Don Terrill’s Speed-Talk

We tried this "trick" carb on the dyno the other day and promptly lost 50 hp when compared to the basically stock Holley UltraHP Domintor that was on there.

The trick carb is a three circuit Dominator (same as the UltraHP that was on there) and it was dead rich for much of the pull. The odd thing is that it didn't seem to want to respond to changes to the air bleeds. We increased the intermediate air bleeds by 4 sizes (63 to 66) and the AF read out showed that it got richer! We increased the main air bleeds from 30 to 34 and the AF readout didn't change. We gave up after thrashing for an hour and bolted the UltraHP back on there and the power came right back. Still scratching my head on how the carb got richer when we put bigger air bleeds in it.

What size jets are it? Did you change any when testing? Air bleeds are for fine tuning. That far off I would be changing jets first.

andyf wrote:We tried this "trick" carb on the dyno the other day and promptly lost 50 hp when compared to the basically stock Holley UltraHP Domintor that was on there.

Must have been a cheby carb

Seriously, either jet way down or assume the carb was set up / flowed improperly or something else went wrong with it. Ultra HP is a great carb. A properly sized for the engine it will make just as much power as a custom carb.

IMHO, the custom carbs shine in a carb rule situation, not so much when you can just size the appropriate carb for the engine.

If the carbs have the same flow, you will get very little if any difference between a trick and an out of the box if you get the metering correct. And sometimes those "trick" carbs are not so trick.....

Sounds like someone molested another carb with some drill bits.

But I can't believe you expected to correct a pig fat carb with air bleeds.
You have to get into the ballpark with jet first.

I had a "trick" Dominator down 60 hp from the old stock Dominator,
pulled it apart and it was 10 jet sizes fatter than the old one. Worked good after rejetting but made me rethink the trick carb deal for the cost.

This "trick" carb had main jets that were 3 sizes smaller than the UltraHP carb which ran leaner and made more power. So there is something funky going on inside the carb.

We started off with both carbs dialed in with similiar jet and air bleed sizes but the trick one just never came around. My guess is that the special billet boosters must have changed the signal so much that the jetting needs to be really tweaked.

andyf wrote:This "trick" carb had main jets that were 3 sizes smaller than the UltraHP carb which ran leaner and made more power. So there is something funky going on inside the carb.

We started off with both carbs dialed in with similiar jet and air bleed sizes but the trick one just never came around. My guess is that the special billet boosters must have changed the signal so much that the jetting needs to be really tweaked.

I had a "trick" carb that had the same color annular boosters in it that wouldn't even run below WOT throttle, even though I was charged for wet flow and "live" testing. One of the problems was later found to be an idle circuit that was completely blocked off when a 3rd circuit was added. This mod was claimed by the builder to be fully developed and tested many times before.


From that point I learned not to use a source for trick carbs that coincidentally uses that color of annular venturi boosters in their builds. A quote from my current carburetor builder "I see where his thought process is in carburetor development....He's not here yet".

RW TECH wrote:

andyf wrote:This "trick" carb had main jets that were 3 sizes smaller than the UltraHP carb which ran leaner and made more power. So there is something funky going on inside the carb.

We started off with both carbs dialed in with similiar jet and air bleed sizes but the trick one just never came around. My guess is that the special billet boosters must have changed the signal so much that the jetting needs to be really tweaked.

I had a "trick" carb that had the same color annular boosters in it that wouldn't even run below WOT throttle, even though I was charged for wet flow and "live" testing. One of the problems was later found to be an idle circuit that was completely blocked off when a 3rd circuit was added. This mod was claimed by the builder to be fully developed and tested many times before.


From that point I learned not to use a source for trick carbs that coincidentally uses that color of annular venturi boosters in their builds. A quote from my current carburetor builder "I see where his thought process is in carburetor development....He's not here yet".

We (BLP Products) are one of a few companies that machine and sell billet annular discharge boosters to many carb builders in the U.S. and other countries. Of the thousands of these boosters that we sell each year; approximately 70% are anodized purple, although black is becoming popular once again. The color has nothing to do with the size, location or number of discharge holes, nor the shape or length. The color is simply a customer's preference. More than 20 carb-builders purchase the purple AD-boosters from us and all of these companies are pretty sharp.

