EDIT - Help with timing please

[swiftb]

I bought a py90 yesterday which is a copy of a pw80 and uses a 4stroke honda c90 copy engine.
You may have seen it in my show & tell thread. I bought it with a running problem. It wouldnt idle very well, ran a bit rough and although it revs up fine from idle to 3/4 throttle, when it s on full throttle it starts a bizzare running problem as if its hitting a 'limiter'. Kind of goes like this BEWAH BEWAH BEWAH BEWAH when holding throttle fully open. Power kind of bogs down then comes back again in that rythmic BEWAH motion.
So it seems the guy who gave up on it has been trying various things and seems to have given the carb a once over so i headed straight for the valve clearances. There wasnt any clearance basically so have reset and running a lot nicer at tickover and it will idle now - but only when on the choke. Still has the full throttle issue.
I plan to have a tinker with the carb tomorrow. From my understanding of carbs (which i admit is a bit limited coming from an FI background) it seems like the carb could be running too lean, hence it will only run on choke and maybe its lacking fuel on full throttle??
Anybody with any good input and experience here would be a good help before I start messing with shit im unsure about
Was thinking of altering the needle height in a trial and error fashion. Would this be a good place to start?

cheers people

[swiftb]

Had some spare time today so got down to work on this bogging problem. So far I have :

Cleaned air filter and repaired airbox
Stripped and cleaned carb - moved needle height to richer setting
swapped exhaust
adjusted valve clearance

However the bike seems to be burning a lot of oil. Doesn't smoke out of the exhaust much on tickover but under load its pumping a fair bit of whiteish/blueish smoke out and stinks. Also after running for a while at tickover started to escape from crankcase and when I removed the oil filler its coming out of there. So its looking like rings are on the way.

One more thing I decided to check was the valve timing - which SEEMED to be out. But I need some reassurance
Basically it was timed up with the 'F' mark on the flywheel lined up with the notch in the crankcase and the mark on cam sprocket was 180degrees out. Now as far as I know the F is for 'fully advanced' and is the point at which the spark is emitted. The 'T' stands for Top dead centre and this is the mark that should be used to time up with.

So I have reset the timing to look like this:
https://i441.photobucket.com/albums/qq135/photobucca/chinktiming.jpg

Im on the understanding that this motor uses a wasted spark ignition so it does not matter that the cam is 180 out. But obviously timed up as it was (with the F mark) it was a tooth or so out.

So timed up as in my lovely diagram it again runs sweet as a nut on the choke (still cuts out with no choke) And it now seems to rev a lot cleaner and the BEWAH bogging seems to be much improved. Still spplutters and runs a bit shitty under load but a definite improvement it seems. I think it inevitably needs the rings doing so that will prob be next.

Although first up i should really reset the valve clearances again and reset the needle to middle setting.

Im looking for some confirmation that I have got this thing timed up correctly - dont have much experience with this style of engine so anyone who knows there shit gets a cookie

[Kickstart]

Hi

I would say that it sounds like you have set the timing correctly.

If the ignition pickup is on the crank it will be wasted spark. If it is on the cam then it is probably a single spark at the right time.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[swiftb]

Thanks Keith, thats put my mind at rest. Can crack on investigating the oil burning now. Cookie on its way, enjoy!