Snow Thrower Engine Backfires At Exhaust

[Jay Chan]

I have an old John Deere 522 snow thrower that has a 5hp 4-stroke engine. When I received it from my father-in-law, I noticed that it occasionally had backfire (or afterfire) from the exhaust. But lately, the problem seems to be more apparent -- either it happens more often or I start paying more attention to this problem.

Seem like the backfire occurs as soon as I start the engine regardless the engine is under load or not. Sometimes, I even see flame throwing out from the exhaust. Consequently, the exhaust is blackened.

Online source says that the air/fuel mixture probably is too rich in fuel and the incompletely burnt fuel ends up getting burnt in the exhaust. According to the operator-manual, the problem must have something to do with the carburetor not adjusted correctly or the carburetor leaks air. Either case, the manual suggests me to bring it to the dealer. But I really don't want to bring it to the dealer in the middle of winter when I need to use the snow thrower. I would much prefer to bring it in in late winter or spring when I don't need to use it any more.

My questions are:

o Can I continue using the snow thrower in this winter? Then I can bring it to the dealer in the spring.

o Is this something that I can fix if I can find a service-manual and a parts list? I am reasonably handy (home improvement...etc) despite the fact that I have never opened up an engine before. Note that the operator-manual doesn't say anything about fixing / checking / adjusting the carburetor. I hope that the service manual may have the necessary info.

Thanks in advance for any info.

Jay Chan

[mm]

I guess you mean afterfire, because backfire is firing through the carburetor. A lot of people have been misusing the word lately.

I don't really know the term afterfire, but I think that's the term you want.

Leaking air would normally lean the mixture, not enrichen it. But that still leaves "not adjusted correctly". Someone else who knows more about small engines will have to help here, but I thought there was generally only an idle mixture or speed adjustment, and you'r not talking about when it is idling, or are you?

Another cause of afterfire would I think be a retarded spark, that fires too late so the gas is still burning when the exhaust valve opens. Does the thing still have the proper amount of power? I would think that quite a bit of power would be lost if this is the problem. I don't relaly think it is the spark timing because every small engine I know, that can't be adjusted. (Although it could be retarded if you hit something that stopped the shaft from turning and partially sheared the key that holds the flywheel to the crankshaft. (The flywheel usually causes the spark as it passes the coil) You should be able to take off a cover or two and see the top of the crankshaft and the flywheel, and you should be able to see the key that way and be able to tell if it is still rectangular in cross section. If it is shaped like a B or 8 but the bottom half is offset from the top half, it is partially sheared. Ask again how to replace it. Getting the flywheel off takes some instruction.

Ask your fil how long it has been doing this. Maybe for years. Is it getting worse?

How much more snow will you have, how many more hours does it have to run?

If it is the carburetor, you don't have to open the engine. Two bolts and maybe a hose or a couple wires and the carb is off. The lawnmower guy wanted to see me about every replaceable part, for a total of maybe 25 dollars. iirc. If he were around the corner, I would have replaced the parts one at a time, in order to learn which one was the problem, but I needed to mow the lawn and he was several miles away, so I bought almost all of them. The jet(s) and the needle valve seemed most important. A needle valve that doesn't shut will definitely cause too rich a mixture, if it will start, because the height of the gas in the bowl will be too much. Jets I guess don't get bigger, they get smaller when gas hardens in them over the summer (in the case ofa snow blower) but a lot of people would just replace everything.

[gpsman]

You could try adjusting the carb. Typically they have a needle valve screw with a spring around it.

**First, turn the screw in, all the way, and note how many -full- turns it takes to bottom out.** You can always use this reference to return to the original setting, and no harm done.

Try backing it out 1 turn, then 1-1/2 turns, etc. and see what effects that has.

I'd check the first reference first, return to the original setting, start the motor and work the valve in from there. -----

- gpsman

[Charlie Morgan]

Often caused by an intake airleak. Make sure that the nuts holding the carb on are tight.

CWM

[L. M. Rappaport]

Another common problem is that the key between the crankshaft and flywheel has partially sheared. The cure is to remove the flywheel, replace the key, and put the flywheel back. It will be a lot easier if you have a flywheel puller.

[Chrisr422]

-------------------------------------

[Hank]

Test post

[Hank]

Sorry about the test post. I was having problems responding.

