My Sons diesel 2 stroke trials bike???

[jimbo]

My Son (age 9) has a Beta 50 Trials bike. At the last event the tick over started to rise and then about 3 mins later the bike started to Rev its heart out. I hit the kills switch, but it had no effect. I then pulled the HT lead off the plug, but it made no difference the bike was still screaming!
Turning off the fuel and lying the bike on its side eventually did the trick and the beast was silenced.
10 minute later after a brief check over, all looked find and the throttle cable wasn't snagged - I re-started it and the bike ran fine from then on.

So WTF is going on. I can only assume that some hot carbon in the chamber is allowing the thing to Diesel rather than otto - although I had the head off not long ago and it wasn't bad in there.

I plan to rebuild the carb on the assumption that something stuck or jambed. Not too sure what actually happened. stuck throttle, excess fuel just pouring in?
Anybody see this before? I'd rather like to sort it did rather spook my boy!

Cheers,
Jimbo

 

[turbine_2]

Okay, one failure mode I know about with diesel engines (usually turbo ones when the turbo oil seal fails) is injestion of oil into the airway, which works as fuel even when the fuel is shut off and causes just what you're talking about. Could it have injested engine oil somehow?

 

[ChasMill]

My Discovery had a cracked cylinder head (a crowd-pleasing £2,500 repair). The "mechanic" at Lookers Colchester said that could have caused what you describe.

 

[brassmonkey001]

Air leak on the carb, probably at the top.

 

[Shep]

I had the same on an old Greeves trials iron I used to ride, it turned out that it was seemingly pumping oil through a failed joint in the crankcase, we reckoned it could have raised the compression enough for the bike to 'diesel' a full engine rebuild was the only cure then.

Mine became evident when the bike was getting unmanagable in a section I pulled the clutch in and away it went all on its own.

2 stroke sh1t

Shep

 

[(RIP) Proff]

Bloody Betas

2 things.
Did you pull the pluglead right off and away from the plug?, as the sparks can travel up to 3" especially down wet plug cap.

Secondly
Beta killswitchs are notorious for not breaking contact. you switch off and the switch stays connected as though magnetised till you actually stop the bugger.

If it happens again, put choke on and stuff rag down inlet pipe.
at least the rich mixture ensures oily bits are being seen to

I am presuming your Beta runs on premix and not the seperate oil tank

 

[Keith Chapman]

Log on to /www.trialscentral.com there is bound to be some help in the forums there. The forums are in the community section, there is a Beta specific area too.

 

[Mzokk]

Needs a decoke.

 

[Brian E]

Intake rubber (Carb to Cylinder) split or not sealing
Crankcase seals worn or not sealing

either way easiest way to check run bike and use a flamible spray WD40 is good and spray onto intake rubber if revs rise problem is there spray onto crankcase seals if rev's alter problem is there.

 

[KAYCEE]

A similar thing happened to a mates Suzuki GT380 (two stroke triple) many,many moons ago. We had been a decent run and the bike was running quite hot. We stopped at a set of lights and his bike proceeded to rev its nuts off. We turned the ignition off, pulled the plug leads off, turned the fuel off and still it rev'd. To get the bike to stop we pushed the front wheel against a wall, stuck the bike in 5th and let the clutch out and stalled the engine.

We still never found out what caused the problem, My mate kept the bike for a few years after with no recurrance. We always assumed it was just because the engine had been running really hot on that particular day.

Kaycee

 

[LiquidLAN]

I hit the kills switch, but it had no effect. I then pulled the HT lead off the plug, but it made no difference the bike was still screaming!

This is a petrol two stroke right? if you've killed the ignition and then killed it again by whipping the HT lead off there is no spark therefor it can't run... if its still running you have to have another ignition source keeping the think going... the only thing i've ever known to do that is carbon build up in the cylinder

next time you have the head off clean it and the piston and you should be fine

Reading the other suggestions about it happening and not reoccuring, I guess there is a chance that you've burn up the offending bit of carbon enough that is no longer provides the heat for further ignition

 

[Hew]

As well as a decoke check that the fins on the barrel are clear. If these are blocked it might be why it was getting too hot in the first place.

 

[davegs]

Diesel

Try a different /new spark plug this is mentioned in old villiers manuals.

 

[jimbo]

pulled it all apart last night. There is something to be said for a 50cc 2 stroke, only a few bolts and its all apart. I've de-coked it, cleaned and lubed the carb. Hopefully all will be well from now on.....

 

[HP2 KENOBE]

Two things not mentioned but might be a help for all two stroke engines.

mabe take the bike for a hard spin yourself every now and then to de coke it the lazy way ..
(trials bikes tend to do alot of ticking over causing corbon build up)

and make sure you use the correct mix ratio of good quality 2t oil/petrol to prevent cokeing of the combustion chamber,,,,,,,,

 

[Mini.Hall]

This is probely sorted now. But I used to ride these bikes when younger and sounds like possibly the float could of stuck, or the throttle has got sh*t in (take off the bars and oil the bar where the throttle sits)

Hope that helps