The ideal ms260 timing numbers.

[thompsoncustom]

Just random thoughts…

If you can get compression way up there, having a higher exhaust shouldn’t be too bad. But I think @drf256 runs very high compression and still finds 105 to be a good height.

Also the shape of your chamber is important. If you could fill the chamber and then cut it to a smaller bell shape you would probably see more efficient combustion. In theory, you could even fill the plug hole and re-tap into the center of the bell. Like a two piece head.

Are you working with a 44 or 44.7 mm jug? Echo 490 pistons are full circle 44mm. I’ve got one here if you want measurements.

Are you planning to leave the dome on the piston or cut it flat?

You have a bit of luck with the AM jug. What are your current numbers? Looks like the exhaust is higher than 105 but the floor is good news.

Ya I think having the exhaust higher with say the same compression as one with the exhaust lower would be a good thing but I'm also not sure it's even achievable. When I filled the head on the wild thing I figured the compression would have been a lot higher than it ended up being.

Something else I've wanted to try but haven't yet it is gas porting a piston which will make that top ring seal better but also decrease ring life but this saw might be a good candidate for that.

it's a farmertec 44.7 jug and I plan on leaving it domed as I don't have the equipment to lower the jug or cut the piston flat accurately. That Echo piston looks just like my wild thing piston and I was able to go really wide with that, could make a good option for someone with a 44mm jug.

Looking at the notes I made from the last time I had it apart my timing numbers are 96/121/67 which would explain why this saw has a crazy amount of spitback. All I did was widen the ports last time and shape the lowers some I believe. I did take the exhaust side to .050 from the skirt and the intake to .100 which definitely helped this saw but it looks like there's a lot of room for improvement on the numbers.

 

[drf256]

Why would an intake of 67 be giving you a lot of spitback?

 

[thompsoncustom]

Why would an intake of 67 be giving you a lot of spitback?

I would assume to much case compression blowing charge back out the intake

 

[drf256]

Usually not the case.

More intake duration usually equals more spitback. Case compression starts when piston starts to fall and builds when floor is closed. At 67, you’re at stock numbers.

Where your uppers are is irrelevant until the floor closes.

 

[thompsoncustom]

Usually not the case.

More intake duration usually equals more spitback. Case compression starts when piston starts to fall and builds when floor is closed. At 67, you’re at stock numbers.

Where your uppers are is irrelevant until the floor closes.

Good to know I figured the opposite would be true. As right now I would be sitting at 54 degrees case compression vs lowering the floor to say 80 degrees and having 41 degrees case compression I assumed the latter would reduce spitback not increase it.

 

[farminkarman]

Good to know I figured the opposite would be true. As right now I would be sitting at 54 degrees case compression vs lowering the floor to say 80 degrees and having 41 degrees case compression I assumed the latter would reduce spitback not increase it.

The longer your intake port is open, the more time there is for charge to get pushed out through the intake.

 

[thompsoncustom]

The longer your intake port is open, the more time there is for charge to get pushed out through the intake.

makes sense I was looking at it wrong I assumed it came from to high of case compression after the piston passed the bottom of the floor.

I'll have to double check into the spitback thing it could have just been the carb that made it look terrible because when I first installed the 460 carb I was dumping so much fuel I had unburnt fuel coming out the exhaust but she ran which alone is impressive I think, also I don't believe it was a issue at WOT.

I'm not gonna change any of the numbers on the first round of brazing so I'll look into it next time it's back together.

 

[Ketchup]

makes sense I was looking at it wrong I assumed it came from to high of case compression after the piston passed the bottom of the floor.

I'll have to double check into the spitback thing it could have just been the carb that made it look terrible because when I first installed the 460 carb I was dumping so much fuel I had unburnt fuel coming out the exhaust but she ran which alone is impressive I think, also I don't believe it was a issue at WOT.

I'm not gonna change any of the numbers on the first round of brazing so I'll look into it next time it's back together.

It’s spitting back because the carb is so big. You may find that carb gives you a lot of difficulty. I have an HD 12 (same bore and jet as hd 16) on a 50cc echo. I had to modify a jet.

 

[thompsoncustom]

It’s spitting back because the carb is so big. You may find that carb gives you a lot of difficulty. I have an HD 12 (same bore and jet as hd 16) on a 50cc echo. I had to modify a jet.

I've had the carb on there for about a year and I like it. it is picky when it comes to the low side tune tho if it's to rich fuel starts pooling in the intake and it will bog when u hit the throttle.

that's good news if the carb is just what's causing it then I won't worry as much about lowering the intake. that much case compression has to be counter productive.

 

[82f100swb]

that much case compression has to be counter productive.

