expansion chamber blues

[gary55]

I have a grubee skyhawk motor. match ported the intake and exhaust. enlarged them as well. also enlarged transfer ports and baveled case for smother flow to transfers polished all to a smooth mat finish rejetted carb to 1 mile elev. decked jug milled head removed skirt impeding intake flow modestly ramped piston at top by transfers.35 mph don't seem like potential. I am wondering if the fmf fatty gold series exp. chamber for a kz125 is just to big for my 66 cc mill to allow it to performe at it's best. It runs good but won't rev out. should I go with a smaller chamber?

 

[Deleted member 12676]

The stock CDI limits revs by advancing the ignition timing too much at high rpm.
Also the stock 14mm carb limits top rpm power because it is undersized.
Correct those two and then raise the exhaust port little by little till you acheive the top speed you want.

 

[gary55]

thanks jaguar you can probably tell i,ve done a lot of my mods based on your performance tips im with you on the carb change and jaguar cdi i have your cdi on my other bike and it runs much smother delorto or makuni are in the future still what do you think about the fatty fmf exp chamber form kz 125 is it too large to function properly on souped up 66 skyhawk thanks again gary55

 

[Deleted member 12676]

well It delivers a motocross powerband which is not what you want with these bikes. But you can weld in a baffle extension for a broader powerband and weld in a belly stinger to lessen the noise and you would be good to go.

 

[HeadSmess]

its for a 125!

whats the ID of this FMF pipe? about 35mm im guessing? and the HT has a port size for 24mm tubing, maximum!

the small taper in the exhaust port acts just like the larger tapers on the pipe! if the port taper is too steep, the reverting pressure wave is reflected AWAY from the port, without ever having reached the piston and/or inside the cylinder! which means it will do DOODLY SQUAT! well, it will do something, it will effectively scavenge the cylinder for exhaust gasses.

the fmf pipe will probably be tuned around the 9000/10000rpm range, or slightly less than twice the desired rpm range.

the internal capacity of the FMF will be far too large, meaning the pressure waves are reduced in strength.

and, finally, have you matched the 66 ports, both exhaust and transfer, to match the stock ports on the 125? the fmf will act like a longer pipe or be tuned for a lower RPM if the port times are less than on the 125. and if theres a large disparity between either transfer or exhaust in comparison to the 125, there will be annoying "dead spots" in the band that no amount of tuning will cure.


and, a lot depends on what you define by "revving out". even a ryobi brushcutter revs higher than a HT, piped or not!

 

[Deleted member 12676]

all good points.
probably if he used 1" header tubing and made it long enough to match his low peak rpm and did my other two suggestions then it would be usable, not perfect by any stretch of the imagination, just usable. That's not to say it wouldn't give any power boost.

 

[HeadSmess]

another big factor can be the stinger size....


sorry i yelled

i get excited

so yeah, ill agree on the belly stinger.

remove existing stinger that, for a 125, im assuming will be roughly 20 mm.

fabricate small cone to extend cone to replace stinger removed and bring cone to a point. or just blank it off...

replace stinger with something around 12 to 13mm ID to increase pressure hence density. more suited to the smaller capacity engine. makes the system more effective.

also fabricate small cone for header side to reduce the large size header to something more like 22mm. match the taper as well as possible, erring more towards cylindrical than conical.


if it does start working well, it will be a fairly peaky pipe, being so fat... fatness being a factor of amount of taper over length. length determined by porting and desired rpm. so it has a steep taper. leading to narrow power band. unfortunately, unless youre good at measuring things that look like slugs, and mathematics, you wont have much idea of where that powerband is.... FMF may have technical info on the pipe dimensions. the 125 will be "stock" ported. then its a matter of cross referencing with your grubee portings and finding out where its estimated peak is easy huh?


then its just tweaking other areas to get it to rev... mixture, intake, and...um... a steep hill