Ms250 timing numbers

[morbius18]

I picked up a really clean ms250 with questionable bearings. I figured I'd widen the ports. I decided to see what the numbers were.

Bore: 42.5mm 65% bore = 27.6mm
Intake: 80° stock port width: 19.9mm
Exhaust: 98° stock port width : 21.5mm
Transfers: 129°
Squish: .039"

I double checked the initial setting TDC after getting 129°




I can go quite a bit wider on the exhaust port, but the intake port has both rings visable and close to the sides of the port. Is it ok to widen the lower part of the port and leave the top narrower? I can also remove the bottom ring as well.

Sketch of port shape on left with dotted sides. Will be similar to the right sketch.

 

[hedge hog]

[emoji1301]


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[blsnelling]

Is it ok to widen the lower part of the port and leave the top narrower? I can also remove the bottom ring as well.

Yes and yes.

 

[morbius18]

Yes and yes.

Thank you.

 

[morbius18]

Yes and yes.

Is 31 degrees of blowdown too high for a quad open port clamshell?

 

[blsnelling]

IMHO, yes.

 

[morbius18]

IMHO, yes.

I did around 20 degrees on my 346 and 372xp. Is this a good general number?

 

[morbius18]

I widened each port about 3mms. I can go wider, but was concerned about the amount of cylinder for the tings to run on.

I opened one side of the intake more because of the upper ring end. It will run with onlythe top ring.

 

[blsnelling]

I did around 20 degrees on my 346 and 372xp. Is this a good general number?

I build 372s with 18-20, 346s with less. I would go for maybe 22 in the Stihl.

 

[morbius18]

I build 372s with 18-20, 346s with less. I would go for maybe 22 in the Stihl.

I've got the numbers written down at home for the 372 and 346, I couldn't remember exactly what they were, but knew they were around 20 degrees.

They seemed a little high. I'll raise the transfers some on the ms250.

Thank you for taking the time and sharing your knowledge.

 

[cus_deluxe]

Any plans for carb upgrade?

 

[morbius18]

Any plans for carb upgrade?

I haven't thought that far ahead. I plan on running it with the stock carb and seeing if it runs out of fuel.
Do you have a suggested upgrade?

 

[blsnelling]

I haven't thought that far ahead. I plan on running it with the stock carb and seeing if it runs out of fuel.
Do you have a suggested upgrade?

It's not going to run out of fuel. That's not the point of a larger carb.

 

[morbius18]

It's not going to run out of fuel. That's not the point of a larger carb.

I'm slightly ignorant about switching carbs. I know you can tell if you have too much carb, you'll kill velocity and will run poorly. How do you tell when you don't have enough?

 

[hedge hog]

No four stroking ?


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[cus_deluxe]

I haven't thought that far ahead. I plan on running it with the stock carb and seeing if it runs out of fuel.
Do you have a suggested upgrade?

Im not super familiar with stihls, so idk whats the best bolt on upgrade, but i know that the carb is gonna be a bottleneck in the system on a 250, especially when ported. Id imagine with a little creativity an ms 290 carb might not be too hard to adapt.

 

[cus_deluxe]

No four stroking ?


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I dunno what would be a good list of symptoms, but i know that chainsaw carbs are pathetically small compared to other high output two strokes of a similar displacement. I know about 1% of what id like to know about carburetors lol, so im not a great source of info. Im not sure ive seen a saw that had a decrease in performance from a larger carb. Every saw ive seen with a bigger carb fitted has benefitted