372OE with 268 pop up timing numbers help

[Yotaismygame]

Readings could be wrong. I’ve done it four times now and with a different piston I got 167.

 

[trappermike]

Why would you raise the intake port roof? No one does it,it won't add power,but runs the risk of combustion chamber pressure blowing back down the intake port,possibly causing your problem.
I'll bet you can see the piston ring in the intake port at BDC,because thats what happens if you raise the intake port. The intake port roof is only raised in rare circumstances with odd engines.

 

[Yotaismygame]

Like I said earlier. Messed up the port shape and went back in to smooth it out, next thing ya know it was bigger than intended. I think what you just said is the problem. I’ve already re-shaped the port with JB Weld but have yet to run it. What kind of intake duration are you seeing?

 

[Yotaismygame]

Cannot see the ring at BDC, intake timing is unchanged except for a thinner base gasket and the shorter 268 skirt.

 

[trappermike]

I forget what stock 372 in. dur. is but it's pretty low,a 1mm shorter piston skirt cannot bump it to 167-170,something is wrong,,,, Even so,170 degrees shouldn't make 372 a dog,I would use that duration in a hotter motor.

 

[Yotaismygame]

Well I don’t know what to tell ya. Are you saying intake duration is on the long side?

 

[trappermike]

If your intake timing is really 167-170,then maybe your low ex. dur. may be making it bad,you have intake timing made for high rpms and ex, dur. made for low rpms,they aren't helping each other. Move ex. dur. to 168 degrees,tighten squish to .020" and try that. But if you have blowback in the intake port you need another cyl.or try JB Weld...

 

[trappermike]

Yes your intake timing is more than I'd like for a work motor of that size but it shouldn't be fatal.Try some of my things and maybe a 7T sprocket to help keep revs up,I used 7T on all my race saws that size,makes a big difference on those size motors with less low speed torque.

 

[Yotaismygame]

I have another cylinder. I’ll give it a shot

 

[trappermike]

When I raced 70cc saws many of my competition's saws bogged like hell in the cut,too much port timing and too big sprocket for a smaller motor.
Since you can't make intake dur. less,mod the ex. to 168 and try a 7 tooth,may make it a beast.
Well don't give up on the cyl. if the intake port is ok(not blowing back),try the mods I just said first,it might really surprise you.

 

[Yotaismygame]

I just went out and measured both a 372 and 268 meteor piston. The 372 is 2mm longer.

 

[trappermike]

OK maybe your problem isn't the intake port,try my ex. mod and try a 7T.

 

[Yotaismygame]

I’ll give it a try

 

[trappermike]

Well I dunno about a 268 Meteor piston skirt,but the factory Jonsered 670 skirt was only 1mm shorter,maybe that explains your longer intake timing,all the saws I built used the factory 670 piston. So as long as you use 268 Meteor pistons your gonna have more intake duration. OK now things are making sense.
Well do my mods,bump ex. duration to 168 degrees and use a 7T sprocket. The jump to 168 in ex. dur. is gonna make top end power much stronger,and the 7T will take advantage of that.
Try what I say,it will never have the really low rpm power again but it should pull WAY harder in mid to top revs. It may work well after all!
If your intake port isn't screwed you need a new 670 piston,not a cylinder.
But I still think you can make good power from what you have!

 

[Yotaismygame]

I just went and measured a oem 268 piston and it’s the same as the meteor.

 

[Yotaismygame]

1mm longer skirt sounds nice

 

[trappermike]

Listen 168 degrees ex. dur. is nothing compared to what I use in race engines,it should work well in yours. Remember in 2-stroke motor you gain hp at high revs and lose at lower revs,no way around that,so adjust for that.

 

[Yotaismygame]

Do you ever raise the transfers when using a 268 piston?

 

[trappermike]

Well all I ever used or recommended was the 670 piston,people wanna use the 268 because it's cheaper,but I never tried them,but I built about 100 motors with the 670 piston.
Anyway no wasting time crying in our beer,just try my mods,you don't have to buy any new parts.

 

[trappermike]

In your case raising the transfers will lose power.