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Pump gas percolation carburetor

General engine tech -- Drag Racing to Circle Track

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BigBlocksOnTop2
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Pump gas percolation carburetor

Post by BigBlocksOnTop2 » Fri May 15, 2020 2:49 pm

69 mustang (closed hood),395 stroker windsor, vic. jr. 249 @ .050 in.,10.4:1, canfield heads, BG 750 annular, aluminum 3 core radiator with shroud, SS flex fan, mechanical pump (110 gph) edelbrock street 1725, no. 8 braided off the pp to the carb. Ambient air temp is 84 degrees. Percolation is real bad, very hard starting engine at operating temp, pull away from the light and stumbles...what a mess. Got home and immediately popped the air filter and worked the throttle. Gas only spurts from the primary shooters. Secondary's has a nice stream. Cracked the fuel line at the carb and gas sprayed. My question is, will a return fuel line (right on the line connecting to the carb. with around a .050 orifice help. I know I cant go to big...the engine is right stout. I will add, the carb was not hot to the touch but the fuels lines are. Engine cold, there are no issues. The carb is sitting on one of those 1/4'' heat gaskets.I had a fellow hot rodder say I may want to go the EFI....no this one!

aquartlow
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Re: Pump gas percolation carburetor

Post by aquartlow » Fri May 15, 2020 5:04 pm

I had similar issues with vapor lock/fuel percolation. Removing the mechanical fuel pump and installing an electric fuel pump near/below fuel tank and intake gaskets with blocked heat crossover ultimately solved my issues. It seems todays fuels are less tolerant of having a vacuum placed upon them, like a mechanical pump would as an example. I also used self adhesive heat shielding material on fuel supply line where it got within 3" of exhaust tubing/headers. A 1" phenolic spacer would also help a bit if room permits. Hope this helps.

F-BIRD'88
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Re: Pump gas percolation carburetor

Post by F-BIRD'88 » Fri May 15, 2020 8:50 pm

Get the Seals-it fuel pump heat isolation gasket kit.
Move and or insolate the feed side of the fuel line from ex heat. Add a Pusher elec fuel pump at the back near the tank to pressurize the fuel line.
It is spring time. You are likely still using winter gas which vapourlocks easier than summer gas. (vapor pressure)

Yes a return line will help.

JDR Performance
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Re: Pump gas percolation carburetor

Post by JDR Performance » Fri May 15, 2020 9:39 pm

X2 on the return line, it makes a big difference with pump gas.

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MadBill
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Re: Pump gas percolation carburetor

Post by MadBill » Sat May 16, 2020 9:50 pm

X3 on the return line. I spent a number of years doing vehicle development and in our hot soak tests, a return line fed by a 0.040" orifice near the carb made a night and day difference.

F-BIRD'88's seasonal fuel reference is also important. In the carburetor days, customer driveability complaints peaked in the spring and the fall when unseasonably warm weather overlapped with winter grade fuel. In Ontario, "Winter" is September through June. #-o (would anyone then be surprised to learn that high R.V.P. fuel is cheaper to produce?)
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Re: Pump gas percolation carburetor

Post by allencr267 » Sat May 16, 2020 10:52 pm

BigBlocksOnTop2 wrote:
Fri May 15, 2020 2:49 pm
Gas only spurts from the primary shooters. Secondary's has a nice stream. Cracked the fuel line at the carb and gas sprayed.
I don't know the carb, but find it strange that it's percing only on the primary side.
I think you've got an acc pump problem. Check its check valve for debris.
Good luck.

GLHS60
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Re: Pump gas percolation carburetor

Post by GLHS60 » Sat May 16, 2020 11:25 pm

I've had great success with these GM heat shields in the past.

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Woodruff
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Re: Pump gas percolation carburetor

Post by Woodruff » Sun May 17, 2020 12:16 am

X2 on a small electric pump on the back. The summer blend is reported 50% vapor @170* F
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Carburetorrestorationyoungstownoh.com

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Re: Pump gas percolation carburetor

Post by jeff swisher » Sun May 17, 2020 7:35 am

Some gas stations here have the same nozzle for the 10% ethanol as they do for the 100% gas.
I was told 4 gallons needs to be pumped out of the gas pump before you get all 100% fuel flowing from it.

