Has anyone seen this before...

When I start my newly acquired 198 Sherpa 250 (pig to start) the throttle sticks on three quarter throttle until the engine warms up, then it's fine - ticks over nicely. I decided to investigate and noticed the carb slide on the intake side has a circle of sticky black goo on it (presumably becomes less sticky when warm). I then went further and removed the head and barrel and the same sticky black goo is coating the intake side of the piston. The bike has a tank of fresh fuel mixed at 40:1, so my question is 'what is causing this build-up of oily goo?'

Bike has good bore and piston and has been converted to electronic ignition.

Any advice appreciated.

The black goo on piston and throttle slide sounds like residue from incomplete combustion to me. If you clean it off the carby slide and it comes back with use, the piston rings are not sealing well.

The lack of ring sealing may self-correct if you get the motor nice and hot in use. I have seen plenty of two stroke trials bikes that suffer from the rings being stuck in the grooves, which prevents them moving as required to seal against the bore. It usually happens on bikes that are only used gently and don't get hot enough to burn the goo out of the ring grooves (and other places).

Riding up a long hill (a continuous minute or so at half to full throttle) should clean it out

Hi: 40:1 is a 2,5%, I think it is too much oil for this bike using a modern sinthetic oil.

In my 198 Sherpa I normally use 60:1 (1,5%) or sometimes 50:1 (2%) when I am riding in a very hot day.

Hope this helps

The rings are getting stuck and are not moving freely, I've also noticed when I look through the inlet port that the cylinder liner has quite a large lip showing around the edges of the opening - that can't be helping the flow of fuel!

When it's cleaned up and all back together I'll try to use it more aggressively to stop the build up and mix the fuel a bit leaner.

Thanks for the advice.

Edited September 9, 201015 yr by BagONails

Something else i have come across is old premix oil in the fuel tank becoming reconstituted with the fresh fuel and gumming up the works. I had to pull the engine , fuel tank and carb and flush it all with solvent and it finally went away over time. This was on a Penton that had been sitting and the gummy stuff looked like cosmolene and really made things sticky. If you keep having peoblems you might want to look into that as it sounds familiar. Depends also on how much the bike has been sitting but just something to look for.

Hopefully it is what you are looking into and will be an easy fix...

Sam

Is that a fiberglass tank?

Is that a fiberglass tank?

It's ali - on close inspection it appears to have some sort of tank seal lining that is breaking up and has a sticky residue on it - maybe this is the cause??.