Cross Pop-up Piston in MS390 topend

I got an old rough looking 029. When I shook it I could hear stuff rattling in the muffler. It had low compression so I thought it had a busted piston. I did not care enough to pull the muffler since it needed a top end. The saw was missing the recoil assembly and spur sprocket.

I tore the saw down and found that muffler was broke and the saw had ingested pieces of the muffler. I rebuilt it with a Cross Performance cylinder and the Cross Performance pop-up piston from H L supply. The Cross cylinder looked really nice. I was concerned about the weight of the pop-up piston. I thought if it weighed more, the weight might offset the bump in compression. My pop-up weighed 10 grams less than the flat top piston that came with the cylinder!

After I got into the saw I realized that the crankcase was busted on it. It was usable but I had bought a parts saw or part of a saw when I bought my 039. Luckily I had a good crank case. It is a late model with flippy caps which I like.

Everything went together without a problem. It started right up. I left it stock, other than the pop-up. The sounded. It had better throttle response than my 039 and 290/390.

I made videos of the tear down and rebuild. Then I did some testing of the muffler mod and some other things. I have videos of all of that as well.

 

I'm uploading the air filter video now.

 

Very cool! You guys always seem to know what I’m thinking! I was just talking with a friend that’s got an old 290 and I thought it might be a fun build. Not being framiliar with those saws I was curious about the case weather magnesium or clam shell style and that just answered my question!

 

Is the crown height the same on both pistons?

 

Cool! I think these saws are fun to mess with.

Yes

 

Air filter testing

 

This looks familiar? Like I have seen it some where before?

I'm gonna take your suggestion and put a popular in my 390. May make me actually get back to it faster.

I tore it down cause a decompression valve that failed. And I sealed with RTV before hearing that was not right stuff. I then set it to the side and haven't used it since. I have Dirko or some equivalent stuff I was told to get for when I tear it back down.

 

That's good info right there. Breathes way better, you can even hear it.!!

 

Did you check comp before and after the piston swap?

 

If I had a working gauge I wold do it before I do the swap. I have to remember to ask my buddy to bring his home.

I'm too cheap to buy one! Haha

Mine worked then one day just started reading really low...like blown engine low on everything.

 

Cool vids, thanks for posting.

 

I changed the cylinder when I changed the piston. I'm thinking about getting one for my 039 then I can see the gains of the piston.

Thanks

 

Deja va? I used Red RTV on a saw a few years ago on the cylinder. It has not given any problems. I've used motoseal on everything since then.

 

I don't have a problem either, it will tune and stay in tune. But this saw has sentimental value. Was my first saw and my wife bought it for me before she was my wife. Had it like 13 years. Don't want to smoke it

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Yeah, you wouldn't want to ruin the cylinder over $1 worth of sealant.

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Carb testing



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