Dual Carb Setup

[Pete Fl/Wi]

I am going to try a dual carb setup for the first time. What I have is a 8BA, .060 over, Max 1 cam, stock heads cut .020, electronic distributor, no vaccum. The intake is a Offy super dual. The carbs are rebuilt with #51 jets and 7.5 power valves. My search shows some say block off the power valves others say for this size engine go with smaller jets and PVs. I am open to advice on this setup.
Thanks, Pete

[JM 35 Sedan]

I think Charlie NY is good at setting up 94's for a multi carb set up. You may want to contact him if he does not jump in here with some info.

[V8 Bob]

I am running #51 (stock) jets in my dual 94 setup on my near-stock '51 and performance and plug color are good, so I would start with the 51s if you are at or near sea level, but I would change to 5.5 PVs.
The best thing you can do is run each carb on the stock manifold, making sure they perform equally and produce the same manifold vacuum levels, before installing the dual intake. Then take time to sync them, and you will have a good running engine.

[waldo]

I'm running a super dual with 2 94's. 8BA with 4" stroke, no vacuum and 26deg. # 46 jets and 3.5 power valves. Gives an off-white plug ( h-10 ) and runs great. I only run 1/4 mile at a time.

[BUBBAS IGNITION]

This could be a huge discussion ... My vote is to block off both with block off plugs. Works for me and you never need a power valve....

[JWL]

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If the car is to be driven on the street follow V8 Bob's advice. Just use the lower number power valves he recommended. Blocking the power valves is OK for racing but not the most efficient setup for driving.

[ken ct]

My choice would be somewhere around .047 MJ's and 4.5 PV in both carbs.OMO. you will get several and not everyone agrees on my choice,ive had good results with this combo.ken ct.

[Tom's36coupe]

Pete, I had Charlie ny set up my 94 carbs on my hexagon intake, mine have the pv's blocked and it has 51 jets. I tried going with 46 jets and lower power valves and I could hardly get out of the driveway, real doggy. I guess it depends on your motor setup and how wild the cam is, mine is pretty aggressive. I have also run the same with larger jets, 54 size and it ran about the same but the plugs were not as clean. My 2 cents!

[Ol' Ron]

Tuning an engine has become a real problem, especially when we don't have all the information. With a street engine running without vacuum is a compromise. WOT has it's ignition requirements and cruise its. With $5 gas around the corner it might be practical to study proper tuning methods and do you really know when the engine is running RIGHT??

[Tom's36coupe]

I have always heard that a street motor should be running vacuum advance. The new Mallory electronic crab mount units don't have it, wish mine did. Don't think anyone makes a new replacement crab distributor that has vacuum advance?

[Ol' Ron]

For those that are interested in a GREAT ignition system for the early Flathead. Ya just can't beat the stock one. Providing itin proper working order. and you can always convert it to electronic.

[Charlie ny]

Pete,
I build multi carb setups however a customer wishes. On 2x2's I
recommend blocking the PV's based on my practical experience. As you
describe your motor I would stay with 51's and block the PV's. Please
be aware that 94's are susceptible to vacuum leaks between the bowl and
base. I cannot emphasize strongly enough the requirement to reface
the bowl to base gasket surface on the bowl. It's not uncommon to find
.020 warp on this face. Whether you go with or without PV's remember
to do a leak down test.
If you were a local guy I'd build the carbs as described then do
a road test being especially keen to feel any tendency towards a lean
condition (slight hesitation under medium load or a reluctance to pull
hard in every gear etc.) If I felt that condition I'd go up one jet size. In
all practical reality 51's will probably be where your motor will be happy.
Having done that we'd go for a drive on the freeway with the spark plug
wrench on the seat next to me. After 20 miles or so I'd pull off and kill
the motor PRONTO then remove a couple plugs and check their color.
A chocolate milkshake hue is what you want to see.
However you choose to proceed a top notch rebuild is a must. It's
way to easy to double your trouble with 2x2's instead of doubling your fun.
I'll close with this...there are many ways to achieve a goal, this
is the way I do it. I respect everyone elses method as well.
Charlie ny

PS
email me off site if you [email protected]

[JWL]

Sparkplugs removed after being run at "cruise" speeds, on the freeway, should be almost pure white in color. Any chocolate milkshake plug color is a result of an over-rich condition. Why waste fuel and emit excess unburned hydrocarbons and contribute to engine wear?? Install the power valves and enjoy better gas mileage and cleaner air. A good dual carb setup, with properly built carbs, can offer more usable street power than any camshaft and can improve gas mileage at the same time. But not when operating excessively rich during cruise conditions.

If you believe the engine can efficiently use exactly the same mixture ratio at wide open throttle as it does during "cruise" then blocking the power valves makes sense. If you understand the cruise condition engine requirements are much different that the wide open throttle conditions you should elect to use power valves as designed by the automotive engineers.

[ken ct]

Sorry but almost white plugs indicate a mixture on the lean side. A primer for burned valves,sorry to disagree with you. ken ct. OMO

[cadillacoffin]

this is what uncle max told me if your going to use 97s

"I find the best 2x2 combo for most rascally flatheads is .043" mains with # 69 powervalves, unless you do a lot of cruising in the Himalayas.
A lot of guys don't realize that in a properly functioning Stromberg - with new parts - that the powervalve starts to come to the party the moment you stab the gas, due to the hydraulic action of the accelerator pump, and becomes fully open at the bottom of the stroke. If you step on the gas less violently, the built in bypass valve in the pump takes over, for a smooth transition. Simple, but effective... That's why it's important to marry the jets and pv's for correct A/F ratio for different applications."

[JWL]

Yes, the "cruise" fuel mixture should be much leaner than the "power" mixture and will leave almost no color with modern, unleaded, gasoline. There should be 10 to 25 percent less fuel in the cruise mixture than in the "power mixture. Extremely turbulent and homogenous mixtures can be as much as 30 percent different between power and cruise.

[uncle max]

Originally Posted by cadillacoffin

this is what uncle max told me if your going to use 97s

"I find the best 2x2 combo for most rascally flatheads is .043" mains with # 69 powervalves, unless you do a lot of cruising in the Himalayas.
A lot of guys don't realize that in a properly functioning Stromberg - with new parts - that the powervalve starts to come to the party the moment you stab the gas, due to the hydraulic action of the accelerator pump, and becomes fully open at the bottom of the stroke. If you step on the gas less violently, the built in bypass valve in the pump takes over, for a smooth transition. Simple, but effective... That's why it's important to marry the jets and pv's for correct A/F ratio for different applications."

Apples vs. Oranges... These guys are talking about Holleys!

[Mart]

Ok here's what works for me.
Twin 1" venturi holley 2100's. (look like a 94)
4" stroke, .045" overbore French motor.
Thickstun PM7.
Isky Max 1 cam.
Stock crab style dist.
In a fairly light 31A roadster.

52 jets. 2 x stock (7.5) power valves.
Must run with air filters.

Any smaller jets, lower number power valves or no filters just makes a big flat spot.
I get 18" - 20" vacuum at idle - about 14" on a light cruise - why would I not want 7.5 power valves??

It might be that twin 94's with 15/16" venturis would be happier with 51 jets.

Just saying what works for me.

Mart.

[hbleadsled]

I agree with Ken Ct ,white plugs do indicate a lean mixture they seem to work best with a tan color for me . Harold central coast Ca.

[flatjack9]

As JWL has said, there is a marked difference in air fuel ratio for part throttle as opposed to full throttle. Full throttle being significantly richer, hence the coloring. If you are reading that at part throttle, you are too rich and wasting fuel.