ms 390 Tuning and tweaking

[justadirthauler]

Hi all, I've got a 390, someone told me ya'll could tell me how to advance the timing and modify the muffler a bit. Most of the mods I've done in the past to various saws haven't changed them much in the big picture so I thought I'd ask some more knowledgeable heads this time.

 

[Lightning Performance]

Search Google for threads on Outdoor Power Equipment Forum.

Welcome
Get the goo off it first inside and out. You can burn it off gently mind you. A good degreaser works also.

Drill two holes under the deflector straight through the front baffle. It will be loud or drill around the baffle but under the cover. The square is the baffle face.
Make damn sure you get out ALL the metal chips in the muffler. Soap and water with some pressure works well. Rinse and repeat.

File 0.030 off the flywheel key.
460 carb... maybe. It requires mods to work well.
Study time.

GL

 

[dmb2613]

440 carburetor is a direct bolt on , but the 390 carb will be fine just richen up the high side , you will need to make the hole larger on the deflector plate on the muffler also , it will run like a raped ape then the key is made to the flywheel make sure you file off the correct side of the key The flywheel will need to be rotated to the left, I take off about 1/4 of the width of the key , if it is too high don't worry just twist it back some it is a tapered fit and needs no key at all to run. I grind 2 slots in the muffler just connect the holes that are there and do the same to the top of the muffler some already have hole there I do this on a regular basis beware of everything you read , some just post without any experience, make sure you have the plate that goes behind the muffler if you put a pop up piston in it as it will run hotter.

 

[dmb2613]

most I have typed since I was in school lol

 

[justadirthauler]

Thanks a lot! That definitely gives me something to go on.

 

[justadirthauler]

This popup piston, where would I go to find one?
That's kind of what I've been looking for.

 

[dmb2613]

H L supply carries them

 

[justadirthauler]

Ok thanks for all the info!

 

[Carhartt]

Isnt the intake rubber tube ID more restrictive than the ID of an oversized carb? I have never tried and could but never felt the need to replace the existing carb. Always thought they put out plenty of fuel if richened.