Blow through carb rebuild

[Paul_G]

After the install of the air fuel ratio gauge I found the carb tune was way off. Too rich at idle, mid 11's, to rich at cruise, low 12's, and stupid lean when given the beans, mid to high 15's. How the carb is built is a puzzle. It does not have a Holley list number. Looks to be a custom built carb. Best guess is it's a Holley 950. Mechanical secondary's, no choke.

The engine it is on, 528ci Hemi with an F1 Procharger. So a custom built blow through carb is in order. But why is it built like this? The build explains how it performs.

Jets, Pri 72/ Sec 86, 31 squirters front and rear.
The power valves on both sides have the vacuum passages blocked in the base plate. They were tapped for a set screw and sealed with epoxy. So the power valves were not working. The bowl vents are extended in to the charge tube, they did good work on that. The metering blocks have been drilled and tapped for adjusting the idle mixture in the idle feed circuit. This work had to be done by someone who knows carbs. A back yard hack does not do these kinds of things to a carb.

My plan is to leave the jetting as is for now. I opened the vacuum passage to the front power valve and put a 4.5 PV in it. The rear vacuum passage stayed closed for this, https://simplestage.shop/products/brpb-boost-referenced-power-valve , an adjustable boost referenced power valve.

With working power valves the carb should be pretty rich with that jetting. Safer to start there.

What do you guys think?

 

[pnora]

What did you do about the blowby issues?

 

[Paul_G]

What did you do about the blowby issues?

Nothing as of yet.

 

[pnora]

Nothing as of yet.

Just wondering if ring seal could affect the readings. There is no PCV on your car right?

 

[diesel_lv]

Blowthru carbs need crush proof floats. They do not use a traditional power valve as they are getting pressure, not vacuum on hard acceleration. I use CSU carbs.

 

[Paul_G]

Just wondering if ring seal could affect the readings. There is no PCV on your car right?

Does not have a PCV system.

Blowthru carbs need crush proof floats. They do not use a traditional power valve as they are getting pressure, not vacuum on hard acceleration. I use CSU carbs.

Both floats are Nitrophyl.

 

[diesel_lv]

 

[Paul_G]

That is a great video series on blow though carbs. It is where I saw the adjustable power valve. Have one on order, should be here this week.

 

[Paul_G]

They do not use a traditional power valve as they are getting pressure, not vacuum on hard acceleration. I use CSU carbs.

I have given this some thought. A power valve works by manifold vacuum overcoming the spring and closing the valve stopping fuel flow through the valve. The spring on the power valve holds the valve open to allow fuel flow. With the engine running, and as long as the throttle blades are restricting air flow in to the engine, at idle and low throttle positions, there will be sufficient vacuum to overcome the spring in the power valve and keep it closed. As the throttle blades open, vacuum goes away, and the spring is then pushing the valve open. With a boosted application pressure in the manifold increases, but the power valve does not respond to this pressure, it only responds to vacuum, and loss of vacuum.

With the boost referenced power valve in Titans video, it does respond to boost pressure. As the pressure increases the valve will open more and more. More boost = more fuel needed, this power valve can do that. That is why they are used in the secondary with a traditional power valve in the primary.

Or do I have this all wrong?

 

[diesel_lv]

I honestly can't answer that. I had a bit of money tied up in the engine for my turbo jet boat. I didn't want to chance it and lean out n ruin it. I told CSU what I was doing n he built my carb. Ran flawlessly. Unfortunately I let it sit for a little over a yr n it got all gummed up. I just sent it back and had him rebuild. It was cheap insurance to ensure it was done right. I had windowed a cheap block before n know how quickly that can happen.