Boiling fuel in the carb... - Team Camaro Tech
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post #1 of 7 (permalink) Old Nov 19th, 01, 05:05 AM Thread Starter
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Location: Austin TX USA
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I'm out of ideas here.

On a recent purchase, a fairly stock 69 350 with a Q-Jet (I usually love Q-jets), after the car warms up (5-10 minutes of normal driving) the car stalls at idle. Open the hood and you can hear what sounds like boiling fuel in the carb. Perhaps it's flooding and wet fuel is simply draining into the intake. The only way to start the car in this condition is to floor the throttle while cranking the engine, as you would for a flooded carb. Also, when carb is 'boiling', fuel has been noticed leaking from the primary throttle shafts...each side. The engine isn't overheating...though I only have a dummy light and visual inspection to verify it's running cool.

1) Is my intake getting hot enough to boil the fuel in the carb? If so, why? (ps..the car has heat riser valve on the exhaust manifold)

2) Is my needle and seat not shutting off fuel supply? I inspected the n/s assembly and it looked good...some very, very, minor galling in the seat but nothing that would cause all this.

What's my next move here?

Thanks in advance!
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post #2 of 7 (permalink) Old Nov 19th, 01, 05:29 AM
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When this happens, look inside the carb and see if there is a lot of vapor coming out of it. Also feel the carb and see if it is really hot to the touch. If it is boiling the fuel, a 1/2" insulator spacer under the carb may help. I have seen heavily worn timing chains cause this, and also a cam that was installed a bit off (discovered this on my 1st cam swap).

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post #3 of 7 (permalink) Old Nov 19th, 01, 05:48 AM
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The fact that you see fuel leaking around the throttle shafts make me think float valve is leaking?? The boiling sound is probably the fuel hitting the intake manifold floor. I would replace the needle and seat and very carefully check/set the float level.

BTW, as for the "boiling noise", I've heard this before with leaky carbs. Remember, fuel will boil away completely at temperatures well below 180 degrees F!!

Paul D.

68RS, 331, 10.5:1, 4spd, 3.90 9", lowered, 245-45-16's all 'round
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post #4 of 7 (permalink) Old Nov 19th, 01, 06:04 AM Thread Starter
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Thanks guys...Hey PDQuick...I'm fairly familiar with Q-jets but what do you mean by "float valve"? Are you referring to the needle and seat assembly or something else?

Anyone else with any ideas?

I'm depressed...I love this old car...need her back in the game. Thanks in advance!
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post #5 of 7 (permalink) Old Nov 20th, 01, 07:07 AM
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I had a case where the needles and floats were not working as they should allowing way too much fuel into the bowls and the fuel would spill over. Thing is it would do this warm or cold. I could come in the garage in the morning and the smell of gas was terrible and it wouldn't start. So I would guess that you are on the right track with fuel boiling since your problem occurs only when the car is warm. A couple of ideas. First is to check the routing of the fuel line as it comes along side the header by the inner fender, and also where it runs up the front of the engine to the carb. If you have headers and the fuel line is anywhere close to the headers, you are probably boiling the fuel before it even gets to the carb. Buy a roll of reflective insulating tape at a good speed shop ($5-7) and wrap the fuel line carefully to insulate. Or reroute the fuel line away from the header. After doing this, if the problem persists, check into putting a "phenolic" spacer under the carb or some other form of insulation that will keep manifold heat from getting to the carb. My guess is with the headers though, the fuel is being heated before it gets to the carb. Good luck and let us know how it works out.
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post #6 of 7 (permalink) Old Nov 21st, 01, 06:13 AM Thread Starter
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Here's an update.

Apparently the ring on harmonic balancer (dampener) has spun. I set my timing at about 10 degrees initial, but since the outer ring had spun a bit (counter-clockwise), I was probably 10-15 (who knows) degrees retarded causing the combustion temps to skyrocket. The crossover/heat riser was transferring this heat to the carb and boiling the fuel.

After I advanced the distributor, a lot, she runs much cooler and with more gumption.

Ol'Sleepy's much happier now!
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post #7 of 7 (permalink) Old Nov 21st, 01, 07:28 AM
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A word to wise......

Always punch mark a balancer for telltale signs later......

Everett 68/350/PG/11.90/115mph
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