Slight stumble off idle. Octane, jetting or sync?(updated)

[PaulVS]

The sad thing is that's all it needs to drive beautifully.

Well... almost.

Add slip-on exhaust and a Stage 1 jet kit... then it's beautiful.  

[rider123]

Ok, here's the thing: I believe my B12 has been rejetted since the caps are missing from the mixture screws and it has a full Yoshimura exhaust, along with the fact that the guy I bought it from said it was. It takes off very nicely with a lot of throttle. But with just a little bit of throttle, it kind of gurgles and sputters a tiny bit. But, no matter where I'm at on the throttle, when I let off, it pops a lot. But, acceleration is very smooth and throttle response is very quick and even. So, that being said, would trying to adjust the mixture screws accomplish anything for me? I've already checked and reset the TPS to the magical 76% thing night before last which didn't seem to have any effect on the aforementioned problem. Would I need to go up a size on the idle jet? Any ideas??

It sounds like you may want to give the pilots a 1/4 or half turn out just to find out. The best idea is to mark where the jets are now so you can go back if you get into trouble. Try little adjustments first. Say 1/4 of a turn which is very minor then see what happens. Remeber always mark where it is now so you can go back. If it seems to get slightly better with a 1/4 turn, try another 1/4 turn, or if you want to be super safe. Do 1/8's of a turn. As long as you have the original settings marked you can safely experiment a bit. I'm sure someone with more knowledge here can give you even better advice.

As far as I know the popping is an indication of slightly leaner mixture(closed throttle) or an air leak in the exhaust. Because of your sputtering/hesitation, I would go with the pilots needing a little tweaking. Hey, it's free!! Give it a try.

As far as my bike is concerned I KNOW that the pilots are too lean. If you give it a bit of throttle at low RPM it just goes bwaaaah! With on occasional PUFT! instead of accelerating normally. Once it's one the mains though > 1500 rpm it pulls very strong with no hesitation. It boggles the mind why Suzuki would do this to a nice bike such as the bandit!

As far as the TPS goes it smoothed out the entire bike and helped the problem because mine was out of wack from the factory. But no amount of timing precision is going to help lean jets. At least I found out a way to measure the distance between the plugs and the mixture screws. The plugs have a hole in them at least on mine so you can stick in a sewing needle or other thin piece of steel then using a magic marker mark the edge. When you pull it out you have the depth from the top of the plug to the screw. So when you put the tape on your drill to drill them out I suggest giving yourself an extra couple of mm's or two to make sure you don't screw up the heads.

[fritobandito]

Thanks rider, I'll try finetuning the mixture screws along with checking for exhaust leaks.

[rider123]

Well I've had enough of this crazy stumbling and whatnot. I called the dealer and talked to the service department. He seemed like a nice guy and even though I'm early(500kms not 1000kms) for the first service he said it was good to come in and they would definatley get rid of those horrble pilot screw plugs and fix the carbeuration and anything else needed looking at. With today being around 65 and sunny the stumbling has improved obviously but boy they sure do come lean from the factory!!! Once it's on the main jets though it's fine. I figure Suzuki must be playing some sort of cat and mouse games with the EPA or whatever made them put these silly plugs on in the first place.

This must be the conversation at Suzuki:

"Well if we make the bandit 1200 run properly the EPA will be all over us, what do we do? I know we'll lean out the mixture on the pilots to reduce overall emmisions throw some easily drillabe plugs on there but make the mains ok so if you juice it or above anything but idle it will work. Once it's left the factory it's not our problem anymore"

[rider123]

Well I got it back from the dealer and they did drill the plugs and fattened it up a bit so its much better but still a little lean if you ask me. At least the caps are off so I can fix it myself. Still a bit of a stumble but not half as bad as before and at least it's not misfiring due to it being so lean from the factory!

Although maybe it's not the pilots. If I give it just a bit of throttle to take off slowly from off idle there is no problem, but if I give it just slightly more(1/8th throttle maybe?) it starts getting mushy. It's weird, does this mean it still needs some mixture fattening? Or does it need a little shimming? When on the mains it's fine so my guess is the mixture screws. What do you think?

[PaulVS]

It's weird, does this mean it still needs some mixture fattening? Or does it need a little shimming? When on the mains it's fine so my guess is the mixture screws. What do you think?

Ivan's or Dale's Stage 1 jet kit.  Cheap & Effective.

Or... just putting a couple shims (radio shack washers) on the needles should make a difference for you with the stock can.

