Another Jetting Question

[shadowman]

Sorry guys, I know there are alot of threads about this, but I can't seem to find my specific problem.
I've got a 1981 Special, with a Mac 4 into 2 exhaust, and pods.
Using the jetting table:

http://www.xs11.com/forum/showthread.php?t=382

1 jet for the exhaust
3 jets for the pods
is a total of 4, minus the 1 is 3.
3 mains requires 1 pilot

So, the way I read it is go up 3 main sizes, and 1 pilot size.
My question is that I have 110s in the outside carbs, and 120s on the inside carbs. Should I be trying for 117.5s and 127.5s? Should I be trying to keep them all the same? I did read that Yamaha used different jets in '81 to try and cool the inside cylinders by making them a little fatter. I'm just not sure what to do now.
As it is now, the bike starts and idles nicely, has a stumble immediately off idle, and then revs out nicely. If I back off the throttle, and then come right back on it, I get a quick shot of black smoke out the exhaust, which sounds rich, although I find that hard to believe.
Any help or input would be greatly appreciated.

[natemoen]

I know a lot of people change the jets so that they are all the same, but don't know where they start from.

[CatatonicBug]

Yeah, all 4 the same is just fine. Years of testing has basically determined no real benefit of running richer jets in the center cylinders. The black smoke may be many things. Has the bike sat a while? Smoke issues often clear up on their own after about 500 miles or so. Either way, make sure you sync and colortune the carbs after EVERY modification.

[DGXSER]

Yes, Yamaha did at some point in 80 or 81 try the two different jets thing. But even Yamaha quit doing it before the end of the run. I've owned two 81 SH models and worked on several other 80-81 style carbs. I always jet them 110 across the board and the bikes run very nice like that.

If it were mine, base the jetting from the 110 mains across the board. I am not so sure I would consider the MAC a "Custom" 4-2 exhaust. So I would try it stock first, then maybe 115 mains with stock pilots. If that is still lean, move up to the 117.5 and 45 pilots.

Hard to say exactly what the smoke is. Running rich is typical for black smoke. Could also be valve guides from the way your describing it acting. Make sure the carbs are good and clean.

[shadowman]

Thanks for the tips guys. I think I'll try it stock first, and go from there. The smoke is definitely black (gas), as opposed to blue (oil).
I've cleaned the carbs once, and they looked pretty clean, but I'll try it again. Maybe there is something to the "triple clean" thing.
I'm just waiting on a new clutch lever switch, and then I can take her in for her safety inspection. Weather is calling for some warm and rain, so maybe we can get some of the salt and sand washed off the roads, and I can take her for a trial run. I'd like to get everything sorted before the weather gets nice.
Thanks again.

[petejw]

what levels did u set ur floats at?
a rich setting could cause the symptoms u describe,

with the jetting ive found jumping to 115 worked really well
with pods for the 81 model, also if u had the carbs apart did u remove and clean the emulsion tubes?

[shadowman]

When I cleaned the carbs the first time, because everything looked so clean, I did not remove the emulsion tubes. I will definitely remove them this time.
My float heights varied from 0.850" to 0.860" (my Clymer manual says 1.012" +/- 0.040", which would make the lower limit 0.972") These values seem so far out of spec. that I figured I must have measured them wrong. I am going to have to re-check them. With the floats set like this, would that not cause a lean condition?

[petejw]

not sure what that is in metric, but for the later
model carbs u want to set them at 23mm,
and have all 4 set exactly the same.
with ur emulsion tubes im not sure but the middle 2
could be different from the outer 2, i think they were set
differently the same way they jetted the inner 2 with richer jets,
so it wouldnt hurt to check, also if u have the carbs off, check
the 3 little holes near the butterflys (off idle ports) arent blocked.


the clymers manual that u checked the float heights for
could be the specs for the earlier model carbs, those float heights
were 25.7mm.

[natemoen]

If you checked them with the carbs upside-down (which is how it needs to be done) those measurements could cause a rich condition.

When you measure the floats you measure from the gasket surface (with the gasket removed) to the flat top portion on the plastic floats. You need to check each side on every float, so a total of 8 measurements total. Both sides need to be level with the other and you can twist the floats to even them out from side to side. There was a thread recently that had some pics showing how to measure, if I see it I will post the link for you.

[shadowman]

Pete, I think my manual is whacked. In the general part, it gives a float height of 1.012" (25.7mm). In the 1980-1981 service info in the back of the book, it gives a height of 0.906" (23.0mm) for the 1980, but a height of 0.012" ( with an obvious misprint of 3mm) for the 1981. The table shows the exact same carbs for the 1980 Special and the 1981 Special (BS34-111 3J6-00). If they are the same carbs, I would think they should have the same float height. Should I use 0.906 (23mm)?
Also, when the manual uses 0.906", it only allows a tolerance of +/-0.020 (23.0mm+/-0.5) If I use this value, my floats are still out of spec, although not by too much.

[natemoen]

Yes the 23 mm is the correct height for your carbs.

Here is the link to the thread with pics on it. The pics are on the first couple pages of the thread.

http://www.xs11.com/forum/showthread.php?t=31490

[petejw]

set ur floats at 23mm it states + or - 1mm
but unless ur at some sort of high altitude set
them at 23mm, nate explained well how to check em.
it doesnt take much if there out of adj, also after each
adj u wanna do a carb sync.


your float heights affect acceleration, to lean the acceleration
will be flat and may lag, to rich and it may bog and be sluggish.

to check that u have them set right after u have adjusted them
find a flat road, put the bike in 5th gear have the revs at 2000rpm
and crack the throttle all the way open, the bike should accept
the throttle with out hesitation.

[shadowman]

Sorry guys, I misread my manual. It does give hugely different values for 1981, but they are talking about using tubes, with the carbs upright. Like this:

http://www.xs11.com/forum/showthread.php?t=76

The measurement they give is from the fuel level, in the tube, to the very top of the bowl.(as seen in the last photo in the above thread) The correct value would be 0.012" (3mm).
I really must learn to read more carefully

[petejw]

thats a good way to check the levels
when the carbs are fitted on the bike
especially to see if there all exactly the same,
if you have them off the bike the 23mm is still the correct height.

[natemoen]

A lot of people, including myself, do the dry set to the 23mm (depending on bike year) and then do a wet check using the tubing to make sure everything is sitting right. It's much easier to set to the 23mm than the other one just because everything is open and easier to set rather than mounting the rack, filling the carbs, measure, dismount, disassemble, adjust, reassemble, repeat over and over.