I think im running lean. Photos enclosed

[edwardtennant]

Hey again, i am having some issues with my 139qmb based scoot with a 72 kit installed. Here in the UK it has just become winter and now I am having some issues. My bike will start first time every time and will idle all day long, no hunting or anything. However a few days ago, my engine has lost its powerband, and no longer has power in mid to high RPM ranges. Before no matter how steep the hill was (within reason of course) the bike would hold at 40mph. This seemed to be where it was making peak torque. Now though it doesnt really have a powerband and just doesnt pull as well as it did before. The bike also doesnt go as fast flat out either. Before i could sit comfortably at 50mph on a flat road at 3/4 throttle. Now i am lucky to even get to 50 down a hill. I pulled the plug and this is what I found. imgur.com/a/0cpdP . This was done while the engine was warm, then whilst WOT, the ignition was killed and plug was pulled. It seems rather white, which makes me think im lean causing the loss of power, right? (Colder air is denser so more o2 in air, so i need up richen up dont i). I have a CV Carb and am running 85 Main jet, which screw on the carb is for the air/fuel ratio? and which way is richer and which is leaner?. is it possible I have caused severe damage to my engine? I have a borescope and took a photo a few weeks before. Here it is. imgur.com/a/gPA4h.

This is my full setup

139QMB Engine with 72CC BBK (Has around 5600mi on it)
Stock variator with 5.5g rollers
Stock clutch,bell, and springs
Bando Belt
High Torque Starter motor
NGK Iridium Plug
Technigas Silent Pro Exhaust
Running 10w30 Fully synthetic, changed every 1000mi
Stock airbox + Stock filter.


This is what I plan to do to resolve the issue.

Check for air leaks
check valve lash
change filters
clean carb 
adjust air fuel mix
rejet if needed
adjust carb needle


Anything else ive missed? Last time i Comp. tested i got around 150-180PSI (this was a few weeks ago so i cant really remember)

[artman4life]

i would replace the plug with a new ngk. that plug looks worn out

[GrumpyUnk]

Quote:
'However a few days ago, my engine has lost its powerband, and no longer has power in mid to high RPM ranges.'

Whatever happened was relatively quick. If you can accelerate as before, up to a point, I think I'd look at the fuel delivery system. A partially plugged fuel filter can limit flow, as could a failing cutoff valve(if you have a vacuum operated valve), or some gunk in the carburetor partially plugging a jet.
Easiest check would be to apply vacuum to the valve, and check fuel flow from the tank into an appropriate container with proper safety - no sparks, smoking, lit matches or candles you are using to search for a gas leak...
Might check the air filter for good flow also. In most cases IC engines being air pumps, are subject to restrictions in their air flow and fuel flow. If you still have good compression, the fuel supply is the likeliest area to examine.
tom

[edwardtennant]

Hi Grumpy, Thanks for the reply, I am going to put new filters in and clean the carb tonight. If the air filter was clogged, woulnt it make it richer, not leaner though?. ill clean the filter anyway, and see what its like.

[uglo]

Plug doesn't look terrible to me. Colder weather demands a rejet sometimes..

Sound a to me like CVT issues. Check your rollers for flat spots.. Check your clutch torque driver for work grooves.

You have a lot of miles on it with stock CVT parts... definitely time to replace some things. If you notice your acceleration is better than it has been but your top speed is terrible.. you need a new belt.

Belt.. rollers.. variator and clutch. Check them!

[edwardtennant]

Thanks for replying everyone. I found the issue wilst cleaning the carb. The clip on the needle had come off so the needle was on its leanest setting. Put it to second from bottom and its all good.