John Deere AR Backfire Question

[Bobl1958]

I have a question on this little AR I am working with. It is a 1948. I have had the mag rebuilt as well as the carb. Both in a shop that knows what they are doing.

The tractor runs nice, no issues there, but the problem is that when you grab the throttle lever to slow it down, the tractor wants to backfire one time pretty loud. When I shut the gas off to kill the tractor, it will backfire as well, even though it is at an idle. The tractor runs fine otherwise, and the governor is working correctly.

Also, I was driving it around last night about dark and noticed the manifold was red with heat.

This tractor sat in a shed for several years. I am wondering if valves out of adjustment would cause those issues? I am assuming it might be firing on an open hole. I know the carb is adjusted good, and the mag timing is right. Any thoughts on this? Thanks - Bob

 

[Mike(NEOhio)]

I would think fuel mixture. My A will backfire if it's too lean. That or late ignition will also cause them to run hot.

 

[Bobl1958]

Thanks for the reply. I would agree with that except the tractor runs fine until you slow it down. Then is when it backfires, as it is slowing down. I have the carb set pretty well by the book, and have played with it a lot. The load needle is about a turn, and the idle is about 1-1/2 turns. If I mess with the idle either way too much, it starts running rough until I turn it back.

Like I said, the carb was done by a carb shop that knows two cylinder carbs very well.

The timing should be right, but if I turn it either way, retard or advance, it still backfires.

This is why I am thinking there must be a valve hanging open and allowing the tractor to fire when the valve is still open, but again, it is only when the tractor is slowing down. Kind of weird to me anyway. Bob

 

[[email protected]]

A lean condition will cause a backfire. It shall be expected when you turn off the fuel. They all will. Also when throttling down as excess fuel is sucked in without enough air to efficiently burn. This is normal. The things that would not be normal is at constant speeds or acceleration. Acceleration backfire is caused by a lean condition or burnt valves. Read up on detonation and pre ignitiin.

 

[Mike M]

Does yours have governor driven live hyd. ? If so run on dyno and put a load on the hyd. just pulling the lever and setting relief valve can do it. If the engine acts up with a slight load on the gov. thru hyd pump or an add on power steering thru a fanshaft driven pump then by all means check the camshaft bearing ! I had one do this.

 

[Ken M]

I'm no expert, but isn't the load needle supposed to be 2 or 2-1/2 turns open?

 

[bill mart]

make sure the packing on the throttle shaft is in good shape. I have an H that has a bad packing and every once and a while it will backfire loud enough to scare the carp out of you. bill

 

[Bobl1958]

My OM says screw load needle in and back out 3/4 to 1 turn open but my book is for slightly older tractors I believe. Possibly 39 - 46 year tractors. Does anyone know for sure the load and idle settings for a 1948 tractor? Thanks again for the advise -Bob

 

[RandyB(MI)]

Did you set the idle mixture screw at low idle or fast idle? If you did it by the book then I assume low idle. If you set it at low idle (300 +/-rpm)then how do you expect it will have enough fuel to run on that same circuit, with two little pin holes for fuel ,at 1000-1100 rpm? It runs off the idle ports even at 1100 rpm (no load). Set throttle lever at 3/4-full throttle and mixture screw at 2t open.Close load screw all the way Slowly close idle screw a couple clicks at a time until you hear the slightest hesitation or change in how it runs. Open back up 1/2 turn and leave it there. Idle back down and re-open load screw 1 1/4 turn. Now see if it still backfires on deceleration . If it only does it on deceleration , then it is either not getting enough fuel through idle ports or getting too much air which could be worn throttle disc / worn bushings/shaft / throttle bore wall worn (trough) from disc wearing on bottom or leaking around brass drill plug (screws). Not judging your builder but I've seen all these problems at one time or another on "rebuilt" carbs. Please do us a favor and post back when you remedy the problem. We all learn that way.

 

[RandyB(MI)]

I just posted down below on how to adjust needles. Forget the book. That is only base line and for a new tractor/carb. Things change in 60-70 yrs.

 

[ET]

When you throttle down unburned fuel is pushed though the engine into the exaust and it is ignited by the hot exaust. It probably is more noticeable going down hill. The red manifold means hot exaust witch is somewhat normal on these tractors at night, but late timing can make it excesive. I would try advancing the timing a little bit and see if it helps.

 

[Bobl1958]

I will try this. Now, when you say mixing screw, I am assuming you are meaning the idle screw? Just want to clarify that. I know the throttle bushing was replaced during the rebuild, and the brass screws taken out and drilled out. This guy does it right. Carbs are his living. When he is done he puts them on some type of a stand and tests them. Normally when he rebuilds one for me, I just put it back on and it is pretty well ready to go.

Having said that, you never know if someone messed with it in the shop or who knows what else. That is the reason I was leaning towards valves.

I'll try this and let everyone know. I do know that when I shut the load valve clear off, it never made a difference in the way it ran. Also, I did set the idle valve at high idle, but I'm pretty sure my idle valve is about a turn and a half open. If I go about a quarter of a turn either way the engine starts to change it's tune, but again the load valve is still at about 1 turn open when I am doing this, so I'll try your method and let you know. Thanks a lot for the help - Bob

 

[RandyB(MI)]

Yes, I mean the idle needle when I say mixture adj. Closing the load needle when doing this just eliminates any chance of fast idle drawing off of the load nozzle to run right. It shouldn't need any fuel from load nozzle at fast idle , just the idle circuit so this makes sure that you are just adjusting idle mixture without any contribution from load circuit. Just make sure to get it up to fast rpm before closing load needle, then adjust idle needle and then idle back down and open load needle and check for rapid acceleration with just a little dark smoke right when you throw it open but not hesitation. One turn should be all you need for load. When at fast idle, idle needle movement makes more difference + or - on engine smoothness than it does at low idle speed. 1 1/2 is pretty close but a 1/4 turn can make a lot of diff. at fast idle.Give us an update when you have one.