More primary compression means more pressure to push charge up the transfers. This is why we do stuffers and full circle cranks.
Knowing my experience with how much 026's don't like having piston windows opened up much if at all, reducing primary compression on one will be counter productive, very quickly, especially since you are very overcarbed. Part of that I think is due to case volume, you have to remember that a 260 is a big bore and a stroker on those cases...

 

[thompsoncustom]

More primary compression means more pressure to push charge up the transfers. This is why we do stuffers and full circle cranks.
Knowing my experience with how much 026's don't like having piston windows opened up much if at all, reducing primary compression on one will be counter productive, very quickly, especially since you are very overcarbed. Part of that I think is due to case volume, you have to remember that a 260 is a big bore and a stroker on those cases...

Thanks for the advice, the intake on this thing seems more than big enough already so it's not like i need more intake duration and if lowering the case compression is going to hurt things i'll just leave the intake side alone.

Pulled the cylinder today and cleaned it up after taking this picture. Still need to find a longer m10x1 bolt for the decomp hole so I can still run the decomp after I fill in some of the chamber with aluminum.

 

[drf256]

Is your plating wearing off at the top, or is that an illusion?

 

[thompsoncustom]

Is your plating wearing off at the top, or is that an illusion?

believe what your seeing is just a reflection as I can't see or fell anything in person.

Took some measurements:

combustion chamber is 3cc
ring gap is .003!!!

going to have to delete the decomp and hopefully can get them chamber down to about 1cc. also the china rings on there are way to tight kind of surprised it hasn't caused any issues but have another set here that are .008 so will be using those.


hope to do some brazing on the cylinder this weekend.

 

[drf256]

combustion chamber is 3cc

How did you measure it?

ROT is 1cc per 10cc saw displacement is around 200 psi. Fine tune from there. Thats what I was told when I was carving the chamber on my 2 piece head saw. It’s still sitting in a bin.

I wish I could get my timing numbers and stay at or below 200 psi. Compression adds a lot of heat to the saw.

You don’t need a decomp on any 1121. They aren’t bad to pull over.

 

[Dolkitafreak]

How did you measure it?

ROT is 1cc per 10cc saw displacement is around 200 psi. Fine tune from there. Thats what I was told when I was carving the chamber on my 2 piece head saw. It’s still sitting in a bin.

I wish I could get my timing numbers and stay at or below 200 psi. Compression adds a lot of heat to the saw.

You don’t need a decomp on any 1121. They aren’t bad to pull over.

You can stay at or below 200 if you desire, just open the chamber up with a boring bar a touch. Lopping the head off is cooler though have you had any issues with detonation on this chassis after long cuts? I know they end up high psi wise for you with a wide band

 

[drf256]

Just leaning out after long cuts. No detonation.

 

[thompsoncustom]

How did you measure it?

ROT is 1cc per 10cc saw displacement is around 200 psi. Fine tune from there. Thats what I was told when I was carving the chamber on my 2 piece head saw. It’s still sitting in a bin.

I wish I could get my timing numbers and stay at or below 200 psi. Compression adds a lot of heat to the saw.

You don’t need a decomp on any 1121. They aren’t bad to pull over.

I just measured where it drops down around the spark plug and decomp that hole. I didn't measure everything above the piston at TDC.

I just used water to measure it.

 

[thompsoncustom]

Well got the chamber brazed up today which turned out ok, there is more material on one side than the other but that's not a big deal I could go in there an even it up. The bigger issue is the fact that the bore warped so at close to TDC the piston is binding on the cylinder.

Anyone know what the clearance between the cylinder and the piston should be?

I'll probably try and sand the piston till it fits and see what happens but it's never gonna be great at this point. If I had to do it again I would probably figure out what the piston/cylinder gap should be and add something thin to the side of the piston and drop it down in the bore when it was still at full temp so when it cooled it would have a better chance of holding it's shape.

Here's a picture of the cylinder after brazing and grinding out the spark plug hole a little, I had planned on trying to countersink the hole some to try and improve flow to the little chamber. If I can get the piston to clear the bore I will go back in and clean up the chamber and run it but project maybe be dead at this point.

 

[drf256]

Are you sure it’s binding and not a booger on the squishband?

Unsure how the heat of brazing would warp the jug. Welding yes, brazing I’m not sure. @srcarr52 welds plug holes often. Thats a ton of heat in the same area without bore issues. I’ve made a poor attempt at what Shaun makes look so easy and had no warp in bore at all. On one jug, I had the entire chamber start to drop out on me. And that was with a long 5XL cup down in the bore.

Did you heat the head from outside the top until your braze flowed, or did you go right in the bore with your torch?

The possibility of warping is why I never tried an exhaust roof. No choice but to have the heat right by the plating.

 

[drf256]

What is this? It’s at same location on various pics of your chamber.