Don Terrill’s Speed-Talk

Alky carb jetting and other settings.

My 750 DP specs...
PV 3.5
Squirters 42 front 45 rear
Jets 94 front 85 rear
Running a 383 on 1/2 mile dirt...7200 max rpms.

Am I in the ball,park? This is an older carb pre-HP small jets. I'll be rebuilding this deal later this week.

Why a 3.5 power valve?

Not much vacuum available...3.5's seem prevalent from what I can see...these numbers are what was in the carb when I bought it. I'll dial in the PV with a vacuum gauge once it's running.

you better check to see what the vacuum is at WOT so the PV is not closing off

pamotorman wrote: ↑Mon Jan 08, 2018 9:45 am you better check to see what the vacuum is at WOT so the PV is not closing off

i don't think that's going to happen with a 750 on a 383 at 7200rpm.

What is the pvcr ,,, I suggest a chassis dyno session that will tell you if your in ball park, I question the jet spread why is the pri jet so much bigger than the sec jet?

Are there holes drilled above the main jets for additional fuel because the jetting seems really small for methanol alky,those jets look like e85 jetting.

1972ho wrote: ↑Mon Jan 08, 2018 10:15 am Are there holes drilled above the main jets for additional fuel because the jetting seems really small for methanol alky,those jets look like e85 jetting.

Yep... it's an older Barry Grant built before the HP style larger jets were used. Forgot to mention that. I haven't measured the holes yet.

This setup ran for about 18 nights or so. It's what was used on the engine I have. Seems to be a bit rich after seeing a little more soot on the piston tops than I expected.

If this is an old model number 9645 Holley, the original extra holes above the main jets were .125", the power valve channel restrictions were .110", and the main jets were #80. If the extra holes and PVCR's are still these sizes, then the main jets would probably need to be in the mid 80's. I suspect that somebody went up on the main jets in the front end by about 10 steps, trying to cool the engine down. They probably lost power doing that.

Make sure the carb has some decent alcohol needles and seats. What type of fuel pump are you using? Depending on the power of your engine, you may need to use a belt driven rotary pump, being that you are on a 1/2 mile track. Whatever you are using , your fuel pressure at idle will need to be lower than the high RPM/full throttle fuel pressure. Something like 4-5 psi at idle and 7-9 psi at power, depending on the size of your needles and seats. Put a pressure gauge in the car, at least temporarily,and place it up high so that the driver can see it while accelerating on the straights.

On your low manifold vacuum: if your distributor has an mechanical advance curve in it, change that to locked timing. That should bring the manifold vacuum at idle up some, and allow you to use a higher rated power valve, like a 5.5 or 6.5. The 9645 carb came with a hi flow 6.5 power valve, in both ends.

to prevent any fuel system problems i would flush the system with gasoline after the races. also make sure the fuel tank vent is large enought to allow the fuel flow needed

Krooser wrote: ↑Mon Jan 08, 2018 1:03 am Not much vacuum available...3.5's seem prevalent from what I can see...these numbers are what was in the carb when I bought it. I'll dial in the PV with a vacuum gauge once it's running.

I can't imagine not having enough vacuum to use an earlier opening PV. If you are setting it by idle vacuum you are doing it incorrectly.

Yes , I agree with pamotorman. There are two main reasons to remove the alcohol from the race car's fuel system between race days. Alcohols will absorb water directly out of humid air, and that water contamination causes problems in fuel metering and combustion efficiency. The second reason is to prevent corrosion to fuel system components, like aluminum fittings and zinc parts (castings) of carburetors.

Alcohol should be stored in full containers, with as little air space in the top of them as is reasonable. This will minimize the absorption of water by the alcohol.

NormS wrote: ↑Mon Jan 08, 2018 1:19 pm Yes , I agree with pamotorman. There are two main reasons to remove the alcohol from the race car's fuel system between race days. Alcohols will absorb water directly out of humid air, and that water contamination causes problems in fuel metering and combustion efficiency. The second reason is to prevent corrosion to fuel system components, like aluminum fittings and zinc parts (castings) of carburetors.

Alcohol should be stored in full containers, with as little air space in the top of them as is reasonable. This will minimize the absorption of water by the alcohol.

also i would use a shut off valve in the fuel cell vent for the same reasons

I have a #10 fuel supply line with a ball shut-off and a #8 return line from the bypass regulator... still have to plumb that all in after the engine gets installed and the new fuel cell bolted in.

No numbers on the air horn since this one doesn't have one but I believe it's a list #4779... has a .110 needle and seat.

Still waiting for Quick Fuel to ship my carb kit...

Hey Krooser, since it has run a few nights chances are it's not a 4779. The booster orifice size would be too small for alky unless it's soooo small it can't hurt itself. From memory, my alky 750 has .180 orifice through the boosters. Check that if you can.