Scooter engine dies under throttle.

[Taz]

Started Uni this year so decided to drop a few thousand cc from the daily driver and use something a bit smaller.

I ended up getting a Yamaha QT50. Theory is no ones ever going to want to steal it.

Anyway, wasn't running when I got it so pulled it to bits, new piston, rings, spark plug, cleaned the tank, cleaned the carb etc.

Now it goes, starts easily, but only for a few minutes then it dies when you throttle on (doesn't die when you throttle on for the first 3 mins or so)

Anyone familiar with these have any specific ideas?

I had left the air box off in my excitement and thought it might be that but does the same with it on.

Cheers, Taz.

[wopass]

running on the choke when its cold then when it comes off its not getting fuel, clean carb again, check main jet and filter in tank and vac lines on carb are in the right places

[Taz]

There is no filter :S

Thats on the list of things to do next.

Even if the choke is left on it does this and it idles nicely if you take the choke off till the 3 minutes or so is up.

Theres only one line coming off the carb that would be vac, and thats on. Might replace that just in case. Only other bits coming off it are choke, throttle, fuel and breather.

Wish Nissan made scooters haha.

Oh and the carb just went back on. I might as well check it again to be sure but there was only sediment right down the bottom by the drain and I blew out the jet a few times even though nothing was visible in there. Nothing was around the float or needle.

[Scott]

Check the exhust valve rocker for enough clearance, as the engine heats up if there's not enough clearance the exhust valve dosen't shut - no compression motor stops?.

[Moriarty]

Check the exhust valve rocker for enough clearance, as the engine heats up if there's not enough clearance the exhust valve dosen't shut - no compression motor stops?.

its a super blue smoke. Exhaust valve does double duty as a piston....

there is a wee filter built into the fuel cock, check that.
The fuel cock also has a vac diaphragm, shuts off when engine vac stops.
TINY wee main jets, it only takes ONE small bubble of water to kill the QT.
check for bits of rubber in fuel? dying vac diaphragm?

Sounds like that is the prob, it WILL run for 3 mins or so on one bowl full of fuel, leads me back to the vac system and that diapragm in the fuel cock.

[Taz]

Check the exhust valve rocker for enough clearance, as the engine heats up if there's not enough clearance the exhust valve dosen't shut - no compression motor stops?.

its a super blue smoke. Exhaust valve does double duty as a piston....

there is a wee filter built into the fuel cock, check that.
The fuel cock also has a vac diaphragm, shuts off when engine vac stops.
TINY wee main jets, it only takes ONE small bubble of water to kill the QT.
check for bits of rubber in fuel? dying vac diaphragm?

Sounds like that is the prob, it WILL run for 3 mins or so on one bowl full of fuel, leads me back to the vac system and that diapragm in the fuel cock.

Yeah I didn't remember there being anything between the cylinder head and where the exhaust bolts on. These things are pretty primitive.

No blue smoke, bit of white but nothing to choke on (guess that confirms not enough fuel as I just noticed it doesn't do it on start up.). Will replace the vac line this weekend and double check the bowl and look at the fuel cock. It must be the worlds smallest filter.

I just had a wee nosey now and there is a bit of fuel on it (more like condensation than dripping) so you may have hit the mark there Bob. Did one of your offspring have one of these?

Should be good once its going. You can easily 'hand' start it with the kick peddle so even the better half can use it.

Appreciate the ideas people.

[Moriarty]

I was parts manager for three years at a Yamaha agency when these things wuz new!!

Its a two stroke.
Yams of that age ran well on castrol super outboard.
Yup, easy start, 45 kph.

We made one go over 75 kph.
Pore little thing cos it had my 115 kilos strapped on the top of it.


first check for a faulty vac auto cutoff in the fuel cock is to turn it to reserve. this over-rides the vac system.
Its true for all yamahas

go HERE

http://www.mopedriders.org/article_view.php?faq=2&fldAuto=45

and download the workshop manual as a pdf.

[Scott]

Is it a 2 or 4 stroke?

[Moriarty]

Is it a 2 or 4 stroke?

2-Stroke.
a bit like a schoolboy.

[DieselBoy]

I can say with 100% confidence that once yo put the airfilter in, it will run fine.

2strokes, especially trhe really small ones are suoper sensitive to mixutre, and with out the air cleaner, it will be running supre lean and die as you open the throttle.

[Taz]

I was parts manager for three years at a Yamaha agency when these things wuz new!!

Its a two stroke.
Yams of that age ran well on castrol super outboard.
Yup, easy start, 45 kph.

We made one go over 75 kph.
Pore little thing cos it had my 115 kilos strapped on the top of it.


first check for a faulty vac auto cutoff in the fuel cock is to turn it to reserve. this over-rides the vac system.
Its true for all yamahas

go HERE

http://www.mopedriders.org/article_view.php?faq=2&fldAuto=45

and download the workshop manual as a pdf.

Must of been a steep hill you rode down!

I'd be happy with 55 to keep up with traffic but I don't think I'll even manage that somehow.

Put a fuel filter between the tank and carb so just have to find the jerry can and try starting it in the reserve position like you said and let you know how it gets on.

Will grab some of that oil to.

Cheers.

[Taz]

IT ALIVE!

Mawhahahaha.

Something stupid, as always, was what was tripping it up. Stowaways in the fuel line. I cut it up a bit to put a fuel filter in and found the unwanteds so pretty happy about that.

I haven't been on a bike/tube with an engine attached for a while so it seems like its going pretty quick, but it probably isn't haha. Will sort the speedo out and find out.

Shes a little smokey still so a bit more playing with the carb to do I think and some fresh oils to.

Cheers for the help, Taz.

[Moriarty]

94


Two stokes are weird things to tune. dont try premixing it, if the auto oiler is working, stick with that, iuts set for about 45 or 50 to 1 ratio and is designed to pollute furiously!!!! if at full throttle, it starts to fourstroke, an unmistakeable sound, its running a tad lean., lift the main needle UP one groove. Aircleaner is a small foam device, wash in petrol, dry and VERY LIGHTLY oil, to much oil will enrich the mixture and make for truly horrible running. A port and polish helps lots especially the exhaust ports, unburnt oils collect in the exhaust pipe and turn to tars etc in the silencing gauze and baffles, and kill performance quicker than a cold shower!!!

keep us posted!!

B

[Taz]

Well shes running all good now. About to pull the exhaust off, attach a potato to the end and fill with kero to clean it out a bit. Flatmate LOVES it when I leave this stuff under her window out the front haha.

The throttle cable is the latest to go due to the last fellow leaving it able to spin in the wind so that's another bit of coin into it.

Thanks for the help, Taz.

[DieselBoy]

94


Two stokes are weird things to tune. dont try premixing it, if the auto oiler is working, stick with that, iuts set for about 45 or 50 to 1 ratio and is designed to pollute furiously!!!! if at full throttle, it starts to fourstroke, an unmistakeable sound, its running a tad lean., lift the main needle UP one groove. Aircleaner is a small foam device, wash in petrol, dry and VERY LIGHTLY oil, to much oil will enrich the mixture and make for truly horrible running. A port and polish helps lots especially the exhaust ports, unburnt oils collect in the exhaust pipe and turn to tars etc in the silencing gauze and baffles, and kill performance quicker than a cold shower!!!

keep us posted!!

B

Four stroking means its rich. Youy move the clip on the needle up on clip to drop the needle further down, leaning the mixture.

If the mixture is lean, the engine will rev smoothly and very high, but will lack power and die. Also runs the risk of heat seizing.