Question for those of you that have rebuilt a Stihl MS290/310/390

[BigRed96]

Good Day All, I am currently fighting a fuel issue on a 290 that we rebuilt using a 390 piston and cylinder. This is the second one I have rebuilt and the other runs great. My question is how easy is it to accidently get sealant into the impulse hole on the cylinder while putting the bottom end together. I have put two different rebuilt carbs on the saw and it does the same thing. I have faith in my carb cleaning and rebuilding abilities as I have done dozens so I don't think that's the issue. I disassemble, ultrasonic clean, blow out with compressed air, and reassemble using a kit. Right now in order to get the saw to fire up I put it on choke and pull it till it pops, then flip it to the high idle and pull till it fires, it will run for a few seconds and if I try to give throttle in just peters out. The saw won't idle either and just dies when it goes from high idle to normal idle. After stalling I have to full choke to get it to fire again, that tells me the saw has a fuel problem I let my brother seal the bottom end as the saw and parts where a Christmas present and he wanted to try doing the rebuilding, I forgot to warn him to be care not to seal up the impulse line plug. I didn't think he used a large amount of sealant but I may be wrong. I have flywheel/coil gap with a business card as I have done al my saws I have rebuilt. The impulse is the only thing I think could be the problem as I checked tank vent by removing it and blowing/sucking (not funny comments) and it seems to function fine, it has a brand new fuel filter and fuel line. What do you all think? Should I just bite the bullet and tear the saw back down?

 

[shawn022]

Could be other things causing your problem but to answer your question I would not tear back into the saw. Set the saw up for a pressure test and test it through the impulse line. Leave the plug out. It should not hold any pressure at all. If that checks out put the plug in and pressure test as normal.

 

[shawn022]

Or hell, just extend the impulse line and try blowing / sucking through it.

 

[BigRed96]

Or hell, just extend the impulse line and try blowing / sucking through it.

Impulse is clear. I hooked up my pressure tester and vacuum gauge pressure leaked off as well as vacuum. Problem is I use a modified blood pressure cuff and I am not sure what the PSI is. Basically I am just blowing air and it leaks off. I need to check the seals but they are brand new. I use bike inner tube between the muffler and carb I don't know if they could possibly be leaking as well.

 

[Roll Tide]

How's your tank vent ?

 

[BigRed96]

How's your tank vent ?

Seems to be in working order.

 

[HarleyT]

Try a new fuel line, they love to crack badly.

 

[hills bay performance]

Check the boot between the carb and jug . If thats good check the spark plug wire sometimes the connection gets loose . I had that problem on my 036 like to drove me crazy .

 

[watsonr]

I disassemble, ultrasonic clean, blow out with compressed air, and reassemble using a kit.

Not a good practice for carb cleaning, you could damage the carb. Just use a little carb cleaner, plenty of pressure.

If you used rubber between the carb and muffler, spark plug in place and you could hear air leaking by and it won't hold vacuum, it's got an air leak.... seals, where the clam shell is sealed, maybe the decomp?

 

[BigRed96]

Try a new fuel line, they love to crack badly.

Brand new fuel line and filter.

Check the boot between the carb and jug . If thats good check the spark plug wire sometimes the connection gets loose . I had that problem on my 036 like to drove me crazy .

I inspected it during the rebuild and it looked ok. I need to examine it closer.

I disassemble, ultrasonic clean, blow out with compressed air, and reassemble using a kit.

Not a good practice for carb cleaning, you could damage the carb. Just use a little carb cleaner, plenty of pressure.

If you used rubber between the carb and muffler, spark plug in place and you could hear air leaking by and it won't hold vacuum, it's got an air leak.... seals, where the clam shell is sealed, maybe the decomp?

I plan on removing the flywheels and oil pump and inspect the seals using pressure test and soapy water.

 

[gritz]

Ok. An easy way to check the impulse line is to put a dab of oil on/in the impulse line, unhook the spark plug, and give it a couple pulls. You should be able to see movement of the oil. If you have a gauge you can hook up to the impulse line, you should be able to see the needle bounce. I always spray around my sealing flanges first to make sure I have the exhaust and intake blocked properly. Definitely check the seals. It's possible to get a little nick in a seal, or have one sitting in there crooked when installing a new one. I've also had seals that were just bad out of the envelope. Another note, never blow compressed air through the passages on these carbs. It's pretty easy to mess up a check valve if you're not careful. Just my .02.

 

[BigRed96]

Resurrecting this one from the dead. I let the saw in question sit for many months due to being pissed I spent money on it and it didn't work. Fast forward a few months I now have a 20ft shipping container for a shed and have electric which is awesome. Before I rebuilt in an old shed with an extension cord ran from my house. Just enough electric to run one small light and my ultrasonic cleaner. Anyway I was rebuilding some carbs for generators one day and had some time to kill while the carbs ran through their cycles in the ultrasonic cleaner. I brought the saw to the shed and started tearing it down. I though that the saw had a fuel problem that was possibly being caused by a air leak. I had noticed an issue with the fuel hose being kinked when I had the saw together and running crappy. I had to cut off shorter to straighten it out. Anyway I pulls the impulse off and pressure tested and it held pressure fine. I test the hoses by pumping them up from one side while holding my finger over the other end. I go to test the fuel line and it holds pressure. I let my finger off the end and pressure remained. I start inspecting further and find that just a bit of air was getting by and the hose had an internal blockage. The hose must have kinked in the factory while it was hot and caused the internal blockage. The line as mentioned was brand new and came in the Hutzl kit with the cylinder, piston, etc. I went ahead and vacuum tested the saw as well as pressure tested it anyway since I had already tore most of the saw back down. It passed both tests. I get a Stihl brand fuel hose from the local shop and put the saw back together. I go to fire it up and it roars to life. It runs like it should and oils like it should. So I go give it to my brother and I fired it up in front of him. He had a grin from ear to ear as this was the first time he heard it running right. So I am happy I spent the money to rebuild the saw for his Christmas present and I learned something's. I feel blessed that I have the skills to take a discarded saw and give it a second chance as life. I also learned to pressure test all lines prior to assembly. I am not slamming the Hutzl parts as inferior to a Stihl line but I think I know which one I will be reaching for in the future.