ms 390 new piston and muffler mod. good? bad?

[boutselis]

I put a new piston and muffler mod on ms 390. good? bad?

This is the first time I have put a new piston in an old cylinder. The picture of the piston here is the old one. The saw had 140 psi with it. It was checked by 2 different people on 2 different gauges (I am one of the 2).

I put in a new piston. The cylinder looked pretty good. light scratching only. couldn't really feel any of the scratches with my finger. just could see them. Immediately after I put in the new piston I checked the compression and it was 150. I was pretty happy. I started the saw and let it idol for a bit till it warmed up and then the compression was only 120. This is really the first time I used a compression gauge to test after a top end. the few other rebuilds I had done only received field testing and visual inspection.

Will the 120 go up after the rings seat and why was it at 150 when cold?

Is this considered a muffler modification? I opened the H as much as I could with the limiters on and it gurgles a bit so its not to lean. Is that the only adjustment I need to make with (if it is) a muffler mod?

By the way: the saw seems like it runs fantastic. Very east to start and feels/ sounds more powerful. No wood to cut at midnight though.

 

[timberwolf]

Compression reading will often drop when the saw is warm, but if you are getting a reading down at 120 psi and the gauge is good then something is not right.

As for the muffler mod, what was done? sometimes if the mod is mild enough adjustment can be had within the limiters.

Typically more adjustment is needed on the L than the H otherwise you will be lean in the low and mid range.

 

[nmurph]

tell us about your testing procedure and gauge.....did you pull until it stopped climbing?? were the gauges made for cars or for small engines? did they have shrader valves at the end of the tube where it screws into the cylinder?
120 sounds low.

here is a pic of the MM on my 310.

 

[boutselis]

tell us about your testing procedure and gauge.....did you pull until it stopped climbing?? were the gauges made for cars or for small engines? did they have shrader valves at the end of the tube where it screws into the cylinder?
120 sounds low.



The guage is from auto zone so I guess its actually made for larger engines but it has short threads so it can screw in all the way and seat down on the o ring and it has the valve up by the gauge.

I pulled till it stopped climbing. I may have been able to get a few more psi out of it.

this morning I tested again. Timberwolf says the reading will be lower when the saw is warm. I figured it would be the other way around. Now that its cold I pulled till the needle stopped completely and I got 159.

159 cold verses just over 120 when hot?

 

[timberwolf]

159 cold is very good.

Something also on the gauges is if you test hot the o-ring on the gauge seat can leak and give a low reading.

Thats quite a bit of muffler mod though there is still a fair bit of restriction right at the outlet.

I would think it best to pull the limiter caps, trim the stops on the sides of the caps and put them back in so you have full adjustment.

 

[nmurph]

if your gauge is an actron, i have one as do several others here. i have had good results but someone recently had a failure. i have verified mine against a snap-on and a mac, and it was within a pound or so of both.
as brian said, cold compression will be higher than a warm reading. maybe you didn't have the gauge screwed in tightly.

 

[oscar4883]

You checked the comp. immediately after the new piston was installed. Was there assembly lube in the cylinder? That can make a huge difference in readings.

 

[boutselis]

You checked the comp. immediately after the new piston was installed. Was there assembly lube in the cylinder? That can make a huge difference in readings.

Yes there was. I never thought of that. There was a light oil coating on the cylinder.
I guess that would increase the compression.

So the very first reading was of no use! the next reading was with after the saw had idol for maybe as much as 10 minutes and a few moments of full rpms. That was 120. this morning cold it was just under 160.

Why does the compression go down when the saw is hot though?

 

[timberwolf]

With heat the oil will be thinner resulting in more leakage at cranking RPM, the warm fit may be looser, and also a big factor if the saw is warm the air it will be drawing in to compress from base and exhaust will be lower density, as a result you get less actual air to compress and a lower compression reading.

Big thing with compression readings is to make them under the same conditions if you want to compare them. Example run the saw so it has freash lubrication, let it cool to room temperature and test it. If you test a saw at -20 in the garage it may well read different than it will hot or even room temperature.

 

[wse8562]

Compression will be slightly lower when warm but thats a big difference! Maybe the gauge was not tight and the o-ring was leaking? If the saw runs well I would not worry. I would pull the carb limit caps though and richen it up some . That is a fairly heavy mufler mod to be able to adjust with caps. Use a tach, take it to about 13,000 rpm and put on new caps.

 

[boutselis]

With heat the oil will be thinner resulting in more leakage at cranking RPM, the warm fit may be looser, and also a big factor if the saw is warm the air it will be drawing in to compress from base and exhaust will be lower density, as a result you get less actual air to compress and a lower compression reading.

Big thing with compression readings is to make them under the same conditions if you want to compare them. Example run the saw so it has freash lubrication, let it cool to room temperature and test it. If you test a saw at -20 in the garage it may well read different than it will hot or even room temperature.

Compression will be slightly lower when warm but thats a big difference! Maybe the gauge was not tight and the o-ring was leaking? If the saw runs well I would not worry. I would pull the carb limit caps though and richen it up some . That is a fairly heavy mufler mod to be able to adjust with caps. Use a tach, take it to about 13,000 rpm and put on new caps.

Ok;
That all makes good sense. some variable I never even thought of. I didn't really think I removed that much of the muffler. You can't see it in the picture but I opened up the deflector a little bit also.

I don't have a tach. If it is 4 stroking when wide open is that rich enough then?

 

[nmurph]

you don't need a tach.....factory specs go out the window when you MM or port. the factory spec on a 390 is 13K rpm, so you can definitely go higher. you have the right idea. listen for four-stroking and you will be good. i think my 310 is turning about 13.7K rpm.

 

[boutselis]

you don't need a tach.....factory specs go out the window when you MM or port. the factory spec on a 390 is 13K rpm, so you can definitely go higher. you have the right idea. listen for four-stroking and you will be good. i think my 310 is turning about 13.7K rpm.

Cool; I'm cutting with it this weekend. can't wait
thanks for the tips everyone.

 

[RiverRat2]

I just finished a MM on a friends Lightly used MS 390 firewood saw and it was 4 stroking @ 14.3K and cleaned up nicely when I put it in tha wood!!!! BTW I had to pull the limiters and the L is setting on about 2 full rounds out off the bottom

Let us know how yours turns out!!!!!! this saw cuts real nice

Good on ya!!!!

I forgot to ask,,, did you determine what caused the initial failure????