Loosing my mind due to Carbs and a P.O.

[MrOzz]

Just a simple question. If I am correct you need a 110 main jet for carbs 1 and 4. You also need a 120 main jet for carbs 2 and 3? You cannot use a 110 main jet for carbs 2 and 3? The P.O. really has messed things up.

[blue giant]

Just a simple question. If I am correct you need a 110 main jet for carbs 1 and 4. You also need a 120 main jet for carbs 2 and 3? You cannot use a 110 main jet for carbs 2 and 3? The P.O. really has messed things up.

I don't think it will make that much of a difference. I have 79 carbs on an 80 engine with 140 across all 4. I did order a set of 120's because of of fuel consuption and running too rich issues but found the po had raised the needles up 1 notch too high. Never did install the 120's.

Describe messed up!

Paul

[DGXSER]

Ozz,

I am inferring that you have the later model carbs in which the pilot jet is exposed to the float bowl, does not have the cover screw on it or the cross connect to the main jet tower.

That being the case, provided you have the stock air box and a stock or similar exhaust, then 110 mains all across is typically not a problem. It is what I am running.

Yes, I have read that some (not all though) bikes came with 120s in the middle, but most folks that have posted here are running one size all across.

[MrOzz]

Ozz,



That being the case, provided you have the stock air box and a stock or similar exhaust, then 110 mains all across is typically not a problem...

Would a K & N air filter make a difference? I do have stock exaust.

[blue giant]

Would a K & N air filter make a difference? I do have stock exaust.

It would give you a bit better air flow. You could run 120's across all 4 with no problems. There is a chart here that gives a guide to jetting for different mods.

[DGXSER]

Hey Ozz,

There is a tech tip around here somewhere on jetting and mods. It tells you what size jet to go up or down based upon what mod you have done.

I doubt the K&N alone would change your jetting.

[blue giant]

Hey Ozz,

There is a tech tip around here somewhere on jetting and mods. It tells you what size jet to go up or down based upon what mod you have done.

I doubt the K&N alone would change your jetting.

Great minds think a like!

[Rasputin]

I believe this is the page the guys were referring to. The images do not seem to be there but the wording will tell ya what ya need.
http://www.xs11.com/forum/showthread.php?t=382

[cajun31]

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

[TopCatGr58]

Hey Rasputin,

I fixed the photos links, I don't know how they've gotten messed up, but I think it happened during the migration to the new software. The links are correct in the Drop Down Menu Tech Tips, but the Forum versions have been having some corruptions in the links!? We'll fix them as we find them!?

Mr Ozz,

The K&N filter will provide a bit better flow as has been stated, but doubtful that it will require rejetting. The later model carbs, at least the specials came with 110's straight across despite what the manuals said. And this is plenty of jetting for stock setup. I ran mine for 9 years with 110's even after putting on 4-1 pipes shortly after I got the bike back to the states, and had plenty of power up to redline, never burned a hole in a piston. Since I've done the little big bore kit, and added Indy filters, I've rejetted to 117.5, but could possibly go up to 120's, but again plenty of power to redline.

SO.....your 110's should be fine for your bike and should provide plenty of fuel for the rpm range to redline.
T.C.

[MrOzz]

Thanks All.

this is why I love this site. So much info here.

[tarzan]

Both my SG's came with 110's straight across.I think I am going up to 115 or 120's this year though.Although it is stock, mine seems to run on the edge of running lean.I will still run the same size jet all the way across.That two different size jet idea was good in theory but in practice doesnt make much difference in my opinion.

[oldyam80sg]

I know alot of people like K& N air filters but not alot will change when you install them.I ran them for years on my XS and my Jeep-no change.I run them on my ZRX as a replacement air filter and have not seen significant improvements one way or the other.I belong to a Jeep message board and will have people swear up and down that they will ruin an engine.Don't know if i believe that, but changing an air filter wont require you to change jet sizes.I rode back in the 80's when i first got my XS 1000 miles with a K & N filter and did not notice a drop or increase in performance.Not saying that they make a bad product i just don't think that replacing that alone will change anything!

[MrOzz]

Im not looking for a performace gain with the K&N filter. I have used them in all my cars for years. I just like that I dont have to keep buying filters. Since my local shop wanted to charge me $35 for a "stock filter". I spent the extra $5.00 on the K&N.

Thanks again all for the info.