need help diagnosing idle problem

[cityboy]

I bought a used Stihl string trimmer, FS250R a few weeks ago, and its starting to have trouble holding an idle setting. I get it idling perfectly and it stays like that for a couple days, then mysteriously won't idle and stalls.

1. I don't have a compression tester, so I can't verify compression, but it appears to run fine under throttle. Its responsive and powerful under throttle, so it seems like its running OK everywhere but idle, but I don't have a good running unit to compare it with. When I pull on the starter rope I feel a pretty good resistance when the piston compresses, but don't know if that means anything.

2. It was running a bit rich when I first got it but I adjusted the H screw a little and the plug looks normal now. So other than at idle I don't see any symptoms of anything being wrong with it.

3. When I get it idling well, and spray a bit of carb cleaner on the gasket between the carb and the plastic plate between the carb and cylinder, the idle slows, or it will even stall if I continue spraying. This would indicate a gasket leak, correct? I pulled the carb and the gasket looks OK, but I'm guessing that doesn't mean its actually OK. So I'm assuming this is part of my problem. I also have noticed the sweet spot on the L adjustment screw is very close to all the way in, which would be consistent with the gasket leak leaning things out.

So I'm hoping a new carb gasket will solve my problem, but have a few general questions.

I'm assuming that if the gasket leak at the carb were severe enough to affect overall running, I'd see evidence on the plug of leanness, which I don't see. It only appears to be affecting idle. So I don't think running it like this would have hurt the P&C at all, right?

If the compression is in fact a bit low, would that also be consistent with the erratic idling symptoms I'm seeing?

If the compression is a bit low, but the P&C aren't actually damaged, is it possible that I could just throw a new ring on it and improve the compression, or would the cylinder likely be shot as well? This unit is something like 7 years old and was run commercially, so its got a fair amount of hours on it, but I don't really know much else about its history. As long as its not seized is it likely I won't need a complete P&C replacement even if the compression is bad?

If the compression was bad, would the thing still rev up and appear to run normally? If it puts out this much power with low compression I'm afraid to see it with full compression.

Other than what I've already talked about, is there anything else to check that would affect the idle?

thanks,

Doug

 

[cityboy]

One more thing I thought about is the fuel tank venting may be plugged. Would that give the symptoms I'm seeing with the idle changing, or would it just die after a short time?

 

[cityboy]

3. When I get it idling well, and spray a bit of carb cleaner on the gasket between the carb and the plastic plate between the carb and cylinder, the idle slows, or it will even stall if I continue spraying. This would indicate a gasket leak, correct? I also have noticed the sweet spot on the L adjustment screw is very close to all the way in, which would be consistent with the gasket leak leaning things out.

I just realized I had this backwards, if air is leaking past the gasket then it would already be lean, so I'd expect to have to set the L screw richer, not leaner, so that part doesn't make any sense.

 

[cityboy]

I just realized I had this backwards, if air is leaking past the gasket then it would already be lean, so I'd expect to have to set the L screw richer, not leaner, so that part doesn't make any sense.

OK, so I realize I'm just having a conversation with myself, but I thought I'd mention my latest progress in case anyone else is chasing similar symptoms.

I cranked up the idle screw to keep the thing idling no matter what, then gave the L screw a bit more of a workout. I'm not sure exactly what happened, but something changed because the L screw now turns a lot more freely, and required me to turn it significantly richer to get the idle at the strongest point. I probably backed it out at least 1/2 turn from where I started, but was turning it a lot in both directions to free up the feel of the screw so I'm not exactly sure. It now feels like its set about where it should be, I'd guess its between 3/4 and a full turn counterclockwise, although I didn't turn it full in and back out to measure it exactly. It then idled strong enough that I could turn the idle back down with the idle screw and put it to good use.

It now idles strongly, and over the course of several days starts easily and idles smoothly once it fires up. I'm still not 100% sure about the fuel cap venting working properly, but the thing runs well enough that I'm happy for now. I'll try and verify the fuel vent later, but I'm not sure exactly how you go about testing that. I took it out of the fuel cap and cleaned some dust off a few surfaces, but couldn't tell by looking at it if it was OK or not. Didn't see anything suspicious on it so I put it back together and ran it again.

I also found a Zama carb manual online, so if I eventually have to rebuild the carb that will help. For now I'm crossing my fingers and hoping that it was simply a case of being out of adjustment, and/or a bit of twiddling with the adjustment screws cleared something up that wasn't right before.

