Help! My moped died and I can't get it to start

[x8rowner]

My Honda X8R 50cc moped died on me on a very hot day after a 15 mile journey. I was heading uphill at 25mph and it lost all power and konked out. It never gets low on fuel (I top it up daily with 2 to 3 litres that it has used), oil is kept topped up too. It has a new battery with lots of life in it. The spark plug is less than a month old. The exhaust is a two month old Leo Vince sports exhaust. It does get pushed and it packed up following a five mile stretch at full throttle. It was extremely hot - could put my hand anywhere near the engine for a couple of hours afterwards.

I have checked the following:
Removing the spark plug, letting it lean against the frame and trying a kick start - there is a spark (or several quite healthy looking sparks)
Swapped out the coil for a spare & checked the HT lead connections - no difference
Removed the spark plug and turned the engine (with the ignition off) and held my thumb over the spark plug mounting - there is compression
Removed the cylinder head and examined piston, cylinder head and gasket - they're all quite new and when wiped down, look perfectly fine - no holes blown in the piston, piston rings in place, piston lubricated with fuel/oil mixture
Checked the fuel connections (as best I could without removing them)
Opened the screw at the bottom of the carb that allows petrol to run all over the floor and ended up with a small pool of petrol
Checked fuses, relays, CDI etc. for loose connections

The starter motor has been swapped out (the old one hasn't worked since I got the moped) and I can turn the engine with the electric ignition, but it doesn't fire at all.

Messed around with the mixture screw and before doing so I would get the odd misfire. Now I don't get that either.

I need some help.
I think the spark is okay, but would know whether it is enough to do the job.
I think the piston & cylinder are okay. They certainly look fine.
Fuel - surely if I was getting misfires then the fuel must be getting through?

I have a few spare parts from a Honda SFX50 that I bought for a fiver, so I have tried swapping them over, but no luck.

[hazza]

what does your plug look like?...i think if you have messed about with the mixture screw it may be worth posting a picture, you can tell alot about the condition of your engine by looking at the plug
____________________
3 Honda NC50's(Sold)-->'72 Puch Maxi S (Sold, want it back!)--> '90 Yamaha RXS 100 (Stolen)--> '87 Honda Cub 90(Sold) --> '95 Kawasaki Eliminator (Sold) --> '98 Bandit 600 (Sold) -->'07 SV650S (Current)

[GearboxGeezer]

Have you tried bumpstarting it or kick starting it? ( im guessing it has gears? )

Also, its a long shot but it may be a coil gone... But normally it starts up for a while then just cuts out. then if you leave it for 30 mins or so it starts up again. Like I said, its a long shot.

Yea post a pic of the plug, that may help.

[plugger147]

The next things I would check would be carb rubbers for splits/holes and the air filter, then a good clean out of the carb.

The spark should be a nice fat blue one although it should at least fire if your getting a half decent spark.
____________________
Tristan the wrote: just whipped off my trousers to find a big bruise on my arse, caused by matt rear ending me...

[GearboxGeezer]

Just noticed you alrady tried a coil.

How about droping some fuel down the plug hole and trying to fire it up? If it fires up but then cuts out then that would indicate a carb problem or somthing.

[x8rowner]

Hi
It's too late tonight to get a picture of the spark plug. I have three and they all look okay. I'd put a new spark plug in about two weeks ago that is recommended when running at high speed continuously. I tried the old one which had been fine when removed - nothing. Tried another that I am told is good - nothing.

Starting - I've tried kick-starting and using the starter motor. Neither are working for me. The starter motor turns the engine but it's not firing.

Air filter - I removed it earlier today and cleaned it, dried it out and replaced it.

I haven't dismantled the Carb. I wanted to seek advice first before doing so.

[x8rowner]

I'll try some fuel in the spark plug hole tomorrow evening and see what happens

[finpos]

I suspect your engine has heat-seized and then unseized itself, which unfortunately can mean you now have satisfactory compression according to the thumb-test but not enough to actually start the engine (the seizure may have have buggered the rings up).

You've had the top off, but you really need to inspect the bore of the barrel and the pistons rings.

Any chance of getting it properly tested for compression?

finpos.

[GearboxGeezer]

Its not so much looking at the plug to tell if it works, its to look at the colour of it, If its white it would mean its running lean, But if the piston looks ok then I cant see this being worth to check really, since if it was due to running lean you would of probably had a heat sieze.

I reckon its a good idea to strip your carb down, and give it a blast with carb cleaner. Blast all the jet holes, and the float pin thingy ( The thing that connects to the float and opens and shuts as the float goes up and do ) thats of course if the carb even uses floats.

Oh and I also blow threw my jets with my mouth.. its a dirty and rather discusting way of doing it, but you can tell if you just unblocked somthing..

[divuk83]

Check the compression of the bike. Did you alter the jetting for the leo pipe at all. If not my guess is that the combination of heat and running a bit lean caused piston rings to go. Did the spark plug have little speccly bits on it after it died?

https://www.divuk83.net/images/bike/plug.jpg

Dave

[cestrian]

The Leo Vince sports exhaust and the comment about it getting extremely hot got me thinking about the carburation too.

Good luck with this one.

g:

[firefox]

thats running mega rich put a new plug in i bet itll start then get it sorted asap

[x8rowner]

[quote="finpos"]I suspect your engine has heat-seized and then unseized itself, which unfortunately can mean you now have satisfactory compression according to the thumb-test but not enough to actually start the engine (the seizure may have have buggered the rings up).

You've had the top off, but you really need to inspect the bore of the barrel and the pistons rings.

Any chance of getting it properly tested for compression?

finpos.[/quote]

Spent all evening working on it and you were right. When I removed the cylinder, I found that the piston rings had welded themselves to the piston.
Replaced piston, cylinder and cylinder head and the bike started. I've got a lot to do on it but at least I know what the problem was now.

You were right, the thumb over the spark plug hole is not a sufficient test for compression. As it had seized and unseized again, there was a gap between the piston and barrel big enough to prevent engine firing.
Thanks all.

[firefox]

mine had a heat seize today, the barrel, piston had a huge chunk out and the head had melted the gaskets away along with the thermostat yet it still fired , going 70 cc now