so i got my truck smogged and registered last week. smogged very easily with the stock 2150. problem is thats its lean at anything above idle. its a 1.21" venturi 2150. do you drill out the main jets and run it or just bolt on a new 500 cfm holley 2 bbl ? does anyone know the cfm rating on a motorcraft 2150 ??
thanks in advance.
-chris

 

Have you tried to turn out your mixture screws? They are the two screws on the front base of the carb. They should be around 1 1/2- 2 turns out.

 

The mixture screws are for idle mixture control. The main jets are what controls off-idle mixture.


As for the 2150, the 1.21 is a 351 cfm carburetor. The 1.08 would be 287. How do you know it's running lean? Wideband O2 sensor installed or guessing based on some sort of drivability symptoms? Personally, I'd lean towards sticking with a 2150 - hard to beat 'em for a 2V carburetor. Great drivability, etc. and not as finicky as an aftermarket Holley.

 

Regarding the jets, you can order new ones - better bet than drilling them out, IMO:


Autolite & Motorcraft Carburetor Jets | 1100 2100 4100


Other suppliers exist too - just the first thing that popped up in a Bing search for me.

 

mixture screws were adjusted with a vacuum gauge, and theyre 1.5 turns out and yielded 19" of vacuum. as far as how do i know its lean? well at the 2500 rpm test on the smog machine, it only blew 38 hc's and im allowed 350. it pings and ive backed the timing off and added Klotz octane boost with no help. yes its lean !

 

From stoichiometric, going lean by any significant amount should actually improve knock resistance ... "pinging" is actually worst very close to stoich. Going rich definitely helps more than going lean does, but knock propensity falls off in either direction. That's not to say it isn't lean, just that the fact it's knocking only tells me it's not overly rich. Not sure what units those HC numbers are in, but 10% of the standard is pretty low; are you running catalytic converters? Low HC would indicate somewhat lean, especially engine-out.

What's your base timing set to now?

Either way, to enrich off-idle, I'd look at what size jets you've got and step up a bit from there.

 

the HC measurement is hydrocabons part per million. even with a small crower cam its lean. my truck is a 8100gvwr so no cats! my base timing is 10* btdc.
how does motorcraft number their jets ? for instance- if i have a #40 jet in it now, will a #50 jet be .010" bigger hole ?

 

38 ppm w/o cats? Dang. That is low! I was figuring it must be some other units!

Yes, the jets are sized in thousandths of an inch, so #50 is 0.050", #55 is 0.055" etc. IIRC, the stock jet size in the 1.21" version was something like #56.

 

toldja it was lean ! thanks for your help brother. hope to pull the jets out and see whats in it. i got the carb from Tx from a company who does killer rebuilds and a wet flow test/adjustment. i'll let you know what i find !

 

so maybe go up 10 numbers ? if i have a 56 jet, go to a 66 ?

 

Hey Chris usually carb tuners suggest jumping 2 jet sizes at a time. 10 is quite a bit, assuming the correct size carb is installed to begin with it "shouldn't" be necessary to jump 10 jet sizes. You can drill out jets to determine what size works best and save a little money, but factory jets are flow tested, drilled jets are not recommended for permanent use.

 

Sorry, I don't have good intuition on how much of a change to suggest. I haven't done a lot of carb tuning at that level (most of my experience is with modern engine technology), just understand the theory. Going from 56 to 66 would be almost a 40% increase in cross-sectional area - seems like a lot to me, but not sure how lean you are and how much you need to enrich.

 

i think i will break out my jet drills and go a very little at a time. once the "correct" size is determined, i'll purchase the proper jets. i also have the original 2150 that came with it, maybe i can read the jet number size on them !!!

 

welp, no luck reading the numbers on my old jets. nothing on top or sides. guess i'll have to take the top off my new one thats on the truck

 

Sounds like a good plan. Or as a first attempt, maybe just grab the jets from that one and swap 'em in to the current one and see if they're in the right ballpark.