Need small engine help

[RocketJSquirrel]

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??? I can't see it, but...

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?

 

[kubmech]

Here's a couple of suggestions:
Snow Kings tend to run kind of crappy unless it's pretty cold out, that's normal, so keep that in mind. But you should'nt have any trouble starting.

Check the high speed metering needle (the one in the bottom of the carb) and look at the needle where it would sit in the seat (the pointy end) and see if there is a slight ridge on it. This ridge generally comes from the needle being turned into the seat too far (overtightened when fully closed off). You may also have a low speed metering adjustment that could also have a ridge for the same reason. Now if a carb kit was installed at some point the needles will look new. But that does not mean that somewhere down the line someone did'nt over tighten them into the seat and damage the seat in the carb housing which will also cause this problem. No matter where you adjust the needles the carb is not right in some other range of throttle which sounds like what you are saying.

With the float bowl removed and the carb off the engine. The boss that extends down from the base of the carb that the high speed needle/assy that holds the float bowl on screws into has a small check ball inside it. This ball needs to be free. With the carb dissasembled you should be able to shake the carb next to your ear and hear the ball moving up and down.(Hold the choke and throttle butterfly from moving while doing this or you will just hear them) If the ball is stuck you will have to replace the carb. You can try a good soak in carb clean to free up the ball.

If the carb has been off before, make sure the right size intake port gasket was re-installed and that it is in good condition. It is possible to install a gasket that has the hole size for the intake port too small, this will cause some interesting problems as well /w3tcompact/icons/blush.gif. Also check for any intake or vaccum leaks at the carb mount.

Valve condition and compression could also be a factor, would'nt hurt to pull the head and have a look.

You should have a nice blue spark (points or electronic mag?)

Proper spark plug and gap?

Proper air gap between mag and flywheel?

 

[RocketJSquirrel]

kubmech wrote:
<blockquote><font size=1>In reply to:</font><hr>

... Now if a carb kit was installed at some point the needles will look new. But that does not mean that somewhere down the line someone did'nt over tighten them into the seat and damage the seat in the carb housing which will also cause this problem.

...This ball needs to be free. With the carb dissasembled you should be able to shake the carb next to your ear and hear the ball moving up and down.

You should have a nice blue spark (points or electronic mag?)



<hr></blockquote>



The needles look great, but I didn't think to check the seats. It is very possible it has been rebuilt with a kit, judging by the receipts.

That item about the ball check needing to be loose could be very interesting. I will look at that.

I don't know if it is CDI or points yet. If the next carb exam does not show anything, I will check compression and ignition.

Thank you very much for the great suggestions.

 

[RocketJSquirrel]

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 great suggestions!