70-72 Qjet boiling gas in Carb body

[Norm C.]

While my original Cater Qjet had not experienced this problem. I'm learning of it now. I've been told, the slow start to Qjets over the last 10+years is related to the dreaded ethynol now allowing the (boiling or evaporating of the gas in the body) after turning the warn engine in summer months. I'm also told that there is a trick to seal 3 different ports in the body of the Carb that will help solve this issue. 3 questions: Have other C3 owners experienced this problem and found a cure? If so, could you share your success? 2. Does my description sound familiar? 3. Are there other ways to solve the problem?

Thanks in advance to all for you assistance!!!

[Mark E.]

Re: 70-72 Qjet boiling gas in Carb body

It sounds like you're not having any of these problems, but you've been told about them?

Fuel with ethanol does evaporate more quickly. For a Q-Jet (which has a small fuel bowl), it typically takes several days for the entire bowl to evaporate requiring extra cranking during start up. Warm/hot starts are not affected by this.

I've heard of fuel boiling in the bowl, but never experienced it myself. I suspect it may happen during extremely hot conditions (like when the engine overheats or during heat soak on a hot day). Chevy does offer a stainless heat shield that's installed between the manifold and carburetor. Years ago, I had a California spec Malibu 400 with a 402 big block with a shield that I believe was factory installed. So maybe they were concerned about the carb getting too hot. I installed one my '70 454 but I'm not sure if any Corvettes had one installed originally.

Starting in 1970 (CA spec cars) and 1971 (most other cars), the external fuel bowl vent was plugged to reduce evaporative emissions. That may have also reduced evaporation a bit.

But if you don't have these problems, you're good to go...

[Edward J.]

Re: 70-72 Qjet boiling gas in Carb body

Norm, look to see on pure gas. Org website to see if you have a station that sells non ethanol fuel near you. as Mark says ethanol does have a lower boiling point. with high temps and long drives can create harder starts after hot soaks.

[Patrick H.]

Re: 70-72 Qjet boiling gas in Carb body

I have not seen this in the base motor 72s that come through the shop, or with my own.
Sounds like more urban legend of Quadrajets?

[Norm C.]

Re: 70-72 Qjet boiling gas in Carb body

Thanks to those who responded. With the jump in temperatures here, coupled with ethanol it was suggested the percolation or boiling was a candidate for the Carb body not holding fuel. There are no signs leakage anywhere on or around the carb. Any other possible reasons to ck for
why this is happening now?

[Mark E.]

Re: 70-72 Qjet boiling gas in Carb body

Thanks to those who responded. With the jump in temperatures here, coupled with ethanol it was suggested the percolation or boiling was a candidate for the Carb body not holding fuel. There are no signs leakage anywhere on or around the carb. Any other possible reasons to ck for
why this is happening now?

Norm, There are three main ways the fuel bowl loses fuel:
- Evaporation, which takes days and generally does not affect warm or hot starts
- Back down the fuel line. Using a fuel filter with an anti-siphon valve helps with this. Some folks remove the clip on the needle so it stays seated.
- Leaking plugs on the bottom of the bowl. These plugs can leak fuel into the intake manifold, so it's not visible. The engine will start like it's flooded- hard to start, black smoke. If bad enough, the engine will run a bit rich all the time.

You can find more detail about these issues in other threads on this forum, CF and Cliffshighperformance.com.

Be sure you don't misdiagnose normal fuel evaporation (common) for a leaky bowl.

[Norm C.]

Re: 70-72 Qjet boiling gas in Carb body

Discovered Gremlin

Wanted to share we finally located the Gremlin long cranks on a cold start. Recap, the Carb received a rebuild kit. After the first 1/2 dozen or so cold starts, the starts became longer and longer. What threw us off when the engine was cold, when extending the accelerator by hand, the carb showed absolutely no fuel being squirting in the primaries. Hence believing the bowl was empty, right? Not this time. Yet there were/are no external signs anywhere of the carb leaking. However when removing the carb lid while on the intake manifold, we discovered gas was in the bowl. After moving the pump a few times and not seeing any fuel movement, we discovered old pump had created very slight scratches or ruffness on the throttle barrel wall that caused the new throttle loss of performance thus allowing fuel past it when initially advancing the throttle. After a few passes with very fine emory cloth on the throttle barrel and another new throttle pump, it starts now almost as fast on cold start, as it does on warm starts. Thanks again to all who offered their experience!!!

[Mark E.]

Re: 70-72 Qjet boiling gas in Carb body

By throttle pump and throttle bore, are you referring to the accelerator pump and bore?

[Norm C.]

Re: 70-72 Qjet boiling gas in Carb body

Yes Mark, I believe I am. Sorry for any confusion or nomenclature errors