Chainsaw Repair

Husqvarna - Stihl - Poulan - Jonsered - Dolmar chainsaws and more => Stihl => Topic started by: jvh24521 on March 04, 2014, 04:43:46 pm

Title: MS290 won't run slow
Post by: jvh24521 on March 04, 2014, 04:43:46 pm
Greetings,
I had been using my saw all day. Started cutting a large standing tree and suddenly it would not run unless the throttle was wide open or nearly so.

It will start in the choked position and then if I can get it started in fast idle and rev it up it will stay lit but it will not run slow.

Fuel and impulse lines are OK. Impulse stem on block is open too. Cleaned the carb, put in a kit, new fuel filter, and screen too. Same problem, just seems like it is not pulling in the fuel since it has to be choked or high RPM.

John, aka, Sixty and stumped.
Title: Re: MS290 won't run slow
Post by: jmester on March 04, 2014, 05:06:35 pm
Pull the muffler and look and see if you have some scoring or seize marks on the piston. Could have ran lean today for how cold it was.
Title: Re: MS290 won't run slow
Post by: jmester on March 04, 2014, 05:11:03 pm
What makes me say that is, had a 660 do that to me before I knew anything about chainsaws and did the same thing you did. Then took it to a shop and they said the piston was scored from a lean seizure. Hope it is not the case.
Title: Re: MS290 won't run slow
Post by: roostersgt on March 04, 2014, 05:21:35 pm
How about checking the muffler outlet and screen? Might be clogged.
Title: Re: MS290 won't run slow
Post by: Cut4fun on March 04, 2014, 06:52:36 pm
WAG what about tank vent?  If it was stopped blocked up with crud would or could it maybe cause this?
Title: Re: MS290 won't run slow
Post by: jmester on March 04, 2014, 09:07:00 pm
The only other thing I could think is the primary or secondary idle jet is clogged in the carb. But don't think that as he said he cleaned it and the saw will start with the choke on. And if they where it would be real lean on the high side. If it is a faulty vent I don't think he would be able to get it started as the vent releases the pressure in the tank as to not vapor lock. Once they build pressure in the tank it is hard for the pump to get any fuel up the line to the carb.
Title: Re: MS290 won't run slow
Post by: 3000 FPS on March 04, 2014, 10:38:43 pm
Look for scoring on the cylinder and then I would check for an air leak.    The carburetor is pulling fuel or else it would not start and run at all.   Sounds like it is very lean from a bad air leak.
Title: Re: MS290 won't run slow
Post by: AKDoug on March 05, 2014, 01:15:03 am
Very likely a bad air leak. You can't run at low RPM's because you have too much air for the mixture. When you are adding fuel it is able to run, but it is likely running very lean. Do not run the saw anymore until you thoroughly check out the saw. It needs to be vacuum/pressure checked.
Title: Re: MS290 won't run slow
Post by: stihls066 on March 05, 2014, 02:11:49 am
Clutch issue is possibility. Had a 066 that stopped idling because a bad spring
Title: Re: MS290 won't run slow
Post by: 3000 FPS on March 05, 2014, 09:55:21 am
Clutch issue is possibility. Had a 066 that stopped idling because a bad spring


You can pull the bar and chain off and start it.    If a bad clutch spring it will run and idle without the bar and chain attached.
Title: Re: MS290 won't run slow
Post by: jvh24521 on March 05, 2014, 12:29:32 pm
Sorry if this is a double post, tried to reply this morning but it has not shown up.

Thanks for the good ideas. If they aren't the cause they will be good to think about next time.Here are more facts I should have given, been working on it off and on as weahter permits so didn't remember all of it when I posted.

Clutch: Bar and chain removed, no effect
No scoring: I opened the case.Didin't know about ring position pins. One new ring cost me $18 at Stihl, now know I can get an aftermarket piston kit for same price as one ring.

Muffler and screen: Very clean

Vent: I started it several times and it would run as long as I held the throttle to a high speed. When I did remove gas line it spurted. Full tank warmed up in sun. Still shouldn't have pressure on it. Tried running with check valve/vent removed. No help.

Air leak: Haven't checked that but have old manifold gauge, tubing, tees, and 60cc catheter syringe. Do I need to block ports or just position the piston to do that and apply pressure/vacuum to the impulse port? How much pressure/vacuum and how long?

I suppose dunking and pressuring carb is called for too but I replaced all gaskets.

I was sure it was the diaphragm since the problem started suddenly but that has been replaced too.

Thanks,
jh
Title: Re: MS290 won't run slow
Post by: 3000 FPS on March 05, 2014, 12:56:23 pm
Yes when doing a pressure check you need to seal the impulse line, the intake port, the exhaust port, and spark plug hole.   I use about 10 to 12 psi and it should hold for 3 to 4 minutes.

If you have an air leak you will know it right away.     

