MS390 won't start

[a. palmer jr.]

I rebuilt yet another 290 which I turned into a 390, new piston/cylinder, seals, sealed up bottom end good with Dirko, put it all back together and now it won't start. It seems to have plenty of compression, spark, fuel and pulse line okay. I tried two new spark plugs but no difference. I squirted a small amount of fuel into sparkplug hole so the cylinder had fuel but it wouldn't hit even a little. This isn't my saw and I don't know what's been done to it in the past except for dirt and grease which I cleaned off. I had the flywheel off when I did the p/c and sure I put it back right but it sure seems like it's out of time. Guess I'll go out tomorrow and check compression and maybe part number on flywheel. Any suggestions?

 

[a. palmer jr.]

I guess maybe I answered my own question. I went out and checked the compression and it was just over 90 psi cold. I put in all new parts so now I'm wondering if there's anything I can do other than tearing it all apart again and checking the engine out?

 

[Lightning Performance]

I guess maybe I answered my own question. I went out and checked the compression and it was just over 90 psi cold. I put in all new parts so now I'm wondering if there's anything I can do other than tearing it all apart again and checking the engine out?

Not really. Try checking the rings for radial tension. If you didn't break them you have one of two issues.

Out of round bore or out of round rings.

It should be a minimum of 130 psi, new, when wet.
Oh btw don't ever trust a compression gauge.
Make sure the rings are wet when you check them. The only other place it can leak is out the spark plug and highly unlikely but possible.

 

[Wood Doctor]

The Dirko seal may have failed. Some guys use gasket sealer rather than a new gasket and eventually it wears out, but usually it starts and lasts several hours of run time. The 390 does want a decomp valve, and that may not be working right. Try a simple block off bolt plug and then check the compression again.

 

[a. palmer jr.]

The Dirko seal may have failed. Some guys use gasket sealer rather than a new gasket and eventually it wears out, but usually it starts and lasts several hours or run time. The 390 does want a decomp valve, and that may not be working right. Try a simple block off bolt plug and then check the compression again.

I already have the bolt plug in it and the compression gauge is the one I've been using all along and has been very accurate on everything else. Dirko is what the Stihl factory uses on the bottom pan on the 1127 engines so since I had some thought I'd use it also. They don't use a gasket on the 290 engines from the factory.

 

[a. palmer jr.]

Not really. Try checking the rings for radial tension. If you didn't break them you have one of two issues.

Out of round bore or out of round rings.

It should be a minimum of 130 psi, new, when wet.
Oh btw don't ever trust a compression gauge.
Make sure the rings are wet when you check them. The only other place it can leak is out the spark plug and highly unlikely but possible.

I was trying to avoid tearing it down again, thought someone might have something to try while it's together. I've installed several rings in saws and don't think I broke anything upon installation but I'll pull the muffler off and see if anything is amiss.

 

[Wood Doctor]

I fear I have never suffered from the same problem you are having and have always upgraded the size to a 390 top end. Now that I think about it, I have always used gasket sealer on the 290 and never had this problem with low compression. Unfortunately, the 290 is not that easy to take down like the 026, 028, 034, 046, etc. So, I can understand why you would want to avoid taking it down to find out why the compression isn't there as it should be. Before doing that, make sure all the cylinder mounting bolts are tight, but I am sure they are because you are an experienced mechanic.

I feel your pain.

 

[cookies]

I was trying to avoid tearing it down again, thought someone might have something to try while it's together. I've installed several rings in saws and don't think I broke anything upon installation but I'll pull the muffler off and see if anything is amiss.

check the squish before you tear it back apart, im betting its massive or the cylinder bore is out of round.

 

[a. palmer jr.]

I fear I have never suffered from the same problem you are having and have always upgraded the size to a 390 top end. Now that I think about it, I have always used gasket sealer on the 290 and never had this problem with low compression. Unfortunately, the 290 is not that easy to take down like the 026, 028, 034, 046, etc. So, I can understand why you would want to avoid taking it down to find out why the compression isn't there as it should be. Before doing that, make sure all the cylinder mounting bolts are tight, but I am sure they are because you are an experienced mechanic.

I feel your pain.

Yeah, I've done several of these, never had results like this though. I usually take my time and make sure bolts are tight, etc. I'm thinking the kit I bought was low quality (Chinese). I bought it from a vendor I hadn't used before. Probably go back to a previous vendor and get a better kit.

 

[Wood Doctor]

Yeah, I've done several of these, never had results like this though. I usually take my time and make sure bolts are tight, etc. I'm thinking the kit I bought was low quality (Chinese). I bought it from a vendor I hadn't used before. Probably go back to a previous vendor and get a better kit.

