Very Small Engine Repair

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I have a small tiller I bought second had - 4 stroke. It ran well the first year and then it developed a small problem. I can start it (fresh gas, clean oil), but as soon as I pull on the throttle it stalls. when it stalls I get a little blow back out of the throat of the carburetor. Also if I take the oil cap off while it is running there is blow back coming out of there as well.
I am not afraid to rip it down, as I can do minor engine repairs. I was just hoping someone might be able to point me in the right direction.
I am thinking a gasket...
It's not worth taking in for repair as the labour would cost more then the thing is worth.
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Ned Flanders wrote: ...

... If so, compression will be shot or at least quite low...
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If it starts and then dies when the thottle is being opened, that means it is running a lean mixture. This can be caused by many things such as a dirty carb, bad gasket between carb and head, loose headbolts or bad head gasket, valves need adjusting.
Start with tighening everything down. If that don't get it, check vavlve clearance, then replace manifold gasket. Since you have the carb off, clean it thoroughly.
Hank
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Ned Flanders wrote:

I would check the timing first.
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LSMFT

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LSMFT wrote:

That was my first thought as well. Check to see if the sacrificial key is at all anything other than pristine.
Jon
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Definitely, especially if it is something you can do yourself. I bought a brand new Murray lawnmower a couple of weeks ago for $20 that the guy couldn't get running, and all I did was clean the float bowl and stick a needle in the jet.
I know nothing about your experience with external combustion engines. All in all, they are fairly simple, and a one cylinder L head lawnmower is about as simple as it gets, yet they ARE cranky, and any small obstruction or problem will cause them not to work, and be very frustrating.
First, check and see if the spark plug is sparking. Take it out, put the cable on there, and sit it on the metal around the hole it came out of. It is much more fun if you have a brother-in-law or kid who will hold on to it while you crank it, but they only fall for it once. Spin it and see if you have a spark. No spark, that's your problem, and go from there.
If it is sparking, try priming it. Sometimes just the attempted starting of a motor, or a short starting will suck loose the obstruction, or at least get it running so you can further diagnose it.
That being said, priming and starting motors can be a very dangerous thing to do. You basically do not want more than one cup of gasoline near what you're doing. Drain the tank, or just tip the mower over, and get rid of the gas in the tank. Smell it first. If it smells like gas, it's probably good. If it smells like varnish, it needs to be dumped. Take off the air cleaner, and with a short hose, or little eyedropper (or any equivalent, like a ketchup squirter) put a LITTLE gas in the opening. Now be sure to put the rest of the gas about ten feet away. Crank the motor, and see if you can get it to do anything at all. If you get a sign of life, try to prime it a couple of times and see if it will run.
If not, the next thing to do is to remove the float bowl. That is on the bottom of the carburetor, and on lots, it is held in place by one central bolt that is also the jet. Unscrew that and CAREFULLY take off the bowl, trying not to screw up the gasket. Look in there and see if there is water or crud. Wash it out with gas. Take a needle and insert it in the jet and wash that with gas, too. On some models, it is held in place by four or more screws, and you have to take the carb off the motor.
Here's where it gets complicated. If priming and checking the float bowl don't solve your problem, you will probably have to take off the carb and clean it. These are not rocket science, but they do have some small parts, and intricate nuances. Sit out a towel on your work bench, and keep it close down on the towel so that if a spring or rod or small part falls out, you don't lose it.
They make carb rebuild kits for almost any small engine. Included in these are gaskets, primer bulbs, and other things. By the time you get the carb apart, you should be able to see something that is amiss, such as rust, crud, a split primer bulb, cracked float, etc.
Now, on to the major components.
Could be cracked ring, burned valve, burned piston, seized piston, stuck valve, broken valve, foreign matter keeping valve open, all sorts of stuff that may either be fatal or easily changed.
Only you know what your capabilities are, but if you have a dead lawnmower, what harm is it to pull the head and learn by doing an autopsy? Learn how they work, and fiddle with the thing until you get how it works.
Most generally, unless the motor has been run without oil, it is a simple carburetor problem. The timing is set because the flywheel is on a keyway, and there is no ignition timing adjustment. It either fires or not. Sometimes removing the flywheel, and sandpapering the components that generate the spark solve the problem, or at least help by making the spark more robust. Rust inside the area that creates the spark is a common problem.
Small motors are simple yet challenging. And many times when you do find the problem it's one of those DUH moments.
Unless you want to spend $50 or more and take it to a shop, most times you can DIY, which is why we're here, isn't it?
No? ....................
Steve
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On Sat, 26 Jun 2010 15:22:23 -0700, Ned Flanders

