I know, I know.....another

[Mr. Schweer]

Okay, before you even think about telling me.....I know there is a search feature and I know I can search carb info.
What I am hoping for SPECIFICALLY is someone with a similar set up as mine, that already has their jetting/tuning DIALED IN to give me some advice.
It's been mentioned that I hurt peoples feelings in my posts because I'm insensitive..or whatever, so please, nobody get offended by me asking for EXPERIENCE here, not conjecture or regurgitation of someone else's tech answers.
That being said, let the flaming begin!!

Seriously though, I have a '79SF with about 57K. Engine runs great, no electrical issues, no fuel leakage issues, in fact, I'm scared because there really is nothing 'wrong' with my bike....compaired to alot of the nightmare senarios that keep popping up over and over regarding these bikes. I think that where this bike has lived it's life has a lot to do with it.
Anyhow stock engine and my mods are listed below. K & N, drilled airbox and a 4-1 Kerker.
Bike runs great, starts easily and doesn't need the choke. When I changed to the Kerker, it lost a little below 3K, but the new K & N and airbox mod brought some power back on the bottom.
Minor popping on decel, no backfire. Not wanting to burn a piston is my main concern. The only place the bike feels 'flat' is around 6-7K.
I have a few specific questions:
Can this be resolved with float adjustment? If so, what's the key?
Do I need to re-jet? If so, any specific numbers?? Which jets??
I am just about at sea level if that helps.
Like I said, I am looking for experience from folks with similar set ups. I don't want to confuse myself with theories or 'try this or try that' answers.

TIA,
James

[latexeses]

I don't have any direct experience with you set-up but I think one of the first questions would be..What are your sparkplugs telling you? Sooty? Tan? Grey?

Just a thought.

Rodger

[Mr. Schweer]

Not too bad. They are a little light. Not 'white' but not very tan. I forgot to mention that I run one heat range COOLER than stock since it gets hot here in the summer. I don't know think the carbs are way off, and I don't want to start fiddling with them and go the wrong way which is why I was stressing the similar set up idea....

[GNEPIG]

Not too bad. They are a little light. Not 'white' but not very tan. I forgot to mention that I run one heat range COOLER than stock since it gets hot here in the summer. I don't know think the carbs are way off, and I don't want to start fiddling with them and go the wrong way which is why I was stressing the similar set up idea....

How does it run on the big-end? If it runs up to redline quick and smoothly and remains smooth, you could be right where you need to be. If you feel surging when holding the throttle steady in the higher Revs or you feel the bike speed up a little when you first chop the throttle= you could be a little lean.

Because you have cooler plugs in your bike and they are near white, you might be a little lean.
Since we don't know what jets you have in your bike right now, we can't give you a specific number you should have. Try the next size bigger main jets and see how it feels to you and see how the plugs look.

Sounds like you are real close tho... As for the after pop of your exhaust it could be the design/ condition of your baffle or you have exhaust leaks somewhere= that part may never go away completely.

If my answer is not what you want to hear, Oops, that how mechanics works, there is no real science to it, only trial and eror. Science only gets you in the ball park, each mechine is a little different, each climate just a little different.

Hope this helps

[Bigfoot]

Had some popping on decel on my bike as well. I had cleaned the carbs well and set the idle mixture screws to 1.5 turns out as per the manual. We used a colortune and ended up around 3.5 turns on the idle mixture screws on all 4 carbs. Popping on decel went away immediately.

This process was replicated on another bike two weeks ago. He ended up around 3.5 turns out as well and popping on decel went away. Both of these bikes were also sync'd at the same time. His mileage did increase as well as better throttle response.

Both of these bikes were pretty much stock, so no, not an identical setup that you have, but it could cure that one specific problem.

[cywelchjr]

Had some popping on decel on my bike as well. I had cleaned the carbs well and set the idle mixture screws to 1.5 turns out as per the manual. We used a colortune and ended up around 3.5 turns on the idle mixture screws on all 4 carbs. Popping on decel went away immediately.

This process was replicated on another bike two weeks ago. He ended up around 3.5 turns out as well and popping on decel went away. Both of these bikes were also sync'd at the same time. His mileage did increase as well as better throttle response.

Both of these bikes were pretty much stock, so no, not an identical setup that you have, but it could cure that one specific problem.

The stock setting is just a starting point, and is intended to get you close to factory setting which is VERY VERY lean. For good running you MUST go richer than the factory settings, especially on todays fuels which tend to give a leaner mixture anyways.

Oh, YMMV.

[Mr. Schweer]

Geez guys......I'm not THAT much of a d*ick.......well maybe........
Anyhow thanks.....very good and to the point and gives me ideas where to start.
What are stock jet sizes and what is a good size to step up to for just a little more richness?

[rpgoerlich]

On my 79 XS11SF with 56kmi on it, still running 137.5mains and 42.5 pilots, needle at center clip position with the 4 into 1 header and a stock airbox. Pilots adjusted out roughly 3¼ turns. Has a little stumble just off idle to 3k but pulls hard from there all the way to redline. Plugs just starting to look tan. I'm getting ready to pull the carbs and recheck float heights as I changed out the brass floats to plastic ones last year and it looks like it might be a bit lean at idle.

