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stainlesscycle 12-07-2012 03:57 AM

jawa 05 scooter lacks power.
so i picked up this mid 60's jawa 05 50cc scooter (i couldn't pass - looks like a mini cezeta to me) it had sat a long time, motor was very mildly stuck - i pulled head - tapped on piston mildly, and it was unstuck.


it starts easy, idles good, pulls nice in first gear. but that's it. riding along and shift to second and it loses power, revs drop and eventually stalls. i can downshift to first and it'll recover and rev/run like it should. it's only 3hp or so - and i weigh 190lbs - so i don't know if i'm being optimistic as it should pull me up a slight hill in 2nd/3rd (it doesn't now)...

so i set the points (didn't adjust timing), made sure the exhaust wasn't plugged, cleaned carb, new plug. no love. same issues.

i'm guessing either the rings are stuck in the lands, the rings are wore, bore is outta spec, or timing is off. there's no air leaks, and compression is so-so......

it also could be - wet side crank seal (that's grasping at straws..) does not smoke excessively, jetting, but i doubt it - i tried moving the needle around, and thoroughly cleaned jets, - it's a jikov carb, so the adjustability is almost nil..... could be worn slide/needle maybe? but it seems like it's not carb related, in 1st gear it pulls fine at all throttle positions. in 2nd gear no throttle position pulls at all. could be sprocket ratio (there are 5 different front sprockets available - maybe dpo geared it so high it wouldn't pull anything but first?)

i'm gonna check the timing and give it a try - otherwise it's a tear down and wait forever for parts - all the motor parts are available for it, it's just a long slow shipping process from eastern european countries..

anybody have any other ideas?

stainlesscycle 12-07-2012 10:32 AM

well timing was way off, but that did not help. plug reads good, it's gotta be compression issue, too high gearing, or dunno. when i shift to second/third it drags the motor way down - it has a clutch, it's fine in those gears when i have clutch pulled, so it's not the trans.

stainlesscycle 12-08-2012 02:19 AM

i think i have part of the problem figured out.

RE: timing
this is what one manual says: (and the spec i used)

"At this moment the piston should be 1,2 to1,5 mm
(7/64") before TDC"

another manual says

"ignition point 2.7 (7/64in) to 3mm (1/8in) before top dead centre"

soooo...... the only consistency is 7/64 which is 2.7mm - which is way more normal...

i was kinda wondering that .047" (1.2mm) btdc seemed way weird to me. 90% of 2 strokes i've done are timed .090" - .110" or so..

nsu max 01-05-2013 11:11 PM

Have you tried decarbonizing the top end especially the exhaust port and you may have to decarbonize the exhaust. Oh, pictures please.

stainlesscycle 01-06-2013 12:24 AM

yeah i did all that.

timing adustement fixed some of it. maybe i'm just being optimistic about what a 50cc scooter should do.

lnewqban 01-08-2013 06:46 PM

Many, many years ago, I owned one of those little Jawa's.

They reach 45 ~ 50 km/h (28 ~ 31 mph) even carrying two adults.

Up-hills were not problem in second or first gear.

I would pressure test the crankcase, those crankshaft seals are probably too hard to prevent leaks (worsening performance at low rpm-wide open throttle).

I would verify the transmission for all gears by turning it by hand.

These little engines are more sensitive to poor rings-cylinder seal that to off-timing.

Best !!

stainlesscycle 01-09-2013 03:52 AM

maybe a leakdown test is in order. might be easier to just replace seals than to leakdown test it though :D

lnewqban 01-09-2013 01:18 PM

Check out this way to do it:

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