48 farmall cub

[G Dorn]

I finally got my cub running.I STIL HAVE TROUBLE WITH GOVERNOR SETTING BUT NOW THE CUB BACKFIRES WHEN I INCREASE SPEED.what is causing it to back fire

 

[old]

I finally got my cub running.I STIL HAVE TROUBLE WITH GOVERNOR SETTING BUT NOW THE CUB BACKFIRES WHEN I INCREASE SPEED.what is causing it to back fire

Good chance the carb is running to lean so it in turn backfires

 

[G Dorn]

Good chance the carb is running to lean so it in turn backfires

When I adjust the carberator to 1 1/2 turns out it starts but if I turn more than 1 1/2 turns it does not start.Turning out makes it less lean correct?

 

[560Dennis]

Suggest this , Put a vacuum gauge on the tune up port of the manifold . If you want to Post results for comment , should be steady needle 18 to 21 inches . Adjust idle screw and watch vacuum change .

 

[G Dorn]

What is the tune up port ? The exhaust on the manfold. How would I attach a vacuum gage

 

[used red MN]

When I adjust the carberator to 1 1/2 turns out it starts but if I turn more than 1 1/2 turns it does not start.Turning out makes it less lean correct?

My understanding is that Cub carbs have a set high speed jet, meaning no adjustment needle. The one idle mixture adjustment screw on that carb has nothing to do with the problem you have. That idle mixture screw is for adjusting for a smooth idle when the throttle plate is closed at idle. Once the throttle/governor opens the throttle plate the idle mixture circuit/passages in carb become ineffective. Have you checked if the mechanical advance is free in your distributor? Remove the distributor cap and with you thumb and forefinger twist the rotor clockwise with light force. It should twist forward against spring pressure with the tip of the spark blade moving about a quarter inch or so and then spring back when released.
The vacuum plug if it has one is on the intake manifold right above the carb. Using a vacuum gauge is “all well and fine” the problem is to do much diagnostics with them takes a person that has used one a lot on engines to see how it reads when engines have certain problems. By doing this they train themselves or use the experience to make a diagnosis. For you, ya you can plug a vacuum gauge on there and it will give you a reading, but in my opinion that is all you’ll have is a number x vacuum at this speed. In my opinion yes there are ways to use a vacuum gauge that are beneficial, but it takes experience working with one on a lot of engines to develop a diagnostic “knack” that is reliable.

 

[G Dorn]

So what you are saying I can not adjust whether the engine is running lean or rich.What causes the engine to backfire?. Could it be a faulty carberator

 

[used red MN]

So what you are saying I can not adjust whether the engine is running lean or rich.What causes the engine to backfire?. Could it be a faulty carberator

If the problem or blame is for sure in the carb you would have to first open the carb and make sure the main jet passages and the power nozzle are free of dirt or restrictions. If that doesn’t fix it you have to increase the size of the jet. Again, if you are nearly 100 percent convinced it a problem in the carb.
What about the mechanical advance? A stuck mechanical advance could produce the same symptoms.

 

[kencombs]

Turning the idle mixture rich or lean should not affect starting at all, at least in my experience. If it backfires when moving from idle to a faster running speed, that could be due to a lean idle setting. But, as others have said, it needs to be clean and dirt free internally first. A float setting to low can also cause lean running.

I don't recall if the Cub has a transition passage like some older cars and trucks. But if it does, that passage being restricted will cause that symptom. I've seen that a lot with 40s/50s carter one barrels.

 

[560Dennis]

What is the tune up port ? The exhaust on the manfold. How would I attach a vacuum gage

How I set up mine on Farmall C . I see 1/8 pipe plug on the cub manifold in photos. Connect vacuum gauge with the resources you have.

 

[grandpa Love]

Have you adjusted the valves? Tweeked the timing? Do you have a manifold gasket leak?.......100% sure the plug wires are on the right plugs? #1 is close to the radiator....1,3,4,2.....

 

[G Dorn]

Do you mean by moving the magneto a little off from where it clicks.Is that tweaking ? I made sure the wires are correct

 

[Powered By IH]

So what you are saying I can not adjust whether the engine is running lean or rich.What causes the engine to backfire?. Could it be a faulty carberator

Is the engine actually backfiring or is it just stumbling (like not getting enough fuel) as you increase speed?

Once you get it up to full throttle does it work fine?

Does it backfire less once the engine is warmer?

 

[used red MN]

moving the magneto a little off from where it clicks.

Okay, I ran through your previous post on your governor linkage pretty quick, I see you have a magneto. It will not have a fly weight mechanical advance I was asking about in my previous replies. It does however advance the timing from static once it starts. Basically if the impulse mechanism is free enough to operate the “advance drive position” comes in essentially when the impulse comes up to speed and the catch levers are held in by centrifugal force.

 

[G Dorn]

Is the engine actually backfiring or is it just stumbling (like not getting enough fuel) as you increase speed?

Once you get it up to full throttle does it work fine?

Does it backfire less once the engine is warmer?

The engine starts at half throttle if I move it back to a lower speed it stops .When I move it to increase the speed the engine backfires

 

[G Dorn]

If I retard the timmig would that help or would cause me more problems

 

[used red MN]

If I retard the timmig would that help or would cause me more problems

Retarded or late timing can cause back fires and sluggish response. Advancing the timing would be the direction I would go as a possible remedy. If you don’t crank start it kick back is not as big of a concern as far as injury but it can damage starters. Pre ignition aka pinging is what to much advance will do this comes on under acceleration and heavy loads. Thatprobably won’t be a problem if it has stock pistons. A lot of advance also produces a “fluttery” idle.

 

[G Dorn]

I seen on YouTube that to increase load for pulling or plowing you should retard the timming and for road driving advancing the timming.Now I am confused

 

[used red MN]

I seen on YouTube that to increase load for pulling or plowing you should retard the timming and for road driving advancing the timming.Now I am confused

I would have to see their explanation to give any agreement to the that statement. If you have a higher compression ratio engine that may be true to a degree. In general retarded timing doesn’t give power, rather it lowers the propensity for pre ignition/ pinging. On the road the load is not constant figuring in hills, so on uphill it may ping but on the coast it would not so the cylinders and valves could cool off. I really don’t see the advantage of running the engine on the ragged edge. If everything on YouTube is the “gospel” we are in trouble.

 

[G Dorn]

Thanks for your insight I will try to advance timming a little and see if it helps.I don't believe everything on YouTube I appreciate your experence