Resurrecting a Stihl KM110R

[MRC01]

A friend gave me one of these that had been sitting unused in his garage for 5 years. It got it running and the steps might be useful for others troubleshooting similar engines.

First, I got fresh autogas having no ethanol, blended in Stabil and 2-stroke oil at 50:1, and fueled it. Cleaned and reinstalled the air filter. It would not start, of course.

Pulled the spark plug. It was wet, oily, and contaminated with debris. I cleaned, dried and gapped it.

Still would not start, but it would fire intermittently like it was trying to start. Pulling the plug, it was wet & oily so I cleaned & dried it.

Since it was too rich, I continued starting without the choke, and while holding the throttle full open. This got it chugging for a few seconds like it was going to run, but it always died within a few seconds.

After pulling the starting cord, while it was chugging, still holding throttle wide open with one hand I immediately pressed the prime button with my other thumb and it actually ran! It would run only so long as I repeatedly thumbed the primer. This tells me either the slow speed/idle jet is obstructed, or the fuel tank has vapor lock.

I opened the fuel tank and it made a "pop" when I opened the lid. So vapor lock appeared to be a contributing factor. Probably an obstructed vent line.

At this point, the engine would run, but only at wide open throttle. That is: pull the cord while holding throttle wide open to start, immediately thumb the primer a few times, then it would come to life and run at wide open throttle. As soon as I reduced throttle, it would die unless I thumbed the primer. This reinforced the prior diagnosis of obstructed low speed jet, or vapor lock. Running at full power it either (A) generates enough suction to overcome vapor lock and draw fuel from the tank, or (B) the high speed jet is not obstructed but the low speed jet is.

I removed the vent hose that connects from the airbox to the fuel tank, blasted it clear and checked the 1-way valves. Also removed and inspected the muffler spark arrestor screen: it was fine.

At this point it ran like a champ. Instant start, no need to thumb the primer, idled smoothly at low RPM, smooth throttle response to full power. Yay! I leaned the low speed mixture adjustment a bit (until it gave a slight RPM rise at idle) to compensate for the rich condition / oily spark plug. Left the high speed mixture at full rich for proper lube & cooling at full power (I am at sea level). Was able to tune the idle nice and slow.

I got lucky that the problem was vapor lock due to obstructed vent line, because the alternative would be to remove and clean out the carb to fix obstructed jets, which would be 10 times more work.

 

[MRC01]

Oh BTW, since it took 50:1 fuel I thought it was a 2-stroke. But when I removed the plastic cover to service it, I saw that the engine has a valve cover, so it must be a 4-stroke.

 

[Brian123]

Oh BTW, since it took 50:1 fuel I thought it was a 2-stroke. But when I removed the plastic cover to service it, I saw that the engine has a valve cover, so it must be a 4-stroke.

I think it is called a 4-Mix!

 

[fantastic]

I think it is called a 4-Mix!

Yes, That's right. It's still a 4 stroke engine though. Just Stihl decided to use a fuel/oil mixture for fuel like a 2 stroke would.

 

[97prizm]

A friend gave me one of these that had been sitting unused in his garage for 5 years. It got it running and the steps might be useful for others troubleshooting similar engines.

First, I got fresh autogas having no ethanol, blended in Stabil and 2-stroke oil at 50:1, and fueled it. Cleaned and reinstalled the air filter. It would not start, of course.

Pulled the spark plug. It was wet, oily, and contaminated with debris. I cleaned, dried and gapped it.

Still would not start, but it would fire intermittently like it was trying to start. Pulling the plug, it was wet & oily so I cleaned & dried it.

Since it was too rich, I continued starting without the choke, and while holding the throttle full open. This got it chugging for a few seconds like it was going to run, but it always died within a few seconds.

After pulling the starting cord, while it was chugging, still holding throttle wide open with one hand I immediately pressed the prime button with my other thumb and it actually ran! It would run only so long as I repeatedly thumbed the primer. This tells me either the slow speed/idle jet is obstructed, or the fuel tank has vapor lock.

I opened the fuel tank and it made a "pop" when I opened the lid. So vapor lock appeared to be a contributing factor. Probably an obstructed vent line.

At this point, the engine would run, but only at wide open throttle. That is: pull the cord while holding throttle wide open to start, immediately thumb the primer a few times, then it would come to life and run at wide open throttle. As soon as I reduced throttle, it would die unless I thumbed the primer. This reinforced the prior diagnosis of obstructed low speed jet, or vapor lock. Running at full power it either (A) generates enough suction to overcome vapor lock and draw fuel from the tank, or (B) the high speed jet is not obstructed but the low speed jet is.

I removed the vent hose that connects from the airbox to the fuel tank, blasted it clear and checked the 1-way valves. Also removed and inspected the muffler spark arrestor screen: it was fine.

At this point it ran like a champ. Instant start, no need to thumb the primer, idled smoothly at low RPM, smooth throttle response to full power. Yay! I leaned the low speed mixture adjustment a bit (until it gave a slight RPM rise at idle) to compensate for the rich condition / oily spark plug. Left the high speed mixture at full rich for proper lube & cooling at full power (I am at sea level). Was able to tune the idle nice and slow.

