I rebuilt the pony on my RD-6 several years ago, had the carburetor rebuilt, installed a new Wico mag and had it running perfectly when I was finished. In the years afterwards I've worked on the rest of the tractor. I took the carb, mag, and manifold off to paint the pony when I painted the rest of the tractor. I'm getting close to finishing my long project. I put the manifold, mag, and carb back on the pony and started it up to start the diesel. For some reason the pony is intermittently backfiring through the carb. I never changed the location of the drive coupling on the mag from years ago. I have double checked my timing and everything checks out. I have disassembled the carburetor and cleaned it. No change. I swapped carbs with another rebuilt carb that I had on the shelf and it runs exactly the same. I made sure that all of my gaskets are good and that I have no air leaks at the carb or manifold. The gas is fresh non-alcohol. Plugs were new when I rebuilt the pony and are AC and have the proper gap.

What I can't figure out is why the engine would want to backfire through the carburetor. It seems that a plug would have to fire in a cylinder when the intake valve is open and the cylinder has a fuel charge in it. But that doesn't seem like it could happen. If I pull the throttle/governor linkage as far back as possible in the idle position the pony will idle and run smoothly. The very instant I try to give it some throttle and the engine picks up some rpm's it starts to backfire. The pony has great compression so I don't think I have an intake valve issue but I probably should check that as I haven't yet. But why would it run perfectly years ago and not now?

This one really has me stumped. I would appreciate any ideas.

-my experience with gasoline engines...backfiring back thru the carb...is 90%+ of the time... the engine is too lean

-does it get better as engine warms up?
-can you adjust the choke...add some and see if it helps or changes...etc..?

Kittylover123 - I know backfiring is sometimes associated with a lean mixture. I tried to adjust the main jet to make it richer but it doesn't really help. I tried to give it a little choke and while it may have a tiny bit more power it stills backfires.

Hi D8PETE,
just wondering if you have the plug leads crossed or someone turned the engine over on you whilst the mag was off and the mag is now timed to the wrong stroke as per the Mag timing mark on the flywheel. Mag timing mark is only correct if that coinciding cylinder is on its compression stroke.
Four stroke engines need to do 2 full revolutions to complete their 4 cycles.

Odd firing means there is 180 Deg. between the two cylinders firing and then a gap of 540 degrees until no 1 fires again and then 180 deg until no2 fires and so on.
The pistons on these engines are spaced on the crank so if one piston is at TDC and other will be at BDC--they are inherently an inherently mechanically balanced engine.

An Even fire engine requires the pistons to both be at TDC at the same time to give a 360 deg gap between firings--these are not an inherently balanced engine and so have periods of vibration moments. They are usually balanced with counterweights to suit a particular designed rev. range.

Or words to that effect as I am recalling from my Motor Mechanic Apprenticeship School days back in the early 1960's.

Regards,
Eddie B.

I vote restriction in main/load fuel passage in carburetor. Since it idles good and slow, I doubt there’s any vacuum leaks and the idle fuel circuit must be clear and properly adjusted......maybe try to make it a bit richer and see if that helps any to get it to transition from idle to load circuit supplying most of the fuel. May have to increase the low idle rpm to match the increased fuel setting. While I like to have a good slow idle speed, sometimes a simple carburetor just can’t transition from a very slow idle to high rpm/high load.

Had a genset I sold to a neighbor many years ago. Big thing with an IH 549 cubic inch V8 running on propane. He had the LP gas company come out and hook it to the tank. Everything was fine for a couple years, but then he started having trouble with it backfiring.....bad!

Backfire, as I had learned on this thing with a big silencer muffler, was like a small bomb going off, but that happens when it was shut down by ignition cut off and the engine coasted to a stop still drawing in propane until the small amount left in the system was exhausted......combustible mixture in hot muffler... BOOM!