Anyway, in response to the OP:

Running it won't hurt it.

In older engines gunk builds up on the valve stem. When this happens, the valve may either sticks open or close slower than normal, therefore (in your case) allowing the engine to fire when the "exhaust" valve is slightly open. If the valve were to stay open, it wouldn't run. It could be a broken/weak spring, but unlikely. After it warms up the stem gets a little oil and runs properly or at least better. This is normal for gunk build up.

Using a lighter weight oil MAY help the problem. But to fix it, the valves have to be removed and cleaned. "lapping" them in at this time would be suggested also.

Hank

[trader4]

[Hank]

You are correct about carbs getting clogged jets after sitting and stale gas. Most people go straight to rebuilding/cleaning the carb, but I don't think this is the case of the OP. Due to the fact that it starts up and runs, points me in a different direction. Esp. since it gets better after it runs awhile. This is all indicative of a stuck/ sluggish valve, which is commonly mistaken for a carb problem, gasket leak and etc.

In reality, we'll probably never know what ends up being wrong. We are both guessing.

Hank

[clare]

[clare]

Sea Foam costs about half that, and a bottle will treat that blower carb 4 or 5 times, at least. And it's a whole lot less work.

Run some in the gas constantly and it won't go skunky, fouling the carb.

[Ralph Mowery]

I have seen the Seafoam but not used any.. I do put Stabil in the gas of all the small engines now and that seems to help alot. I have had to tear into the carb on a 5 kw generator several times in the past when I did not run it for several months in the past. I think the added ethenol in the gas is causing lots more problems also. Seems that they can add about 5 % or so with out even stating that at the pumps.

I don't have a snowblower as I am in the South, but do have a tiller, mower, pressure washer and chain saws that are only used in the warmer months. Well maybe not the saws.

[trader4]

Having tried a variety of alleged "carb and fuel injector cleaners" and haven taken apart carbs to clean them, I'm very skeptical that any carb cleaner that you just mix in with gas is gonna cure a fouled carb in an engine. Upon taking them apart you find varnish and gunk that is difficult enough to remove with carb cleaner sprayed on full strength and rubbed with a brush. Plus, especially with today's gas, and cheaper carb parts, you often find needle valves that are rusted, etc.

With the carb rebuild kit you get the new needle valves, gaskets, o-rings, etc. And doing the work is maybe an hour and a half.

I agree. But I think it also depends on the particular carb. I've had no problems with my mower, blower, trimmer, etc. But for some reason the Tecumseh snowblower engine can foul within just a couple months.

[Jim Elbrecht]

Takes me at least 4hrs of work and an overnight soak to remove, tear down, soak, re-assemble- re-attach-- but I'll better your 1 1/2 hours anyway. Once I've invested that time, effort & money- I don't know what the outcome will be. did a tiny ball go missing? Did a seal slip? Did I break a piece & now have to buy a new carb? Did a Welch plug hide something important-- or did I remove it and not get the new one in just so? [all things I have done in the past 40yrs of carb-adventures]

I add a few tablespoons of K100G to a full tank - start the machine and go do something else. This stuff goes wherever gas can go, and it cleans all the crap without disturbing seals, seats, or plugs. I rarely have to repeat it, but if I can get an engine to run for 5 minutes- it is well worth the effort to try.

My neighbor is a small engine repair guy. He's been doing it for 40 years or so. He turned me on to the stuff.

This fall my snowblower didn't like last winter's gas. It was running rough and searching for a speed. A tankful later, it was running pretty good. I repeated the treatment- and now it runs as good as it did in 1975.

Jim

[clare]

You can be sceptical untill you try it. The darn stuff works, and it works well. If you have one that is TOTALLY mucked up, you may still need to take it apart and rebuild it, but in MOST cases the greenies dissapear and the jets open up, making a balky engine run like new.

One that won't run, period, is a different story.

That's where I found the SeaFoam to be terribly effective - 35 year old (or older) Tecumseh SnoKing. I replaced it with a little Briggs powered blower and the SeaFoam got it running like new within minutes this fall.

[clare]

Another similar product is BG 100K - we used to say it was strong enough to eat the backside out of a dead skunk from 100 yards.