372 Pston swap/mod

[old guy]

I don’t see any reason why the specs mike wrote on the first page wouldn’t work with the hyway pop up. You would end up with a pop up on a pop up. No clue if there’s enough room for that in the cylinder

I'm thinkin that's gonna tap the sparkplug.

 

[Yotaismygame]

So I finally got ugly betty #2 put together.


Here's the build specs:

Meteor 268 piston machined into pop up via Mike's specs.
.025 squish with base gasket
used 372 jug
RWj-4 carb(372xt) with 390xp boot
widened and raised exhaust port
modded and matched muffler
un-shielded lower transfers and smoothed divider

First start up took me awhile to get a ball park tune. I'm sure the jets are around 2 turns out. Throttle response is instant! I didn't tach the wot rpm but it screams. Had to richen it up a lot to get it down for break in. Sadly my compression landed at 150psi. I'm sure it'll go up some after a few tanks of fuel but I was hoping for at least 170psi. I didn't think I took the exhaust up very much but maybe I did. Actually I thought I took off so little that I was anticipating having to raise it more. Ive got a tach on it and a sharp chain for some test cuts tonight. Depending on how that goes I might take the squish down to .018".

 

[old guy]

That sounds about like mine, my squish is .020, I raised the ex, about .080, widened in. & ex. about a m/m on each side. I used an oem 670 piston.
I used a 390 carb., first comp. 148, after one tank of fuel comp. was 158.
I too had to open the main jet to get the saw to calm down, it wouldn't 4 stroke till it got down around 13000. I've run another tank thru it now but it's been too cold to mess with it, it seems to pull hard from rev 1 rite to the top.

 

[Yotaismygame]

So I tore the saw back down and lowered the squish from .025 to .018, squared up the intake and widened it just a tad. Compression went from 150psi to 162psi. It free revs to 15k now. That's a 500 rpm increase after lowering the jug. Before I was having some idling issues where it would load up and then almost fall flat. I'm not sure if it was the carb or not but after opening the intake the saw idles and runs great now.

If I build another one like this im not going to raise the exhaust at all and see where that puts compression and max RPM.

 

[Deleted member 150358]

So I tore the saw back down and lowered the squish from .025 to .018, squared up the intake and widened it just a tad. Compression went from 150psi to 162psi. It free revs to 15k now. That's a 500 rpm increase after lowering the jug. Before I was having some idling issues where it would load up and then almost fall flat. I'm not sure if it was the carb or not but after opening the intake the saw idles and runs great now.

If I build another one like this im not going to raise the exhaust at all and see where that puts compression and max RPM.

I think that's pretty much what trappermike recommends in his initial posts.

 

[Yotaismygame]

Mike talked about getting 170psi with a looser squish and raised exhaust. I’m not sure how that was happening. I’m curious what comp would be with a stock jug. I’m going to do some more cutting with it today. I’m feeling like I’ll be impressed this time around.

 

[Deleted member 150358]

Hmm. Thought he said widen only but I've been wrong often enough!

 

[Yotaismygame]

With the mod I do compression with a stock ex. port is around 200 psi(Far too much for my liking),with the ex. port raised 2mm the comp. is about 170,which is perfect for good midrange power and top end power from a 72cc saw with more ex. duration.

Btw I’m not complaining about the outcome so far. Just discussing my results. I didn’t measure the roof of the exhaust before or after but it seemed like i barely ground on it. The intake originally looked like it was maxed out already in stock form which is why I left it alone at first. I maybe took 1-2mm off the sides but mainly squared it up the second go around. My 162 comp could be from the lower transfer work I did or the used cylinder I’m using.

 

[Yotaismygame]

Just finished making more cuts with it. Buried 24” in fir, holds 10.8k. Didn’t skip a beat. Strongest saw I’ve got now. I will say though my hyway BB with hyway pop up isn’t far behind. And that saw only has a modded muffler and no base gasket with around 026 squish.

I need to get it in some bigger wood

Thinking about my compression, I bet it will be close to mikes proposed 170psi by the time this new caber ring seats.

 

[Yotaismygame]

Well I jumped the gun on my compression. I have about 3/4 tank of fuel through it and I could feel it had gotten stronger. Just put the tester on it cold and it read 180 psi.

 

[Spring1898]

I’m not sure about the hyway piston. This thread is just about the normal 268 piston. I did bring it up earlier because I was curious but mike said even with the regular 268 piston you’ll get about 200psi but he likes to raise the exhaust to the point compression comes down to 180. With the hyway you would get a mega pop up

What if you made the mega pop-up and dished the center to bring the compression to a reasonable level?

