180cc running then overheating and dying.

[dazza1974]

Hi all, I've just put in a new 180cc bore kit into my old 125 direct bikes Scooter. I had trouble yesterday getting it started and after a lot of head scratching figured out I'd timed it up wrong. So, I get the bike running today only for it to shut off after about a minute or 2 of running. It starts up lovely and runs real nice, sounding lovely with an aftermarket exhaust on it. You can just hear a slight misfire every now and then but it will suddenly become tappetty almost and starts knocking then dies. I'm thinking it may be overheating due to the timing being out slightly.....am I right?

[bull]

Since that brand is a UK marketed, did not know much about them. Seems like it has a Chinese db152qmi engine. Here is the link I used and viewed the specs pg:
http://scooter.co.uk/125cc-scooter-spyder.html

The noise and running you are experiencing would indicate that somehow you have reduced or eliminated oil flow / pressure to the engine-- but that is strictly a WAG-- if the exhaust is loud enough it might be masking the noise until it shuts down.

Could you remove the cam cover, start the scooter and check to see if oil is flowing to the head?

[dazza1974]

I checked that as the rocker cover seemed to get hot quickly. Opened up the rocker cover, started it up and oil flew out so it's definitely getting oil. A mechanic suggested gapping the valves at 0.008 inlet and 0.016 exhaust and change the plug. I did this and it ran for a little while longer all be it a little noisy obviously. Still stumped.

[dazza1974]

http://www.scooter.co.uk/125cc-cobra-scooter-p-45.html This is my one.

[bull]

To deal with your assumptions of overheating:

did you correctly reinstall all the cooling shields around the engine? Did you remove or alter the cooling fan?

please reset the valves to a more correct setting, a guess is around 0.005"-- you could check with the seller for exact specs and maybe a manual- if you don't have one

the short amount of run time, cold to 2 min, suggests that the scoot might be running lean-the 2 minutes is about enough time for the auto choke to cease operations. rereading what you have done-- increased engine displacement by 50%, and new exhaust (freer breathing??) -- both of which demand more petrol

you did not say whether you had increased the main jet size or decreased the A/F ratio

also, when you installed the new exhaust, did you get a good seal of the pipe flange to head? if not, more petrol required.

might consider reinstalling old exhaust and check changes.

if it is really running lean, that translates to more heat.-- think blowtorch

[dazza1974]

For the time being I have taken the 180 bore kit off and bought a new 125cc piston and rings and put the old block back on. In answer to your question I upped the jet to 125 from 110 and had a k&n style cone filter on it. I'd say your probably right but I don't have the time to play with it as I need it for commuting but I will bear your comments in mind for when I rebuild my other engine that is kaput.