Well that wasn’t the problem....it was upon initial startup. As soon as the engine started it would begin backfiring in the intake manifold and propane mixer.....really more of a violent stumble than a backfire but obviously small explosions and the engine couldn’t come up to speed. My first attempt was to install an Oiler to drip 2cycle oil into the intake for possible sticking intake valves. That had no effect but I did discover that I could get the engine up to speed by restraining the governor to prevent it going to wide open throttle. Once it was up to speed it ran great until a big electrical load was dropped on the generator. Then it would start the spitting and stumbling again until I grabbed the throttle and closed it off some before gradually letting the governor have it back, by which time the governor would not be opening it all the way.

Upshot of this was the owner had changed LP gas suppliers and the new guys had installed a tank regulator that was too small. The regulator could not flow enough gas to keep the mixture from leaning out during wide open throttle events.

I just laugh at how some stupid people are about to shut off there riding lawn mowers, I yell bang! then they shut the engine off right after running it wide open on a hot day and it goes bang! I laugh and say I told ya so! Got to let it idle and cool off, it's an air cooled engine, duh!

ccjersey - I hear what you are saying and that would normally be my first diagnosis. The thing that leads me to rule out the carb's mixture is that the spare carb I have makes the pony run exactly the same. I find it difficult to believe that two rebuilt carburetors, one with about an hour of operating time on it and the other with none, would have the same blockage restriction in them and cause the engine to operate identically.

One thing I did notice is that the no 1 cylinder's spark plug is tan in color while the no 2 cylinder's plug's tip is tan but the rest of the body is covered in black soot.

My starting engine on my 75 wants to do that too, tried timing, carb settings and still pops and cracks, I'm thinking it's in the mag, i have a RC-2Q on it now but i dont think you can get condensers for them any more, i think that may be the problem. I set the carb richer but then the exhaust manifold will almost get cherry red. The exhaust is always clear, it never smokes. I dont know if it's this gas nowadays or what.

Sticking/slow closing valve???? As rpm increases above dead slow idle, intake valve floats and allows it to backfire????

On the carburetors, especially cast iron but really all carburetors can have passages plugged that must be reamed out with a drill bit. Unless that is part of your procedure when rebuilding, I still think it could be your problem. Only upright pony carb I’ve ever done was on a D7E but it was cast iron and had a top and bottom split so it didn’t take special long drill bits to reach through.

I learned my lesson when I finally drilled out the soft plugs on a D6 pony carb I had been spraying and blowing through the passages with compressed air. Once I reamed out the main passage across the bottom of the bowl, i was finally able to run it without choke on all the time and tune it properly. Old worn out pony motor wasn’t actually as worn out as I thought!

Don’t forget the gas supply into the bowl as well, any restriction there or between tank and float would mean choking it wouldn’t help.

"ccjersey" wrote:

Sticking/slow closing valve???? As rpm increases above dead slow idle, intake valve floats and allows it to backfire????

On the carburetors, especially cast iron but really all carburetors can have passages plugged that must be reamed out with a drill bit. Unless that is part of your procedure when rebuilding, I still think it could be your problem. Only upright pony carb I’ve ever done was on a D7E but it was cast iron and had a top and bottom split so it didn’t take special long drill bits to reach through.

I learned my lesson when I finally drilled out the soft plugs on a D6 pony carb I had been spraying and blowing through the passages with compressed air. Once I reamed out the main passage across the bottom of the bowl, i was finally able to run it without choke on all the time and tune it properly. Old worn out pony motor wasn’t actually as worn out as I thought!

Don’t forget the gas supply into the bowl as well, any restriction there or between tank and float would mean choking it wouldn’t help.

good suggestions...i had another brainfart:smokin:

-with the pony engine running, can you squirt additional fuel into carb inlet...sort of what a accelerator pump would do on a larger carb? and see how engine responds...does it speed up momentarily...slow...start backfiring...what?

-if it speeds up without acting up, even some of the time.... it may rule out other mechanical possibilities, problems etc...

-what is the plug gap set at? you may be able to close it up a bit...just to help weed out if the spark is weak...i think you can go down to .020" easily...