Determining correct jet size

[Petescj]

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.

 

[jeepardy]

What size jets you running now. The MC2150 I picked up actually had 53's in it and ran okay, idled like 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
​

 

[Purple77CJ7]

Here is something I found that may help. Basic Jetting Theory and Procedure - Aircooled.Net Tech Article
If you read down a ways it has some formulas.

 

[Old Dog]

I always used the redneck theory, buy some 2 sizes smaller and see what happens.

 

[dalefan88]

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.

 

[CJ007]

I always used the redneck theory, buy some 2 sizes smaller and see what happens.

x2 on anything thats does with redneck theory

 

[elwood blues]

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...

 

[Petescj]

I always used the redneck theory, buy some 2 sizes smaller and see what happens.

x2 on anything thats does with redneck theory

You laugh but thats exactly what came to my mind.

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.

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

 

[Petescj]

What size jets you running now. The MC2150 I picked up actually had 53's in it and ran okay, idled like 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 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

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...

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.

 

[Petescj]

Mine were 48s. Gonna drop down to 46s like the rest of yas.

The redneck theory wouldve worked perfect .

 

[dalefan88]

You laugh but thats exactly what came to my mind.



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

no problem. i did alot of reserch on this when i did mine. i didnt want to do it twice.

 

[Petescj]

Apparently parts stores in this area only sell kits. Pretty bad when they tell you your better off going online.

 

[Petescj]

Apparently parts stores in this area dont sell just jets. Rebuild kits ( probably with the wrong size) and a buttload supply of different sizes collectively. Pretty bad when they come out and tell you your better off online. Off to http://www.summitracing.com/search/Department/Air-Fuel-Delivery/

 

[elwood blues]

Autolite & Motorcraft Carburetor Main Jets


Ford Motorcraft 2100 2150 2 barrel carburetor Parts Page


MOTORCRAFT/FORD/AUTOLITE MAIN JETS


Summit doesn't carry them, they only carry Holley jets

 

[Petescj]

Autolite & Motorcraft Carburetor Main Jets


Ford Motorcraft 2100 2150 2 barrel carburetor Parts Page


MOTORCRAFT/FORD/AUTOLITE MAIN JETS


Summit doesn't carry them, they only carry Holley jets

Thanx yet again.

 

[elwood blues]

Thanx yet again.

 

[Petescj]

Ordered from Mikes. Ordered 2 pair. This should end my what seems to be never ending carb saga.

 

[johnbilt]

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.

 

[elwood blues]

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.

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.

 

[dalefan88]

i had to get my jets online to. holley jets are to small.