1937 B is backfiring

[Matt Ammentorp]

My 37 unstyled B misses and backfires when lightly loaded or when engine braking downhill. It runs just fine when under load or pulling uphill. I have been through all the timing elements multiple times and am convinced this is not the problem. Good spark at each plug, which are both new Autolites, and right on the LH Impulse mark. The carb was rebuilt by Roberts and I have adjusted load and idle screws per the operators manual multiple times with no impact on this problem. I have also confirmed the float setting is correct. A neighbor suggested that there could be a flaw in the manifold allowing intake gases to escape into the exhaust chamber. Seems possible, but I would like to get some more input before tearing into it to investigate that line of thought.

 

[Anonymous-0]

Does it sound like its only running on two cylinders?

 

[coonie minnie]

As you can see, not all the posts here are helpful.

I'd start by looking for any leaks in the the exhast manifold gasket, or between the manifold and muffler pipe.

Backfiring down a steep hill is not uncommon- I'd say normal. Just loping along though, that is not.

It is difficult to tell without hearing it run. You are sure about the timing? No leaky valves?

Good luck.

 

[David G]

It sounds to me that it is just running a little rich, building up unburned gas in muffler and backfiring.

 

[Lee B]

In my experience, if they are running too rich you will at least see some signs of black smoke. Also if they backfire it's because they are running too lean and the mixture never catches in the cylinder, it gets halfway out and the fire runs back into it thru the open exhaust valve. Technically that's not a backfire, but it is a loud pop just the same and so often passes for a backfire I'm willing to consider it a case of semantics. A real backfire would leave the carb dribbling gasoline out it's vents and air cleaner oil might even make it out the intake stack as well. I've never seen that myself.

In my opinion, Matt needs to loose the operators manual for a moment or two and adjust the idle screw a little more open until the backfire goes away. If it runs good at full tilt, the load screw is pretty much good to go where it's at, especially if it doesn't falter when you open up the throttle quick. If your adjustments don't have an effect then you aren't doing it right or the carb needs more work. I'd give that idle screw a 3/4 turn to the left and take another ride up and down that hill, I'll bet you DO see a real difference.

 

[Matt Ammentorp]

That makes a ton of sense to me. You're correct - it's not technically a backfire I guess. In extreme cases flame shoots out the exhaust stack, but nothing bad is happening at the carb. I will try what you suggest. It may take a while before I can visit the farm again, but I will report back. Thanks!

 

[JRSutton]

I'd agree with all the advice to adjust the carb mixture properly - and that some popping is normal going down hill.

FAR from a JD expert here, and just guessing - so for what it's worth:

If your governor is closing the throttle plate while you're speeding down the hill, it'll pop a lot like that. The engine's going to over-speed by the gear action of gravity pulling you down hill - and the governor's going to want to fight the increase in speed by closing the throttle plate.

If the throttle plate closes too quickly and completely, it can cause lots of popping by leaning the engine too much.

I'd imagine there's an idle setting on the throttle plate that allows for the plate to stay a crack open when it's "fully closed".

I'd check that. Or perhaps the JD carbs use an idle circuit that may be clogged? Either way, I'd make sure the engine can still breath while you're engine braking.

 

[Matt Ammentorp]

I opened up the idle screw as recommended by Lee B with positive results. The manual says open it 1.5 turns, I've got it more like 3.5 turns and everything is working WAY better! Not perfect, but not sure I expect perfect from a 75 year old tractor.

I believe JR's comment about the governor shutting down the throttle plate completely is also relevant. When I head down the hill slowly in a lower gear, things work well. If I let the tractor really get rolling in a higher gear, the engine is overspeeding and I still get some popping - nowhere near as much as before the idle circuit adjustment however.

I'm still getting some missing at lower RPM's. Watching the plugs fire (grounded on the manifold) it does seem as though the spark is a little inconsistent. I may try some new plugs and wires just to see if that cleans it up. Thanks for the input!