Topic: Carburetor Jetting Guidelines from www.motorcyclecarbs.com

[obnoxshuz]

This document was taken from :  http://www.motorcyclecarbs.com/Jetting_Rules_of_Thumb_W43C551.cfm



This document is based on Mikuni Carburetors. But the theory follows through on all carburetors. Be aware that Mikuni Main jets increment in steps of 2.5, Hitachi Main jets increment in steps of 2.0. Hitachi Main jets (screw in style [Yamaha XJ's] are available in sizes from 110-134 in increments of 2.0. All this means that if you find you need to go up 4 steps on the main jets according to this chart (2.5x4) the step up would be 10.0. For Hitachi mains the step up would be 5 jet sizes (2.0x5) since following this chart for Hitachi mains would put you at 8.0 which is very close to 7.5 which would be 3 steps according to this chart.

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

2 jet sizes for both single K&N and drilled airbox

2 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 Color Tune. (See "Color tuning 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 acceleration from off idle to 80+ mph, and steady pull past 120 mph. My "F" has 65K mile on it, of which I have put 33K with this jet configuration.

Happy Jetting

Additional info: Decrease main jet size one step per 2000' above sea level. Decrease pilot jet every 6000' above sea level.

[colab]

I don't understand any of that. Is there a dummies guide you could suggest?

[750goes]

Ok, here is my setup - can you tell me what jet sizes I should have....

1975/76 750F

Stock carbies
Jet sizes - Mains 105, Idles 40

4 into 1 pipe - baffled
Individual pods

next to sea level in height

bike currently runs RICH - idles at 1000
carbs synched, and clean as a whistle..
air screws about 1 1/2 - 2 turns out

what's your suggestion for jet sizes

thanks

[crazypj]

Ok, here is my setup - can you tell me what jet sizes I should have....

1975/76 750F

Stock carbies
Jet sizes - Mains 105, Idles 40

4 into 1 pipe - baffled
Individual pods

next to sea level in height

bike currently runs RICH - idles at 1000
carbs synched, and clean as a whistle..
air screws about 1 1/2 - 2 turns out

what's your suggestion for jet sizes

thanks

The pods allow the back of carbs to 'sink' and thats probably causing rich condition, it raises fuel level in float bowl
I found you have to drop needles one notch (clip towards top) and go up at least 2 sizes on main jets.
 CB550 likes 112 mains, 750 likes 117.5 with this set up.
 
check float levels while you have carbs off and set to highest level of range ( eg, if float height is 21~23mm set to 23mm)
PJ

[apetersonboy]

So, if I have wide open 4 into 2 exauhst (no baffles), should I go up one or two?

[NewOldSchool]

Ok so here's what I've got as my base line.

1974 CB750K4: Stock Main = 110 and Stock Slow = 40

K&N Filter                          +1
Holes Cut in Stock Air Box +1
New Kerker 4 into 1          +2
Standard Subtraction        - 1
                                         = 3 Sizes Larger
One Size Larger on Slow   +1

So Main Jet = 117.5
So Slow Jet = 42
Pilot Screw = 1 Turn Out

I've done a lot of reading and many "jet kit" suppliers ran cylinders 2 and 3 (the inner cylinders) a step richer.

Is it worth trying 117.5's in 1 and 4 and 120's in 2 and 3? Or should I try 115's in 1 and 4 and 117.5's in 2 and 3?

[Jerry Rxman Griffin aka MuthaF'er]

Fine print from home boy states that EACH increase in HIS jets is 2.5 WHEREAS ours is 5. So by his analogy if you did, say for instance, a Kerker 4 into 1 would be one normal jet size for us, ie +5. BUT, which Kerker baffle you using?!! I'd say street baffle would be one size while comp baffle may be 1+.

NewOldSchool, I'd say that if your's is NOT running slightly rich at 110 then I'd go 115 first. I'd look for slightly rich irregardless of actual size and decide based on how it runs that way.

