Dumps fuel when hot

[Daniel G.]

My 67 427/435 center carb dumps fuel when the engine is hot. I set the floats and tightened numerous screws that had come loose but it still sends vapor out of the center carb when I stop the engine and requires a "pedal to the floor" and lots of cranking to start when warm. Any help is appreciated.
Daniel Grunwald

[Jack H.]

Re: Dumps fuel when hot

You may be experiencing a 'natural' consequence of today's alcohol blended pump gas... We had several tri-power cars that owners complained of poor WOT performance. The common denominator was they'd fail to climb much above 5200 RPM.

We decided to get SERIOUS about troubleshooting one (a '69) whose Holley carbs had just come back from a professional overhaul by Jerry Luck (good reputation). After being frustrated by various common cure paths, we rented time on a chassis dyno and immediately duplicated the engine's failure to run above the 5200 RPM threshold (+/1 200 RPM).

On the dyno, the car was outfitted with spark analyzer and exhaust gas sniffer. When the 'problem' occurred the exhaust gas sniffer fluxuated SIGNIFICANTLY (CO2 and HC levels).

The dyno operator was a vintage race enthusiast and had his son don a flame proof suit + race helmet with protective plexiglass. The air cleaner was removed and the son 'rode' the car's fender with the hood on his back and a flashlight in hand...

As soon as the problem re-occured with the engine stumbling/missing at +5000 RPM, his legs kicked wildly telling his dad to shut 'er down! He came out of the engine compartment laughing so hard he could barely stand...

He'd seen the three carbs, randomly spewing spurts of raw gas from their vent stacks at high RPM meaning the fuel bowls were BOILING. Raw gas that errupted was being sucked into the carb venturis causing the air/fuel mixture to be unstable and artifically RICH in random pulses.

He put wet shop towels in the freezer and wrapped the fuel lines with the cold towels. The car ran FLAWLESSLY up to red line three times in a row before the towels heated up and the carbs once more began to boil (max RPM = max engine BTU output).

Then, the tank was drained and refilled with pure race gas. Once the pump gas had been driven out of the fuel lines/carbs, the car ran up to red line without a hitch every time it was dyno paced.

Our conclusion: the packing density of the tripower setup fosters poor heat dissipation (compared to a single 4 Bbl). With today's alcohol blended gasoline, we've crossed over the tri-power's thermal dissipation limit...

[Dale S.]

Re: Dumps fuel when hot

We used to take the methnol out of gas at the races by shaking a gas and water, 50-50 mix in a gradutated cylinder (It was called the water wash test). The methonol would combine with the water. It told you how much methnol was in the gas (this was a long time ago). IF you ended up with 40 ML of gas and 60 ML of water= 10% menthnol. To bad someone could not invent a machine to do this at home. I will get some coments on this one. Peter L your comment? Dale

[Stuart F.]

Re: Dumps fuel when hot

Sounds like another "Crappy Gas" story. A real doozer! Yes, there is a great market for some simple fixes out there. I've done mine on my 63, but it won't pass judgement. Still, you have to look close.

Stu Fox

[Clem Z.]

Re: Dumps fuel when hot

this is caused by the exhaust hear riser passage in the intake manifold directly under the center carb. that part of the manifold is seeing exhaust temp heat when you shut off the car. try wiring you exhaust heat riser valve open but the best fix it blocking the crossover in the intake with stainless steel shim stock to keep the exhaust heat out of the intake manifold. you will loose your auto choke feature as you will have to disconnect the linkage because there will no be enough heat to operate the choke

[Stuart F.]

Re: Dumps fuel when hot

Clem;

I believe it was Duke that recommended blocking just one side of the heat riser passages which would allow using the divorced choke. I've never tried this, but I do know from past experiences that it can be a trade off, i.e. you may have to sacrifice some level of low speed or warm up drive ability (bogging or hesitation) in order to address your heat soak problem. I think, as you say, start with wiring open the heat riser valve, then perhaps block off the left side riser passage and, if necessary, then the right one. Back in the day, we used to do this by just loosening up the center bolts and cut a piece of tin can to match the passage in the gasket. Snip the corners off and flatten it real well, then bend the rolled end up to make like a handle to grasp it with. The tin will last usually a full summer season. It's not permanent, which is what you want for a test anyway. It is better than removing the entire induction system.

Stu Fox

[Clem Z.]

Re: Dumps fuel when hot

Clem;

I believe it was Duke that recommended blocking just one side of the heat riser passages which would allow using the divorced choke. I've never tried this, but I do know from past experiences that it can be a trade off, i.e. you may have to sacrifice some level of low speed or warm up drive ability (bogging or hesitation) in order to address your heat soak problem. I think, as you say, start with wiring open the heat riser valve, then perhaps block off the left side riser passage and, if necessary, then the right one. Back in the day, we used to do this by just loosening up the center bolts and cut a piece of tin can to match the passage in the gasket. Snip the corners off and flatten it real well, then bend the rolled end up to make like a handle to grasp it with. The tin will last usually a full summer season. It's not permanent, which is what you want for a test anyway. It is better than removing the entire induction system.

Stu Fox

i also drained down the rad when i did this to make sure you do not loose any coolant into the engine

[Keith G.]

Re: Dumps fuel when hot

I asked a similar question 2 weeks ago, got a couple of replys, finaly took it to race mechanic, he came up with the same answer, stay away from ethonal. Just drained the tank today but haven't test drove it yet. My symptoms were, when floored at idle it would almost die then take off, at 3000 when floored it would spit and sputter, backfire threw the side-pipes, then take off.

[Daniel G.]

Re: Dumps fuel when hot

Thanks to all for the info. My car runs fine once it starts when hot. It's just that annoying cranking time. I want it to always start immediately when I turn the key- boom- like in the old days. I think I might try some avgas.
Daniel