I would suspect an engine problem first, because of the blow back. However, my mini-tiller has a centrifugal clutch and geared transmission. If yours is similar, are you sure the gearbox is filled with grease to prevent seizing?
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wrote:

They all blow back if the oil cap is off. The downstroke of the pston does it. And blowing back through the carb is not that odd either. If it stops on an up compression stroke it can turn backwards.
s another poster mentioned, sounds like a fuel problem. If it's starting then it's not ignition, the spark improves as the rpm go up. Also as another person mentioned low compression makes them hard to start. If you get one started they will run with pretty low compression.
I'd remove the float bowl and check the main jet.
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All good advice James.
wrote:

They all blow back if the oil cap is off. The downstroke of the pston does it. And blowing back through the carb is not that odd either. If it stops on an up compression stroke it can turn backwards.
s another poster mentioned, sounds like a fuel problem. If it's starting then it's not ignition, the spark improves as the rpm go up. Also as another person mentioned low compression makes them hard to start. If you get one started they will run with pretty low compression.
I'd remove the float bowl and check the main jet.
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Johnl wrote:

Ok, that makes sense (all the blow-back).
I appreciate all of the input. I plan on getting to it sometime this week. I have to get a few things off my Honey-do-list first.
Thanks again everyone!
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On Sat, 26 Jun 2010 15:22:23 -0700, Ned Flanders wrote:

Hone the cylinder. Replace piston and rings to match the new diameter. Might need to re-seat the valves using lap compound. Have to admit I've never had a really small 4 stroke apart but it should follow the agenda of larger 4 strokes.
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On Sun, 27 Jun 2010 14:27:06 +0000 (UTC), Chief Two Eagles

A little extreme I think. It may need just a fuel / air mixture adjustment.
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That is not extreme at all, well maybe you don't have to hone the cyl. much but you def. Have to clear it smooth an the piston and rings need be checked n replced according. A space age motor flush n lube could do the job if there is no lead to suspect extreme wear or tear inside.
Years messing with ninibikes :-) an mowers makes anyone an expert in 4stroke jobs et al. We always get them going strong again. Good Luck !
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On Sun, 27 Jun 2010 13:57:10 -0700 (PDT), Roy Quijano

The OP did not mention changing the spark plug. Just reading the plug can tell you about serious internal damage*. If the plug is fouling he my very well have unburned gases coming from the carb/exhaust....
I would change / gap a new plug and try again. I would not be going into the engine without some minor checks or adjustments (less costly). YMMV
http://www.verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htm
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Sure, a basic tune up could be the solution...
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A compression test is the cheapest and easiest thing to do to find out about a motors condition and life left, If I were shopping for something used I would carry a tester and wrench. Smoke out of oil means nothing yet, it could be many simple things like a dirty carb, flywheel key, coil, ignition module, and as simple as a bad plug wire or plug that fails to carry full current under load but will idle, ive had all these things happen. Was any gas left in the carburator over the winter, empting the tank doesnt empty the carb, if gas was left in, then the carb is what I would do first.
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If it starts and runs, if only for a minute, the timing is ok. Timing doesn't change under normal circumstances. Spark plugs can operate under a "no Load" condition, and fail under a load. He obviously has an air/fuel mixture problem.
Hank
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He said it fails under throttle, to me that means many things that it could be, but the most logical is he left gas in the carb.
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That would be my first place to look too. As I stated, an air-to-fuel ratio problem, which is usually caused by a dirty carb, or some restricted fuel flow problem. But, we'll probably never know.
Hank
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Hustlin' Hank wrote:

Well I reset the spark plug gap, rocker arm clearances, cleaned and re-oiled the air filter - so now it idles fantastic. Unfortunately it still stalls on full throttle.
I plan on pulling off the carb next. The spark plug was covered in carbon so its probably a fuel mixture problem.
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