750 DP alky settings

[NormS]

Are you actually sure this is an alcohol carb? The easiest way to tell is to measure the feed holes in the boosters, and the angled passages near the top of the metering blocks that feed the boosters. For Methanol. the booster passages should be at least .180", and the metering block passages even larger. The 4779 gas carb came with the booster passages at.160", and the metering block passages at .140".

The needles and seats you use will need to be much larger that .110". The actual size will depend on what your fuel pump is.

What type of fuel pump are you going to use?

[Krooser]

Are you actually sure this is an alcohol carb? The easiest way to tell is to measure the feed holes in the boosters, and the angled passages near the top of the metering blocks that feed the boosters. For Methanol. the booster passages should be at least .180", and the metering block passages even larger. The 4779 gas carb came with the booster passages at.160", and the metering block passages at .140".

The needles and seats you use will need to be much larger that .110". The actual size will depend on what your fuel pump is.

What type of fuel pump are you going to use?

Yep...the early BG carbs were converted gas carbs. It has 18 nights on it + or -. I have to dig out my number drills and see how big those passages are. The metering blocks have been modified.

I'm using a mechanical tandem fuel/power steering pump thats belt driven off the flywheel. The original owner used a modified stock style fuel pump.

[pamotorman]

here are the barry grant alky specs

[Krooser]

This carb was built before the Demon series was produced... 1993. Been on the shelf since '95 except for two nights in '07.

The booster passages (F) are .148... the PV passages (J) are .111. Looks like 50cc accelerator pumps. Same specs in both front and rear blocks.

I'm not sure if this was a shelf alky carb back then or someone switched it to alky... I doubt the PO did it as he farmed out all of his motor work. Could be the original engine shop did it...I do know the PO never spun this deal over 6500 so MAYBE the carb was tuned to his conservative style?

http://st.hotrod.com/uploads/sites/21/2 ... C671%3A419

[barnym17]

I don't think the booster passages are big enough at .148

[NormS]

Yes , definitely the booster passages are way too small. They need to be at least .180"

The tandem pump is a good addition, I'm glad you're not using a pushrod pump.

[JDR Performance]

It sounds like it's pieced together or built wrong. Does the body have the "BG" stamp on it? It'll be on top. If the main body was set up for gas it won't have the right air bleeds or squirter nozzles either.

[pamotorman]

why not just follow the BG specs on the carb you have as that should get you very close

[Krooser]

It sounds like it's pieced together or built wrong. Does the body have the "BG" stamp on it? It'll be on top. If the main body was set up for gas it won't have the right air bleeds or squirter nozzles either.

Stamped BG...

I'll talk to the PO about the history of this carb.

[Krooser]

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.

I've run a few 4779 on gas... after a second look it's likely not. Stamped BG on top...never had a choke tower. Front and rear metering blocks stamped 9924.

[In-Tech]

You stated a few threads back the limiting factor of your boosters orifice size. I "treed" Norm by a few seconds stating the same thing about the booster orifice. Your straw is too small, change boosters.

btw, all my circle track customers love your sig line.

[Ks Fats]

X3 on the booster feed orifice; are they drop leg or straight? If straight then use a ream or core drill to take them to 180-190. If they are drop leg then replacing them would be easier; usually use the stepped drop legs from B.L.P.; available in both 180 and 190. Before I got the tool to roll them in I secured them with epoxy and 4-40 brass set screws,never had a problem with that method. Your new kit should come with .150 needle and seats.

[Krooser]

Thanks in-tech... A lot of my younger friends question me getting back in the saddle at my age and budget. Someone has to finish last!

Fats... Drop leg boosters.

I have a good friend that does great carb work but he's expensive and doesn't do alky carbs. Redid my son's 4412 carb...$450.00 and I bought the parts! Too rich for my blood.

[pamotorman]

if you are going to epoxy the boosters in place make sure you use methanol proof epoxy.

[Krooser]

Funny how things work out... My friend who's in the race carb business left a phone message for me last night. He found a 4412 that I dropped off at his shop four years ago... It was going to be a spare for my kids b mod but wasn't gauge legal so we just parked it for another time.

He was cleaning the shop and found it on the shelf.

Might be a good opportunity to ask him about changing the boosters in my carb.