Blow through carb?

[Paul_G]

My new to me 69 Roadrunner carb has a problem. The primary needle and seat is not holding back fuel, fuel dribbles out of the boosters at idle. Tried to lower the float and it just gets worse. Carb needs a rebuild, but that is not the problem, what could be a problem is the carb that's on the car.

Specs; 528 Indy Cylinder Head Hemi with a Procharger, and water meth system. Now this should be a blow through carb. The number on the carb is 6R 7867 B, which I can not find on Holleys site. I did find that that part number is also listed on some sites with #80496, which is a 950 CFM, race double pumper. But thats not a blow through carb. So maybe someone modified it?

I know my way around a Holley, but never played with a blow through. Can some one school me a little on the differences and what to know when rebuilding.

 

[Beekeeper]

First thing what is your fuel pressure?

 

[Paul_G]

6.5 psi. Boost referenced, and regulated with return.

 

[RemCharger]

Did you put new needle and seats in.

 

[diesel_lv]

Is it doing this at idle w the 6.5 psi fuel pressure? If so, needle n seat or floats are ruptured which is what happens to unmodified floats in a blowthru application. Blowthru needs different floats and power valve. I had mine built by CSU. But many articles online about them.

 

[Paul_G]

Did you put new needle and seats in.

Not yet. Looking for the correct renew kit.

Is it doing this at idle w the 6.5 psi fuel pressure? If so, needle n seat or floats are ruptured which is what happens to unmodified floats in a blowthru application. Blowthru needs different floats and power valve. I had mine built by CSU. But many articles online about them.

Yes at idle with the hat removed. Going to take the bowl off tomorrow and see what float is in it. I want to get the correct needle and seat as well.

 

[Curiousyellow71]

It sounds like the needle and seat is bad. Take the bowl off and blow through the line and move the float up and down to see if it's stops the flow.
This is a decent article. We converted a 750 holley to blow through in the mid 90s.
https://www.motortrend.com/how-to/build-blow-carb-less-50/amp/

 

[Paul_G]

I am going to pull the front bowl off today and take a look. I have some used needle and seats around. I will swap one in and see what happens. I dont want to change anything related to the tune of the carb. I dont want to go down that rabbit hole.

I found this video. They say a picture is worth 1000 words, a good video is a picture with 1000 words. This guy explains it pretty well, if like me you don't know anything about how a blow through carb works.

 

[dvw]

Make sure the O ring around the seat assy is not torn. Weigh the floats on a gram scale. One could be heavy.
Doug

 

[Paul_G]

Make sure the O ring around the seat assy is not torn. Weigh the floats on a gram scale. One could be heavy.
Doug

BINGO! That is it, the O-ring. Hard as rock, crumbled. Trying to find another.

 

[Paul_G]

Picked up a renew kit. Replaced the needle and seat in the primary bowl. That took care of the over filling the bowl problem.

This car is new to me. So now I have this other problem. Burning the tires at will. it can be a problem. But not the problem I found on the test drive.

When really on the gas hard the engine lays down after just a few seconds. When I let off the peddle it drops to idle and all seems well. First impression is it is sucking the float bowl dry. How to determine if it is too small needle and seat, pressure regulator problem, or fuel pump problem?

 

[dvw]

Assuming a good fuel system. Single needle and seat bowls will flow enough fuel to make a little over 850hp on E85. On gas probably 1000hp. Get close to those numbers and you'll need dual needle and seat bowls.
Doug