Twin Carb "issues"

M

magzboy

Junior Member
Hi Team. I recently installed two "new" Holley 94's on my 59a. #51 jets and 7.5 PV as a base set up. Float levels are set at recommended height / drop. The car starts easily first pop and idles sweetly. The "issue" i have is that i have the idle screws wound right in closed - is this normal ?
 
Looking forward to this thread as I’m also now looking into putting an Edmunds 2 x 2 intake I recently acquired on my C59A. Info on the interweebs seems to be all over the place
 
A couple of issues here to be aware of. First, the NEW Holley 94''s may have issues not present in original carburetors, as reproductions nowadays are suspect.
Second, .51 jets are way too big for dual carburetors, as they had been furnished on stock engines with only single carburetors. Try going down to .46 jets and see where that puts you: One clue to jets being too small would be experiencing a hesitation when calling for rapid acceleration from idle speed.
Your large main jets overcompensates for the idle circuit being closed off. You need all three circuits for proper operation of the carb(s), so when you change your main jets, you'll find that you cannot idle until you open and adjust those screws.
Having two carbs is in itself a pain to synchronize functions, so good luck with that.
 
If you can turn both sets of idle mixture screws in completely and it still idles, then the engine is getting fuel from another circuit in the carb.

The most likely cause is that you have the throttle plates open enough for the main jets to supply fuel. I would (being careful!) look down the carbs and see if you see any fuel coming down the main dump tubes. Also, take whatever linkage you have on the carbs completely off (to ensure the throttle plates are closed) and then adjust your idle stop screws and the idle mixture circuits.
 
Bored&Stroked said:
If you can turn both sets of idle mixture screws in completely and it still idles, then the engine is getting fuel from another circuit in the carb.

The most likely cause is that you have the throttle plates open enough for the main jets to supply fuel. I would (being careful!) look down the carbs and see if you see any fuel coming down the main dump tubes. Also, take whatever linkage you have on the carbs completely off (to ensure the throttle plates are closed) and then adjust your idle stop screws and the idle mixture circuits.
Click to expand...

In addition, you may have a leak at the base of the PVs. From V8 Bob

"The fact is dual 94's have and will work great on 200 ci "A"s and stock 239 ci flathead Fords, as in my '51, so unless your engine is really small, the 94's should work OK. The stock jets are #51, so I would not go any leaner than the 49s. I think, as others have said, leaking power valve(s) are the problem. The late Holley valves do not have the same flat seal area as the early valves do, and require slight machining and/or special washers to keep them from leaking."

Your PVs are probably too high too. With a dual setup you have less vacuum, ditto with any type of hotter cam, so with 7.5s, the PVs will open too soon. The Holley 94s are vacuum operated. Stromberg 97s are mechanically operated BTW.

I was aware of this PV seating issue and when I decreased the size on my dual 97s, I bought correct ones from Charlie Schwendler who either machines them or purchases them this way.

Charlie Schwendler
5845 Cole Rd, Orchard Park, NY, 14127
(716) 662-9159
cas5845@yahoo.com
 
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I installed new PV's from Vintage speed but have some 4.5 ones coming. I also have #49 and #48 jets awaiting. Being in NZ makes it frustrating as the time to get stuff, the poor dollar exchange and crazy shipping costs. Two PV 's, one set of jets and 4 gaskets come out to NZ$203 ! All in the joy of the flattie.
 
"new" as in new repro or "new" as in they are new to you but are original 94s?

If it runs with idle screws in, fuel is getting in by some other means. needle valve not seating, too much fuel pressure, float level too high, punctured float and the one that catches lots of people out, a vacuum leak between the base and main body in the area of the power valve chamber.
If you have old carbs the bodies can warp and create a leak there.
How can a vacuum leak cause over rich? you ask.
Because vacuum holds the power valve closed. Low vacuum lets the power valve open and gives you the fuel so it runs with the idle screws in.
My personal experience is that 51 jets and 7.5 (stock) power valves are fine in a twin setup. But there must be no fuel enrichment by other means - hence the thing about vac leaks in the power valve chamber.

I have videos showing inspecting 94s and machining the bodies to sort the leaks.

I have two 94s on a stock 239 and it runs great.

I tested both carbs on a single manifold before pairing them up.

If I can find the relevant videos I will post a link.

Mart.
 
Mart brought up a good point . . . what does "new" mean? If they are new 94 style carbs from Edelbrock, then the power valves should be correct (as far as not leaking). If these are rebuilt 94's, then only God knows . . . we have no idea as to what is in them.

Where did you get these carbs from?

The fact that you've bought new PVs from Vintage Speed is probably a good thing, but until you know the source of your "added fuel", it is hard to say.

Even if you had 7.5 power valves, you should not have your idle issue . . . unless there are other problems to uncover (causing an extremely low vacuum).
 
I have two original 94’s on my ‘53 and have gone thru a number of combinations of jets and PV’s. Originally I had stock (51) jets and 7.5 PV’s in the carbs, and it ran rich and fouled plugs in a hurry. Called Daytona Carbs and talked to their tech advisor and he suggested smaller jets (can’t recall exact sizes…47’s???) and 4.5 PV’s. That was a minor improvement. Also had “hot restart” issues, and it always seemed to be flooded after sitting for more than a minute. Only cranking for 5-10 sec with gas pedal to the floor would get it to start. If I started it immediately after shutting off, it would be running after half a revolution. A couple years ago, I corresponded with Charlie, and he suggested blanking off the PV’s and going with larger jets (again, exact size I can’t recall). Installed them and no fouling, but it runs really rough stumbles badly until it’s really warmed up. And it also has some stumbles at initial acceleration. And there’s no improvement in the hot restart issue, so I can’t blame leaking PV’s. So I’m still shopping for a better jet and PV combo. May also check again for vacuum leaks per Mart’s video.
BTW… Charlie also helped me (with extreme patience and effort) with Mallory ignition issues I was having. He’s a great resource!!!
 
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