Stihl MS290 inconsistent mixture

[mowmow]

Have had my MS290 for several years and it's been good to me. But it's sat for nearly two years with little use, mainly because other saws fit the need over that time.

She fired right up a month ago, but over the next few tanks of use the mixture has become increasingly finicky. It becomes too rich, so I lean it. Then it becomes borderline too lean or begins to bog, so I enrich it. Then in the middle of a cut it bogs way down from being too rich. The high speed mixture is the biggest issue, but the idle is affected too. It's basically unusable now as I don't want to score the cylinder. I just rebuilt the carb, replaced the fuel pickup and vent lines, replaced the spark plug, air and fuel filters, and this made no difference except where the high and low needle nominal settings are, which my guess is mainly due to the new spring and diaphragm. The big change seems to happen when I'm actually cutting. I can tune it and shake/point it all around and she'll run great, but once I get cutting - it all goes south.

My next guess was the boot/intake manifold is maybe dry rotten and cracked, but I'd think that would KILL the idle. Maybe the impulse hose or am I missing something else? I've never had this saw apart past the service that I just mentioned, so any pointers would be greatly appreciated.

Thanks!

 

[HarleyT]

Did you put in an OEM fuel line?

 

[HarleyT]

I see that you live in the flatlands!

 

[mowmow]

Did you put in an OEM fuel line?

Yes - fuel line is OEM. The one I removed looked new actually - almost put it back in.

I see that you live in the flatlands!

I can see windmills 20 miles to the west...

 

[HarleyT]

My granddad lived near Defiance.

 

[HarleyT]

Who has a kit with a metering spring?

 

[mowmow]

Who has a kit with a metering spring?

That kit was aftermarket, but for the life of me I can't find where I bought it. Just installed that today and if anything that spring was the biggest item of concern in that kit as it seemed possibly too long/strong relative to the original and I almost reused the original. Everything else looked quite impressive. I swear I bought it on Amazon but don't see it in my purchase history.

Wait! Here it is...

https://www.amazon.com/gp/product/B00RWWOVFE/
Did that link work?

 

[HarleyT]

That kit was aftermarket, but for the life of me I can't find where I bought it. Just installed that today and if anything that spring was the biggest item of concern in that kit as it seemed possibly too long/strong relative to the original and I almost reused the original. Everything else looked quite impressive. I swear I bought it on Amazon but don't see it in my purchase history.

Wait! Here it is...

https://www.amazon.com/gp/product/B00RWWOVFE/
Did that link work?

Put in the old one.

 

[mowmow]

My granddad lived near Defiance.

I just drove through there today. I don't live far off Route 6.

 

[mowmow]

Put in the old one.

Will do. Won't be able to test until tomorrow. Pitch dark here now.

Not sure how that explains the unchanged behavior though.

 

[HarleyT]

Just takes out excess variables.
The spring in there is the incorrect one.

 

[mowmow]

OK, with the original spring back in, the mixture screws are closer to their original positions. I also cleaned the spark arrestor, which was maybe 2% clogged - I'd say this was the first cleaning FWIW.

So today I had a harder time getting the problem to show up, but it eventually did. This time after a cut, the idle drifted upward due to leaning. I couldn't open the idle mixture screw enough to get the idle back down - I'm talking 5-10 turns had nearly no effect. So, I took the opportunity to experiment, and with the engine idling high, I dropped the unit onto the thick grass from about a foot up. The idle immediately dropped and the engine stalled from being overly rich. I had to keep my finger on the throttle to keep the engine running while I leaned it back out. The saw ran great again, and the varying of mixture was now happening to a lesser degree. Repeated drops/jostling had no effect. This sound's like a kinked fuel line or something, but it is not kinking. This was also a freshly filled tank. I also got a decent look at the piston from both intake and exhaust ports and saw no scoring.

I took the unit down further and both the intake manifold and impulse hose are fine as far as dry rot or cracking is concerned. The manifold hose clamp screw was not very tight though - I would say it wouldn't have been difficult to turn using the shaft of a screwdriver. I was not able to move the manifold before the clamp was removed though - it was stuck to the cylinder along the top edge. The impulse hose spins easily on the engine's barb fitting but takes some effort to remove. Maybe these are both due for replacement regardless?

 

[rupedoggy]

Air leak

 

[struggle]

You need to pressure test and vac test the engine. Its really the only way you'll find the problem. They can develop very small air leak where the two halves of the crank case come together at the crank seal. Only way I found mine was using a mighty vac pressure/vac tester and a bottle of soapy water when at 5lbs of PSI it showed up.

I put a piece of bicycle tube with holes in it between muffler and block. Plug impulse hose end. Then can operate vac tester while watching gauge with thumb covering the intake boot. If the gouge drops at all you know its leaking. Then proceed with water testing it to find where its coming from. At least that has worked very well for me in the past.

 

[drf255]

I’d say airlleak, but I recently had a saw do the same and it was the choke lever binding. Run the saw without rear cover if thats possible, or one cut with the AF off to see the plate.

Wouldnt affect idle though.

 

[mowmow]

Thanks for the suggestions guys. I'm hoping I was experiencing a manifold leak given the looseness of the clamp. That would also explain the leaning out and sensitivity to jostling. I have some parts arriving on Friday and will give things another go this weekend, including leak tests if it gets to that.

 

[mowmow]

OK, so I changed out the impulse hose and intake manifold and same thing. Then I tried a $18 generic carb and so far it's running fine. I think it leans out bar-down more than it used to so I'd like to get the factory HD-18C carb back in there. I'm wondering if the o-rings around the high and low adjust screws need replacing. The carb kit didn't include them, and they seem pretty flattened. The screws aren't moving on their own - just wondering if air gets past. Either that or there's a turd still in the carb somewhere. But for now, it hasn't changed in 30 good cuts. I ran out of tree.

 

[HarleyT]

1127 120 0604 CARBURETOR HD18A $34.01

 

[mowmow]

Actually more like $42.00 in my neck of the woods. That said, I installed a new factory carb (Walbro HD18D) and have put some time on it. It does perform better than the knockoff (maybe better than ever - I bought it used some time ago), with more consistent mixture across various orientations, and that allows me to run a little leaner. I was glad to see that both high and low mixture adjustments are still there. I have to say that the aftermarket carb ran surprisingly well - just not quite as well as the real deal. Either way, I'm happy to have my 290 again. Thanks everyone!