Andyf: As otherwise stated - for a carburetor to be 50 HP lower than another carb of like size; there has to be something significantly wrong. I'd suggest that you give the original builder a decent chance at correcting the problem because we've all made mistakes. Chances are the builder has a strong desire to make it right for you.

Mike

It's hard to tell, but those boosters look red, not purple. I have to agree with ccb, why were you changing air bleeds, not jets? And if I am not mistaken, Tuner lives somewhere in your neck of the woods, maybe he can help you get it close...

Switch it to a 2 circuit carb. Do a search on the Innovate Motorsports bulletin board and you will see how to do it. The 3 circuit metering is your problem.

Mike Laws wrote:

RW TECH wrote:

andyf wrote:This "trick" carb had main jets that were 3 sizes smaller than the UltraHP carb which ran leaner and made more power. So there is something funky going on inside the carb.

We started off with both carbs dialed in with similiar jet and air bleed sizes but the trick one just never came around. My guess is that the special billet boosters must have changed the signal so much that the jetting needs to be really tweaked.

I had a "trick" carb that had the same color annular boosters in it that wouldn't even run below WOT throttle, even though I was charged for wet flow and "live" testing. One of the problems was later found to be an idle circuit that was completely blocked off when a 3rd circuit was added. This mod was claimed by the builder to be fully developed and tested many times before.


From that point I learned not to use a source for trick carbs that coincidentally uses that color of annular venturi boosters in their builds. A quote from my current carburetor builder "I see where his thought process is in carburetor development....He's not here yet".

We (BLP Products) are one of a few companies that machine and sell billet annular discharge boosters to many carb builders in the U.S. and other countries. Of the thousands of these boosters that we sell each year; approximately 70% are anodized purple, although black is becoming popular once again. The color has nothing to do with the size, location or number of discharge holes, nor the shape or length. The color is simply a customer's preference. More than 20 carb-builders purchase the purple AD-boosters from us and all of these companies are pretty sharp.

Andyf: As otherwise stated - for a carburetor to be 50 HP lower than another carb of like size; there has to be something significantly wrong. I'd suggest that you give the original builder a decent chance at correcting the problem because we've all made mistakes. Chances are the builder has a strong desire to make it right for you.

Mike

I'm aware of BLP's offerings for parts to various carb builders and how colors are selected. In my case I was talking about red boosters that were installed by a builder who generally is considered as being very sharp. Then I had to have the subject carb "repaired" for my customer at my expense.

My point in mentioning this is nobody should ever be surprised that a part like a modified carburertor doesn't work as well as expected. In the old 390 carb days one of the best carbs we ever ran was an out of the box Holley, and some of the worst were provided by builders who otherwise provided championship-winning pieces.

Everyone has a bad day once in a while.

What I've found, for what it's worth Holley has done quite a bit of R&D on it's carbs and from my experience, they are hard to beat if you know how to tune them and give them a chance. I've seen guys take a carb right out of the box, and change the jets based on something they read in a magazine or what some old-timer told them will work....(that was not meant to be offensive to the older guys here since they are probably alot smarter than I am).
I've had great results with Holley's right out of the box, and some dyno time tuning!

We tried a few more things today but nothing could bring the carb around. We changed jets, plugged the power valves to see if that was the problem, returned the air bleeds to out of box setup, etc. This carb just sounds ratty and it runs horrible. We took everything apart and couldn't find the problem. Gaskets are new, float level looks correct,etc.

As a final check we put the UltraHP Dominator back on there (which is also a three circuit carb) and the motor went back to making 680 hp instead of 630 hp.

One very odd thing happened that I can't explain. The carb had 6.0 power valves in it which should be fine since the motor has about 15 inches of vacuum at a fast idle. We blocked off the power valves to see if that would solve the problem and the motor wouldn't run at a fast idle. It sounded horrible and shook the dyno so bad that we shut it off after just a few moments of running. We put some new 6.0 power valves in it and it went back to running the same as before.

So I'm thinking that there is something going on with the power enrichment circuit. Even though the power valves should be closed with 15 inches of vacuum they must be doing something since blocking them off causes the carb to go into a complete fit. Beats the heck out of me.

Seems like fuel is leaking past the power valve...maybe when it was plug the carb was way too lean...

Did you try a new power valve and gasket?