I got lucky that the problem was vapor lock due to obstructed vent line, because the alternative would be to remove and clean out the carb to fix obstructed jets, which would be 10 times more work.

Which one is the vent line? Is it in the fuel tank? Did you go ahead and replace the fuel filter while you were at it?

 

[Opelman]

The 4 mix likes frequent valve adjustments also.

 

[MRC01]

Which one is the vent line? Is it in the fuel tank? Did you go ahead and replace the fuel filter while you were at it?

The gas tank vent line comes up from the fuel tank into the air intake behind the air filter. It also has a crankcase vent that is connected to the bottom of the tiny little carburetor. The fuel filter is integrated into the weighted pickup that swings around in the tank. It's fine.

Turns out this engine need a little more work. It started running like crap again, full throttle only, no idle. That tells me clogged idle jet or old spark plug. The plug measured infinity so I got a new one which measured 7.5 kOhm but that didn't fix the problem.

The owner told me it sat for over a year with untreated pump gas so I figured the carb was all gunked up. <sigh> so I removed the carb and completely disassembled it (required wearing an 8x magnifier on my head) and soaked the parts in Berryman B-12 for a couple of hours. Reassembled everything, ensuring to oil all the contact points of o-rings and gaskets. The throttle plate was tricky because it's not quite round and has to be carefully aligned so when closed the idle gap lines up with the tiny bores in the top of the throat which let in fuel from the idle jet. And the throttle axle has a cutout that opens another spring loaded jet when it's full open, which was fincky to reassemble.

Anyway that was 2-3 hours of work but now it runs like a champ from idle to full power and everything in between.

 

[super20dan]

these motor are junk -avoid them too many problems

 

[MRC01]

You may be right. After all that work the engine started running like crap again - no idle. It started up on first pull and ran strong at full power, but that is all. No idle. Clearly, the idle jet passage is clogged somewhere, and clogged so hard that no amount of carb cleaning solvents and using a thin wire to ream it out would fix it. So I bit the bullet and replaced the carb with a cheap ($16) aftermarket kit. Took all of 10 minutes to install and now it really does run like new again. Smooth low idle, full power, and everything in between.

Moral of the story: don't make the mistake my friend did - never let untreated pump gas sit around in your little engines.

 

[super20dan]

stilh and husky use zama carbs-these are junk. walbro is the good carb . its its got a zama-dont but it

 

[97prizm]

The gas tank vent line comes up from the fuel tank into the air intake behind the air filter. It also has a crankcase vent that is connected to the bottom of the tiny little carburetor. The fuel filter is integrated into the weighted pickup that swings around in the tank. It's fine.

Turns out this engine need a little more work. It started running like crap again, full throttle only, no idle. That tells me clogged idle jet or old spark plug. The plug measured infinity so I got a new one which measured 7.5 kOhm but that didn't fix the problem.

The owner told me it sat for over a year with untreated pump gas so I figured the carb was all gunked up. <sigh> so I removed the carb and completely disassembled it (required wearing an 8x magnifier on my head) and soaked the parts in Berryman B-12 for a couple of hours. Reassembled everything, ensuring to oil all the contact points of o-rings and gaskets. The throttle plate was tricky because it's not quite round and has to be carefully aligned so when closed the idle gap lines up with the tiny bores in the top of the throat which let in fuel from the idle jet. And the throttle axle has a cutout that opens another spring loaded jet when it's full open, which was fincky to reassemble.

Anyway that was 2-3 hours of work but now it runs like a champ from idle to full power and everything in between.

If you have access to a marina or airport get a gallon of high octane fuel. The Trufuel is way overpriced. My Stihl runs super smooth on 100 octane, and I only go through a gallon over the summer.

 

[MRC01]

With new replies I'll update this thread. The cheap aftermarket carb was a little too cheap. Back in Feb, the priming bulb tore internally after being used only a few times. I still had the old carb since it might be useful for parts, and tried moving its priming bulb to the new one. But that wasn't possible - they were different sizes. So then I installed that old carb again just as a last-ditch effort. While I was doing so and inspecting all the connections, I noticed something wonky with the tank fuel pick-up. I removed it to find that inside the fuel tank, the flexible hose's upper connection to fuel line was loose and pinched. I re-connected it and applied a small zip-tie around it to secure it to the pickup nozzle. After this the engine ran perfectly from idle to full power. Since then I've used it quite a lot and it's going strong with that original carb.

Note that the valve cover seals are important with these 4-mix engines, so I checked this too. It's the original cork but it's in good condition, not cracked, seals nicely.

 

[Fordiesel69]

MRC01,

The 4 mix engines are a nightmare but do produce excellent torque and a dull sound that is less offensive to some lawn care companies (less neighbors complain).

These engine are unique and have 4 failure areas:
-The valve cover gasket must be air tight. If they leak the engine does not run.
-The cam pin and side plate. The camshaft pin usually does not wear bit the hole in the cam does and the hole in the block and late oblong and thru the valve lift off.
-The cam itself suffers premature wear. Inpect and replace.
-The wristpin wears on some of them.