Muffler modding the 1127 series Stihls. (MS290, MS390, etc)

[fearofpavement]

I rebuild a lot of this series of saws and have a pretty standard muff mod that I do. I have a Bridgeport mill set up so it takes me less than 10 minutes to do one. That said, I rebuilt an MS290 into a 390 this week. This series of saws all fit the same muffler but different models have different amounts of "ports" or exhaust vents. I assembled the saw with the stock MS290 muffler figuring I'd test it and then do the muff mod later. The saw ran awful so I messed with the carb some and finally put a different one on. It was "better" but not typical. Compression was good, flywheel was tight, I re-gapped the coil, tried a different spark plug but the saw just wouldn't rev up right. I finally decided to mod a muffler and install it. That solved the problem. Apparently the limited amount of exit ports on the MS290 muff was choking the saw (like a plugged spark screen would)
Just thought I would throw this out there in case anyone else encounters a similar problem when upgrading one of these saws to the 49mm cylinder kit.

 

[Onan18]

You mean you actually reassemble a saw without poking extra holes in the muffler? Herecy I say!

Joe

 

[Bilge Rat LT 20]

You need to get a Foredom. It must be rough having to use a Bridgeport for a MM

The 290/390s respond well to MM
Restricted exhaust kills performance. I've been seeing a few saws, weedeaters etc that have Cats with carbon restricted exhaust ports. They usually won't rev up. The Cats are clean but the port restriction kills rpm.
I clean the carbon and all is good again.

Get the exhaust out so you can pull the air/fuel charge in.

 

[fearofpavement]

Don't need a dremel grinder to mod an 1127 series muffler. They have plenty of holes in the internal baffles, it's just the outlet that's restricted. I make a couple grooves and then drill two holes in the depressed part and use a narrow wheeled bench grinder to shave back the deflector cover. As stated, less than 10 minutes including cleaning the burrs and chips out of the muffler. No carbon build up as the cylinder is factory new.

 

[Bilge Rat LT 20]

I have an 039 that is close to 20 yrs. old. It has been my rough use do everything saw.
Just chains, carb kits, rubber parts and a few sparkplugs. It still runs strong and dependable.
Did a very mild MM on the outlet years ago.

The 039 is pretty open compared to some of the newer 390s.
The 1127s are good saws, just a little heavy.

I use an old die grinder and burr then a sandpaper roll. Stuff i used porting Chevy cylinder heads years ago.

I know you are getting the heat, are you getting any rain?

 

[StihlmanCasey]

Another thing I've noticed about a modified 039-ms390 is if you use a Ducati ignition coil it will take a timing advance to a whole new level. I've built a few hot saw 1127's and the best one yet had the Ducati coil. I love these saws they are great.