Scooter Running Problem

[regsmyth]

I know this isn't a scooter forum, but I'm sure there are plenty of expert mechanics around here.
I have a Chinese 50cc 2 stroke scooter. It's about 3yrs old and done around 3000 miles.
I replaced the cylinder and piston recently, following a seizure after it had been running really well, and afterwards, although it has been ticking over reasonably well (missing a bit), it's got no power when you turn the throttle and dies.

I think it's an electrical problem.
The carb is clean, the petrol vacuum valve works.

I bought a new spark plug, and although I get a spark from the old one, when I put the new one in - no spark. How can that be?

The ignition is a cdi thing, with coils and a magnet on the flywheel. The ignition coil is a sealed unit. I don't want to randomly start swapping bits until I have more of an idea of what the problem might be. Any suggestions anyone?

Thanks

 

[Burnerman]

Even Chinese cdi units are reasonable, but their electrical connectors aren't - so spray some contact cleaner into the sockets and protect from the wet. Disconnect any cutouts that you can.
Check for your spark in the dark - chances are it is there, just very small and hard to see.
Check the coil connections haven't been reversed.
When you replaced the piston and barrel, did you put the arrow on the piston towards the exhaust port, do you remember?
John

 

[Mursal]

Definitely not an expert, on anything, not even scooters ..............

So it starts, but wont rev up?

Usually those CDi units stop working, that's it. Or they will work for a few minutes and then stop, unlikely its stopping it from revving up.

What way is the cold start (choke) its not stuck on?

 

[Burnerman]

A couple of other things to do......run the engine without the air cleaner, and then try it without the exhaust - just in case its choked a bit.
John

 

[Dave54]

Had almost exactly the same problem with a Kohler engine on a ride on mower a couple of years ago. It was quite mysterious, it'd run alright for a bit, then lose power, stop, and be difficult to restart. There was a spark there, which fooled me for a bit. I reckon it was too weak.

It was the ignition unit. Check the other stuff suggested here first though.

 

[regsmyth]

Thanks for your thoughts.
What I'm sort of stuck on is the spark.
When I connect the old plug and lay it on the cylinder, I get a spark, visible in daylight.
When I put the new one in, I get nothing. What could make this happen?



It doesn't have a manual choke. Is that the tall cylindrical thing next to the fuel inlet?

Yes I put the piston in properly. I've had it ticking over quite a few times now.

It's all dry, been in a greenhouse for the last few weeks.

All the connections seem ok and clean.

Have done it without air filter and without exhaust, and it ran same - a bit lumpy, but ticking over.

With a little throttle, nothing happens for a few secs, then the engine picks up and gets faster all by itself.

I think whatever it is is getting worse. A couple of weeks ago, I could ride it up a small incline for 20metres, then coast back down and do it again - several times. But when I got onto the road, and gave it a bit more welly, it just died. Started again straight away, but did the same thing.

I don't think it would even do that now.

In fact I dont think it's even going to start now.
I'm going to see if it will now.

 

[Burnerman]

I think its fuel starvation Reg - how do you feel about stripping the carb down?
Your symptoms are typical of weak mixture....revving up by itself, no power etc.
Try another plug....dud new plugs are very rare but possible!
For the life of me I can't recall how these auto chokes work - maybe a close up of the carb on the bench would jog my memory!
John

 

[Dave54]

As Burnerman says, not totally unknown for new plug to be duff. rare though. Have you set the gap?
That Kohler was exactly the same. I stripped the carb on that and blew it out, thinking it was that. Certainly can't hurt to do everything else you can before spending money.
There was a spark there on that too. I can only assume it was either not enough voltage to spark under pressure, or the spark was coming at the wrong time.

 

[regsmyth]

Thanks John - yes it sounds and feels like fuel starvation. But I don't think it is. I've taken the carb apart a couple of times. It's as clean as a whistle, the float works, the needles are clean....

I've also tried two new spark plugs - neither of them get a spark, but the old one does. Surely the problem must be something to do with that?

I'm tempted to replace the ignition coil, but the spark issue doesn't really make sense to me.

I'd be tempted to say the timings out, but it seems on these cdi type ignitions that there's no adjustments to make, and anyway, it was running fine till it seized.

Maybe the auto choke?

Can't start it at all now anyway.

Thanks for the suggestions.

 

[Belle427]

Id double check all the connections to and from the cdi unit including earths etc.

 

[Mursal]

Put the old and new plugs side by side, any difference?

Like the old one having a top and the new one having only threads?

Just asking, because its an easy mistake, when things are not going well.

Timing is preset, unless there is something fouling the pickup sensor, or as already mentioned, you have a bad connection?

Those CDi boxes give a bit of trouble, but double check everything else first.

 

[regsmyth]

Thanks for all the ideas. I'm not ignoring you I just can't muster up the enthusiasm to mess about with it any further at the moment.

But I'm still looking for ideas as to why the new plug won't spark but the old one will (yes it's identical, yes i've tried two new ones)
Also why it's "missing" while ticking over.
And why it dies under load.
Maybe crankcase seals are leaking? How would I check whether that's possible

And now I don't seem to be able to start it at all.

Hey ho.

 

[Dave54]

Digging back in memory for days of 2 stroke bikes. I doubt if the crank seals have gone on a 3 year old / 3000 mile bike unless you have either run it without oil (and that would depend to some extent how the main bearings are lubed on that particular engine) or have left it standing for some considerable time without running it.
You can pressure or vacuum test the crankcase, to check the integrity of the seals but you need the equipment to do that.
If my memory is right engines with poor seals will usually start, but don't run well.
Have you tried fresh fuel?
Are you using the right oil?
Long shot with a modern 2 stroke, but if the plug isn't firing after it's been in a running engine it could be "whiskered" Run a feeler through the gap to make sure.

 

[Burnerman]

I'd agree with Dave regarding the crank seals....naturally they are vital for primary compression but the symtom of their failure is that the engine will start but its impossible to get a reliable tickover - its fine one moment, hopeless the next......it doesn't cause a misfire.
I don't understand the bit about the new plug not sparking, but the old one does.....I don't follow that one but if you can be bothered, try altering the gap of one of the new ones......wider in particular.
I hate to say it but I guess its an igniter issue, if you've ruled out the kill switch connections and the like.
John

 

[Dave54]

I think you're right John.

Three things you need for an engine to run, compression, fuel and spark. Compression should be obvious I'd have thought from the way it turns over.
Fuel. If the engine isn't firing, does the plug get wet at all?
Spark, as already said you should have a spark at the plug. Otherwise it's one of the parts of the ignition system, or something on the switch side. Could be cutout, ignition switch, or a safety interlock fitted somewhere.