MZ/MuZ Riders

Hi
Had it running but sooty plug, stripped carb but now cannot get any fuel into the combustion chamber the plug is bone dry.
Tried a spare carb with no luck, there is good suction at the manifold and have unscrewed the throttle stop.
Any ideas please.

Yes, as Puffs said.


What did you adjust on the carb when you rebuilt it?

Did you take out all the jets and clean with brake cleaner/jets of air? Did you adjust the float height at all? Did you test that the float was doing it's job (blowing through the fuel inlet and actuating the float)?

Do you know what the adjustments of the carburettor were before you took it apart? I might not have needed to be touched other than adjustment of the pilot fuel screw which may have been wrong.


According to the official service manual (of which I have attached), the 'slow running air screw' (pilot screw) should be at 1.5 turns from fully seated. It's quite possible that originally it might have been something silly like 3 turns?

So if the plug is bone dry it sounds like no fuel is getting in from the carb.

You say you had it running so the fuel tap must be OK - even so I would pull the pipe off the carb to check, how much fuel is in the tank? Perhaps you need it on reserve!!

Anyway if fuel comes out the pipe it must be the float valve / float setting that is the problem - take the float bowl off and have a look.

There is not much to go wrong so should not be too hard to find out the problem.

Good luck.

Thank you for the advice and I will strip and check the carb again, I checked the float height which was ok and I have tried it with increasing the height but still the same problem. One time the float stuck open and flooded but still had a dry plug. Thanks again and will come with with the results of the next check.

"One time the float stuck open and flooded but still had a dry plug"

That sounds like crank seals, if the carb is flooded yet the plug was dry cant see many other reasons why the plug would be dry unless the rings are not sealing / stuck in the grooves - does it have decent compression?

You did say that you had it running but with a sooty plug - had it been layed up for some time?

If layed up for a long time thee crank seal lips could have been stuck to the crank and if it just fired up for a short time the lips could have been ripped off the seals whichh could explain why the plug is dry, you could stil have good suction on the carb side but if the seals have gone there would be no crankcase compression so the fuel would not find its way into the cylinder.

Just few thoughts

Fair comment Puffs, but if the jets are not plugged......

Cant disagree that gravity always wins.

If I were spark26 I think I would take the carb off and:

1. Check that the float valve is working correctly
2. Check fuel level is correct in float bowl
3. Check that the float is not punctured
4. Check all jets to make sure they are not clogged
5. Make sure that the needle jet is fitted correctly - spring clip fitting
6. Makes that everything is correctly assembled

Think that spark26 has already said that fuel is flowing from the tap to the carb.

If that all checks out OK I would be looking at crank seals.

Put the original carb back after increasing the float height and checking the jets and airways, now have petrol in the combustion chamber and a decent spark but does not fire. The bike was laid up due to getting the Vape ignition checked over by the manufacturer as it previously had a weak spark, and it took several months to get it there and back from the manufacturer.
After getting it back together it started second kick, took it for a ride but it soon started missing due to the plug shooting up. So after swapping the two carbs I have and making various changes to the float it’s not run since.
The engine was rebuilt about three years ago but has not done more than two hundred miles as it kept seizing above 30 mph and also had problems with the carb flooding and the weak spark problem.

OK so you have sparks & fuel getitng to where it should be.

Couple of things to check:

1. Air filter condition - is it clean and dry
2. Choke Plunger - what condition is the little rubber pad in on the plunger, if it is in poor condition fuel wil leak past it making for a very rich mixture
3. Choke again - remember slack cable = choke off, tight cable choke on
4. Make sure that the choke pluunger is not sticking open
4. When the choke is off make sure that the slack cable has some free play

It does sound to me that this might be the source of your problem, I had a problem with the cable routing of the choke on my TS250, when you turned the handlebars to the right it cold pull the choke on sometimes, these engines die very quickly if you have a choke that sticks on, finally did you say you had a new spark plug fitted?

Sometimes they will spark outside the engine but put then under compression and they dont - I had 2 plugs that did this to me on my TS250.

P.S. If you have flooded it, take the plug out and leave it overnight with the piston at BDC, that will usually dry it out enough to get it going.

Hi again,
When it started second kick after its layup it ran very rich because the rubber choke bung was missing, I replaced it with a new rubber but still ran rich.
I have removed the rubber carb connector to make it easier to take off the carb and eliminate the filter.
I hope you are right about the vape ignition as it’s proved very expensive so far as I was advised by the supplier that the coils sometimes go so I bought one for £110, plus the shipping and testing at the vape factory, there’s not much change out of £500.
I will try adjusting the float level and check on the choke cable run.
Thanks for all your advice.