 

[a. palmer jr.]

Usually, a used item like a trimmer or a chainsaw will not hold a perfect idle unless all things on it are brought to new specs. This includes compression, crankcase pressure, carburetor cleanliness, new carb kit, properly adjusted carb, fuel tank venting, spark plug, exhaust needs to be clean, proper fuel/oil mix with no old fuel. There may be even things I left out.

 

[Simonizer]

For the amount of time and money, I would be inclined to put new seals in it. Trying so sort out a perceived carb problem when you have an air leak is really like chasing your tail. They are nice to work on.

 

[cityboy]

For the amount of time and money, I would be inclined to put new seals in it. Trying so sort out a perceived carb problem when you have an air leak is really like chasing your tail. They are nice to work on.

Question for you Simon. For the time being I'm ignoring the possibility that there's a small air leak in the carb gasket, as the trimmer is running perfectly for me now. I may have compensated for it by over-adjusting something else, but it runs and idles perfectly for me. I'm a homeowner and will use this thing once/twice a month. If I'm not leaking air badly enough that its causing a lean condition (checked the plug, its perfect) is there anything I'm doing that will cause further problems/damage by just running it as it is now?

I'm mechanically inclined enough to pull the carb and rebuild it, but I'm also inclined to take the 'if it ain't broke don't fix it' attitude for now. But I don't want to do something stupid and make it worse by ignoring a relatively simple maintenance item now, only to do a more expensive repair later.

 

[andrethegiant70]

There's really no great way to tell whats up, you just have to go through it all, as an earlier poster said.

I've had saws that ran terrific one day after a piston change and carb kit and wouldn't take the throttle a week later. My opinion is that the saw had sat for a while before I got to it and fresh fuel in the carb broke free some deposits. An ultrasonic cleaning of the carb fixed my problem. I do believe these things CAN have transient issues... like kidney stones... and them be ok.

If it's running fine for you, I'd keep running it and be alert for any other issues. If it begins to run oddly for you, the manner in which it does will be good clues as to the issue.

 

[cityboy]

If it's running fine for you, I'd keep running it and be alert for any other issues. If it begins to run oddly for you, the manner in which it does will be good clues as to the issue.

That's my plan. With mechanical devices, especially older ones with unknown history, Murphy's Law is of primary concern. I will wait until it doesn't work right, so that it demands attention. If I tinker with minor issues, and create even bigger problems in the process, then I'll feel pretty stupid for not just leaving it alone.

The other part of that plan is I bought a used one because I just didn't like the idea of spending $500 on a trimmer that I'd use once a month. But it wasn't because I couldn't afford $500, just that I felt more justified spending $200 considering the intermittent use. What I'll like even less is owning a used one that eventually costs close to $500 after I get done removing 'warts'.

 

[a. palmer jr.]

That's my plan. With mechanical devices, especially older ones with unknown history, Murphy's Law is of primary concern. I will wait until it doesn't work right, so that it demands attention. If I tinker with minor issues, and create even bigger problems in the process, then I'll feel pretty stupid for not just leaving it alone.

The other part of that plan is I bought a used one because I just didn't like the idea of spending $500 on a trimmer that I'd use once a month. But it wasn't because I couldn't afford $500, just that I felt more justified spending $200 considering the intermittent use. What I'll like even less is owning a used one that eventually costs close to $500 after I get done removing 'warts'.

Plus all the labor and aggravation you'd put in it.

 

[cityboy]

update, problem appears to be solved

bought a carb rebuild kit and that seemed to fix the problem. Starts and idles much better now, and has held setting for a couple of days. I'll keep checking it for a week or two and hopefully it will hold these settings and I can call this done.

thanks to all for the help.

 

[Fish]

Wow!!!!

Sorry I missed this one.....

 

[7hpjim]

which zama series carb do you have on it, my 250 has the self regulating "intelli-carb" and I would like to replace it with a fully adjustable carb.

 

[cityboy]

which zama series carb do you have on it, my 250 has the self regulating "intelli-carb" and I would like to replace it with a fully adjustable carb.

Mine had the C1Q-S83, which is one of three different versions I've seen associated with the FS250. (You did notice that I'm talking about an FS250R trimmer, not a MS250 chainsaw, right?)

How old is yours? I took a look at a new one at the local shop and it still had all three adjustment screws, even here in EPA/CARB controlled California.