For those who have done the 1196.. or beyond.

[trbig]

If the bike was running fine carb-wise with a stock motor, then you had it bored and put in the 1196 kit, would the carbs need re-jetting?

One train of thought says that the air/fuel mix the carbs are sending into the cylinder hasn't changed.. just a bit higher amount of it with the big bore.. and wouldn't need to change anything on the jets.

The other train wonders if it is going to be the same amount of fuel with a higher volume of air and leaning it out. BUT with the way a carb works.. air drawing the fuel.. more air would mean more fuel also.

I changed the jetting on mine for the pod filters a little bit, but I never changed anything after the 1179 kit. Has anyone that's done the bigger bore had any problems and need to re-jet the carbs? Not that it really matters, but the project is going to be done to an XJ.


Tod

[Ken Talbot]

Not speaking from experience yet, but I would think the bigger bore would pump more air through the carbs and into the cylinders. If right on before the big-bore, the main jet would have to be able to flow just enough fuel to achieve the perfect air/fuel ratio. After the big-bore, I think the main jet would still only be able to pass the same amount of fuel as before, which would result in a lean mixture.

YMMV, JAT, TMSAISTI, etc, etc

[trbig]

I see what you're saying, but the jet isn't giving max flow until max HP.. and maybe not even THAT is the max flow rate? I don't think you'd achieve that until you got the max AIR flow rate possible from these carbs. From what Dan Hodges has said, that isn't possible to do with the CC's we're talking about here. At 4k rpm, you are running strictly on your mains, but you aren't flowing your max rate of fuel. Air being drawn through the carbs is what draws the fuel. At 8k rpm, you're drawing more air, which draws more fuel. Same mixture ratio as 4k..(Hopefully) just more volume of it.

Maybe my thinking is flawed... it sure wouldn't be the first or last time. As I stated, I went up a size on my mains when I put pod filters on, and did nothing more after I did the 1179 bore. I do have some popping on decel, but my pipes stay chrome and plugs look good???


I do appreciate the input, Ken. I threw this out there for opinions. Thanks.


Tod

[DiverRay]

Tod, I think what Ken is getting at is the main jets are sized to flow "X" amount of fuel. If with the big bore kit the fuel needs go to "Y", you will run lean. One thing to think about is this bike was designed with 95 CRANK HP in '78, the most powerful of the series. The carbs are designed for that 95 HP, and may start to lean out as you start pushing 100 to 110HP with a big bore kit. I would suspect that a one size larger main would keep things running well, even at 8K+ RPM. JMHO

[oseaghdha]

If the bike was running fine carb-wise with a stock motor, then you had it bored and put in the 1196 kit, would the carbs need re-jetting?

....................
..................

BUT with the way a carb works.. air drawing the fuel.. more air would mean more fuel also.



Tod

Tod, I think you are right on.
If the carb can't keep up, it's not able to flow enough air and jets won't cure that. I imagine there is more than 8% extra flow in those Mikunis.

Reducing restricions is different from increasing the swept volume.

[DGXSER]

..... One train of thought says that the air/fuel mix the carbs are sending into the cylinder hasn't changed.. just a bit higher amount of it with the big bore.. and wouldn't need to change anything on the jets.

The other train wonders if it is going to be the same amount of fuel with a higher volume of air and leaning it out. BUT with the way a carb works.. air drawing the fuel.. more air would mean more fuel also. ....

If you think about why you install larger jets, the idea is that a larger opening causes less flow restriction so you get more fuel for the same air volume. But it is not the air flow that causes the fuel to be drawn in, its the negative pressure created by the air flowing through the venturi. More air flow will equate a slightly lower presure, or more negative pressure. This will induce a higher fuel flow. However, as the rate fo fuel flow increases the friction it creates on the walls of the jet and the emulsion tube increases which increases the amount of pressure required to draw it up. The same way raising your floats creates a deeper pool of gas in the bowl and therefore less pressure required to lift it so you get a richer mixture with a higher float level. (higher being relative here as it actually is a lower setting since you set them upside down.) So with any jet, I believe you are getting a slightly leaner mixture the more air you draw through. It just may or may not lean out enough to matter, and the carb and engine have been designed so that stock it does not show up.

So, depending on how much more air is being drawn, there is a law of diminishing returns at work where you will reach a point of creating a high enough friction loss to create a lean enough mixture to matter. Will you reach that point with 1196cc over 1179cc? That is really your question and I do not know the answer. but it was fun thinking this through anyway.

[81xsproject]

Speaking with very little big bore experience, I bet the carbs will bolt right up and be fine. For example, TC and I run the exact same setup on our carbs, yet he has a big-bore kit. Both bikes run great.

