372OE with 268 pop up timing numbers help

[Yotaismygame]

I'm on my second saw that I ported with a 268 windowed pop up piston in it via trappermike's instructions. The first one came out nice and strong. But now this second one I ported the same except I messed up on the intake and now I think its too big and causing issues. The saw overall is lacking power compared to the first one, especially down low. After a lot of tuning I realized it would be very rich down low, and lean out up top. All I did was cut a 1mm pop up, set the squish at .025", and comps at 175psi. I widened all the ports, raised the exhaust, and smoothed out the lower transfers. I believe the intake is "hogged" out now since I took more off the roof than I meant to.

Here's the numbers I got.

EX:152
IN: 170
Blow down: 22

Im not really sure where to go from here or if I can salvage this cylinder. I'm wondering if going back to a 372 piston and some tweaking could turn this dud to a stud.

 

[psuiewalsh]

If the intake has too much off the roof, couldn't you get the base turned down?

 

[Yotaismygame]

That would change everything. I don’t know if that’s good or not. Maybe if I used the 372 piston. Right now intake duration is pretty long since the 268 piston has a shorter skirt.

 

[Deleted member 150358]

JB weld the intake to get this 5° back.

 

[Yotaismygame]

I thought about that but I’m unclear exactly how to do it. Would the jb weld go all the way up to the bore edge to the point the rings ride over it?
My other thought is going to a 372 piston. The intake duration would be less and case pressure would go up. Maybe cut the base or get a 372 pop up

 

[Deleted member 150358]

I'd use an old piston with a release agent. But others have used masking tape. Just use acetone or something to degrease it and rough up the surface so it can grab well.

It's pretty forgiving stuff to work it.

 

[Yotaismygame]

Just to be clear, I would put jb weld all the way to the lip, then smooth it out like a normal port? So the rings riding over it will be fine?

 

[Yotaismygame]

Another member posted this in another thread and it sounds exactly whats going on with this jug.

" you have very aggressive intake timing, your transfers had better be up to the job of flowing it too, otherwise you have excessive residual charge left in the case.. If you have excessive charge in the case, it means your peak intake velocities will suffer, and especially with stuffed/low volume cases, will quickly cause carb reversions, leading to tuning nightmares at lower RPM's... (way rich and boggy at the low end while leaning out on the top)."

His remedy was opening up the lower transfers a lot more

 

[Deleted member 150358]

Just to be clear, I would put jb weld all the way to the lip, then smooth it out like a normal port? So the rings riding over it will be fine?

Yes.

If you search a little there are a lot of examples. It will be a good repair. Its often used to change transfers to get more velocity too. It stays put. Easy to form.

 

[trappermike]

170 degrees is a little high for a worksaw,but I don't think fatal. When you widened the intake port did you leave 2-3 mm sealing of the piston skirt on each side? Did you really raise the roof of the port(?),what of the piston do you see in the intake port at BDC,can you see the ring completely?
152 degrees on the ex. port is still low,if you raise it 1mm you will increase blowdown time(and power) and create less pressure in the cylinder and crankcase to cause blowback and poor low speed running. Maybe try that first,also muffler should be free-flowing and not holding pressure back in the cyl./crankcase.
I never tried JB weld in an intake port,hope it holds.

 

[Yotaismygame]

The 170 intake is just what you get with the 268 piston. I only widened 1mm from the skirt and raised it a little. Muffler is free flowing.

Hope you’re doing well. I’ve been waiting to hear from you on this 268 stuff.

 

[Yotaismygame]

Mike what is your ideal numbers when using the 268 cut to your specs?

 

[Yotaismygame]

I did another saw with 157 ex duration and that saw was weak too.

 

[trappermike]

Are you telling me you get 170 duration from using a 268 piston with no intake mods?

 

[trappermike]

How do you mean "raised" the intake port,did you grind the lower(closing) edge? Think of top and bottom of a port as if it's(the cyl.) sitting upright,not upside down on the workbench. There would be no need to raise the top of an intake port,the bottom edge yes sometimes in bigger saws.

 

[trappermike]

Anyway buddy don't grind anymore from the stock intakes opening and closing edge,it will add duration a worksaw does not like.
But 152-157 degrees ex. duration is still pretty low,I use a lot more than that,but HOW people determine that with a degree wheel can vary a lot. I use a much simpler method that take all the measurements and math out of it and gives a far more accurate reading. I'm not afraid to put 168 degrees ex. duration in a work motor. Which gives you far more blowdown time.

 

[trappermike]

And contrary to what some say here porting a motor does not mean "Hogging" all the metal you can out of the ports and taking photos of big piles of grindings,(That is butchery)porting is precision work,just altering timing and keeping ports straight and smooth.

 

[trappermike]

I admire your hard work,but smaller motors don't like too much intake duration. First I would re-check intake duration using my simpler more accurate method.
If it is 170 then try some other mods to work with it.

 

[Yotaismygame]

I’m talking about I literally raised the roof of the intake port if the cylinder is on the saw. Only because I messed it up a bit and before ya know I took more off than I intended. I’ve never lowered the intake floor. 170 is what I’ve got on three different cylinders using the same piston.

 

[trappermike]

Putting a 268 piston with a 1mm shorter skirt will not jump intake duration to 170,either you mistakenly ground the bottom of the intake port too far or you degree wheel readings may be wrong...