Scooterotica

replaced piston on long rodded 186 Mugello after owner blew a serious hole in it.

but I can't seem to keep it from getting too hot....

JL3
30 PHBH with
50 slide
X7
AV266
125 main [too big probably, but that's not the issue here]
55 pilot


can't find any air leaks
timing checked and set at 16 BTDC
tried X13 & X2 needles on various clip settings, and the mixture screw as far as 2½ turns out.
tried B8 & B9 plugs
made it run completely rich, but still seems to get too hot too quickly.

it pinks just where the owner says he likes to ride, between ¼ and ½ throttle.
realistically he probably doesn't need a motor with such a spec...

anything else to check for ?

thinking maybe change the slide to a 40, although it's more than ¼ throttle where the issue is..

try a different pipe ??

I can't give it back to him like it is, although [apart from the heat] it does ride ok, cos he will blow another hole in the new piston....

thanks

i think you,ve answered your own question here...
and i agree that that slide is far too weak..

deffo try a 40,and of course get it dynoed if poss to be sure..

sean brady scooters wrote:i think you,ve answered your own question here...

thanks sean. do you mean more than fitting a richer slide ?

not really ,everything else is prob ok,but the fact thats its pinking at the point where he likes to ride and cruise indicates that its probably very weak in that area...
the 40 slide will help a lot i,m sure,but as i say ...best to get it set up on a dyno for peace of mind...

cheers, definitely worth trying then.
I'd set it up originally to be ridden more like a tuned two-stroke...

Jet it for a B8 plug too Jarv (or NC3 OR 24 Nippon) esp if he intends to only do 1/2 throttle. check float valve and height too.
What is the squish?

soullad wrote:Jet it for a B8 plug ....

yeah, that's probably a good plan, given the way it is ridden

soullad wrote:What is the squish?

in a bid to keep the heat down, I fitted a thicker head gasket than I normally would, so it's up to about 1.7mm...

soullad wrote:Jet it for a B8 plug too Jarv (or NC3 OR 24 Nippon) esp if he intends to only do 1/2 throttle. check float valve and height too.
What is the squish?

yes good points,a 250 tap jet is a must...and a squish between 1.2 to 1.5mm...

byron wrote:

soullad wrote:Jet it for a B8 plug ....

yeah, that's probably a good plan, given the way it is ridden

soullad wrote:What is the squish?

in a bid to keep the heat down, I fitted a thicker head gasket than I normally would, so it's up to about 1.7mm...

NO NO....to keep the heat down ,you need a tighter squish........

1.7mm is on the point of big for a performace engine.

As Sean says 1.2-1.5mm is about right.

You could if all else fails and no air leaks are found, try a wide squish band and a deeper combustion chamber, machined to give the desired compression ratio, sometimes less compression is equal to more power.

The cylinderhead And piston rings are responsible for taking most of the heat away from the piston, which most ignore when trying to reduce heat issues, and instead play with carb jets or ignition timing usually in the end running over retarded or over jetted in an attempt to offset heat, but loose sustained power as a result.