Carb Jets
 

Has anyone had to install larger carb jets for their stock 239? My '41 Merc (totally stock) seems to be lean at cruise, all speeds. The carb has been professionally rebuilt and the rebuilder suggested I may have to go larger. The stock jets are #50's. I had a pair of 51's that I tried with a possible slight improvement. Unfortunately, my stock of jets (of which I have well over 200) go from #64 to #99, obviously for higher displacement engines.
Vacuum leaks are nonexistent. As soon as the power valve comes into play it pulls very strong. As I am a bit of a perfectionist, I will be satisfied with nothing other than a perfect running car! Next step I will probably hunt for some #52's.

Re: Carb Jets
 

Tom, Eastern Tennesee? Smoky Mountains 5000' elevation? If anything, I'd think too rich with 52s. There must be another answer. You know what they say, half of all carburetor problems turn out to be ignition.

Re: Carb Jets
 

I had Charlie NY do the carb and fuel pump for my "46 Coupe, 59AB, because I was getting 12-15 MPG. He service them both and ran them on his test engine.
When I got them back, I bolted them on, made no adjustments to idle or idle mixture, and the engine idled at 500 RPM, with good vacuum. He found that a previous owner had drilled the main jets oversized. Mileage now ranges from 17-20 MPG. and the plugs look good, at least to my eye, after 9500 miles.
Don't know if this information helps, but that is my story.

Re: Carb Jets
 

https://www.fordbarn.com/forum/attac...4&d=1749755328

https://www.fordbarn.com/forum/attac...5&d=1749755328

Re: Carb Jets
 

Ford did recommend jets for high altitudes for the 21A Carb. : 50 is the standard, 5.K' to 10.K' .048 and 10K' to 15K' .046

Re: Carb Jets
 

Drilled out jets are not that uncommon. A set of new jets from a trusted source wouldn't be a bad place to start.

Re: Carb Jets
 

We are only at 1200' as I remember. I do have an altimeter As I recall. But it is running lean at light throttle, not rich.

Re: Carb Jets
 

Tom, you have three throttle rod positions on your carb. You might have it set in the lean position.

Re: Carb Jets
 

My jets are not drilled out, I measured them. I will say my compression readings are not that great, 70-80#, vacuum is low too at maybe 17-18, but have not checked after sticking valve loosened up. I have good power, no smoke, blowby or oil usage. Thanks, I value your experience.

Re: Carb Jets
 

I have it in the middle. It does not lean out when throttle is increased, only at steady throttle.

Re: Carb Jets
 

Not ignition, runs great at all RPM's except at steady throttle, where it is lean.
I had a scope on ignition, not easy considering the unique access to the condenser. Dist has been rebuilt.

Re: Carb Jets
 

Forgot something - When the car is surging, which is not dramatic, if I pull the choke out a bit to richen it a bit, it stops surging!

Re: Carb Jets
 

Hows your air cleaner. need a clean. Try running with it removed,
Phil NZ

Re: Carb Jets
 

OK, I got it... your fuel pump is not delivering. The problem can be the pump itself, an air leak before the pump, a restricted fuel line before the pump, or a non-vented fuel tank. Surging is the key word in your problem. I take you at your word that your carb has been correctly rebuilt and float adjusted properly. Your choke pulled out slightly helps pull the fuel through the pump.

You should have from 1 to 4 lbs pump pressure.
An air leak or a restriction will reduce volume and pressure. Do you have a glass bowl pump? An air leak may show bubbles there. Use cork gaskets there, soaked in oil. Check connections and hose clamps. Wire spring hose clamps are no good, replace with worm gear clamps. Fuel lines can deteriorate and develop pinhole air leaks. A fuel line leak before the pump will never leak gas, but will suck air.

A restriction can be dirt in the fuel lines, a fuel filter needs replacing, or an internally collapsed flex line. These are reproduced fuel resistant, advertised in the V8 Times under Shewman. Do not get the check valve type.

Non vented tank requires a vented cap. Have you replaced fuel caps lately without regard to venting?

Re: Carb Jets
 

Phil, He's running lean, not rich.

Re: Carb Jets
 

2 things that get forgotten and effect everything 1 float level will effect mixture more than anything else and 2 is your vacuum brake set correctly [if its dancing around it effects timing and will cause surging ]

Re: Carb Jets
 

Have you adjusted the idle mixture to get the vacuum as high as possible. 17-18 is a tad low. If you can not increase the vacuum with the idle mixture then check for vacuum leak. The summer hot setting on the accelerator is #1 #2 is average temp.

Re: Carb Jets
 

I will admit I have not checked fuel pressure. I should do that, but have not bothered since it pulls strong and reaches for the stars at full throttle. Otherwise I have covered all the bases you mentioned. The car has an electric pump as well as the original. I only use the electric one to prime It has no effect on the running otherwise.

Re: Carb Jets
 

Interesting comment. Do not see a problem on the scope.

Re: Carb Jets
 

Will revisit idle mixture, but I do have screws turned out further than I ever have on my cars. I have suspected vacuum leak, but found none listening with an instrument and introducing propane around all surfaces on top of engine and around carb. None found. My carb was rebuilt by Charlie. He has been helping me trying to figure this out.