Carb fine tuning

[Crazcnuk]

Ok, another carb thread, we should be compiling a pretty comprehensive guide for these bikes by now!

If you remember, last year I rebuilt my carbs about a dozen times, trying to get my std to run right AND get fuel economy. Then that motor died.

So, I now have these carbs on my special, and they are running pretty good. I am getting ~43mpg so far.

The bike starts nice, although it's a bit picky about whether I hold the throttle open or not when I crank it, depending on hot or cold.

What I have is aftermarket 4 into 2 turnouts (possibly MAC, not sure) and a K&N filter in the stock airbox.

The carbs have 115 mains, #45 pilot jets and the 'upgraded' Canadian emulsion tubes and 5 step needles from Mike's XS. Floats are set to the stock 23mm.

All the specs for what has been done has been covered in this thread:

http://www.xs11.com/forum/showthread...ight=find+cure

Now on to the current question.

The bike runs well, starts well (far better than the other motor) It's a bit sluggish at the bottom end, and will 'load up' if I idle it around a u-turn or whatever, then bog for a 2nd or two on acceleration.

It lays down, ever so slightly at 7500+ rpm (but nothing like the old motor). Mid range works great, and as I said fuel economy is not bad.

I am thinking that it's a bit rich and needs my floats down a bit? From what I have read, this should help both top and bottom end?

Does this sound right or do you think my jet/pilot combo is not right.

[DGXSER]

Crazcnuk,

I doubt you made the same mistake I did, but I'll share in case it is hurting you. I had one cylinder that would foul up running at idle or low RPM less than 3K. After 3K it took off like a raped ape! After months of fighting this and turning my mixture screw about fully seated, I recalled pulling out one of my needles when I cleaned them, I had reassembled it backwards, with the spring in first and the needle on top of spring. I switched it around last night and it runs smooth all the way through. That plug came out looking cleaner than it ever has. Not sure it will help you, but there it is.

[foreda]

Rich?

Definitely sounds rich to me. I pretty much have been going thru the same thing although my setup is for a 78/79 Standard. I think you're headed in the right direction.In the beginning I thought the float height would not be that critical but after dicking with these carbs I have found that in fact it is. You would think +/- 1 mm would not be a big deal? Wrong!! If raising does not help maybe 112.5?? I have a set if 112.5 that I will not ever use so if you need em let me know?

[Ivan]

Which part number are the needles? I have been thinking about pulling the ones out of my 79 carbs to try and get a little leaner on the needles.

I had the same hesitation from off idle to about 3500. Dropping the floats definitely helped the situation. The problem is that tonight I dropped them enough that off idle pulled like a tractor, but the upper end was lean. Therefore I think I need to get a leaner needle setting, and one that would be adjustable would be nice. I looked at the site and it says the canadian spec needles for the 80-85 carbs are a bit richer profile, which is not what I want. So, does the adjustment make enough of a difference to actually lean out the mid range?

[TopCatGr58]

Hey Cnuk,

If it lays down at 7500+, sounds like you're actually lean at the top end.

But loading up at the low rpms does sound rich in that range.

Due to a pilot jet being welded in my carbs 7 years ago when I was first doing my rebuild, I kept the stock 42.5 pilots, and just have them turned out a few turns, starts easily, no dead spot, just not super strong till 3K+ due to 4-1 pipes and Indy filters, but I rarely run that low rpm anyways! I'm also running 117.5 mains on 81 model carbs, floats at 23mm.

I remember the gurus saying tune first for top end...mains, then mid transition and then idle/pilots. Do you have any larger mains to try for the 7500+rpm range? Getting the 40+ mpg also looks like it's lean in the mid/upper range.
T.C.

[Crazcnuk]

Actually the mileage indicates it's slightly rich in the midrange. It should get better than 43mpg, cruising in the midrange.

I was reading some of the other posts around here, and, for some it seems lowering the float helped across the board.

I am saying this because, to me, the 45 pilots are a bit large (rich) for my application, but both sets of 80 carbs I have came with 110's and 45's. Also my new needles/emulsion tubes are supposed to be richer than stock, and I live at 3500ft above sea level (another cause for richness).

I do have some 120's, but with my minimal upgrades, they shouldn't be needed.

That being said, it does sound like it may be lean at the top end. Besides sooty plugs what does rich in the top end feel like?

