moby porting ?

so ive been working on porting my stock moby cylinder lately, and had a couple thoughts i wanted to run by you dudes.

First of all, attached is the portmap of the cylinder im working with... all the dimensions are +/- .5 mm. Its a stock 50cc jug i got from QK, i believe the ports have the same dimensions as the "fast" cylinders.

basically my question is whether or not itd be wise to widen the exhaust port at all. the wiki recipe calls for 2mm on each side of the intake, but the intake is already wider than the exhaust stock. i feel like widening the exhaust a bit to help it catch up could be beneficial. yes? no?

i already have the exhaust done and it went very well... didnt touch it with a dremel, just files, so it went slow and steady till it came out damn near perfect. im about to start the intake, and thats what got me thinking about widening the exhaust.

educate me.



Re: moby porting ?

I have found that if you get the exhaust walls parallel to each other, meaning equal widths on your exhaust window to the inside of the ring for the crush washer. thats about where you want to stop. any more and you get really close to the ring ends, looking at your measurement its already pretty close.maybe its not and just looks that ways cause its flat, makes any sense?

Re: moby porting ?

mark your ring ends on the top of you piston and stick that in there to see...

Re: moby porting ?

Man, are my English skillz and spelling in the toilet today!

Re: moby porting ?

oooooh ok i get what you mean. make the walls paralell... simple enough haha but it took me a couple reads to get it.

yea that makes sense... i havent actually even taken a good look at my ring end locations, i'll have to check that out.

is it safe to say i should widen as much as possible w/o conflicting with the ring ends?

thanks man.

Re: moby porting ?

I just ported my stock cylinder (but yet to run it) and I followed the blueprints in the wiki. I was in contact with some very knowledgeable French tuners and they all recommended following those instructions. See this page for some before and after shots of following the blueprint instructions.

Widening the exhaust is recommended for better gas flow and I assume allows it to rev higher.

Re: moby porting ?

cool man. i'll put up some before and after port maps once im done.

do you have a port map of your cylinder before you ported? my biggest thing is that the ports on all the stock cylinders are _not_ the same. so say you had the "slow" version, and ported it to the wiki specs... the end result would not be the same as if you ported the "fast" version. this is obvious, and the wiki specifies to start with the "fast" cylinder... i want to know _the_ definition of the fast cylinder in terms of port sizes.

i assume since the cylinder i bought was a repro, it is the fastest model they had... why reproduce a crappy cylinder? also Nick at QK told me it was... i just want to compare my stock port maps to other people's.

Re: moby porting ?

The 'fast' cylinder is not that fast at all...

I never calculated the port timings or mapped them but I assume it was a 25mph cylinder. It had an intake that was the same size as the one on my B speed motor, fitting the 9mm stubby intake perfectly (idk if the larger bendy intake had a larger opening, but on the before pic on that site, his looks MUCH larger then mine stock) and the transfer ports are the same shape and size. I snagged the cylinder from a pile at the kzoo 1977 moped fire sale and didn't know it's history.

The cylinder transfer ports appear to have similar width to the stock ports in the French article so they might be the same. I cant get any measurements from it now (all bolted together and not redoing that).

Talk to Jesse S. about porting out the moby to specific timings, that way you no longer have to talk about raising them X mm anymore just degrees. Still have to figure out optimum width another way...

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