MZ/MuZ Riders

Hi
I have been having backfiring on deceleration. More so in coming of the throttle in 4th and 5th. Its too much now! its loud and annoying. I don't mind the odd gurgle or backfire. just checked the plugs and this is what i found. This looks a little lean to me, but am hesitating because of the blackness round the end of the thread. What do you guys think?

Also i have noticed that the exhaust has a couple of small joins that need taping, so i know there is a leak there! all the Carbs seem to be vacuumed sealed to the engine and airbox. The bike is a baghira 2002 with 1800 miles on the clock!

Apart from this the bike starts first time. It ticks over fine. It pulls and runs fine, however When you are pottering along at 30mph in 3 or 4th it sometimes seems a little jumpy/hesitant if that makes sense?

Anyway your thoughts would be very welcome....thanks in advance

oh and i only bought this bike a week ago!

It should be a light tan/brown colour when its been for a good run. Looks a bit lean to me . I'd try turning the low speed screw out a couple of turns and seal the exhaust, see if it improves. is it a standard can? Perhaps you might need a carb clean to clear a blocked jet or to go up on the jets to cope with a non standard exhaust.

There's a lot more to "reading plugs" than just looking at them. You need to do a "plug chop" http://kawtriple.com/mraxl/carb/plugchop.htm (this link talks about doing a chop to read the main jet at WFO). To modify for your issue, you'd get into 5th gear and ride in the trouble-zone for about 30 seconds, then chop (it's important to hit the kill, clutch and close the throttle at the exact same time), come to a stop and remove the plugs. I've never heard of anyone cutting the plugs open before, that's completely unnecessary ~ you can see just fine with sunlight.

If you simply remove the plugs after a normal ride, you're reading Mostly what's going on for the last 30 seconds... typically idling or slow-speed stuff, but it'll be muddled and mixed with ALL the running conditions the plugs have ever encountered. This is why the first few steps in the link are important.

It's a convoluted and complex process, mostly forgotten (it's tuning knowledge I learned 16 years ago from a guy who is probably now dead) and what little people do remember isn't doing much good.


As for your actual issue, I have no experience with the Muz carbs (I'm here doing research as there's on for sale near me, I'm hoping it sells soon so the temptation to buy goes away.. heh), but that would Normally be a condition that's addressed by a deceleration circuit. What's happening is that as you close the throttle you're creating a lean condition (cylinder is still creating a low pressure that allows a fuel-mix to be drawn in), causing the backfire. I'd read somewhere else on here that it's normal for these bikes with the stock car too, so I'm thinking there isn't a decel circuit on the carbs. It's not an issue that a change in the main jet would address:



You're looking at adjusting the air/fuel mix screw, or Possibly going up on a pilot jet (cleaning it first ~ just a chemical won't clean it like just swishing with mouthwash won't clean your teeth, you need something to physically abrade the gunk, copper wire is safe as it won't damage the brass jets) which Might also require other changes such as going in on the air/fuel mix and/or lowering the CV needle as it affects the whole fuel delivery range.

I have *no* clue how the dual-carb setup works, splitting the duty of delivering fuel, but carbs all do the same essential job.

Good luck.

+1 I also think its the pilot jet circuit. It probably just needs a clean out. Quite easy to do, the hardest part is taking the tank off and putting it on again, its a bitch. Don't know much about yours, mines a Skorpion but is it the standard can on it? Could be the cause.

If it still backfires once the exhaust joints are sealed up, then check the small enrichment diaphragm on the side of the CV carb for splits/perished - that can cause popping too

dandywarhol wrote:If it still backfires once the exhaust joints are sealed up, then check the small enrichment diaphragm on the side of the CV carb for splits/perished - that can cause popping too

Ah, so they do have the decel circuit ~ with that working properly there should be no backfire.

Thanks for all the replies. All super helpful. So it's not a standard exhaust so it may need rejetting. I'm still waiting for the paste and aluminium tape to turn up to seal the gaps.

I screwed the air/fuel mixer all the way in then 6 turns out. Took it for a spin today and it was marginally better but still backfiring in 4th 5th on deceleration. strangely if you take it to the top of 2nd gear then roll it off it doesn't back fire. Just 4th and 5th mostly. Might give it another turn on the screw to see if it helps. if not a rejet will be in order and a clean.

So what's the deal with the fuel tank? What are the pitfalls of taking it off? Does it need to come off to get the carbs out?

also what are the Jets people are recommending for stock carbs ?

watch that 6 turns out as 7 or 8 my let it fall out
fix something so you do not loose it
i lost one out of my rt125 and got lucky digging in a dealers junk box

DAVE

Nabsteel wrote:Thanks for all the replies. All super helpful. So it's not a standard exhaust so it may need rejetting. I'm still waiting for the paste and aluminium tape to turn up to seal the gaps.

I screwed the air/fuel mixer all the way in then 6 turns out. Took it for a spin today and it was marginally better but still backfiring in 4th 5th on deceleration. strangely if you take it to the top of 2nd gear then roll it off it doesn't back fire. Just 4th and 5th mostly. Might give it another turn on the screw to see if it helps. if not a rejet will be in order and a clean.

So what's the deal with the fuel tank? What are the pitfalls of taking it off? Does it need to come off to get the carbs out?

also what are the Jets people are recommending for stock carbs ?

I think you should stop feckin' about with the jet until you get the joints sealed - you're just masking the problem. Like I posted earlier - check the enrichment diaphragm on an engine that age - the slightest cracking of the diaphram WILL cause popping. The reason it's doing it in higher gears is because the revs are hanging around high for longer - in a low gear the revs drop faster and popping will be less.
Just trying to help

Valve clearance?

When were they last done? I'd get them done, made a world of difference to mine.

If your having the exhaust and headers off, change the headers to cylinder head gaskets changed as well.

With the carb tuning, let it get fully up to temp (take it for a ride) before setting the idle and screw, otherwise you'll be all over the place.

I cannot recommend you getting the valves and the bike on a dyno enough, will save you a ton of hassle, make sure the exhaust is all sealed up too.

Enjoy!

Thanks
I think I will see how much it will be to get valves checked and dyno. How much are garages chairing at the moment to check valves and dyno? I'm not doing it! Then if it's still persistent problem I'll strip the carbs and check the diaphragm and the general insides of the carbs.

Will keep you posted.

Valves are not hard to do, any shop will do them for you, perhaps you should provide them with the SZR660 workshop manual so that they know what to do if they are not a Yamaha main agent. If your near Grahams in Taunton, they would be the best. The 660 engine is a lovely one and quite rare, you won't see many MZ's with them around so its worth sticking with it. Just a word of encouragement

Thanks
Im defiantly going to stick with it. Its sounds really sweet when its ticking over and running. Its only done 1800 miles and for a bike of 14 years old is quite rare!

So i patched up the exhaust and there are no leaks anywhere now. Its still backfiring so its going in to a very trusted bike shop for them to take a look and get it on the rolling road. They will rip the carbs out/re jet/fix diaphragm etc if necessary and get it running as sweet as possible!

Then hopefully its just a routine service and thats it....apart from a bit of chrome polishing!

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Plug colour guide.

Thanks. That's a great chart to help you out, at least just to give you a guide to what could be wrong!
A bit like the Bristol chart!