If your pressure test checks ok then something is wrong with the carb.   Maybe the order of the diaphragm and gasket are backwards.   I have done it myself.   Same for the other side of the carb.   I would double check those.
Title: Re: MS290 won't run slow
Post by: KilliansRedLeo on March 05, 2014, 01:02:08 pm
You need to block both the intake and exhaust ports. A strip of inner tube cut to fit between the muffler and carb bolts. Tighten up the bolts enough to compress the rubber and form a seal. Put 80kPa or around 11.6 psi of pressure in the saw and watch it for 30 seconds, if at the end of 30 seconds the pressure is 60kPa 8.7 psi or above you are good to go. If you cannot get pressure into the saw either your intake and exhaust ports may not be sealed well enough, or there is a huge air leak somewhere. Use some soapy water around the crank seals and cylinder base to find leaks. Also may be leaking air past the crankcase gasket.
Title: Re: MS290 won't run slow
Post by: 3000 FPS on March 05, 2014, 01:04:10 pm
You need to block both the intake and exhaust ports. A strip of inner tube cut to fit between the muffler and carb bolts. Tighten up the bolts enough to compress the rubber and form a seal. Put 80kPa or around 11.6 psi of pressure in the saw and watch it for 30 seconds, if at the end of 30 seconds the pressure is 60kPa 8.7 psi or above you are good to go. If you cannot get pressure into the saw either your intake and exhaust ports may not be sealed well enough, or there is a huge air leak somewhere. Use some soapy water around the crank seals and cylinder base to find leaks. Also may be leaking air past the crankcase gasket.

There you go very good info.
Title: Re: MS290 won't run slow
Post by: KilliansRedLeo on March 05, 2014, 01:11:32 pm
Roger, didn't mean to step on your post. Did not realize that you had already posted the info before I hit the post button!
Title: Re: MS290 won't run slow
Post by: 3000 FPS on March 05, 2014, 01:56:21 pm
No problem the more info the better for the original poster.
Title: Re: MS290 won't run slow
Post by: Fish on March 05, 2014, 04:19:45 pm
Look closer at the fuel line, look for cracks right under the carb above the tank, even do a pressure test.  That is a real weak point on this model...
Title: Re: MS290 won't run slow
Post by: fossil on March 07, 2014, 08:38:43 pm
Is there a carb boot on the 290?
Make sure you do a vacuum check as well. The pressure check will check for all leaks except for crank seals which may or may not leak under pressure. I've had saws that hold pressure but will not hold vacuum due to bad crank seals.
On saws with impulse lines I apply the vacuum and pressure through that.
The vents on the fuel tanks are one way and only allow air in to prevent a vacuum from forming. The tanks will build pressure and that is not a concern for running.
Title: Re: MS290 won't run slow - pressure/vac test update
Post by: jvh24521 on March 11, 2014, 11:02:50 am
Hi all,
Sorry for slow reply, had to go get rabies vaccine yesterday.

I sealed off exhaust and intake ports a pressurized to 4 lbs. It quickly bled down to 2 lbs, about 8 seconds. It then took 25 seconds to leak to 1 lb and 48 additional seconds to bleed from 1 to zero. So I would say a moderate leak. I was using a manifold vac/pressure gauge. I bled off to zero then pulled the starter cord. Needle swung quickly but seemed to oscillate between about four lbs and several inches of vacuum. I reasoned that with the ports unblock this might be enough pressure differential to move the fuel/air charge and provide impulses to the fuel pump.

I had earlier tried adjusting the jets after cleaning the carb but nothing radical. The low speed jet calls for 1/4 initial. So just to make sure I tried something ridiculous. I opened it 2 turns and it would run at low speeds, rich and 4 stroking though. I screwed it in to 1.5 turns and the saw operates normally.

From experience about 30 years ago, low speed jets seemed to control low speed only but this one seems to have effect up to near mid-range.

So I guess the question is whether the ls jet is compensating for the poor compression test. Should I use the saw or pay the ridiculous Stihl price for some shaft seals to make it 'right'?

BTW, I got by on the cheap for the pressure test. I used an old manifold pressure/vacuum gauge, some vacuum line, a vacuum line Tee, and a 60 cc syringe with a Luer Slip tip (about $1.00).

thanks,
jh
Title: Re: MS290 won't run slow
Post by: aclarke on March 11, 2014, 12:04:22 pm
Low speed contributes to the gross fuel supply at full throttle as well. Richer low speed will = more on top too
Title: Re: MS290 won't run slow
Post by: jvh24521 on March 11, 2014, 12:13:43 pm
Low speed contributes to the gross fuel supply at full throttle as well. Richer low speed will = more on top too
Low speed contributes to the gross fuel supply at full throttle as well. Richer low speed will = more on top too

Yes, quite correct. I should have said I didn't think it would contribute as much as it does at higher speeds. But then it doesn't take much gas to run fast under no-load conditions, does it? Especially since the chain is off.

Apologies,

jh
Title: Re: MS290 won't run slow
Post by: aclarke on March 11, 2014, 12:20:56 pm
Have you tried opening up the low speed needle to where the saw idled and accelerates well and cut wood with it? 
Title: Re: MS290 won't run slow
Post by: jvh24521 on March 11, 2014, 12:53:11 pm
Have you tried opening up the low speed needle to where the saw idled and accelerates well and cut wood with it? 

No, again you make a good point.

I was waiting for my wife to sharpen my chains before I put them back on. I hate to admit it but she has a lot more finesse with a file than I do. An old time wood cutter around here let her place her hand on top of his as he sharpened a chain and she picked right up on it. I can never get a chain as sharp as either of them. Too clumsy and heavy-handed I guess.

Nonetheless I should at least put a dull one on and try it out and will do so when I get home.

Thanks,

John
Title: Re: MS290 won't run slow
Post by: Fish on March 11, 2014, 03:03:40 pm
You better watch them old time wood cutters......
Title: Re: MS290 won't run slow
Post by: wisecobandit on March 11, 2014, 03:20:03 pm
Sounds like a leaking impulse line to me.  ;) Runs with rpm but no bottom end tickover or power.
Title: Re: MS290 won't run slow
Post by: jvh24521 on March 11, 2014, 03:29:25 pm
You better watch them old time wood cutters......

Dunno, I could use a night off...