Chart Drill Size~Jet Size Standard~Metric Jet Values.

Reference Chart for Drilling-out Jets to Next-up or Different Size.

 

Rick,

I have a question.

I replaced my stock exhaust for supertrapp exhaust. 4 into 2. That is the only mod I have done. The calculations calls to upgrade the main jet from 120 to 122. I have almost 4 turns out on the pilot screw.

Now the question is: should I go for 123 instead of 122? and Once I do this I supose to go down in turns for the pilot screw?

Thanks,

Carvall

 

I may be going out on a limb here Carvall, but usually if you are increasing the the output of the exhaust you may want to consider increasing the air going into the motor with say a K&N air filter. Now that it's breathing out better, you'll want it to be able to breathe in better too.
I'm thinking the stock air filter might be too restrictive and that may affect your tuning of the carbs.
You will probably have to do some re-jetting and possibly move the needles. Unless someone on here has done the same as you it'll be trial and error or throttle chops or dyno runs. Just sayin'.
All that being said, iff you add more plates to the super trapp to bring it back to stock back pressure then you may not need to re-jet at all.

 

Putting more plates will makr it leaner! I think with 5 plates is pretty close to stock. But if I add more disks as far as I understand, correctme if I'm wrong, I need to increase the main jet in order to compensate for the air input and have a richer mixture. Right?

 

I would only recommend that you try what others who have the same Mods have done.

But, be prepared to experiment with different settings. I wish there was a definite answer to how to re-jet.

 

I swapped to a shorter float needle and seat. Now its running rich to the point that I've almost turning in the mix screw completely closed.
Would this chart help me out?

 

what should I use to get an exact measure of the jets in my bike. My mpg sucks so I'm thinking the po may have enlarged the jets. How much would it take to decrease mpg to about 31mpg, and pilot screws having to be set between 1 1/4 and 1 3/4?

 

new jets

 

WHOA!!!!!!
Holy old thread revival Batman!!!!!!!!!!!!!
Almost a year and a half.

 

The Fuel Economy is LESS DEPENDENT on Jet Sizes and Pilot Air Screw Settings than General Engine Condition and "How you ride".

Decreasing a MAIN Jet Size could possibly result in a Critically LEAN Condition causing Overheating and potential catastrophic damage such as "Holing" a Piston. Burning through the Piston Crown because the HEAT generated by a LEAN Mixture MELTS the aluminum piston.

Invest in a COLORTUNE Spark Plug.
Set the Air-Fuel Ratio to be optimal for the state that your engine is capable of producing due to its age and mileage.

Alterations to the Stock Intake and Carbs may result in performance problems not easily rectified through re-jetting, installing aftermarket performance-related items or Stage-Kit modifications.

 

i just got my 81 xj750 its completely stock but im looking to put a four in to one exhaust and k&n unifilters.. im guessing i should definitely run bigger jets right? and if so is it just the main jets or the piloits jets also ?

 

When you embark on the modifications you plan to make; take every precaution not to damage the Stock Parts you remove in the process.

To this day, XJ-Owners are still experimenting with processes to eliminate "Fine Tuning Issues" related to BOTH Intake and Exhaust Modifications.

There is a distinct possibility that you will encounter Performance Related issues after installing PODS and Exhaust Components that dramatically reduce Engine Back Pressure.

In the event that you are unable to find a degree of performance you find satisfactory without the lean conditions and mid-range tuning issues that occur subsequent to PODS and ALL -to- One's, ... having the Original Parts saved instead of destroyed will be a big plus.

 

Hey all.... any idea what bit if possible you would use to drill out a pilot jet to a size 42?

 

is chart for hitachi carbs or mikuni oer keihin, cuz i need a hitachi chart, and these weird sizes are beging to make me think this chart is for mikuni to muy knowledge hitachi only did even numbers

 

Those are the Metric Sizes for Carb Jets.
They are Industry Standard.

 

okay so i justy finished drilling out my mains like one size bigger i think i cant tell this chart is not all that helpful. I need to know wat 0.055 is in inches, cuz i don't have any drill bits that say 0.055 inches or 1.30mm, so i am confused as to to what the actual siz e of the drill bits are in standard. Any who i digress i now have a problem with at half throttle it chokes itself out and chugs and pops, so know i reckon i need to bump up the pilot which is a 40 to like a 42 or something?
any thoughts?

 

"I need to know wat 0.055 is in inches"

That actually is "fifty five thousanths" in inches.

"or 1.30mm"

and it's 1.40MM , which is too big for a Main Jet on any XJ -

"i now have a problem with at half throttle it chokes itself out and chugs and pops"

Sounds like you went too far, and need to start over - start a new thread with all details. XJ4EVER has all sizes of new jets.

 

This Thread should be limited to discussion of:

Drill Sizes -and- Jet Sizes

 

i did start a new thread ,but i new it was 55 thousands of an inch but like is it a 1/16 or 3/68? my drill bits don't go by the thousandth.

 

If you have FRACTIONAL Drills, ... you don't have the right size.

But, ...

If you had NUMBER Drills, ... and you need a .055 Inch Hole ... you would select a Number 54 Drill.

http://www.csgnetwork.com/drillsizeconvert.html

http://www.harborfreight.com/60-piece-t ... 34627.html