Basic advice for selecting main jets?

[Anonymous]

I've got the Autolite 2100 running pretty good, other than some rough idle when warm and held against the brake. The jets that came in it were 51's, but I drilled them out to 62's thinking I was too lean. Since then I've ordered 3 more sets--52's, 56's and 60's. Question is, what exactly am I looking for as I try to find the optimum size? In other words how do you tell a rich condition from a lean condition at cruise?

 

[CZLN6]

Howdy 65 six:

The rough idle you describe is likely not the main jets size. That is the idle circuit. We typically have to turn the air screws out three full turns to get away from a too lean idle condition of 2100s on 200s and 250s.

You didn't say what 2100 you are using. What car did it come off of? What distributor are you using? The idle circuits varied greatly even in the same size 2100. These carbs may be the simplist carb ever to rebuild.

Assessing main jets is one of the last carb tuning steps we do. More often than not, the main jet does not need to be changed, and then only a size or two. Before you start on the carb, make sure the ignition is in the best shape possible. 1st assess and tune the idle, next transitions, cruise and then Wide Open Throttle (WOT). The main jets are not much visible until highway cruise speeds are reached or at WOT.

Before you begin assessing main jets, again, make sure your ignition is in tip-top shape.

At a cruise, at steady throttle, if your engine surges or "hunts" as in won't stay at a steady rpm you are likely lean. Verify this by checking the spark plugs for signs of glazing or white hot ashing. Before you decide to change jets, make sure the carb is clean and all orifaces and channels are open.

For too rich, look for black smoke out the tail pipe and black or wet plugs.

When assessing for WOT, it is best to start with a fresh set of the correct heat range spark plugs. Have the engine completely warmed up and no choke. Then do a WOT acceleration run, in 2nd gear from 2,500 to 4,500. Immediately shut off the engine, stop the car and pull several plugs to assess. You are looking for a light tan color on the porcelain. Lighter is lean, darker color is rich- or possibly a sign of other problems, like oil contamination.

Many factors effect optimum jetting- things like temperature- ambient and engine, barametric pressure, humidity, elevation, quality and condition of ignition and engine. So don't get too anal. In general, it is best to jet slightly on the rich side, but optimum power tends to be achieved on the lean side- BUT- so does engine damage so be careful.

That should get you started.

Adios, David

 

[Anonymous]

my autolite seems to run well at high speeds with 52 jets and I have found my best idle comes at a little more than 1 and 1/2 turns of the idle screws, but like czln6 the carbs vary a good deal

try starting with the idle screws two turns out then when the car is fully warmed and with the choke fully off and the aircleaner on slowly turn the idle scres out maybe a 1/4 turn at a time untill the idle speed starts to drop from a rich mixture, then turn them back in untill it drops from lean, then back out again untill you reach the fastest idle speed, try to keep the two screws at the same number of turns not more than 1/4 or 1/2 different from each other

 

[Anonymous]

Thanks David and 64Falcon for the thoughtful responses. Sorry for the missing info.

The 2100 is a 1.14, bought rebuilt on Ebay from a reputable supplier (kpraxl). Ignition is a new store bought DS-II. New Accel wires (cut to fit) and new plugs, Autolite 46's.

I don't ever see any smoke, and the engine pulls steady at cruise and WOT--no surging. It actually runs very, very well with the exception of struggling a bit when sitting for a while at a red light (it's an automatic). The idle is set at 900 in Park, 550 in gear.

The timing does perplex me a little bit. I've got it set at about 12*, and it still detonates if I'm cruising uphilll and mash the throttle--rattles pretty bad if it doesn't downshift, which it does sometimes. I mixed in half a tank of premium this weekend to see if it helps, but I haven't been able to test it again since. Retarding the timing below 12 makes it idle rougher in Park, but it makes the stop light problem better--In other words:
At 14*--purrs like a kitten in Park, detonates like crazy at WOT, struggles at red lights and gets worse the longer it sits there.
At 10*--idles rougher in Park, doesn't detonate at WOT, and idles at red lights without getting worse the longer it sits.
At 12*--best compromise I've found between the two conditions.

