1939 Lean mixture

FlatheadTed

FlatheadTed

Well-known member
Been thrown a curve ball here , , Car spits and back fires on rapping the throttle , have gone through the distributor coil condenser ,cleaned out the carb (Ford ),changed the Power valve /Gaskets , checked the pump, all checks out good ,tightened the Manifold ,I am thinking a bad manifold gasket or crack, or carb jets to small .broken spring or sticking valve ,Thanks Ted
 
Ted, you say it's lean, but are you sure it isn't rich? A new power valve and gasket may itself be a problem if it doesn't seal well. The modern power valves have a radius on the gasket surface that prevents a good seal. This must be machined flat, or, some guys say using two gaskets afford the thickness needed to bridge the radius.

If your guess of a vacuum leak is the cause of reduced pull at the venturi, you can find it quickly with a spray can of carb cleaner at the suspected spots.

If the jets were too small, you'd have a flat spot on acceleration. A more likely cause of running lean would be crud in the carb bowl. Take the air horn off and you'll see the crud clogging the main jets. This can happen even if using a fuel filter, so check it.
 
Thanks Alan for the quick response , it does have a flat spot on acceleration, it back fires through the carb specially when you jerk the throttle to 3/4 ,Its had a complete clean out all jets blown out .power valve seat looks ok ,one cylinder is fowling the plug ,Ted
 
Ted I recently had a similar problem with my truck which I have cured by installing Petronics electronic ignition and leaning off the mixture and check tightening the carb bolts, I also discovered that the fuel pump was delivering too much pressure to the carb and have fitted a pressure regulator. All is now very sweet.
 
Ted, A couple things are going on with your engine:
The flat spot, if not plugged jets, can be caused by a malfunctioning accelerator pump. Some rebuild kits come with a bad piston. The best to use is the original leather, just oil it up and go. Adjusting the piston throw to the longest position can also help smooth out a flat spot.
If the carb had previously been used in a dual carb setup, then it may have smaller jets than normal. Stock jets should be 50 or 51.
The fouled plug is probably the cause of the backfiring through the carburetor, if the intake valve is stuck or fails to close. This condition can also blow out your new power valve in a big hurry.

mjdunn, carb cleaner or any flammable sprayed at a vacuum leak will be sucked in, causing the engine to momentarily surge.
 
A while back,Ol' Ron had a good suggestion for determining if the jet sizes are correct. He said to run your hand over the carb.If the engine speeds up,it's too lean. If it slows down, it's too rich.
 
Thanks for all the replies .I checked out the possibility a malfunctioning accelerator pump and changed to a nice leather one , one fouled plug is probably the cause of the backfiring through the carburettor, I cleaned the plug and it seem to run smoother, If the intake valve is stuck or fails to close.have had this before were the valve keeper was not in place ,but this one checks out at 100 PSI ,I did spray started fluid around and it smoothed out, that was at the base of the carb, at that stage I replaced the carb gasket. pump pressure is spot on, 2,1/2 I have tried the hand and choke over the carb ,I know a drop in voltage can cause a similar condition but now after 3 coils the spark stays strong jumping to the head ,After the cheeking the jets I will lift the manifold , Ted
 
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I have improved it a lot by changing carbs ,I think they re kited wth small jets (50)s.I know the Model As need the jets flow checked in the rebuild kits ,Wondering what size hole should be in the Gasket (pic ) this lets Exhausted gas into the idol circuit .still a irregular idol .Thanks to the F Barners .Ted
 

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Ted, if your intake manifold has a heat riser hole, it is incorrect for use with a ford carburetor. It can still be used, but will deliver way too much heat to the carb and should be blocked off. A manifold of that type is used only with the Stromberg, not the Ford.

As I said earlier, a 50 or 51 jet is stock.

A Ford carb is not plug and play. The idle screws need to be tuned to idle correctly:
Turn the screws in by hand until seated, then turn them out 3/4 turn.
With the engine warmed up, set the idle speed at 500.
Then Adjust one idle screw only, the goal being to gain rpm, running smoothly.
Then readjust the idle speed down to 500 again, and follow the same procedure with the other idle screw.
This process should then be repeated until both idle screws are adjusted correctly.
 
Alan Thanks again I wondered about the heat riser and gasket hole didn't look right ?? This manifold was put on in the past .it has a copper blank of gasket that I will put back on ,this could be the exhausted gases leaning out the idol mixture . Yes I understand the process on the mixture screws . Ted
 
Ted, as stated, the ford carb is not compatible with the manifolds that have the exhaust heat riser hole. The small hole in the carb base is the vac supply for the cavity that holds the power valve. You might do better to swap to a non hole manifold if you have a suitable one handy.

Mart.
 
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