Autolite 2100 Carburetor Stock Jet Size?

[nosaj2k]

Wow, information on these carburetors is getting scarce. I am running an Autolite 2100 carburetor, 1.08 venturi on a relatively stock 1970 Bronco. For a baseline, I am trying to determine what the stock jet size was in these carburetors. Did jet size vary based on sales region? Were all vehicles jetted for sea level? I thought for sure I would be able to find a standard "jet sizing chart" on the inter-web. The jets that were installed in this particular re-manufactured carburetor are 48's. I am kind of running on the assumption that these were a common "universal" jet size for these carbs. I removed them and installed 46's for altitude (6,400').

Thoughts?

 

[DirtDonk]

Sounds reasonable, but my memory is sketchy at this time. I seem to remember 46's being in my '71 but maybe it was the 48 instead. Too close to call.
It was living in Montana prior to it's being brought here however, so maybe the 46 was correct and it also had been modified for higher altitude? Seems plausible.

Be good to find a reference guide for sure.
Good luck.

Paul

 

[Rustytruck]

47 and 48 were common. 46 would be for high altitude
My smog encrusted 1974 los angeles delivered 302 had 47F jets

 

[mimmonen]

I am running 46 jets living in Reno and running between 4,400ft & 8,000ft altitude. Carb had 48s in it when I bought it.

 

[tirewater]

Start at #48 and tune for drivability. Check your spark plugs to see where you stand, and go from there.

Right now I have #52 jets. However technically my 302 is a 306, and I have an aftermarket intake. Everything else is stock including air filter. My spark plugs are clean as a whistle, so I'm going to try #54s to see what happens. I'm at sea level, and have tried #48, #50, #52 jets.

How do the #46 jets drive?

 

[nosaj2k]

Great input. Thank you all for the information! Tirewater - It seems that for as long as I can remember, the Bronco has had a nasty hesitation upon acceleration, this while using the 46 jets. I have replaced the accelerator pump a few times (the last two I put in backwards, so yeah - I'm sure that had something to do with it) %). I guess aside from this issue, and hard starting (cold starts are fine - the choke system works property), the 46 jets seem to be OK? I pulled a few plugs, and they actually indicate a rich condition. So... I went through the carb, installed the accelerator pump correctly, and re-installed the 46 jets. I actually haven't had a chance to tune/drive because the weather here has been so nasty. Mostly doing spring cleaning inside the house ;D.

 

[DirtDonk]

Definitely need a good working accelerator pump, but a hesitation off-idle and under acceleration is usually a sign of an over-lean mixture.
Unfortunately hard to determine by the plugs if certain circumstances are present. If they've been in there a long time and you have not looked at them before this, there's no telling what else they were put under. But an over-lean condition can lead to what's called "lean burnout" where the flame front just does not do it's full thing and leaves unburned fuel. This can sometimes look like a rich mixture with sooty smoke coming out the pipe(s).
Which means that in theory it can also leave deposits on the plugs that looks like it's running rich. More often than not you'll get the correct reading on the plug so that if it looks dark and sooty, it does mean that it's too rich. Just that it's not universally so.

Naturally! I mean, we don't want our Broncos giving us the correct clues every time after all. Right?%)

Of course, back to your symptom, even with it being rich you can get a bogging if your pump is not squirting enough.
So you still have to get the whole carburetor dialed in.
The cold starting does not mean the jets are too large either. Remember the choke is closed/on at that time, so even a super lean setup can start easily in the cold and run like poop when the choke is back open when warm.
Just another thing to think about.

Paul

 

[tirewater]

Another variability is the carburetors themselves. The stock carburetors are 40+ years old and many have been through multiple rebuilds. Some carbs just won't work regardless of the fiddling as things such as throttle shaft wear can start to appear.

My first 'reman' carb I received was the 1.21 sized 2150, which had a hesitation on initial throttle. I tried to get it to work, but ultimately sent it back for a 1.08 which has worked fine for the most part.

I've had a slight hesitation after I bought some fancy brass idle screws. Putting the stock screws back in eliminated the problem. I'm not sure what was wrong with the brass screws - whether they let too much air in, the ends were too pointy, size was off, etc...

Point is, perhaps try another 2100/2150 carburetor to see if that helps or hinders your driving issues.

 

[nosaj2k]

All great points - thanks fellas. Why go to modern fuel injection when you can toy with these old carbs? Actually, I’m trying to keep the engine and engine bay looking stock. So, I will start fresh with the plugs, tune the carb and go from there. I’ll keep everyone posted as to how things play out.

 

[nosaj2k]

Update

So just a quick update. I decided to use #47 jets as opposed to the #46's, along with a correctly installed accelerator pump %). After dialing things in, using a vacuum gauge to set the idle mixture screws, and driving, I am very impressed. No hesitation, and improved acceleration & power. The spark plugs are no longer completely black, but a light tan. Crazy. Thanks for the input!

 

[BrettKoenecke]

This place is a treasure trove of information. Thanks to all of you.

 

[DirtDonk]

Thanks for the follow up report nsaj. Good info. And with you at the 6000-ish foot mark too, that's another data point for others to reference in their search for the right tune.

Glad it's working out.
Now you can play with different PCV valves as a "controlled vacuum leak" for when you go to higher altitudes!%)
It's what I used to do with mine. Being tuned for sea-level but playing at 5,000 to 9,000 feet used to give my carb fits. Using the "wrong" PCV made changing jets a thing of the past at least for me.

But that's another story... In your case, you might be good all the way with what you have.

Paul

 

[jckkys]

. Hesitation is usually caused by slack in the pump linkage. ANY movement of the throttle needs to have a corresponding stream of fuel from the nozzles in the booster venturi. The accelerator pump can be working perfectly once the linkage presses on the diaphragm and the engine still stumbles. The jets are only a small part of the picture and they were set at the factory for a particular carb. They can vary by a couple sizes in carbs made for the same altitude. There no particular jet size for say, sea level. They sometimes used 2 different size jets in the same carb. A mean or average of several 2100s will get you close, but it's only a starting point to begin tuning. A/F gauges or plug color reading are needed to see where you are. There are instructions on Holley carb tuning in books and web sights. The principals are all same. The difference is a lack part selection for the 2100s.
I keep a second carb for over 5000 ft. and one for sea level to about 5000 ft. There are too many changes; jets, choke, power valve, accelerator pump linkage, and idle mixture/speed to use the same carb. So in the west you get to play with carbs a lot more than eastern states.
Timing can also be advanced to keep some power as the altitude makes the air thinner and compression is decreased.