Carb Jets

TomC750

TomC750

Senior Member
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.
 
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.
 
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.
 

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51woodie said:
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.
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568074


568075
 
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
 
Drilled out jets are not that uncommon. A set of new jets from a trusted source wouldn't be a bad place to start.
 
ford38v8 said:
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.
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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.
 
Tom, you have three throttle rod positions on your carb. You might have it set in the lean position.
 
tubman said:
Drilled out jets are not that uncommon. A set of new jets from a trusted source wouldn't be a bad place to start.
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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.
 
ford38v8 said:
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.
Click to expand...
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.
 
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!
 
TomC750 said:
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!
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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?
 
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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 ]
 
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.
 
ford38v8 said:
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?
Click to expand...
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.
 
aussie merc said:
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 ]
Click to expand...
Interesting comment. Do not see a problem on the scope.
 
Terry said:
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.
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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.
 
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