MS290 rebuild now running too hot

[muddy_cj5]

After much reading on this forum, I decided to buy a worn out ms290 on Craigslist and convert it to a ms390 using a huztl top end (cylinder, piston & rings) Assembly of the motor went great and it held a 15" vacuum for at least an hour, including the impulse line. I also did a compression test and got 140 psi. I finished putting the saw back together and rebuilt the carb at the same time. The saw fired up on the third pull and I adjusted the carb, both idle and high speed to get it to 4 stroke at wide open. I checked to make sure that the saw was oiling and then I cut a few 18" disks with a 20" bar and a sharp chain. I stopped it and the motor was extremely hot (smoking from the cylinder head and I could barely touch the plastic above the cylinder). I cut three more disks the following day and it seemed extremely hot again.

I tested the vacuum again and it was still holding 15" for at least an hour. I then borrowed a pressure tester and found out that it would only hold to 1 psi. Anything higher than that and it would bleed back to 1 psi.

I have been doing a bunch of reading about this and it seems that it is running too lean. I have very limited time now (newborn baby girl) and I am debating pulling the saw apart again to figure out where it is leaking. I was reading that most people pressure test to 10 psi and I was wondering if I should be concerned about only getting to 1 psi. I believe that the bottom end works under vacuum so I don't know how important the 10 psi is. I believe that my only other option is to rebuild the carb again (I already pressure tested the fuel and impulse lines)

Thanks in advance

 

[HarleyT]

See where the extra 9 psi is leaking off at.......

 

[huskihl]

Remove the clutch and flywheel and while under pressure spray some soapy water around the crank seals, intake, and muffler looking for bubbles

 

[fearofpavement]

My guess is that the pan to engine seal is leaking. Check there and also around the bolt holes that hold the engine together. If it's not holding pressure, there's something wrong that needs to be remedied. A spray bottle with soapy water can help you locate the problem...

 

[Wood Doctor]

You said, "I checked to make sure that the saw was oiling..."
May I ask how you did this?

Just for the heck of it, remove the bar and chain, and see if oil comes out of the oiler hole. And, even if it does, it may not align with the bar. The MS 290 is noted for this problem. Take a look at what I ran into:

Note that the saw's oiler is not aligned with the bar's oil hole. That built up heat beyond belief because less than half the oil from the pump made it to the chain.

The only solution was to increase the size of the bar's oil hole (not easy) or to enlarge the groove on the saw case (also not easy) with a sharp chisel.

 

[muddy_cj5]

Thanks everyone for the quick reply, I finally got a chance to work on the saw today. My pressure leak was the metal exhaust gasket. Once I removed it, the pressure dropped from 10 psi to 7.5 psi in 90 seconds which seemed to be within spec. The only leak was a very small leak on the perimeter of the flywheel side. I also rotated the crank shaft to make sure that it didn't leak in different spots. So I put the saw back together and got it running.

My new thought is back to the carb. When I was adjusting the H screw, I discovered that even if I snugged it close ( I removed the limiter tabs), the saw continued to run at WOT.

My carb rebuild consisted of removing the old gaskets, fuel needle and arm. I also removed the H and L needles and blew compressed air and carb cleaner thru all passages. I installed new fuel needle and arm and the gaskets in the proper order. I am going to check to make sure that the needle is properly aligned tomorrow.

Any ideas on what I need to clean next to get the saw to stall out when I snug the H screw closed?

Joe

 

[muddy_cj5]

Wood Doctor,

I checked to make sure that it was properly oiling by putting a piece of paper on the ground and I saw it sling some oil when I revved it up.

Joe

 

[strtspdlx]

My understanding if it is if you have enough fuel on the low side you can almost close the high side and it'll still run. Not right but it'll run. The high speed circuit is almost like having an accelerator pump. But instead of a single shot of fuel at wot it's a steady flow. But it doesn't come into effect until very late in throttle use. What base settings did you start with on the carb?


Regards-Carlo

 

[Wood Doctor]

Wood Doctor,

I checked to make sure that it was properly oiling by putting a piece of paper on the ground and I saw it sling some oil when I revved it up.

Joe

Good, Joe. I'm not sure if all engines do stall out if the hi-speed set screw is closed all the way. Seems like some that I have tuned will run anyway, but likely too lean.

 

[muddy_cj5]

I started H and L both at 1 turn out. I am getting some nice 4 stroking at 1-1/4 turn on H and I believe that L is still at 1 turn. Most other write-ups listed the carb tune sweet spot at the same numbers that I am at.

I have also been reading some other posts that say that the 290, 310 & 390 run "hotter" than other saws.

I guess that my next question is how hot is too hot? I was reading that cylinder head temps should be below 200f, but I don't have a good way of checking that. My only other point of reference is my husqvarna 55 and I know that the plastic above the cylinder gets warm, but not hot. The plastic on the 390 definitely gets hot.

I have run 1 tank of gas through it so far and I have checked the cylinder and piston through the intake, exhaust and spark plug and I haven't seen any scoring yet. I just want to make sure that I am not running the saw too hot.

 

[Chainsaw Jim]

Did you lose some of the clearance when you stuffed the larger cylinder under the plastic cover? It might not necessarily be a temperature problem as much as a clearance problem overheating the cover.

 

[muddy_cj5]

The 290 and 390 cylinder are the same size, I didn't loose any clearance.