From the sheer number of people who have found lowering their floats a few mm helped, I am wondering if the specific gravity of gas hasn't changed since the 70's...

[Ken Talbot]

If you're at 43 mpg now, you've got to be pretty d@mn close to perfect. I would try resetting the floats just a hair leaner. Actually, it would be a bit more than a hair, even by the standards set by an earlier post about how much one type or colour of hair is compared to another.

Do a throttle chop test before you adjust anything to get a good read on the plugs for the rpm range you're interested in. Move the floats all by 1mm - in this case leaner, and do another chop test. If that proves to be wrong, and TC is right, move them 2mm the other way to richen up as TC suggests. Don't touch anything else as you do this, other than to ensure the carbs are properly sync'd.

[petejw]

Hey Craz,
My bike is the 81 sports over here, came out with 110 mains and 40 pilots with out the bleed holes.

Im running a fairly open 4 into 1 exhaust, the adj needles mikesxs sells i believe came stock with our bikes over here.

Ive been playing with these carbs for years.
The best set up i found was 115 mains, the slide needle 1 up lean from middle, stock pilots, 23 mm float height.

When i raised the mains to 120 i got a slight miss at off idle and again around 3/4 throttle.
when i went to 117 the miss in the top end went but the off idle stumble was still there.
with the larger jets the mid range also wasnt as quick as wat it was with the 115 mains.
I also played with the float heights, this fixed the off idle stumble but was flat through out accelleration.

The best thing i found was stretching the slide springs a quarter inch longer, this fixed the off idle stumble.
the 115 mains i found is on the lean size at 6000 and above at wot but was the best for the rest of the throttle range.

Ive now gone to dyno jets and they are a big improvement, but thats another thread.

sorry for the long story, but i hope it helps.

[Crazcnuk]

40 pilots? Most came with 42.5, both my canadian models came with 45's.

When you say needles up 1 lean, do you mean up 1 notch, so the needle is DOWN 1 notch? Derwat did this on his otherwise stock SF and it jumped his fuel mileage from mid 30's to low 40's.

Mikuni says that these carbs could see emulsion tube wear in as little as 5000 miles. By dropping the needle one notch we may just be compensating for wear.

Dropping the floats may be making up for changes in how our gas is made these days. Ethanol, additives, and no lead may make the floats sit differently in the fuel.

Rejetting is as always, making up for changes we make to the system.

My bike actually pulls strong from off idle to redline in gears 1-3. The stumble only occurs if I lug the motor under 2500rpm for a few seconds, then open it up.

The flatness in the upper rpm is only noticable in 4th and 5th gear.

[petejw]

yeah the pilots were 40, but had no holes in the side,
those 45s u have, do they have the air holes in the side?
my set up had the needle up 1 notch from the middle
(lean).

[Crazcnuk]

I am pretty sure mine have 2 holes one way and 1 hole 90deg to that.

[petejw]

sorry i meant the clip up 1 notch dropping the needle.

[81xsproject]

Try a 42.5 pilot to work on the low end hesitation. Whether it was stock or not, your set-up shouldn't need a 45. I agree with TC that your top end sounds lean. It should be pulling like freight train at 7500. You wouldn't really notice it being rich up there unless you were running something rediculous like 125s/130s. And I would also say that unless you want to run the bike crazy lean, don't expect to get much better than your 43. When it comes to economy AND power from carbs, you just don't get to have your cake and eat it, too.

[pete_s]

Is the emulsion tube the needle jet?

If the tube, jet is worn, wouldn't the needle be worn too?

I'm at sea level and blame the lean spot of idle on the 10% ethanol in my state. I say it is lean because it is not as noticable when the weather is warm and the engine is hot.
Also, I think CV carbs adjust for air pressure or altitude?

Just another theory!

scr#w it I'm switching to fuel injection, fuel pumps, pressure valves, O2 sensors, injectors, butterflys ......( just kidding)

[Crazcnuk]

I would think so, I replaced both the needles and tubes (to the matched ones recommended by MikesXS)

Funny part is, the mid range is where it runs best, with the 45 pilots.

Since Mikes says the new needle/tube combo is richer than stock, maybe that would make the 42.5 more usable?

I think the test will be to drop the floats 2mm, to 21mm, and see what it does. I suspect it may clear up the low end, but take away from the top. If that happens, then going up on the mains should clear it.