750goes, Your's does not compute  You already had a 4 into 1 so that doesn't count. Sea level does not count since most of Japan is near sea level. My 105's ran fantastic around 300' or so. So, PODS should increase your size BUT you are rich at factory settings with slight mods 

I was running 115's vs stock 105's at 6900' in my 75 750F. Kerker with street baffle in place of factory 4 into 1. K & N with 4 holes drilled into air box (looks cool anyway). 812 Yoshi kit with Yoshi cam. Needles in stock middle position. Ran slightly rich but like a scalded ass ape when I cranked it. Slight elevation increases at this setting got a little too rich.

[NewOldSchool]

Fine print from home boy states that EACH increase in HIS jets is 2.5 WHEREAS ours is 5. So by his analogy if you did, say for instance, a Kerker 4 into 1 would be one normal jet size for us, ie +5. BUT, which Kerker baffle you using?!! I'd say street baffle would be one size while comp baffle may be 1+.

NewOldSchool, I'd say that if your's is NOT running slightly rich at 110 then I'd go 115 first. I'd look for slightly rich irregardless of actual size and decide based on how it runs that way.

750goes, Your's does not compute  You already had a 4 into 1 so that doesn't count. Sea level does not count since most of Japan is near sea level. My 105's ran fantastic around 300' or so. So, PODS should increase your size BUT you are rich at factory settings with slight mods 

I was running 115's vs stock 105's at 6900' in my 75 750F. Kerker with street baffle in place of factory 4 into 1. K & N with 4 holes drilled into air box (looks cool anyway). 812 Yoshi kit with Yoshi cam. Needles in stock middle position. Ran slightly rich but like a scalded ass ape when I cranked it. Slight elevation increases at this setting got a little too rich.

Thanks for the advice. Yes its the Kerker 4 into 1 with the street baffle. Right now I have 115 mains in carbs 1 and 4 and 117.5 in carbs 2 and 3. I have stock 40 slow jets and the stock needles are one clip lower (richer) than stock.

I am still reassembling the motor, I just did a light hone and new rings as well as had the head rebuilt with new guides and valve seats cut.

I'll check the plugs after it breaks in.

[bigcity81]

I have a 1975 cb750f with a 4-1 no muffler open pipe and no air box or pods. Bike hasn't been started since I've owned it I know it has 125's in it last I looked any suggestions? 

[Gizmo]

Looking for tips;
73 CB 750
bored out to 836cc
K&N pods
4-1 exhaust
130 mains
40 idles
needle clip 4th from top
mixture screw currently 1 full turn out


Having synced the carbs, i started playing with the mixture screws to compensate for the excessively rich state it is currently sitting in. Despite having backed all mixture screws out in 1/16 increments to the point where they were all 2 full turns out the bike is still excessively rich. My instinct is to change needle position to choke some of the flow of gas.

thoughts?

[HondaMan]

Looking for tips;
73 CB 750
bored out to 836cc
K&N pods
4-1 exhaust
130 mains
40 idles
needle clip 4th from top
mixture screw currently 1 full turn out


Having synced the carbs, i started playing with the mixture screws to compensate for the excessively rich state it is currently sitting in. Despite having backed all mixture screws out in 1/16 increments to the point where they were all 2 full turns out the bike is still excessively rich. My instinct is to change needle position to choke some of the flow of gas.

thoughts?

Change the mains to 120, drop the needles to center notch, and put the air screws to 1 turn. The air screws only adjust between 3/4 and 1.5 turns: more in than that does not make any further lean-ness, and out past that doesn't make it any richer - UNLESS something else is wrong, like leaking vacuum hoses (then it will be rich at low RPM on the affected cylinder). Pods will cause it to be rich from 1500-3000 RPM, and then it will fall lean until 5000-5500 RPM, after which it will straighten up again: this is due to the disruption of the airflow across the emulsifier's air jet in the bell area of the carbs. Until you do something about that (which is mighty hard to do!) the problem of uneven mixing will always be with you.