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We tried to run our 1st ROMCO race last nite. We had just bolted on a new BG 750 Race Demon. It came from the factory with 4 #80 jets, #34 air bleeds, and I can't remember the power valve size, but this is the baseline setup BG recommends for our motor size, rpm and track length. We only had 1 practice and the carb was loading up thru the turn and missing off the turn. It would clear up toward the end of the straight, but then load up again when he got off the gas.

We went from #80's down to #76's and went out for the race. It did exactly the same thing----no change.

Someone suggested a bad power valve, another fuel pressure.

I need to hear from some really good carb tuners--amateurs need not apply!

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  • 2 weeks later...

OK, for all that are interested, here's what BG found:

1. The float in the secondary was a drag race float, not a circle track. That made it look like the level was correct in the pits, but in the turn the level was way too high.

2. Even my 76 jets were too big. They replaced with 74's.

3. The power valves were 45's. Our manifold vacuum is less than 8, so these are slightly too big (rule of thumb is 1/2 the manifold vacuum). We are going down to 25's.

They flowed it and looks OK. We are going to run HMP saturday so I'll let you know if it works.

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I was thinking about the PV numbers you mentioned. I've never heard the "1/2 the vacuum" rule of thumb. The one I've always gone by is 1.5 to 2.0 inches below idle vacuum. If your idle vacuum is just under 8, I think a 2.5 PV is possibly going to open too late. It may not open at all, thus causing a lean condition.


Personally, I'd probably use a 5.0 PV with the vacuum reading you mentioned, unless you took that reading at a really high idle RPM.


As cheap as PV's are and as easy as they are to change, the best plan would be to have a few on hand beginning with a 5.5 and going down to a 3.5 or so and find the one that nets the best pull out of the corners. Start with the highest number and step it down until you feel the motor bogging out of the corners. Then step it back up one notch.


Good luck

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Most of the Holley HP series carbs don't come with secondary PV's. I wouldn't run one there. The idea is for the PV to richen the circut during transition to WOT. Since you go from idle to WOT just after the apex you should be able to set it up like a drag car. PV around 4.5 in primary and bump up the jets in the secondary by 8-10 and block off the PV.


I would make fine tuning much simpler. Kinda like locking out the timing, one less variable without any performance penalty.

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Here's what BG tech said:

1. They DON'T recommend eliminating the PV in the secondary, and they DO recommend square'ing the jets (the same in both primary and secondary).

2. BG jets flow different than Holley jets, so stick with one or the other, but don't mix.

3. PV of 2 inches below idle vacuum was a spec recently printed in Circle Track Mag regarding a BG carb but is not what BG recommends - 1/2 idle vacuum is what they recommend. They are sending me both 45's and 25's. Different length tracks may require different sizes (how long you stay at full throttle).


Jay, They didn't change the air bleeds, but said the 74 jets will be on the high side and we should wind up in the 71 - 74 range depending on conditions and track. Jason will definitely be watching the temp gauge (at least in practice - I don't think he knows what a gauge is in the race).

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You might consider tuning your carburetor on a chassis dyno (using your actual race car). Select the jetting that gives you the proper air-to-fuel ratio and maximum power. Try different ignition advance settings for best power. The dyno operators have an AFR sensor that they can place inside your exhaust pipe.



See the following website to locate a business near you.




Best $150 to $200 you will ever spend. Some tips are:



Use rear tires with same air pressure and no stagger.



Make a sign for your dashboard that reminds your driver not to touch the brakes!



Bring a portable fan to blow into the radiator to keep the motor temperature the same from run-to-run.



Bring earplugs.



Get a good baseline by making the first 3 runs without any changes.


Have your crewchief or car mechanic drive the car while it is on the chassis dyno. It's a rush.

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Carb worked fine, but had to go down to 25 power valves. Then had electrical and timing problems. Car died on about lap 38, coasted in, found short in dist wires, thought we fixed, ran 8 more laps and died again on lap 46. Couldn't refire. Entire electrical redo before next race.

By the way, when I finally found a tech (Mike) at B/G that knew what he was doing, they were very helpful, and even put my carb ahead of the normal schedule since they knew we had a race.


Thanks for the website. I found a place in Austin that will do the 1st pull for $75 and then $25 for each pull after that.

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