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Non GS won't start. Got me stumped.

hampshirehog

hampshirehog

Forum Sage
Past Site Supporter
My mate runs a rat MZ ETZ251 - a 2 stroke single. Yesterday the points melted - classic ignition coil failure. So he replaced the points with new and popped on a replacement coil (unknown make 12v). Good spark but no go. Plugs are getting wet so fuel is getting through plus it's got good compression. Battery topped up fine. Tried new and old capacitor - no change.

We tried tipping fuel in the plug hole - nothing. But still have a good spark (and ignition timing is fine). Took the carb to bits - everything clean so no problem there. Even the crappiest lawnmower I ever owned ran if it had spark and fuel tipped in the plug hole!

We swapped over 4 known good plugs - all sparked fine. We did manage to get backfiring and occasional chuffing out of the exhaust plus on 3 or 4 attempts it ran ok for about a second and stopped.

I didn't have my multimeter with me so couldn't check out the replacement coil but the only thing I can think of is that although there is a good spark with the plug out but that it's struggling to spark under compression. I've seen this with individual 'duff' plugs but not 4 at the same time.

There's a new coil (proper MZ one that fits) on its way in the post but other than that has anyone got any thoughts on this?
 
Has it got compression, and being a two stroke crankcase back pressure?

you dont say, checkin the basics...


John
 
Standard flywheel / points ignition (not a separate mag) plus it's got good compression. 'Sucking and squeezing' exactly how a stroker should do.

It was running fine before it melted the points and the only thing replaced are the points (standard MZ) and the coil (no idea who made it but it's 12 volts).
 
Almost sounds like a timing issue...I don't know anything about 2-strokes, and very little about points ignitions, but is it possible that something timing-related got taken out in the meltdown? Condenser, perhaps?
 
Ignition timing was checked and it's spot on - can't be out much anyway due to the design. Fitted a new condensor plus tried the old one and there was no difference.

Excellent spark with the plug out. Is it sparking properly with the plug in? The thing is old Eastern bloc simplicity personified which is why it's so baffling.
 
Have you tried a new plug? Failing that, put the business end of the plug over a gas ring till it's glowing. Then pick it up with a pliers and fit it quickly while it's still hot and you should get it to fire up. Very simple motor on them.
 
Well here's a thing. The (secondhand) new MZ coil turns up today and Gary fitted it - bike starts first time. So he swaps back to the coil of unknown make and the bike won't run - but still the plug has a good spark. MZ coil back on again and the bike runs like a champ.

Anyone got any idea what's going on here? Does the bike not like non-communist sparks?
 
hampshirehog said:
Well here's a thing. The (secondhand) new MZ coil turns up today and Gary fitted it - bike starts first time. So he swaps back to the coil of unknown make and the bike won't run - but still the plug has a good spark. MZ coil back on again and the bike runs like a champ.

Anyone got any idea what's going on here? Does the bike not like non-communist sparks?
Click to expand...
Why don't you check the resistance of the two coils and end your anguish?
 
It could be not starting because it's totally loaded up and is literally drowning out the plug almost immediately. I'm a little rusty on my 2 stroke stuff but if there is drain plug on the bottom of the crankcase (it will look like an oil drain plug, maybe smaller) remove it and see if if any fuel/oil mixture comes out. With the drain and spark plug out kick it over a bunch of times to clear out any excess fuel mixture and maybe let it sit a bit. Put everything back together and try to start it.
 
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