Author Topic: MS290 won't run slow  (Read 1188 times)

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Offline jvh24521

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Re: MS290 won't run slow
« Reply #10 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

Offline 3000 FPS

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Re: MS290 won't run slow
« Reply #11 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.
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Offline KilliansRedLeo

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Re: MS290 won't run slow
« Reply #12 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.
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Offline 3000 FPS

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Re: MS290 won't run slow
« Reply #13 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.
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Offline KilliansRedLeo

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Re: MS290 won't run slow
« Reply #14 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!
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Re: MS290 won't run slow
« Reply #15 on: March 05, 2014, 01:56:21 pm »
No problem the more info the better for the original poster.
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Offline Fish

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Re: MS290 won't run slow
« Reply #16 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...

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Re: MS290 won't run slow
« Reply #17 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.
Tim

Offline jvh24521

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Re: MS290 won't run slow - pressure/vac test update
« Reply #18 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

Offline aclarke

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Re: MS290 won't run slow
« Reply #19 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

 

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