MC2100/2150 with Altitude Compensator

[kg0rp]

Hello to the forum. I've searched around a few jeep forums and haven't been able to find much in the way of opinions about altitude compensators so I thought I'd fish for info. Here's the scoop:

I have an '81 CJ5 with a 304. It have what appears to be a 2150 with a 1.21 venturi in it. From everything I've been able to gather, that carb belongs on a 360 engine. It had #50 jets in it when I bought the Jeep and it ran REALLY rich. I swapped a set of 45 jets into it and it helped but it still burns rich.

Well, I just found out that the power valve is blown and that probably explains a lot right there, but I still wonder if I wouldn't be better off with a 1.08 venturi 2150.

So, here are my questions; I hope someone can help me out here. I'd love to get above 13 mpg and a little more power would be nice!

1 - Is the 1.08 a better choice than the 1.21? If so, what's a good jet size to start out with? The Jeep is never driven below 5,300 ft of altitude.

2 - Is the altitude compensator worth having? Does it really work? This Jeep get's driven all the way up to 13,000

3 - I see that some 2100's have the vacuum port for the power valve built in to the base of the carburetor body. Others have an external vacuum nipple on the power valve cover. In the case of the external nipple, where does it connect? Full manifold vacuum?

4 - Rebuild kits don't seem to contain components for the altitude compensator. Do they have parts that wear? Is the thing more of a problem than it's worth? What should the initial setting be for 5,000 feet of base altitude. The Jeep will never be operated below that, at least not while I own it.

Thanks in advance for any help,

Tom

 

[elwood blues]

1. Yes, the 1.08 venturi was the size of the factory MC2100 on the AMC 304's. 45-48, because of your altitude it's difficult to say on the correct jet size. It's something you'd have to play with. I've heard of guys in the mountains running one 46 and one 47 jet, they just had to keep playing with the setup until they dialed it in.

2. In theory, yes it works. As you go higher in altitude, you have less O2 in the air so the standard carb runs richer.

To off set that, the carb has an atmospheric pressure sensitive circuit (HA circuit) that is normally closed at sea level and as you go higher, the reduction in pressure gradually allows the vavle to open and it exposes an air inlet port in the carb. This then leans out the mixture in the carb by injecting more air to compensate for the thinner air at high altitude.

3. Going to need a picture to answer this one, there are so many different MC2100/2150 variants out there, heck some of them had 2 stage power valves, I'm going to have to see it to give you a positive answer.

4. Is there a tag with a number on it on your carb? If so, I have a bunch of websites to look stuff up just for MC2100's. The only difference between a 2100 and a 2150 is the altitude compensator, so you have to make sure you're looking up a 2150.

EB

 

[kg0rp]

Thank you for your information. I'll post a picture after I remove the 1.21 v from the engine to replace it with a 1.08. I have found a couple of them on eBay with the altitude compensator. They want a couple of hundred bucks for a freshly rebuilt one, substantially less for one that's freshly off an engine.

I'm going to try to get one on the cheap and rebuild it myself thanks to a most excellent posting on this forum as to the procedure.

Now that I've decided to replace the 1.21 with the proper carb, the only remaining question is calibration. I realize that jet size is a matter of dialing it in and it's much easier said than done!

I'll start with a set of 45 or 46 jets and work from there. Here in Colorado we have "air care" from which my vehicle is exempt because of it's collector plates. However, they will give an O2 reading for free.

The remaining question is the altitude compensation. It seems this could be an important influence on jet size depending on the initial setting.

For instance, setting it up with the compensator completely closed as though we were at sea level makes no sense since I'm at 5,300 feet for my baseline. I have no way of simulating sea level.

So where to set it for it's initial setting? I could set it completely closed and experiment with jet sizes until it's dialed in and then depend on it working when I go up in altitude. It would burn rather rich if it ever went to sea level, but it's never going there.

Any thoughts?

Tom