Yamaha Tiara Running Less Than Perfect..

[ClareQuilty]

I have two Tiaras, both as far as I know with the same 120cc two-stroke one finds in all Tiaras, one happens to run fantastic with loads of power and very 'revvy', as in it eagerly revs from take off. The other Tiara was a fixer-upper, and I had the engine rebuilt, which did improve the running greatly, and it ran nicely, good idle, but didn't really have much power. As a friend said, 'it feels like a four stroke'. It just didn't seem to want to rev up much at all, with no higher end power.

The mechanic said it was an 'oversize piston', so I guess it had been bored out or something before. What could be causing this poor power? Could it be the carburetor? Could the fact that the engine may have been bored-out previously mean that the carb or 'jets' should be different?

[Lickey]

Useual thing with 2 strokes that are 4 stroking is a breathing problem, no need to change carb jets if a normal +10/20 re-bore, easy stuff is air filter and exhaust, to clean out exhaust, remove from bike, block end and stand on end, fill with a strong solution of caustic soda, leave about 1 hour then hit exhaust with a flat piece of wood, remove bung and it should remove all the carbon, a quicker way is put some petrol in, light this and stand back, it will crack and spit a lot, once its calmed down, hit with wood, again, all the crap will fall out, if this doesnt help, then its time to look at carb, crank seals ect..

[ClareQuilty]

Thanks Lickey, that sounds like a good theory, and though it is beyond my ability I'll try to get the mechanic to do that soon.

Reminds me of when I first bought a little Honda Smile a year or two ago - it had only ever been driven very slowly put-putting around the town, and there was a lot of black gunk around the back of the exhaust pipe. Well as soon as I got it I drove it about 80 kph for about 16 kilometers on a warm night, and a when I stopped I noticed there was a lot of smoke coming from the tailpipe even after I turned it off - I think it was all the gunk burning in there.

Btw, when one takes off the exhaust pipe, do you have to cut the pipe and re-attach it by soldering?

Edited January 17, 2012 by ClareQuilty

[Lickey]

Remove it as a unit, from the 2 bolt flange or screw type from the cylinder barrell, then clean as suggested, if as you say the engine has been overhauled recently, the securing bolts should undo easily,

[VocalNeal]

First as it is easiest replace the spark plug with the correct one or for a test use the one out of the one that runs OK.

No improvement?

Take the air filter out and go for a ride.

No improvement?

Clean out the exhaust.

CQ,

With all your project bikes maybe buy a few basic tools and try a little bit yourself. Whatever you do won't be expensive and the DIY reward is OK. Of course maybe fishing or reading is more your thing but...

[Kwasaki]

First as it is easiest replace the spark plug with the correct one or for a test use the one out of the one that runs OK.

No improvement?

Take the air filter out and go for a ride.

No improvement?

Clean out the exhaust.

CQ,

With all your project bikes maybe buy a few basic tools and try a little bit yourself. Whatever you do won't be expensive and the DIY reward is OK. Of course maybe fishing or reading is more your thing but...

Right on, try getting a compression test as well.

[ClareQuilty]

Thanks guys. I have yet to try the cleaning the exhaust pipe solution, but I intend to. As for the air filter or compression, or the spark plug, I'm 90% sure he did all that when he rebuilt the engine - for sure he always puts a new air filter on when he gets a fixer-upper of mine, and I usually ask him to put a new spark plug. As for the compression, unless he did the rebuild badly it should be OK.

VocalNeal, you're probably right it would do me good as far as understanding to work on the bikes a little bit more myself, but its so cheap to go to the repairmen here I haven't been motivated. The real hurdle is communication - and my Thai keeps getting gradually better, though its still pretty poor.. such is the case when one is a teacher and surrounded by English speakers most of the time.

[MaiChai]

2strokes are very sensitive to their exhaust (as Lickey mentioned). If you think its this, put the exhaust on the good one on the duff one.

Compression test: remove the spark plug, put your thumb over the plug hole and kick it; if you thumb blows off, compression is ok.

Could be spark plug, dirty carb, broken reed valves, air cleaner?

Since you have a good bike, you can swap bits around until to find the part that is causing the problem?

I once had a 2stroke that had a worn cylinder; it would run for a while and it would die. Basically it was smothered in oil it sucked up from the crankcase such that it would not spark. Solution was a cylinder rebore and slightly larger piston (cost me about 600 baht for the rebore and piston; I did all the other work myself). Cylinders wear at the bottom of the cylinder.

Just remebered: I had another 2stroke that died like you speak of. Turned out to be a faulty spark HT coil. It was a tricky one to track down!

Edited January 19, 2012 by MaiChai