• Hello Guest, we are proud to now have our Wiki online that is completely compiled and written by our members. Feel free to browse our Jeep-CJ Wiki or click on any orange keyword when looking at posts in the forum.

Determining correct jet size

Determining correct jet size
Petescj

Petescj

Member
Lifetime Supporter
Posts
23,102
Media
190
Solutions
1
Thanks
184
Location
PA
Vehicle(s)
1982 CJ-7 258 / T-18 / Dana 300 31 in All Terrains.
1978 CJ-7 304 / TH 400 / Quadratrac 32 in Mud Terrains
Is there some formula out there to determine what is the correct jet size for your carb?

Im running a Mc2100 on a AMC 258 i6 / 4.2l . While Ive been able to get it to run super smooth now it'll burn your eyes and stink up the garage if I leave it idle for a bit . Ive turned the idle mixture screws down but it doesnt perform well at all.

I think Ive found the happy spot on the carb and that the jets are just too big. Must I determine how many degrees above sea level Im at and go from there? Any additional info and pictures would help. :D
 
What size jets you running now. The MC2150 I picked up actually had 53's in it and ran okay, idled like :dung: and gobbled gas. Had a pair of 50's switched to and runs better, just not where I want it. From what I've read AMC 258 i6 / 4.2l 's need 48's or 47's with a 6.5hg power valve to run clean. If anyones got a specific formula for this calculation, please post link
​
 
I always used the redneck theory, buy some 2 sizes smaller and see what happens.:D
 
i got 46 jets in mine and it runs fine. mine came with 53 jets but i screwed them up when i rebuilt the carb.
 
Old Dog said:
I always used the redneck theory, buy some 2 sizes smaller and see what happens.:D
Click to expand...

x2 on anything thats does with redneck theory :punk:
 
Here's what I've found for you Pete;

"Holley carburetors are calibrated for sea level operation and an inlet air temperature of 70 degrees Fahrenheit. Once you know the correct stock jetting for your particular Holley carb, you can determine whether you live or race at an altitude above sea level. For every 2000 feet increase in altitude, you can reduce the jet size by one size. If you had a carb which has a stock jet size of 80 and you live or race at 2000 feet above sea level, then you would use a #79 Holley jet in the carb. Similarly, a change in the carburetor's inlet air temperature may require a change in the jet size from the stock calibration. Many racers go a step further by combining all of the weather variables, temperature, barometric pressure, dew point and humidity with the altitude of the track they are racing at to determine the "density altitude". This is a "corrected" altitude above sea level. From there they can determine whether a jet change is necessary to maintain performance or whether to change their "dial in", if they are bracket drag racers."

Now, because the Autolite or Motorcraft 2100 carb is a Holley design carburetor this should, be the correct answer. The #1 problem though is that very few of us know what our original "stock" jet size is. So, it really turns in to trial and error. I think I'm running 46's. But heck, I remember reading of guys that ran one 46 and one 47 or something crazy like that to get it to work the best.


Found this little tidbit of info on a random website;

"Since main jets are replaceable, its size does not necessarily determine carb's matching engine size. Stock 302 carbs came with 48F jets while all other engines (351/390/429) except for manual Transmission 351 came with larger 54F jets. Manual 351s came with 56F jets"

Again, not a ton of help, my carb came off a 289...
 
Last edited:
Old Dog said:
I always used the redneck theory, buy some 2 sizes smaller and see what happens.:D
Click to expand...

CJ007 said:
x2 on anything thats does with redneck theory :punk:
Click to expand...

You laugh but thats exactly what came to my mind. :D

dalefan88 said:
i got 46 jets in mine and it runs fine. mine came with 53 jets but i screwed them up when i rebuilt the carb.
Click to expand...

46 - 48 seemed to be the numbers that were tossed around most in what Ive dug up lately. Thanx Dale
 
jeepardy said:
What size jets you running now. The MC2150 I picked up actually had 53's in it and ran okay, idled like :dung: and gobbled gas. Had a pair of 50's switched to and runs better, just not where I want it. From what I've read AMC 258 i6 / 4.2l 's need 48's or 47's with a 6.5hg power valve to run clean. If anyones got a specific formula for this calculation, please post link​
Click to expand...

I got mine off Ebay rebuilt , but havent determined the jet size yet. I have one on each Jeep and Im willing to bet both are over jetted. 46-48 is what Ive been coming up with as well

elwood blues said:
Here's what I've found for you Pete;

"Holley carburetors are calibrated for sea level operation and an inlet air temperature of 70 degrees Fahrenheit. Once you know the correct stock jetting for your particular Holley carb, you can determine whether you live or race at an altitude above sea level. For every 2000 feet increase in altitude, you can reduce the jet size by one size. If you had a carb which has a stock jet size of 80 and you live or race at 2000 feet above sea level, then you would use a #79 Holley jet in the carb. Similarly, a change in the carburetor's inlet air temperature may require a change in the jet size from the stock calibration. Many racers go a step further by combining all of the weather variables, temperature, barometric pressure, dew point and humidity with the altitude of the track they are racing at to determine the "density altitude". This is a "corrected" altitude above sea level. From there they can determine whether a jet change is necessary to maintain performance or whether to change their "dial in", if they are bracket drag racers."

Now, because the Autolite or Motorcraft 2100 carb is a Holley design carburetor this should, be the correct answer. The #1 problem though is that very few of us know what our original "stock" jet size is. So, it really turns in to trial and error. I think I'm running 46's. But heck, I remember reading of guys that ran one 46 and one 47 or something crazy like that to get it to work the best.


Found this little tidbit of info on a random website;

"Since main jets are replaceable, its size does not necessarily determine carb's matching engine size. Stock 302 carbs came with 48F jets while all other engines (351/390/429) except for manual Transmission 351 came with larger 54F jets. Manual 351s came with 56F jets"

Again, not a ton of help, my carb came off a 289...
Click to expand...

Yeah I dont know what motor mine came off of anyway. Im gonna play around with that calculation though and see if I come up with a number in the 46-48 range just for fun.
 
Mine were 48s. Gonna drop down to 46s like the rest of yas.

The redneck theory wouldve worked perfect . :laugh::laugh:
 
Petescj said:
You laugh but thats exactly what came to my mind. :D



46 - 48 seemed to be the numbers that were tossed around most in what Ive dug up lately. Thanx Dale
Click to expand...

no problem. i did alot of reserch on this when i did mine. i didnt want to do it twice. :D
 
Apparently parts stores in this area only sell kits. Pretty bad when they tell you your better off going online. :mad:
 
I've been tempted to install one of those air/fuel ratio sensors with the little led display. There are several listed on summit racing. You can instantly tell if your motor is running rich or lean going down the road. Then you can adjust to fit and make sure it's exactly right.
 
johnbilt said:
I've been tempted to install one of those air/fuel ratio sensors with the little led display. There are several listed on summit racing. You can instantly tell if your motor is running rich or lean going down the road. Then you can adjust to fit and make sure it's exactly right.
Click to expand...


The only thing with those air/fuel gauges is that they connect to an O2 sensor, and if you don't have an O2 sensor in your exhaust system already, then you have to install one.
 
i had to get my jets online to. holley jets are to small.
 
You must log in or register to reply here.

Jeep-CJ Donation Drive

Help support Jeep-CJ.com by making a contribution.

Help support Jeep-CJ.com by making a contribution.
Goal
$200.00
Earned
$0.00
This donation drive ends in
0 hours, 0 minutes, 0 seconds
  0.0%
Contribute
Back
Top Bottom