Go BottomSmall Engine Help Needed
 | DonM
Join Date: Posts: 1 
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2002-10-25 44204
I need help from a better mechanic than me:
I am working on my Ariens snowthrower with 8HP Techumseh Sno King motor. The previous owner had the same problem I am having. He spent a lot of time trying to figure it out, and paid good money to a shop also. No one has fixed it.
If I let it idle for more than 10 or 15 seconds, then move the throttle up slowly, at mid-throttle it loads up, sputters, and belches a few puffs of black smoke. Then it dies or else stumbles back to a smooth RPM. OTOH if I
let it idle just for a few seconds, then move the throttle, it does not do this, it only has a slight stumble in the midrange but nothing you couldn't live with. If I let it idle for a longer while, then move the throttle rapidly to full, again it only has a slight stumble in the midrange but
nothing you couldn't live with. Most of the time it will idle forever, it just won't accept mid-throttle after that. So far, you could say: "Don't let it idle", and "if you do, then don't move the throttle to mid-way". :)
And, so far, that's the best anyone can do.
It has fresh gas.
Ultra clean fuel system. The inside of the carb is spotless, looks like new.
Welch plugs and ball plugs do not look leaky.
I changed the needle and seat. Pressure tested it, it works.
Gravity feed, no fuel pump.
Float is hollow brass but not leaking.
Tried leaner float positions.
Tried different idle mixture settings.
Tried different high speed mixture settings.
Tried combos of the last 3 things.
There is a stock primer connected to the carb by a hose. It seems to put pressure on the float bowl when you push it. I disconnected the hose at the carb, no difference.
There is no air filter.
Like most small utility motors the throttle cable actually moves a spring loaded lever which is connected to the governor which is connected to the throttle. I cannot come up with a governor theory to cause the problem. ??
I'm not sure on this next clue, but I *think* it doesn't cold start well. Tonight it took forever to start. It has a 110v starter so it can crank pretty good. I kept thinking it must be flooded but I think I got it to
fire with the choke, then it was loaded up and I had to floor it to start it. So that could be just poor technique on my part tonight.
I have not checked compression or ignition. It is like new and runs fine when you work around the loading up problem. Could ignition adjustment lead to this problem??? It actually is a like new older unit, and I think it has points and condenser.
I don't like to be a "parts replacer", but I am tempted to buy a whole new
carb. It seems like there must be a circuit leak somewhere.
Any suggestions?
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 | Peters
Join Date: Feb 2002 Posts: 3034 Northern AL 
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2002-10-25 44208
Oh boy where do you start. It sound like the high speed jet is allowing too much gas and flooding out in the transition. With these small engines especially the small ones the high speed jet is often fixed and can not be adjusted.
I say it sound like, but can not here the engine. One thing that can act simularly is if the carb gasket is leaking and allowing air to suck by. In the transition the air pressure will be the greatest and can cause a similar failure at transition.
The spark advance in the fly wheel if sticking could also cause a similar problem.
This is a tractor board, not Techcumseh, but hope this helps? ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2002-10-26 44221
How about this for pure theory? It's weak spark. When running above idle gas consumption keeps the float level down a bit, a few seconds of idle lifts the float to it's shut-off level, which makes for a richer mixture that a weak spark won't ignite. Intake manifold vacuum at full-throttle is less than at mid-throttle and the spark is better able to handle the cylinder charge.
My usual small engine trick is cleaning the flywheel. It's about my only small engine trick so I always think that weak spark is the answer. I suppose I'm generating a nonsensical theory to fit my one and only trick.
....
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| DonM
Join Date: Posts: 1
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2002-10-26 44229
For some reason, I cannot log on to the board. I will have to get ahold of Dennis and see what's wrong.
Thanks for the replies. You'd think with all these forums there'd be a right place for a small engine question, but there wasn't so I picked this one as the one most likely to have the good mechanics reading it. :)
I am going to go over the carb one more time with a fine tooth comb. Then onto ignition and compression.
-Don M
registered member
JD 755, Ford/NH 1320, & lots of small engines :)
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| DonM
Join Date: Posts: 1
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2002-10-26 44238
I think I got it fixed. The problem was, it was idling with the throttle butterfly completely closed. Based on the tuning instructions in the service manual, that is not the way they want it. It looks like the airflow past the idle and intermediate fuel ports is screwed up with the butterfly closed. That was my first clue, when I saw that. The book says to adjust it slightly open as a starting point, that was my second clue. :)
I did have a hard time with the governor linkage adjustment. The book was not too clear on it. I did it by trial and error and think it is close enough. I will see when I get put it under load.
I am going to wait for winter to fine-tune the jetting, otherwise I'm declaring it "done". Thanks again for the suggestions!
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| Peters
Join Date: Feb 2002 Posts: 3034 Northern AL
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2002-10-26 44241
Glad to here of your success. I guess I had the right cause but the wrong reason for the problem. Your low speed adjustment was to rich to allow the opening of the valve and addition more fuel from both jets. I had a Briggs acting the same on a leaf vacuum a few years back. ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2002-10-27 44263
Glad it's fixed too and maybe you got a good deal on the blower since neither your neighbour nor his shop figured it out.
Never being one to entirely give up on even an admittedly contrived theory, I'll say that a fully closed butterfly valve would create abnormally high intake manifold vacuum. Having said that and not really being serious about it, I think Peters 'right cause for the wrong reason' is far better than mine. I really don't know if high vacuum actually would change the idle mixture but maybe somebody knows.
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