Trials Australia Forums

Hi again,
Thanks to Austini letting me borrow bits off his recently refreshed machine (that looks absolutely fantastic), I could now problem solve my bike further, but still not to any conclusion, your help please.

I got hold of a flywheel puller, revealed points and condenser to be in good condition (looked fairly new), cleaned anyway. Replaced flywheel and checked gap ok. Have new plug also.

Using Austini's carby, it started not easily (with choke), but better than my carby, then off choke. When getting to run needed a few more revs to 3/4 to keep running. For about 30 secs its sounds ok, then starts to misfire and more so under more revs. (done this every time it starts)
Pull plug to reveal not too wet, but black coating from running. Not much smoke from exhaust, but a puff when misfires.

Used another HT coil, same prob.
Disconnected kill button also, still the same.

Having previously checked reeds and manifold for leaks, is it sucking air leaning mixture; fuel starvation; elec prob still; or the worse outcome - crankcase leak via base gasket or engine seals?????

Any tips here to prove the above further?

Thanks Roger

I think I may have answered this one myself, after a quick google I found this, it was the best article found that explained about a leak down test, many other sites refered to it, this explained it (even though its about personal water craft - still a 2 stroke)

http://www.groupk.com/airleak.html [/url][/code]

A failed conder can look fine but be doing nothing. I suggest you have a look at the ignition points while the motor is running to look for arcing. If you see arcing, the condenser has failed.

If you tested the timing with a timing light, you would also see the timing was going all over the place due to the failed condenser.

Air leaks usually affect the running at low throttle openings not high throttle openings so it doesn't sound like an air leak.

David, you are right on the money!! It was a crook condenser. Thanks for you help.

Couldnt get my hands on a new one, Yam shop dont seem to be very helpful so far, but thanks to Austini, he had an old DT175 engine laying about, and used the original looking condenser from that. Measured correctly at 0.3uF, where the car type one fitted was 0.4uF, leaving me with the common car one 0.22uF as a backup. The main fault was the wiring insulation burnt and shorting to casing due to a shorted lighting coil. It had an exposed terminal that caused the lighting coil to short then melt the wiring a little and bare it to casing. Now starts easy as after repaired.

Then after getting a good healthy spark, it was starting then not running well due to the previous owner fiddling with the carby, the needle needed raising and mixture adj, then ran fine. It seems lean under hard throttle, may have to go to bottom clip position, but main jet is a 23 not a 24 as original.

I did a Leak Down Check also. Made up some blanking flanges during the week and made up an attachment with an old sparkplug to fittings, hose and gauge and hand pump and valve etc. Pumped to 7psi, dropped approx 0.75 per min. Quite ok and happy knowing this now too. Engine sounded good, not rattly etc. Hard not knowing this when buying from unknown owner and wasnt running when I bought it and its a 30 year old bike!

Cheers, Roger