Today I noticed a fair amount of difference in the compression
between my stock Zen 260 and one with top end mods
Is this due to the mods on the ports
Both motors are low hours/257 carbs
Tap

Ok .... which has the perceived higher compression ?

Scott
the stock motor feels crisp when pulling over slowly
they've both got about the same amount of time
( two 24 oz. tanks)
thanks Tap

Most common issue causing a compression loss in a mod motor is whats called ring clipping. We need pictures within EX port to tell ? ... but what it is, when exhaust roof of cylinder gets raised it's done straight across allowing ring to come out into port slightly. As ring gos up to seal port it is shoved back into the groove all at once and edge of ports roof wears away on rings sealing face making it seal less effectively.

If you remove EX system and look into port make note of rings sealing face, it is flat and even looking or ... have secondary bevel / wear pattern on top edge ?

Thanks Scott
headed out to run the boat now
Will get a picture and post later
tap

Scott
this motor was on the shelf for 6 months or so
and after running it today seemed to be fairly nornal
my old eyes don't realy see much on the ring, but under magnifacation there is some discoloration on the top edge. Don't know if these pics are good enough to tell.

Can't definitively tell ? ... but rings light reflection does seem to be such it may be the case. Taking a better picture in close using macro may be best if your worried about it ?

On the other hand you saying it's running fine. If that is the case don't sweat it.

Scott
the top two pics , the ring apears to have to reflective surfaces
slow puling the motor felt OK after it ran
but the performance was about par with the stock Zen
Is there a remedy , can that edge be beveled?
Thanks tap

In a chrome bore engine prevention of ring clipping is in the profile the exhausts roof when recut. We want roof to be a gentle arch so ring starts to get pushed back into it's groove on outside edges first and continues to get pushed in with the middle of EX port roof last to close.

If Ex port roof is FLAT across now, NOTHING you can do but recut the roof increasing EX duration more ( Pending where set at now ? ) or live with it knowing periodic ring replacements necessary when rings getting torn up

Scott
I see what your talking about
it almost looks as if the ring is hitting in the center before the edges/a reverse crown
How about I send it to you, and see if it's repairable?
mailing address
tap