I have ran 10% ethanol and have ran a mix of 50% E85 to 91 octane.

I noticed and many friends noticed in the heat the gas will look like it is boiling in the fuel filters with the Ethanol fuel.
100% gas I do not get that issue.

Really bad issues with boiling at the track when you have a lot of hot soak time.

Datsun had issues with fuel boil or vaporizing back in the 70's-80's and installed an auxiliary cooling fan to blow onto the injectors for the 280Z cars.
A fuel return was the best fix for those cars.

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Re: Pump gas percolation carburetor

Post by BigBlocksOnTop2 » Sun May 17, 2020 8:13 am

MadBill wrote:
Sat May 16, 2020 9:50 pm
X3 on the return line. I spent a number of years doing vehicle development and in our hot soak tests, a return line fed by a 0.040" orifice near the carb made a night and day difference.

F-BIRD'88's seasonal fuel reference is also important. In the carburetor days, customer driveability complaints peaked in the spring and the fall when unseasonably warm weather overlapped with winter grade fuel. In Ontario, "Winter" is September through June. #-o (would anyone then be surprised to learn that high R.V.P. fuel is cheaper to produce?)
A .040. I was going with a .050 but I'll do .040 and try. The smaller the better. Thanks for the input.

BigBlocksOnTop2
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Re: Pump gas percolation carburetor

Post by BigBlocksOnTop2 » Sun May 17, 2020 8:16 am

allencr267 wrote:
Sat May 16, 2020 10:52 pm
BigBlocksOnTop2 wrote:
Fri May 15, 2020 2:49 pm
Gas only spurts from the primary shooters. Secondary's has a nice stream. Cracked the fuel line at the carb and gas sprayed.
I don't know the carb, but find it strange that it's percing only on the primary side.
I think you've got an acc pump problem. Check its check valve for debris.
Good luck.
I think because the engine has been operating mostly on the primary. So the ''hot'' gas is constantly going in the pri. I am going to check the pri. shooters...engine cold will answer the question.

Bill Chase
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Re: Pump gas percolation carburetor

Post by Bill Chase » Sun May 17, 2020 8:21 am

Out of sheer curiosity, is this becoming more prevalent with modern ethanol blended pump fuels and the seasonal blend changes?

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Re: Pump gas percolation carburetor

Post by GRTfast » Sun May 17, 2020 8:31 am

Had the same issue on my big block. Mechanical pump dead head feeding a 850 Holley that was bolted to the manifold.

Went to a 1 inch phenolic spacer and electric pump with return line. Carb is actually cold now, as are the lines.
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Re: Pump gas percolation carburetor

Post by treyrags » Sun May 17, 2020 9:19 am

MadBill wrote:
Sat May 16, 2020 9:50 pm
X3 on the return line. I spent a number of years doing vehicle development and in our hot soak tests, a return line fed by a 0.040" orifice near the carb made a night and day difference.

F-BIRD'88's seasonal fuel reference is also important. In the carburetor days, customer driveability complaints peaked in the spring and the fall when unseasonably warm weather overlapped with winter grade fuel. In Ontario, "Winter" is September through June. #-o (would anyone then be surprised to learn that high R.V.P. fuel is cheaper to produce?)
This is spot on. I run the .040 bypass with a mechanical pump and it helps a bunch. Its also important this time of year to get gas at a station that sells "top tier" rated fuel and moves a lot of it so that you can make sure you are getting fresh gas that is the summer blend. Even with all that there are still times during very hot running conditions when mine will percolate a little after a hot soak. After start up it takes a few miles of driving to get the cooler gas and some air flow to make it act right. I'm also going to try mounting a couple of computer type mini fans under my air pan to blow directly on the bowls. An electric pump with a full bypass would be even better.

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