[Dan Hodges]

Cubic Inches, Vacum And Jet sizes

When you increase the volume of the cylinder or change the rod ratio you will change the vacum signal at the carburetor and because a carburetor works off vacum, the vacum signal (less or more) will effect the jetting and rate of slide lift. Increased volumetric effiency achieved by a more efficient exhaust, more compression, better combustion chamber design and higher flowing head and more camshaft will move more air through the motor and consequently will require a like amount of fuel (larger jetting) however VE will not have the same effect on jet sizes like more displacement or a shorter rod. If you have two motors exactly alike and you increase the displacement by 10% or shorten the rod ratio the larger motor or the short rod motor will generally require a smaller main jet because of the increased vacum signal at the main fuel circuit. This increased signal will also cause the slides in a CV carburetor to rise more quickly which will sometimes cause the A/F ratio to be rich in the 2500 rpm-3500 rpm area. Camshaft overlap causes the greatest problem in this rpm area, i.e. (going from the late cams to the early ones) using the 80-81 carburetors both Special and Standards. If you are using good flowing pod filters (K&N) the main jets will need to be much larger and I will give Dyno Jets recommendation and I'll quote DJ numbers which are different from Mikuni's. A bone stock Eleven gets DJ 124 mains, Eleven with a good flowing 4 into one pipe gets DJ 128 mains, Eleven with same pipe and K&N pod filters gets DJ 138 mains. 128 DJ jets correspond to a 120 Mikuni and a 138 DJ jet corresponds to a 130 Mikuni. When you remove the stock air box, you get rid of the velocity stacks/tubes contained in the airbox which decreases the velocity at the carbs and requires a much larger main. The pod filters do flow more air than the airbox however they don't flow enough more to require that much more jet if the carbs were seeing the same velocity as they do with the stock air box and so part of that big increase in main jet size is due to decreased velocity at the mouth of the carb not volumetric effiency. Pod filters also alter the signal at the slides and are generally a tuning nightmare however they can be worked out but you will need a dyno to do it unless you can copy someone else who has already been there. To make a long story short, if you have a box stock 81 Special with stock pipes and factory spec carbs and you install a Wiseco 1179 kit, you will need 112.5 Mikuni main jets for a good runner at sea level. It will run better at idle, off idle and mid range with a DJ stage I kit. The 80-81 Standards had different emulsion tubes, needles and main jets than the Specials and you can get this from the shop manual and adjust accordingly. Before I close I would like to mention the needles again in the 80-81 Special and Standards. Both are slightly different (different part numbers) but both are too short and have the wrong taper to get the low and mid range correct. You get the correct needles which are adjustable in the Dyno-Jet kit and will make a world of difference in tuning these carbs. I dicked around with those stock needles for the better part of a year and never did get them right. I put the DJ kit in with DJ 130 mains and it completely cured my 2500 rpm glitch and made 2 more horsepower on the top end. The DJ kit was the best $118 I ever spent. Attached is a picture of a stock 81 Special needle compared to the applicable DJ needle and you can see what I'm talking about. Lastly, the 34 mm Mikuni carbs flow more than enough air to support a motor putting 100 horses to the tire and yes I'm familiar with so called modern Susuki carburetors and so forth, particually those from a 1200 Bandit. A 1277 or 1311 cc XS will like some new (old) 36 mm FJ carburetors (more air flow).

[81xsproject]

Dan, a great read as always! However, I wanted to mention that, unless I read the manual wrong last week, the 80's standards and specials use different jet needles but the same needle jets/emulsion tubes (Mikuni 300-x2). I cross checked the yamy parts catalog and all 80's carbs call for the same part (3H5-14941-02-00). I wonder if it had anything to do with the smaller rear tire on the specials...?

[Dan Hodges]

Emulsion Tubes X-2

Dan, a great read as always! However, I wanted to mention that, unless I read the manual wrong last week, the 80's standards and specials use different jet needles but the same needle jets/emulsion tubes (Mikuni 300-x2). I cross checked the yamy parts catalog and all 80's carbs call for the same part (3H5-14941-02-00). I wonder if it had anything to do with the smaller rear tire on the specials...?

They all used the X-2 tubes but the number of holes in the tubes varied with each model

.

