.

Thanks for your help.

I'll snap a picture and post it up. Maybe something is not right with that vent.

This is an issue I have had for ever. I was running a Holley blue pump and regulator. It would start out 7psi and slowly drop to 3psi and then go to zero. I bought a new pump and regulator and still had the same problem. Car still ran good though and didn't run out of fuel. I thought my gauge might be bad. I recently changed to a mechanical pump to eliminate that as a part of the problem. I have an in line pressure gage but just haven't put it in yet. I'll report back with a number.

Timsride

If Your sitting at 5hg I would put a 2.5 power valve in thats what I had in My camaro and it worked great. As far as the air bleeds they are on either side of accelerator squirters, when You look down the top of carb they look like brass inserts beside squirter. Someone also mention the whistle that goes on fuel block that may help too.

1971 LS-5

Holley makes a heat shield you can get from Jegs for about 25$. I bought a GM reproduction from Heartbeat City for about 30$. Both of these raise the carb about a 1/4". If you are running a thin paper with steel core gasket between your carb and intake like I was, you need one of these heat shields.

68sting

I switched to a 2.5 PV yesterday.

I don't have much clearance left with the L88 airbox and base. Less then a 1/4" for sure. Its hard to believe with aluminum heads and intake that one more small piece of aluminum would help much. You never know though.

I was trying to pin point the problem a little more tonight and here's what I found. If I keep under 2000rpm I'm fine. If I get over 2000rpm it floods out. I still think it is coming from the vent tube mostly. Hard to tell with the hood on. I cruised around under 2000rpms for a while then went to 2500 rpms and in a 100 yds it was dead. I popped the hood quick and there was gas all over the air cleaner base, carb and lid. I did a quick temp test of the gas with my finger and it was cold to the touch so I don't think its a heat issue. Again it doesn't start to do this until the car gets a little warmed up.

68sting

bump

bashcraft

I know you said you checked the float levels, but mine was acting very similar to yours. I have a 502 with an 850 DP. I had to lower the floats slightly below where you would normally have them and it completely cured all the problems.

BTW, about your fuel pressure gauge, I have an oil filled gauge that does exactly what yours is doing. It's just a POS gauge.

chevymans 77

The only way for fuel to shoot out of the vents is if the needle and seat allows fuel to get buy.

Have you checked fuel pressure?

Mine would do this after a while of driving and it would do it randomly.

I have a fuel pressure gauge mounted where I can see it while driving.

I disconnected the return line and ran without it for a while, this caused a my fuel pressure to become unstable. I ran two different fuel pressure regulators and they both did the same thing without the return line connected.

Does yours have a return line? If so check to make sure its connected and verify its not plugged.

Neal

Timsride

I just watched the video again and was wondering if there is either a vacum or pressure build up in bowl. It seems like something is disturbing fuel in bowl. I was wondering what would happen if You placed Your thumb over vent and feel for pressure? Also have You tried running car with sight plug removed? But I would be leary about reving it with sight plug removed. I am going to look at My holley manual on primary side to see if I can see something that can cause that. I have a similar carb and it works great.