[rider123]

Well I'll probably look at aftermarket cans and jetting next year, it's just very bizarre to me that Suzuki would take one of their most popular selling bikes and make it demented right out of the factory. On the weekend it's going to be 14 and sunny so I may try turning out the mixture screws a little bit more after I mark where the dealer set them to. I have a feeling that the dealer would probably be very conservative on any adjustments so as to not boost emissions up too much. It's getting as good or better gas milage it seems than my 750EF with a Yoshimura pipe so I have a feeling it's really lean. Ironically, once on the mains it drives perfectly. Very very weird Suzuki!!

[PaulVS]

Keep in mind that it's also about $2000-$3000 less than any of it's competitors.  They gotta cut corners somewhere.

And it's not a popular bike really... it's an inexpensive UJM that isn't even available in the U.S.A. anymore after this year because of slow sales. I got mine new in '04 for $6500 out-the-door.  Nothing else came close.

But put a few $$$ and it's as good as anything on the road, IMO.  There is no shortage of quality, inexpensive aftermarket parts to make it 'world-class'.

[rider123]

Yeah maybe you're right it's popular here in Canada and VERY popular in Europe more than the States. I don't mind that it needs a bit of tweeking coming out of the factory but covering the plugs and jetting it very lean out of the factory is just weird if you ask me. I guarentee you once I get to those mixture screws tomorrow its going to clean up the last little bit of stumble off idle. I don't mind doing it, but at least they should allow me to!! THank god the dealer left the plugs out. Thanks for all your guys help.

[Red01]

I don't know how long Canada has been going with the US emissions standards, but US carbs have had "tamproof" adjustments for DECADES - as part of federal regs. This is why US B12's don't have adjustable needles like their European counterparts - as well as the pesky plugs on the mixture screws... and why there's a big market in jet kits.

The excessive lean condition is also a finction of getting it to pass US EPA emissions standards.

[rider123]

Wow! Decades? Hmm must be a new thing here then. Ever since NAFTA Canada has been trying harmonize things with the States to make it easier for standards and such. I guess this was one of those things that wasn't "easier"   Still no harm done at least I can fix it now. Because it's semi-cold I won't go too crazy untill the spring, maybe 1/4 turn shouldn't be too much. Although just from this board people seem to have positive results and reviews of the holeshot jet kit and others but I'm not sure it would really help with just the stock muffler so I'll hold off for now.

Here is from Fastlarry's site on pilot screw adjustment:


# Adjusting the Pilot Screws

    * Warm the engine to full hot with a 15-20 min. ride.
    * Using a hand held tachometer or deft ear track the idle rpm as you turn out (counter clockwise) the pilot screw on one carb.  Slowly turn out in 1/8 turns until idle rpm rises no more.  Adjust idle speed to spec (1200rpm +/- 50).  Repeat this step for all carbs.
    * Turn in the pilot screw on one carb until there is an idle drop.  Turn it back out 1/8 turn.  This is your lean-best idle. Repeat for other carbs.
    * You may need to enrichen slightly if you still have an off-idle stumble when full hot.


It makes sense to me any thoughts?

[elofdahl]

Wow! Decades?

Yep...In the US, the big Hondas (most familiar to me) stopped using adjustable needles in 1978 (unless manufactured in 1977 or earlier).  The use of pilot plugs and limiter caps were fully implemented by 1980, as were plastic non-adjustable floats and non-threaded pilot jets (they were pressed into place).  All this was to comply with the US EPA "tampering" laws.

Eddie

[rider123]

Well I fooled around with the mixture screws a bit and it didn't seem to change the idle at all. It's at night so I only turned one turn either way. Hmmmm......Maybe I've maxed out my pilots screws? I'll count the turns tomorrow and see. Or it may be that it is affecting idle but I can't hear any difference as it's slight. I guess what I could do for fun is put the mixture screws out a 1/4- 1/2 turn and see what happens. I'll mark where it is now so I can go back.

[rider123]

Ok well I turned it out a half a turn to richen up a bit and everything seemed ok but occasionally I get this. Sometimes, I'll blip the throttle at idle and the idle will go down to 1000 rpm then back up again to 1250 where I have it set.  It seems to be more previlant at full hot. Maybe a tad too rich? It's intermittant, usually if I stay at idle for a while. It almost as if one of the cylinders it slightly rich. It weird, it idles fine, nice and steady but still is slightly mushy off idle not much now though. I may just bite the bullet and take it to someone other than the crappy dealer. Just for reference on a 2G bandit, the standard screw turns out from stop are 2.5-3.0, is that correct?

[Red01]

More like 2.0-2.5 for a bone stocker.
When I installed my jet kit, I found 'em all over the map from 0.75 to 2.25.