Get a refund immediately and use that money to buy another kit. Let them know exactly what happened. That's my advice. I have done this myself rather recently while rebuilding a Stihl MS201T M-Tronic saw. Several of the parts I received were worthless and I believe the supplier knew that before they were shipped. They refunded my cost. Parts suppliers sometimes ship worthless parts because they figure very few buyers will apply for the refund.

 

[stihltech]

Ah yes, the chinese parts blues.
Check the flywheel cast in key. You say it acts out of time. First place to look.
Do that compression test wet. If it comes up, try starting it.
I have seen these flooded so bad the won't fire.
Also, aftermarket fuel hose? Does fuel come out the hose? Had three so far that would not pass fuel.
Diagnose before teardown. Once it's apart, you lost 75 percent of the ability to find the issue.

 

[HarleyT]

Not torquing the flywheel nut enough the last time that you had the flywheel off may be your problem.

 

[Wood Doctor]

Not torquing the flywheel nut enough the last time that you had the flywheel off may be your problem.

That happened to me once when working on a 290 to 390 upgrade. HarleyT must read my mind or may also have done this. It only happened once. You learn fast after it does. My solution was to use a 12" galvanized steel pipe extension with the socket wrench to help supply sufficient tightening torque. I can usually add new top ends to Stihl's professional line of saws without removing the flywheel.

 

[a. palmer jr.]

I tightened the flywheel nut like I usually do but I can check it again. The only parts that I know of that are Chinese are the piston/cylinder kit, everything else, I believe are OEM stuff. I checked fuel and pulse line before installing, same with intake boot. I was getting ready to check the flywheel index but then I noticed the low compression.

 

[a. palmer jr.]

I went ahead and ordered a set of Caber rings for the 390 for when I tear it back down, I've used them in the past and they're pretty good. Next I have to look around to see where my roll of solder is so I can test the squish. I may look around, possibly have another 390 P/C kit hidden somewhere in my shop. If so I'll put it on next.

 

[cookies]

I went ahead and ordered a set of Caber rings for the 390 for when I tear it back down, I've used them in the past and they're pretty good. Next I have to look around to see where my roll of solder is so I can test the squish. I may look around, possibly have another 390 P/C kit hidden somewhere in my shop. If so I'll put it on next.

I've been curious what your go to kit is since you seem to do this pretty often and never complain of issues other than this different kit.

 

[a. palmer jr.]

I've been curious what your go to kit is since you seem to do this pretty often and never complain of issues other than this different kit.

I really don't have a favorite kit, I just have kits sold by my favorite vendors, ones who have proven to be good over the years I've built saws. Two ebay vendors I've used are vostore and Wangluo18, never had a problem with either of them but I didn't see them advertising them when I needed this last one.
The good news is that I went out to the shop this morning after a couple of days of letting the saw sit, tried to start the 390 and it started right up! I guess I had too much fuel and oil in it and had it flooded. I used a slightly leaner mixture of fuel to oil this time and I guess that's what did it. I'm so glad I don't have to tear this thing down again...

 

[a. palmer jr.]

Uh, I may have spoken too soon about not having to tear it down again; I ran it a bit a couple days ago and although it did run I was unable to get it to idle for very long, that's with a good fuel and pulse line and a new carburetor. I replaced the old Walbro with a new Zama. I could adjust the high side and the low but it wouldn't idle good for very long then it would die. I tried running it on high speed and it died after several seconds also. I'll take the new carb apart and see if it's set okay and if it is it's probably not running right because compression is too low. If it is that I'll probably buy another P/C kit.

 

[a. palmer jr.]

Another thing I found accidentally; I was getting ready to work on the 390 and decided to dump the fuel out of it so it wouldn't leak everywhere I poured the fuel into a transparent jar and noticed the fuel wasn't pure, it had some gray or silver colored gook in it and a bunch of it! Not sure what it was in the gas tank but I'm guessing it wouldn't help the running of it. My fuel in the can doesn't have anything but gas and two stroke oil so the junk must have been in the tank beforehand.
So, what I said in a previous statement must not have been true; the part about it having good fuel..

 

[Lightning Performance]

Another thing I found accidentally; I was getting ready to work on the 390 and decided to dump the fuel out of it so it wouldn't leak everywhere I poured the fuel into a transparent jar and noticed the fuel wasn't pure, it had some gray or silver colored gook in it and a bunch of it! Not sure what it was in the gas tank but I'm guessing it wouldn't help the running of it. My fuel in the can doesn't have anything but gas and two stroke oil so the junk must have been in the tank beforehand.
So, what I said in a previous statement must not have been true; the part about it having good fuel..

Start washing all your new parts and checking ring gap at the ends up in the bore. Shinning a light from behind the ring will show any lowspots while your checking end gaps. New aftermarket parts are rarely washed after machining. Check the ring gap in the ring lands while your checking specs. If you don't have build tolerances you should. Checking the skirt to cylinder clearance is also a very good idea.