660 Carb settings?????

[krusty]

Right here is one for you guru's

2004 MZ Mastiff completely stock apart from a GPR exhaust system with the baffle still in.
Brand new spark plug fitted, fresh Shell fuel, carb mixture setting is as it should be stock (2 & 1/2 turns out).

I have ran it with a stock mixture setting and it pops and bangs on the over run really badly and very loudly!!!
I removed the plug to check it and it is white on the ground electrode which suggests running lean despite the electrode tip itself was a nice brown/grey colour so the mixture could not of been that far off.

Obviously the popping and banging during the over run does suggest a lean mixture anyway!
So to typically richen the mixture, you turn it in on most motorcycles HOWEVER I have came across 1 or 2 bikes in my time that work the other way round :-/
So I turned the mixture screw in a 1/2 turn from stock (so 2 turns out) which has made no difference on a small ride so I have turned it a 1/2 turn out from stock (so 3 turns out) also to no avail... it still pops and bangs loudly on the over run ???

So my first question is-
1- what way does the mixture screw work on this bike? Does it need to be turned in or out to richen or lean the mixture?

And 2- is anyone else running a stock bike with an exhaust system and still using the baffle? If so, what mixture setting have you found best for your bike???

All info is much appreciated

Thanks fellow MZ'ers

[Hajok]

Hi Krusty, before blaming the carb for this popping and banging: check the header gaskets if they're the culprit. If there is an air leak this gives you the biggest bang for the buck as well Old reused gaskets are a no-no and new gaskets sometimes need some tightening after the first heat cycle.

About your question: 2,5 out should do with stock setup. Turning the screw out is richer, look at #6 in the drawing:

The pilot screw adjusts the amount of extra fuel mixture through it's secondary fuel orifice.

I have to admit i'm running a far from stock engine with Mikuni's and the my little Teikei knowledge from practice is fading...

[krusty]

Thanks Hajok

this helps a lot! I can see now that I should be turning the screw out rather than in (like on most carbs!) to 'richen' the fuel mixture.
I definitely have no exhaust leaks... all new gaskets have been fitted when the system was installed and I have actually nipped them up since so I can verify this is not a problem

Like I said, I have it set back to the stock 2 and a 1/2 turns out right now so I will give it a run in a second before it goes dark with my little mixture screwdriver haha and if its popping still then I will pull over and turn it out 1/2 a turn and so on until it is the best it can be.

obviously any more than say 3 and a 1/2 turns - 4 turns and I am starting to worry
at the same time I would happily run it on a slightly too rich mixture rather than too lean! basically I would rather use a little more fuel than damage my motor

Is this something you would rather do?

[edfmaniac]

The stock pilot jet is too lean, it's a known issue. Unscrew the mixture adjustment one turn at a time until it falls out. Note where that point is, screw it all the way back down, and then back out to where it's two full turns short of falling out again. If the engine still pops during overruns, you'll need to go to a bigger pilot jet. The stock jets are nearly impossible to find but the Mikuni VM28/486 #40 will work even though it looks completely different. My bike was actually a little rich with the Mikuni #40 so you might buy a #38 while you are at it. But it started great and ran fine on the #40.

FYI - I ran my screw at about 6 1/2 turns out until I replaced the jet.

[krusty]

The stock pilot jet is too lean, it's a known issue. Unscrew the mixture adjustment one turn at a time until it falls out. Note where that point is, screw it all the way back down, and then back out to where it's two full turns short of falling out again. If the engine still pops during overruns, you'll need to go to a bigger pilot jet. The stock jets are nearly impossible to find but the Mikuni VM28/486 #40 will work even though it looks completely different. My bike was actually a little rich with the Mikuni #40 so you might buy a #38 while you are at it. But it started great and ran fine on the #40.

FYI - I ran my screw at about 6 1/2 turns out until I replaced the jet.

WOOOOW!!!! 6 and a 1/2 turns out ey... that's a lot!!!

I'm not expecting mine to be that bad as I am still running it with the baffle!
so I am hoping to just sort my mixture setting out so it stops popping and banging or AT LEAST calms it down

[krusty]

4 and a half turns out and I have a very slight 'pop' once or twice! this only happens if i full throttle it (using 2nd gear) right through to the red line and then just throttle off completely and let it decel back down! It will pop once or twice (not bang) when it gets down to around 3500rpm-2000rpm.
If I just ride it normally and potter about with the odd blip out of bends etc... sounds perfect with no popping or banging

Sound right to you guys?

[breakwellmz]

Mine came from the dealer 6-7 turns out.

[edfmaniac]

If it runs good, stick with it. If it becomes hard to start after sitting unused for a few days, but runs fine otherwise, it could be the lean pilot jet.

[krusty]

In that case, I think I will leave it in the garage for a few days and then fire it up.
If it starts fine then we are on to a winner haha and then after a little run out and about again to see how it performs, I will whip the spark plug out and have a look at its running condition

[krusty]

I have left my Mastiff for a full week without touching it!!!
I came out to it today and put the ignition on, choke on full, pressed the starter with no throttle and it turned over 3 or 4 times before firing up nicely
It warmed up nicely and then with the choke off completely, it idled perfectly also with no crackling, popping or banging throughout

I took it for a quick spin down the road and back when it was hot and didn't notice any banging or popping that is worth mentioning.

Left the bike for an hour and returned to it in the garage, I took the spark plug out and everything seems to be a nice grey/light brown colour which suggests a happy running motor
after putting the plug back in, I pressed the button again to see if it started whilst still a little warm and it fired right up on the first turn of the motor with no choke or throttle at all and it settled straight in to a lovely idle again

Just want to say Thanks for all the help and advice you guys gave me.

[edfmaniac]

Nice!

[Tarwetijger]

The stock pilot jet is too lean, it's a known issue. Unscrew the mixture adjustment one turn at a time until it falls out. Note where that point is, screw it all the way back down, and then back out to where it's two full turns short of falling out again. If the engine still pops during overruns, you'll need to go to a bigger pilot jet. The stock jets are nearly impossible to find but the Mikuni VM28/486 #40 will work even though it looks completely different. My bike was actually a little rich with the Mikuni #40 so you might buy a #38 while you are at it. But it started great and ran fine on the #40.

This post is more than 2 years old and I might be in the wrong place on the forum but: thank you so much for this great tip!
I ordered a 28/486 #37,5 and the bike runs so much better now. Nearly perfect.

Here's the new pilot jet next to the old one (#50).


At least it is a proper little sports bike right now.