I have at least half a dozen sets of 80-81 carbs and the ID numbers are acid etched on the main body of the carbs and the Standards are different from the Specials jet wise. I have a (new) never used set for an 80 Standard and it is apparently a bastard as the pilot jet tower has the rubber plug covering it. My Special came from the factory with 110 mains straight across and my NOS 80 Standard carbs have 115 in the middle and 120's on the outside. I have a used set of 80 Specials that are identical to my 81 Specials mechanically. I was told years ago by a Yamaha tech rep that Yamaha was dicking around with the emissions thing which caused the variation in jetting at that time. The short 16" wheel/tire deal was a bogus styling deal which also lowered the gear ratio slightly however there was a greater amount of increased tire technology devoted to 17" tires because all the early to mid eighties sport bikes used 17" tires, i.e. Susuki GS 1150 ES and so today you can get some good sporty compounds in those 17" sizes compared to the touring stuff in the 130/16" donuts. I have a 17/130 tubeless on my Special with a Bridgestone Battlax V rated tire which is the hot set up for an 85 GS-1150 Susuki which was putting 100 horses to the ground, two more hp than my Special but not as much torque. Oh yes, I have a cigar box full of XS emulsion tubes............I wonder what will ever happen to all my sh--?

[Guy_b_g]

Wouldn't that 80 setup have the 120's on the inside cylinders and the 115's on the outside two? Just checking 'cause I thought the idea was for the richer jets was to cool the inside cylinders.

[Dan Hodges]

Typo

Wouldn't that 80 setup have the 120's on the inside cylinders and the 115's on the outside two? Just checking 'cause I thought the idea was for the richer jets was to cool the inside cylinders.

You are correct, sorry about the typo. The 120's are on 2 & 3 and the 115's are on 1 & 4. I believe the emulsion tubes are different also on the center two. I suppose Yammie figured the Standards would be wearing fairings and so forth and in 81 they debuted the XS Eleven Venturer which was a Gold Wing with muscles. In 80 they had the same thing but hadn't named it yet. I'll correct my original post, thanks.

[Dan Hodges]

Typo Correction

They all used the X-2 tubes but the number of holes in the tubes varied with each model

.

I have at least half a dozen sets of 80-81 carbs and the ID numbers are acid etched on the main body of the carbs and the Standards are different from the Specials jet wise. I have a (new) never used set for an 80 Standard and it is apparently a bastard as the pilot jet tower has the rubber plug covering it. My Special came from the factory with 110 mains straight across and my NOS 80 Standard carbs have 120's in the middle and 115's on the outside. I have a used set of 80 Specials that are identical to my 81 Specials mechanically. I was told years ago by a Yamaha tech rep that Yamaha was dicking around with the emissions thing which caused the variation in jetting at that time. The short 16" wheel/tire deal was a bogus styling deal which also lowered the gear ratio slightly however there was a greater amount of increased tire technology devoted to 17" tires because all the early to mid eighties sport bikes used 17" tires, i.e. Susuki GS 1150 ES and so today you can get some good sporty compounds in those 17" sizes compared to the touring stuff in the 130/16" donuts. I have a 17/130 tubeless on my Special with a Bridgestone Battlax V rated tire which is the hot set up for an 85 GS-1150 Susuki which was putting 100 horses to the ground, two more hp than my Special but not as much torque. Oh yes, I have a cigar box full of XS emulsion tubes............I wonder what will ever happen to all my sh--?

No I'm not smoking but I made a typo on the jet sizes and it's the center two cylinders on the Standard that gets the 120 jets and the outer two get 115's. You gotta be on your toes on Channel Eleven lest someone will nail you on a mistake. Damn, I hate it when that happens to me!

[Pat Kelly]

Now you have me thinking about how my MNS is running.
It has the 1196 kit, K&N filter in the airbox, and a 4 into 1 exhaust with SuperTrapp baffle/discs.
The carbs had 110 or 115 main jets in all 4 carbs. I increased the jets to 125 and it took forever to warm-up and would die at idle unless it was hot outside and the bike was completely warmed-up. I swapped to 120 jets and took some discs out of the SuperTrapp and it got better. I might now try going back to the original jets.
The PO had done some terrible porting to the head. Cylinder #2 has some issues. I have 2 earlier heads, from a 78 and a 79. I know they have the smaller valves (higher velocity / less volume) yet 'hotter' cams. This bike is my 'hot rod' and when I'm in the mood, pushed for accelleration to 100 MPH. Usually I just cruise around town with it (the 78e is for highway cruising).
Does this make sense..... early head (small valve) with later (80) cams equal more torque for my short-burst forays? I do take it to nearly redline in all but 5th gear when I'm feeling frisky.
The only other mod I have planned (eventually) is to make a 'Y' pipe off the exhaust collector to run 2 SuperTraps. Cosmetic reasons only, I don't anticipate any performance differences (able to adjust with discs).

[Guy_b_g]

Hey Dan,

Don't feel bad, seems like every time I forget to re-check my posts there is a typo in there somewhere (as there "was" [twice] in my prior post). I enjoy reading your posts for the wealth of performance info, and the work you've done on these bikes.