Mastermind Revisits The MS261CM Type II

[jake wells]

Sounds easy but getting 0.000 runout is a beeeotch

That is why I leave it to the professionals.

 

[tree monkey]

OK, guess I've mostly seen 461s and 661s, which look a lot different.

there not strato

 

[MustangMike]

So, forgive my ignorance, but with strato two ports have fuel and two just have air???

Also, any idea why Stihl uses a different technology on their larger saws than on their smaller ones?

Doesn't Husky use strato on all???

 

[retro]

Knowing a great machinist is a rare privilege indeed!

I'm really impressed that you guys have figured out how to port the 261 strato jug (for gobs of torque) without gutting it!!!

Is it just the carb that has to have the divider removed on these critters?

EDIT: I'm guessing the carb boot divider must get cut as well?

 

[tree monkey]

i'll do my best to explain how it works and why it's done.

 

[Deets066]

i'll do my best to explain how it works and why it's done.

 

[tree monkey]

all strato engines have an intake that looks like this. the top port is the air/ fuel port like a standard engine. the lower port/s is the fresh air port/s.
the fresh air does not enter directly into the crankcase.

it goes through the extra ports that look like transfer port. these are lower then the transfers and are never open above the piston crown.

the piston has closed cut outs on the side that open and close a path from the fresh air ports to the transfer ports. this allows the fresh air to go down the transfer backwards. all 4 transfers are filled with fresh air. so now the first thing to come out of the transfers is fresh air. the fresh air is supposed to flush out the spent charge ahead of the fresh air/fuel charge. this is supposed to reduce air / fuel charge losses out the muffler.

the truth is that this dilutes the spent charge with fresh air so when the epa sticks there sniffer up the tail pipe the percentages of hydrocarbons is low enough to pass.


hope this helps

 

[tree monkey]

the 461 and 661 are clamed to be delayed scavenged, this means that some of the spent charge is forced down the transfers. long transfers are needed for this to work.

 

[Tor R]

all strato engines have an intake that looks like this. the top port is the air/ fuel port like a standard engine. the lower port/s is the fresh air port/s.
the fresh air does not enter directly into the crankcase.

it goes through the extra ports that look like transfer port. these are lower then the transfers and are never open above the piston crown.

the piston has closed cut outs on the side that open and close a path from the fresh air ports to the transfer ports. this allows the fresh air to go down the transfer backwards. all 4 transfers are filled with fresh air. so now the first thing to come out of the transfers is fresh air. the fresh air is supposed to flush out the spent charge ahead of the fresh air/fuel charge. this is supposed to reduce air / fuel charge losses out the muffler.

the truth is that this dilutes the spent charge with fresh air so when the epa sticks there sniffer up the tail pipe the percentages of hydrocarbons is low enough to pass.


hope this helps

Crystal clear, well, I dont know how to play with those timings they has

 

[Tor R]

and more then one thread. lol

hope he doesnt drop his cellphone down in the box of vaseline again, would be embarrasing for him to go through it again

 

[retro]

all strato engines have an intake that looks like this. the top port is the air/ fuel port like a standard engine. the lower port/s is the fresh air port/s.
the fresh air does not enter directly into the crankcase.

it goes through the extra ports that look like transfer port. these are lower then the transfers and are never open above the piston crown.

the piston has closed cut outs on the side that open and close a path from the fresh air ports to the transfer ports. this allows the fresh air to go down the transfer backwards. all 4 transfers are filled with fresh air. so now the first thing to come out of the transfers is fresh air. the fresh air is supposed to flush out the spent charge ahead of the fresh air/fuel charge. this is supposed to reduce air / fuel charge losses out the muffler.

the truth is that this dilutes the spent charge with fresh air so when the epa sticks there sniffer up the tail pipe the percentages of hydrocarbons is low enough to pass.


hope this helps

Thats the best explanation I've seen! Looking at that restricted looking divided intake duct and seeing the tiny muffler outlet, its a wonder the stock 261 runs as strong as they do. I can't wait to get mine ported by one of you guys.

 

[Stihl working Hard]

Such a tease

I was thinking the same thing myself

 

[Stihl working Hard]

Cutting the squish and cylinder extension.

View attachment 89729 View attachment 89730 View attachment 89731 View attachment 89732 View attachment 89733

Lovely work as usual Randy

 

[Chainsaw Addict]

Now for the bases......

View attachment 89736
View attachment 89737
View attachment 89738

So where do you use the belt sander?

Sent from my SM-N900P using Tapatalk

 

[Mastermind]

So where do you use the belt sander?

Sent from my SM-N900P using Tapatalk

Exactly.

 

[Mastermind]

Knowing a great machinist is a rare privilege indeed!

I'm really impressed that you guys have figured out how to port the 261 strato jug (for gobs of torque) without gutting it!!!

Is it just the carb that has to have the divider removed on these critters?

EDIT: I'm guessing the carb boot divider must get cut as well?

It's pretty much everything but the carb on this model. If you remove the divider from the carb it throws the low pressure signal off to the point that it doesn't run well.

 

[Adirondackstihl]

It's pretty much everything but the carb on this model. If you remove the divider from the carb it throws the low pressure signal off to the point that it doesn't run well.

You so smart n stuffs

 

[CR888]

I remember when quite known forum builders would remove carb dividers as they thought they restricted flow without really grasping the concept of why they were there in the first place.

 

[MustangMike]

Thanks TM, very clear. I also saw something in the pics you posted that I previously missed, a lower transfer inside the cylinder. I previously thought they were just being fed through the bottom like a 461/661.

Also, knowing that the fresh air does not flow through the crankcase helps me understand why the bearings don't blow out.

So much more going on than a plain old 440/460/660!

 

[Keith Gandy]

all strato engines have an intake that looks like this. the top port is the air/ fuel port like a standard engine. the lower port/s is the fresh air port/s.
the fresh air does not enter directly into the crankcase.

it goes through the extra ports that look like transfer port. these are lower then the transfers and are never open above the piston crown.

the piston has closed cut outs on the side that open and close a path from the fresh air ports to the transfer ports. this allows the fresh air to go down the transfer backwards. all 4 transfers are filled with fresh air. so now the first thing to come out of the transfers is fresh air. the fresh air is supposed to flush out the spent charge ahead of the fresh air/fuel charge. this is supposed to reduce air / fuel charge losses out the muffler.

the truth is that this dilutes the spent charge with fresh air so when the epa sticks there sniffer up the tail pipe the percentages of hydrocarbons is low enough to pass.


hope this helps

So the stratos actually do what fingers try to accomplish opening a few degrees in advance?