Refresher course in carburetors

[MrFurious]

It's been a LONG time since I'm messed around with a carburetor on a car, and now I'm in need of a refresher course in vacuum etiquette since the PO of my CJ5 left me a real mess and I just changed from a MC2100 to a MC2150 and not everything is the same.

Here's what I found for a stock vacuum diagram.


Presently the only real (functional) vacuum line connected to the carb is the one running from the base plate of the carb to the vacuum advance on the distributor (no spark CTO switch). The PVC port and the front manifold port (shown as manifold vacuum above) are just connected to one another forming a loop. The only other connected line is the TAC signal port which is connected to the rear upper port on the back of the carb (to inside of air filter), though I don't think this is correct.

That leaves me with 4 unused vacuum ports on the carb.

- Large port on top of float bowl
- Small port on bottom of power valve cover (old carb didn't have this)
- Small port on bottom drivers side of carb
- Large port on rear bottom of carb

So my question is what's supposed to run to these unused ports, if anything?

In the factory diagram, it looks as if the PVC hose should run to the large port on the bottom rear of the carb. Should I "T" this into the current loop, or run the PVC direct to it and just cap off the front manifold port since I don't have the air pump, diverter valve or spark CTO switch?

The bottom two front ports (marked S & E in the above diagram) appear to be looped via the EGR CTO switch. I don't have an EGR (blocked with plate) or the CTO switch, so should I loop these two with a "T" to the distributor or just leave the one plugged as it is now?

I'm guessing the large port on the top front of the carb would be to supply vacuum to a power brake booster. If that's truly the case I'll just cap it, but want to make sure it's not supposed to connect elsewhere.

No clue where the small port on the bottom of the power valve cover is supposed to connect to as it's not shown in the factory diagram.


Here's two pic's showing my current setup to make it easier to visualize.

 

[Cheepjeep85]

i honestly cant tell you what goes where but as long as you have your vacuum advance, pcv valve, and brake booster (assuming you have power brakes) hooked up i'd plug the rest of em off. excpet the one on top of the float bowl which i'm guessing is a vent, maybe its supposed to have a return line to the fuel tank. i have a mc2100 so i don't know much about the 2150

 

[elwood blues]

MrFurious,

Here are two home made vacuum diagrams I have, the first one I stole from somewhere else and helped me, the second one I made up in paint and it's my current configuration.

As far as the four ports that you have unused on your carb:
- Large port on top of float bowl CheapJeep hit it, this is the bowl vent, it should be hooked up to the vapor return line going back to your gas tank.
- Small port on bottom of power valve cover (old carb didn't have this) This is because you have a 2 stage power valve in the 2150 and the 2100's don't. If I remember correctly you need to "T" the vacuum advance coming off the distributor for this to work correctly.
- Small port on bottom drivers side of carb I also have a vacuum port here, but I plugged it.
- Large port on rear bottom of carb This is where I ran my vacuum line to my power brakes from.

 

[MrFurious]

Thanks Elwood!

The PO ditched the canister as well, so I guess I'll need to figure out where he hid the vapor return line for the fuel tank. Assuming it's nowhere to be found (i.e. terminated at the tank end), would it be reasonable to assume one could just run this up to the air cleaner assembly?

Other than that it sounds like I just need a cap and a T-fitting.

 

[elwood blues]

I'm not exactly positive, but I thought that the fuel system on the CJ's was such that it needed the vapor return line to the fuel tank to prevent vapor lock. I might be wrong though.

If the vapor return line was removed/plugged prior and it ran okay, I would say stick with it, but it may be something down the road you want to reinstall....

The two stage power valve, I'm not extremely familiar with them, I've read a lot about them and I know that they are harder to find and were only used for a few years on the larger Ford motors. I do know that if you don't want to screw with it, you can take the power valve and cover from your 2100 and install it on your 2150, it should work no problem. And if you need a new power valve, you can get Holley power valves from Summit, the Holley power valves are interchangable with the Motorcraft/Autolite carbs. The thing with power valves is to get a good vacuum reading from a good (not vacuum advance) vacuum port, something off the exhaust manifold would be best. Take that number (typically 16-19) divide by 2 and that number is the setting you should get for you power valve. I'm not sure what the formula is for a two stage power valve.

 

[derf]

The bowl vent port is strictly for evaporative emissions control. The line is supposed to go directly to the charcoal canister to reduce how much is evaporated out of the venturis. There's a solenoid that's activated once the engine is running and up to temp to suck the fuel vapors out of the canister so they can be burned rather than just spewed out.

Legally speaking, it needs to be used if the vehicle came with it originally. If not, you can just cap the tube.

 

[Cheepjeep85]

i learned something about power valves today thank you elwood. didn't know that was how you figured the power valve.

 

[elwood blues]

No problem

 

[Rambone]

MrFurious,

Here are two home made vacuum diagrams I have, the first one I stole from somewhere else and helped me, the second one I made up in paint and it's my current configuration.

I see this is an old thread, I'm going to ask my question anyway and maybe it will get answered...

The first diagram has the "CTO" hooked up. The 2nd shows no "CTO" did you just eliminate that?