Scooterotica

My scoot (a Lui Cl converted to a 125) suffers from excessive spit back, which means I get loads of unburnt petrol over the engine casing. What could be causing this and what can I do to stop it?

Its an SH1/20 carb with a fairly std Vega style air filter fitted.

port timing ?
Are you using the original cylinder ,piston, crank & rod, gasket etc ? have you had the cylinder ported ?
Sounds like there might be some unwanted overlap between your exhaust and inlet ports .
Tell us what work was done and what engine components were used.

Its a cento barrel bored out to 125 with a stock J125 piston. I did have some work done on the ports but it was a long time ago and although I did have a record of what I asked for I'll have to have a look around and see if I can find it. I do remember that it was on the conservative side of things (apparently).

I guess that if it is the port timing that is causing the problem then there is little that can be done other than starting again with a new barrel.

Unless you can get a reed valve to fit by adapting one of the aftermarket manifold?

It is possible to fit read valves to vega barrels at least, although obviously a bit of a pain.
Why would a read valve help - probably the answer is obvious to anyone who uses one - but I've never paid much attention

A reed valve basically operates as a one way valve, allowing the air/petrol mixture into the engine, but blocking it coming out (spit back). However if you fit one without modifying the engine other ways (boost port, windows in piston, longer inlet timing, longer exhaust timing) you are likely to see a reduction in performance, after all it is abloody great obstruction in the inlet path . A reed valve intake system is also generally recognised as allowing you to have a longer exhaust port timing without making the engine to "peaky".

Minority wrote: ↑Wed Mar 28, 2018 5:47 am A reed valve basically operates as a one way valve, allowing the air/petrol mixture into the engine, but blocking it coming out (spit back). However if you fit one without modifying the engine other ways (boost port, windows in piston, longer inlet timing, longer exhaust timing) you are likely to see a reduction in performance, after all it is abloody great obstruction in the inlet path . A reed valve intake system is also generally recognised as allowing you to have a longer exhaust port timing without making the engine to "peaky".

I think I could do a lot of that, not too sure about the boost port. That's an extra port in the barrel?

So I found what I asked for:

140 in, 160 out, 125 for transfers. So nothing excessive, and to an extent I left it up to the guy who did the work with the instruction that it shouldn't be so. I guess I will have to lift the barrel and measure what I actually have to be sure though. That will have to wait for the time being as its currently my only running scoot and access to the barrel means dropping the engine ( as I cant remove the engine cowling without doing so )

Those timings sound quite conservative and I wouldn't have thought would be a cause of excessive spit back.

Minority wrote: ↑Wed Mar 28, 2018 10:56 am Those timings sound quite conservative and I wouldn't have thought would be a cause of excessive spit back.

So, and I will check them at some point. Any other causes?

What sort of exhaust have you got on there? Expansions will cause spit back at certain revs.