[Bigfoot]

Stock jets are different by year. Mine is an 80 and I thought it was supposed to have 120's in the middle carbs and 110's in the outsides. When I checked them it had 110's across. Some P.O. had probably changed them not knowing what they were doing.

[TopCatGr58]

James,

As Richard just posted, stock is 137.5, and the mains go up in 2.5 increments, so the next step is 140. Many folks with 4-1 or aftermarket pipes and modded intakes, either K&N drilled OEM boxes or Indy filters have found they can adjust the pilot circuit with just a few turns out, as was mentioned a few folks have found their sweet spot with over 3 turns out with stock pilot jets. However, it's been stated that IF you need to go to over 3 turns out on the Pilots that you should probably go up 1 size there as well. But most aftermarket pipes pop a tad on decel when the throttle is fully closed, and tweeking the pilot screws richer, as well as setting the idle a little higher can help it not lean out so much when you slam the throttle shut!

Sounds like you're just a tad lean for the top end to be fading, and going up 1 size in the mains should help that without harming your bottom end. A float height change will change the whole range which is something you may not want.
T.C.

[Mr. Schweer]

Thanks TC...good info. Where's the best place to get jets? Are used jets a good idea?

[lakecountrydave]

I have no hands on experience to assist you, but I found this in the Modification section. It should get you close.

xs11
Super Moderator


Jetting Recommendations
Jetting Recommendations
by Denny Zander

Here is a simple set of jetting guidelines that have worked for me. For those considering jet changes, this might help select a starting point.

1 jet size for custom 4 into 2 exhaust

2 jet sizes for 4 into 1 exhaust

1 jet size for K&N filter (single inside airbox)

1 jet size for drilling out the bottom of the airbox

3 jet sizes for individual filters

2 jet sizes for no muffler (open header)

1 pilot jet size for every 3 main jet size increase

Add up all the jet size increases and subtract one. (Remember they go in steps of 2.5 for each jet size)

Under a mismatch condition, like individual filters and stock exhaust or 4 into 1 header with stock filter and air box, subtract an additional 1 jet size.

Check plug color often, sync carbs after each jet change, make sure the floats are set correctly, and seriously consider purchasing a Colour Tune. (See "Colortuning Carburetors" in the Maintenance Section).

Make sure your carbs are in perfect working order before making jet changes.

Example from my '79 XS1100 F:
Stock main jets: 137.5

Stock pilot jets: 42.5

2 sizes for 4:1 exhaust (Jardine)

1 size for single K&N

1 size for drilled air box

4 (main sizes) - 1 = 3 or 137.5 + (2.5 * 3) = 145.0
1 (pilot jet size) or 42.5 + (2.5 * 1) = 45.0

With this jet configuration I get 32-38 mpg on the open road, Smooth idle, very strong exceleration from off idle to 80+ mph, and steady pull past 120 mph. My "F" has 76K mile on it, of which I have put 44K with this jet configuration.

I have applied this to the '78-'79 carb set with great success. These guidelines should work for the '80-'81 carbs also. The only part I have not tested is pilot jet changing on the second generation carbs.

here are jet pictures for reference

a air jet - b main jet - c pilot jet (some have different # of holes)
d various needle jets (emulsion tubes) - e two styles of float needle valves and float needles

(note: there are several styles of Mikuni main jets - the ones of interest to us are the 'large round' style)


Needle (jet needle)


Happy Jetting

[DiverRay]

My old "daily ride" had the same set-up as you are now running. I used a 142.5 main, 3 turns out on the mixture screw, and about 1 1/8" float. Bike ran great and I was getting about 140 miles per tank before the 750 mod. This is a STANDARD model, so I have 4.3 gallons on main. I also use the California excuse for gas, that is the reason for the float setting.

[Crazcnuk]

Not sure if you mean you are running hotter plugs (that run cooler) or colder plugs (that actually run hotter)??

Idle mix great place to start. You can go up 2 jet sizes, if you like, it won't hurt either way. Lots of these bikes had a 2 jet size discrepancy from stock.

Light tan plugs are fine. if you richen up the idle mix screws a touch you should see them darken just a tad.

[T99Ford]

Dyno...

Hey Schweer--
There is a one stop shop to tell you all of the questions you need to know. I may have mentioned this earlier, but one dyno run with an Air/Fuel sensor in the pipe will tell you everything you nned to know with no conjecture. Most sport bike places can do this. You will get back a chart showing your air/fuel in relation to rpm and horsepower. It is exact and shows what your bike is doing under actual circumstances. I don't know what they would run in your area, but I have found osme sport bike places around here that will do a run for about $20. It will save you a ton of time and foolin' around, and let you tune optimally. It is hands down the most accurate and fast way to do what you are trying to do.But you have to be careful...it is also very addicting chasing after the almighty HP