quadrajet heat soak

[Sam71395]

Hi all, sorry this doesn't involve a square body, but my chevelle is having some issues with heat soak, It has an aluminum headed 400 and aluminum intake. It starts perfectly when it's cold and never gives me issues with vapor lock while driving or idling, but after I shut it off and restart it after 10-15 minutes I have to put the pedal to the floor to start it and clear it up. Ive done research on these wood phenolic spacers and never tried one, I was just wondering if those work or if there is something else I could try. Thanks

 

[Ricko1966]

Hi all, sorry this doesn't involve a square body, but my chevelle is having some issues with heat soak, It has an aluminum headed 400 and aluminum intake. It starts perfectly when it's cold and never gives me issues with vapor lock while driving or idling, but after I shut it off and restart it after 10-15 minutes I have to put the pedal to the floor to start it and clear it up. Ive done research on these wood phenolic spacers and never tried one, I was just wondering if those work or if there is something else I could try. Thanks

The problem I've had with qjets not running a return style fuel pump. Is they hold pressure in the fuel line,the fuel heats and pressure increases sitting hot,the pressure blows the needle and seat open,and floods the engine. Run a filter with a return line or a pump with a return line.

 

[nvrenuf]

Are you sure this isn’t a choke adjustment problem?

 

[Sam71395]

The problem I've had with qjets not running a return style fuel pump. Is they hold pressure in the fuel line,the fuel heats and pressure increases sitting hot,the pressure blows the needle and seat open,and floods the engine. Run a filter with a return line or a pump with a return line.

Your talking about one like this correct

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[Matt69olds]

Your talking about one like this correct

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Does your fuel pump have 3 ports? One is a feed (from the tank) one is the supply (to the carb) and the 3rd is a return. The return line has a tiny orifice, that provides a carefuy calculated internal leak that constantly circulates fuel to keep the pump cool, and to provide a vent to relive pressure during heat soak.

If the fuel filter in the pic has an orifice in the 3rd fitting, that would work also.

 

[HotRodPC]

Hi all, sorry this doesn't involve a square body, but my chevelle is having some issues with heat soak, It has an aluminum headed 400 and aluminum intake. It starts perfectly when it's cold and never gives me issues with vapor lock while driving or idling, but after I shut it off and restart it after 10-15 minutes I have to put the pedal to the floor to start it and clear it up. Ive done research on these wood phenolic spacers and never tried one, I was just wondering if those work or if there is something else I could try. Thanks

I haven't read all the responses, but I'm going to go with 90% chance it's leaking well plugs. 2 main issues to fix a Qjet for good. Seal the well plugs so it doens't drip fuel into the intake causing this problem, and after sitting awhile, you don't have this problem, but you have to crank til you refill the bowl before it'll start. 2nd, get the throttle shafts bushed with brass bushings to fix the vac leaks around the throttle shafts. Now you have a carb that is repairable with a "kit". Rebuild "Kits" do not make Qjets like new again. They have to have these repairs done FIRST. Once you do, then you can weld the hood shut cuz you won't have any problems.

 

[Sam71395]

Are you sure this isn’t a choke adjustment problem?

I haven't read all the responses, but I'm going to go with 90% chance it's leaking well plugs. 2 main issues to fix a Qjet for good. Seal the well plugs so it doens't drip fuel into the intake causing this problem, and after sitting awhile, you don't have this problem, but you have to crank til you refill the bowl before it'll start. 2nd, get the throttle shafts bushed with brass bushings to fix the vac leaks around the throttle shafts. Now you have a carb that is repairable with a "kit". Rebuild "Kits" do not make Qjets like new again. They have to have these repairs done FIRST. Once you do, then you can weld the hood shut cuz you won't have any problems.

the carb was rejetted and built by Sean murphy induction (SMI) and if i remember correctly the well plugs where epoxied, but they could possibly be leaking. I will double check them tomorrow. and I will double check the choke.

