Merc 99 with 2GC- Main jet starting point?

B

bobscogin

Senior Member
'42 Merc "99", .060" overbore, stock 29A heads, Schneider 260º/.355 cam, stock crab distributor on 12 volts, late Mercury intake manifold. The Rochester 2GC is from a '66 Buick 340 and currently has #52 main jets. Intuitively, I'd think the #52 jets would be too large simply because of the size engine the carb came on. Realizing that there's no exact answer, can anyone offer a ballpark opinion on what jets may work best. I'm at sea level. I'm ready to fire this thing up.What say you guys?

Bob
 
I think you have the wrong 2GC carb. the 2GC came in 3 sizesL: Small on the 265.283 chevy medium like on the Buick and larg like I don'r know what? Whe using the Merc intake the intake should be bell mouth to eliminate the mis match between throttle bores. I bore the intakes to match the carb the #52 jet usually works fine in a mild engine. The biggest problem you face is the stock distributor. It will not work on this carb. you must convert to a machanical adv distributor like the SBC.
 
Ol' Ron, thanks for the response. My 2GC is the small base version with 1 7/16" dia. bores. The holes in the intake manifold measure 1 5/16" so they're a little smaller but with the thick insulator under the carb the throttle opens all the way without interference. When I pull the intake first time to check the valves. I'll open them up. As for the crab distributor, it utilizes manifold vacuum for the vacuum brake, correct? I've hooked it up that way on the Merc intake, so shouldn't it see a similar vacuum signal? I've read that the brake can be mostly backed off anyway if pinging's not a problem, so that's what I planned on doing.

Bob
 
Sorry I thought you were running the late style distributor. Guess my brain isn'r up tp speed yet.
 
I am running a 2GC carb from a 65/66 Jeep with a Buick V6 engine on a 51 merc engine, running a 77B Isky cam and just changed my jets from 51's to 49's , the 51's were running too rich, I am running a stock distributor with vacuum advance. I may even try a set of 45's that I have if the 49's prove too rich
 
Your distributor is not working correctly. the 2GC vacuum is not the right kind, so your advance curve is not correct. Convert to a SBC unit, big improvement.
 
For the sake of comparison - I just built a 276cu. in. 8BA with a 2GC carb. The engine has milled EAB heads, an Isky Max1 cam, a modified Chev. point type distributor and Reds headers. When I built the carb, i used the smallest jets I had at the time which were #55. The engine runs fine at higher RPMs but is a little rich at low speeds. FWIW
 
I used to be a SBC fanatic in my youth so I had a lot of parts and jets, any local carb rebuilder should have them, also I used to get them at NAPA. Ron, If the advance is not working correctly, how can you tell--??
 
The load-a- matic distributor requires a special vacuum signal from an 8BA 94 carburetor. Without that you don't get the proper advance. The only distributor that can furnish the proper advance curve are the MSD with Vacuum or a modified SBC unit. You not only get more power but better gas mileage as well
 
no aftermarket....yet. edelbrock only has the jets for the quad, not the same as the 2 jets.

I think 52/53 jets will work good.
 
Bobscoggin try these guys Carbshop-Eric Morris-864-292-3692
Old Carb Doctor-Jeff Dreibus--800-945-carb
Rudy Faltemeir-in Spartanburg, SC he is a v8 club member so his name should be in the book
 
I believe the load-a-matic dist uses the pressure change across the carb venturi for dist control. More engine speed = more carb air flow = higher pressure differential across venturi = more advance.
 
Barn Junk what you are saying is the vacuum port is above the throttle plates, as the plates open more vacuum is introduced into the vacuum advance on the dist.
 
Actually you have a combination of manifold and venturi vacuum. As the throttle opens, manifold vacuum drops, but the increase in airflow creates a larger pressure drop in the venturi resulting in an increased vacuum signal.
 
Back to the original topic. Today I ran the engine (on the test stand) with the #52 jets. I realize that a real road test is the best way to check jetting, but the engine didn't show any signs of being overly rich. Crisp throttle response and the exhaust smelled right. I'm going to leave the carb as is until I git it installed then I'll fine tune it.

Bob
 
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