Actually now that I say that, I wonder how it would affect flow

 

[Yotaismygame]

Personally I don’t see any reason to go more than 180 on comp with this setup

 

[Huskybill]

Just thinking out loud. When the piston moves down she pressure charges the crankcase. Then allows the charge from the crankcase to enter the transfer ports then into the cylinder. When the piston is at tdc the intake from the carb is drawn into the crankcase. I haven’t checked the piston skirt at tdc in the intake port to see if it can be windowed or not. If we raise the intake port we open the intake sooner. If we lower the intake port we change the duration of the port timing on how long the intake is open. By adding a halfmoon window or one round window that’s about it. At onetime we did cut 1/8” off the bottom of the piston to speed up the opening of the intake port. This works on piston ported dirtbikes too I tested it on. There’s a lot to test with the intake port timing. Raising and widening the exhaust port is next. I been thinking of doing a light bulb shape in the middle of the exhaust port so that initial whoosh of the exhaust gasses can get a pre boost before the pipe. Advance the timing.

 

[Yotaismygame]

This weekend I found out some things not to do when using the windowed 268xp piston and I have some questions about making a stronger saw.

Don't raise the exhaust too high. Personally I think stock height or just a tiny amount off the roof is good. Too much off the roof and compression will come down too far and the saw looses way to much torque.

Don't hog out your intake. It caused all kinds of tuning issues and again had no low end. Not sure what was going on here but it wasn't good.

What i found does work well is to widen the intake and exhaust. The intake can only be widened maybe .040" its more about squaring it up. This I've found does add power. Leave exhaust roof alone or take off a hair. Comp should land around 180-190. The strongest one Ive built yet had the highest comp. Also smooth out and lower the transfer tunnel wall, and case match.

Mike has the piston machined down to have a squish of .025" with stock base gasket. I'm curious how the saw will respond with a popup of .035" instead of .040" and a squish of .020". Or is the .040 pop up and lowering the jug down to get a tighter squish the way to go? The intake duration is already pretty long at 170. Would lowering the jug down .005"-.007" which adds duration a positive change or negative? Or is that not even enough to make a difference?

What about using the 268xp non windowed piston? I think it would increase case compression and increase transfer velocity. But it's probably a bit heavier of a piston. I have one on order.

What about using the Hyway 268xp pop up? You could make a .060 pop up with that piston. .020" is added to the top already, then cut off .040". I have one at the machine shop now. Not sure it will even fit. Could make comp upwards of 180 psi with a high exhaust roof. Good or bad?

Last, what about the upper transfers? I haven't touched them yet but I wonder if raising them would be a benefit. Maybe even just extending them towards the intake. With the 268xp piston they are at 125 degrees untouched.

Where's Mike?! He hasn't been on the forum for a few months. I'd really like to see what he has to say about all of this.

 

[Huskybill]

Before I port a saw I’d like to see if I can get a non windowed piston. If I port the exhaust, the intake and open up the transfer ports with a side windowed piston I feel there’s too much flow not going in the same direction as the crankcase into the transfers. I want a non windowed piston so I can port the piston my way and change the intake port timing with the piston.

The factory salesman ported a 2100 that I assembled but first measured everything. He cut 1/8” off the bottom of the piston too. Plus he widened the ports, raised and lowered ports. I think reworking a smaller cc saw it would scream rpm even more.

Being we got into dirtbikes we took a piston ported ‘72 suzuki ts 125 and cut 1/8” off the bottom of the piston. My son beat a newer Kawasaki kx80 down the straights. I took s ‘81 husky cr250 and did all the porting to the max. Nothing touched this bike in the open areas in the trails.

 

[Yotaismygame]

I had the highway 268 pop up turned down .040” to make a .060” pop up. It does fit. Ran the saw in the garage for a few minutes. Hopefully I can make some cuts with it tomorrow.

 

[Yotaismygame]

Well the whole point of this big poppa pop up was to bring comp back up after raising the exhaust too high. After running the saw I have realized I raised the exhaust so high it doesn't matter what pop up is in there it's never going to be a strong saw with a 268 piston in it. Pretty certain I can have the base cut down and run a 372 pop up in it to salvage it. Next I'm going to run the big poppa piston in a stock jug and see what happens.

 

[wcorey]

Wasn’t the original point of the 268 piston to just get some modest gains without porting or any machine work to the cylinder?
Basically a bump in comp and a bit more intake duration...

 

[Deleted member 150358]

Lighter and cooler running too.

 

[Yotaismygame]

Yes to all. Very cheap bolt on power. $35 piston, few bucks for machine work. Trappermike seems to think a windowed piston works better in the 372. I can’t say that’s true or not. 268 piston with minor port work will blow the socks off a xpw all day any day.

In a few weeks I’m going to compare the 268 windowed piston to the 268 non windowed piston.