I do know that I have a 170 head on my 200; I measured the chamber and intake volumes to confirm, if that makes any difference. I'm using a Felpro gasket from the parts store, not the steel one. Sparks plugs are dry with black/brown insulators, no discernable difference from plug to plug.

At this point I'm just trying to dial it in to the best it can be, but I'm about at the end of my technical knowledge. Thanks for the help.

 

[Anonymous]

changing the timing by itself could cause the rough idle, you will need to readjust the mixture etc.. when you change it, prolly you know that already. It sounds like you have the exact same setup as me with the exeption of the 170 head (that is where your detonation problem is coming from) on mine I can turn the advance up even higher than that before I get any detonation. even running the cheapest gas I can find.

I would try keeping the advance at 10 and maybe turn the idle speed up a little then re tune.

another thing. how do you have your vaccum advance hooked up? the ported vaccum on the carb or full manifold?

also what kind of coil are you running with your DSII? (just out of curiosity)

 

[CZLN6]

Howdy Back 65 six:

thanks for the update. It sounds like an off idle transition problem rather than a main jet issue. Things to try-
*turn the idle screws out a half a turn more than ideal on a vacuum guage.
*Run a can of Techron through the tank to eliminate the possibility of carbon deposits.
*Review your vacuum source to the distributor as per 64 falcons querry. You should be using the ported vacuum source with a stock cam and automatic trans.
*Verify that your TDC mark is really at top dead center.
*verify that your accelerator pump is squirting when levered.

What holes do you have the accelerator pump rod in? You may need a bigger or quicker squirt to cover the off-idle transition. You should have two holes at the pump lever and 4 at the over-reach/throttle shaft lever.

The FelPro gasket should have brought your CR back to the 8.5:1 range so I don't think that is the problem.

Keep it coming.

Adios, David

 

[Anonymous]

David and 64,
Vac advance is connected to a ported source--above the throttle plates, on the choke spring housing side. I have the Clifford adapter, which has a vacuum port below the throttle plates. I've tapped into that and then T'd off to the PCV and the brake booster (Stainless Steel system). The port on the manifold itself goes to the transmission. Vacuum holds pretty steady at about 18.5 inches at idle.

The coil is a relatively new Flamethrower, 12 volts to the terminal.

My idle screws will cause a lean stumble only when they are almost completely closed. I can't seem find the rich stumble point no matter how far I back them off. Once they're backed off of lean stumble, vacuum readings don't really change. They are currently set about about 2 3/4 turns out from lean stumble.

I will run some Techron through, but I doubt that carbon deposits are present. I had the head off this winter and cleaned up all the cylinders, chambers and valve faces. Also installed a new 1.6 rocker arm assembly from FSPP.

I verified TDC when I had the head off; it's right on the mark.

Pump squirts when levered.

Accelerator pump rod is in the second hole from the top on the throttle linkage end, and in the inboard hole (the closer to the pump of the two holes) on the pump end.

Another thing I didn't mention already is that I get a stumble when I accelerate out of a right turn. I assume this is because the fuel sloshes to the front of the bowl, which is mounted sideways because of the Clifford adapter. I think this is an unrelated problem that I can fix with some sort of baffle later.

Thanks again for the help.

Mark

 

[rbohm]

8) mark, just because you had the head off and cleaned the valve faces doesnt mean here isnt any carbon build up. you may find that there is carbon build up on the backside of the valves. this will cause a few problems;

1: it will cause a lean mixture at times, and a rich mixture at others, making for a major headache when troubleshooting problems like you have.

2: it will make the intale valve run hotter as carbon tends to be an insulator, and keeps more heat in the valve head, and that can cause detonation, especially when the mixture goes lean.

3: carbon buildup will also reduce airflow into the cylinder, thus causing a slight power loss.

it plays havoc with fual economy as well.

and while i think about it, move the pump rod to the outer hole to get a bit more pump shot.

 

[Anonymous]

OK, that makes sense. I'll try the Techron this weekend and see what happens; if it's no better I'll adjust the pump rod. Thanks.

 

[Anonymous]

is techron the best fuel cleaner... I dont recall seeing it before?

 

[rbohm]

8) actually i think that bg44k and seafoam are better fuel system cleaners than techron, but techron does a decent job as well.