 

[Sam71395]

Does your fuel pump have 3 ports? One is a feed (from the tank) one is the supply (to the carb) and the 3rd is a return. The return line has a tiny orifice, that provides a carefuy calculated internal leak that constantly circulates fuel to keep the pump cool, and to provide a vent to relive pressure during heat soak.

If the fuel filter in the pic has an orifice in the 3rd fitting, that would work also.

My fuel pump does not have a 3rd port, and I believe those didn't come with a return at that time, I beleive GM started implementing a return in the mid-late 70s, but I don't think it would take much for a guy to add a return

 

[HotRodPC]

the carb was rejetted and built by Sean murphy induction (SMI) and if i remember correctly the well plugs where epoxied, but they could possibly be leaking. I will double check them tomorrow. and I will double check the choke.

OK, so consider it done and the throttle shafts done too. He knows what's up and he'd never let a carb leave without those things done. So yes, you do have another issue then. No need to even check, I'm sure your plugs and throttle shafts are handled already and you have a good carb. So consider other options, like that choke. Do you ever park on a slope, even to the side one way or another? Like if the floats are adjust a tad to high, and on a slope, allowing fuel to flow out 1 side? I'd say doubtful, but it has happened.

 

[HotRodPC]

BTW, not sure if it's been mentioned yet either, but another thing that will cause this issue you're talking about, is initial timing is to far advanced. Sometimes, just a 2 degree retard will solve the problem.

 

[Sam71395]

OK, so consider it done and the throttle shafts done too. He knows what's up and he'd never let a carb leave without those things done. So yes, you do have another issue then. No need to even check, I'm sure your plugs and throttle shafts are handled already and you have a good carb. So consider other options, like that choke. Do you ever park on a slope, even to the side one way or another? Like if the floats are adjust a tad to high, and on a slope, allowing fuel to flow out 1 side? I'd say doubtful, but it has happened.

I will double check the choke tomorrow, im a little foggy on the method to set an electric choke, I will do some more research tonight, and if that doesnt work i will add a return to my fuel system with that fuel filter with a 3rd port

 

[Ricko1966]

I will double check the choke tomorrow, im a little foggy on the method to set an electric choke, I will do some more research tonight, and if that doesnt work i will add a return to my fuel system with that fuel filter with a 3rd port

You don't even have to use a fuel filter. You can use a tee in the fuel line with a restrictor in the return line,we use to use a.piece brass with an orifice drilled,but I have used a lawn mower fuel shut off valve as a restrictor. GM used a 3 port filter on big blocks late 60s early 70s. I'll get you a pic you might like it BIAB.

 

[77Dmax]

Those aren't heat soak symptoms. If you are holding it to the floor and having to clear it out, it's flooded. Heat soak shouldn't be much of an issue with aftermarket heads and intake that don't have an exhaust crossover either. That carb must be dripping fuel or its pig rich to begin with.

 

[Ricko1966]

Those aren't heat soak symptoms. If you are holding it to the floor and having to clear it out, it's flooded. Heat soak shouldn't be much of an issue with aftermarket heads and intake that don't have an exhaust crossover either. That carb must be dripping fuel or its pig rich to begin with.

Yep with no return the fuel in the line expands blows the needle off the seat and they flood.

 

[HotRodPC]

Those aren't heat soak symptoms. If you are holding it to the floor and having to clear it out, it's flooded. Heat soak shouldn't be much of an issue with aftermarket heads and intake that don't have an exhaust crossover either. That carb must be dripping fuel or its pig rich to begin with.

Or advanced timing, but that suggestion kinda takes a back seat since he says holding it the floor starts it. I didn't even realize it had aftermarket heads, assuming aluminum? If so, that also negates the heat soak and vapor lock theory. Aluminum discipates heat quick so yep, I'll go PIGASS Rich adjustment and or float adjustment. He said the carb is from SMI, I'm certain they are aware how to seal up well plugs and bush the throttle shafts, so it's not that.