MZ/MuZ Riders

I had an issue with the carburettor on my 251, which is characterised by the bike seemingly running out of fuel. And then, a moment later all was OK again and the bike ran fine. It continued to run fine, which could be for quite a while, until the next time this happened. It made me go to the pump, but that did not help. As described elsewhere, fitting another carb solved the problem, but to diagnose this issue I took the BFV apart.

After taking out the fuel valve, I tested it with my mouth & finger, in the orientation it is found in the carb. The valve sealed fine, but then, when the finger stopped pushing the needle up, it often (if not always) requires a bit of pressure to open. Obviously in a running engine there are always vibrations, but it seems plausible that the needle stays stuck for too long, thus lowering the level in the bowl & starving the engine of fuel (which is not good).

So I took the valve apart:
The valve needle has wear marks:
and the seat has lost it's sharp edge, and has become tapered to pack onto the needle, also due to wear:

Obviously the valve assy should be replaced, but then probably also the throttle slide & maybe the body. So before I do that, I had a look if this sticking issue could be alleviated.

The seat was marked 20, and indeed it was 2.00mm - except of course for the sealing end, which had become tapered. I drilled the seat up to 2.10 mm, starting from the non-sealing edge, to get a sharp edge on the seal end. The needle was placed in the (stationary) drill, and rectified, first with a very fine file (which gives a flat surface), and then with 1200 sandpaper:
Maybe I should have drilled the seat bigger, but I didn't want to go much higher; increasing the bore will change the level in the bowl.

After reassembly the fuel valve was tested again, with mouth & finger, and no stick at all was perceived, and it seemed to seal fine.

The fuel level in the float bowl is a concern, and also checking that the re-worked valve seals properly, so I rigged a test setup:
The level was indeed found to be a little high (about 12mm below the mating surface), so the lip was bent a bit to drop it to 13.5mm:
The wooden wedge keeps the carb horizontal, and the seat of the blanking screw over the pilot jet is at 14mm.

The bike runs excellent with the PWK, and the BVF also has some wear to the throttle slide, but at some stage I'll test the BFV again to see if this all was successful.

Interesting!

I should imagine I might have come accross this issue myself.

But when I got my 251, it was totally seized including the carburettor.

So I decided to use one of these, and although the kit didn't include an idle jet, I used everything else including the float needle and seat!:

Yes, the bike's done about 35k mi now, with that BVF, which as we all knows is fitted 'hard' onto the engine. There is (some) thermal isolation, by means of that plastic spacer, but it still shakes & vibrates. Now the engine has had work (rebore, crankshaft) but the carb has had 0 maintenance. So yes, it might need a needle, needle guide, fuel valve, throttle slide, and actually also the shaft the float pivots over was worn by 0.03mm (but I don't think that matters much). It's surprising it still ran fine with it, it's only that fuel valve appears to be the weakest link (if that was it - not yet tested). But if I'm going to buy all that, it should maybe also need another throttle body...

By contrast, the Koso is fitted via a rubber sleeve, a more common/modern/Japannese way of doing, and that does filter out abrasive high frequency vibes. And it has a chromed flat slide (--> should wear less quickly). In theory.

Very true. I think it might be worth persuing though just to see if the wear is really doing anything- after all, I have a few 'high' mileage vehicles- and i've never renewed the carburettor components.
The only reason I did so with my BVF 30N3 was because it was unused for 13 years and the jets were all blocked up. It took me ages to clear out the channels in the carb and get the idle jet clear (once again- I don't know why that wasn't in the set).

Well, looks like on your 251 you fitted exactly that part that gave me hassle - the fuel valve. Aka needle valve/float valve.

Anyway, yes, at some stage I'll put the BVF back & see if that really fixed it. But sofar, when it ran well, it ran fine; all the other wear did not seem to impact performance.

Btw, just had a look at the throttle slide. Unfortunately I do not have a micrometer for this range, but measurements with a digital calliper indicate wear of just a few hundredths of mm (~0.03mm, from comparing the area with wear marks & without). So that shouldn't be too bad.

In order to find out if that sticky fuel valve was the issue, and if I had fixed it using the above method, I took the Koso PWK off and put the BVF back. Then I went for a 20 mile test ride, of which the first half with the fuel cock on 'Reserve', for lack of fuel. Never did it give the impression of starving of fuel. So that repair worked.

Compared to the Koso there is no significant difference in how it runs, yet on my test stretch it was about 2MPH slower than with the 32mm Koso.

The plug chop (so done by switching off the engine right after that, and freewheeling home), is shown here:
One of these was taken in the late afternoon sunshine, the other in the shade. The plug picture looks very similar to previous pics with the Koso.