2800CS Zenith Carb Differences from What Manuals State

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jjs2800cs

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Our 1970 2800CS (2205990) is running very rich at idle and warm up. It spits black soot until it gets warm. Carbs were rebuilt a few years ago and otherwise runs great for 130,00 plus miles. Now are trying trying to figure out why it is so rich during startup and to eliminate the soot on the driveway when starting.

The carbs do not have the electric shut off valves so it does diesel a lot when shut down. The carbs do not have the 'thermostart' cold start valve either. Also it does not have the 'chimney' style vent referenced in the manuals.

The car we believe was bought in Europe as it as the speedo in kilometers. Question is why the carbs are different (missing stuff) that the manuals reference. There are threaded plugs where the shutoff valves are supposed to be. The top portion is not drilled and tapped for the thermo start valve. I do believe the carbs are original in content. The former owner did remove the emssion speed sensor and vacuum damper unit.

I have done a few searched to help fix the problem, but before we dig into the carbs we need to know a little more. We do have a set of carbs from a 72 3.0 CS that do have the shut off valves and the cold start units. If anything is there any reason why we cannot install the shutoff valves and eliminate the terrible dieseling problem?

Thanks,

jjs2800cs
 
carb soot, dieselling

Also curious as to other's thoughts but US spec; Same soot, same dieselling, but Webers....runs good otherwise. Dave V. in NC
 
some things

Dave, you can check the shutoff solenoids as follows. Remove from car. connect 12v hot line to terminal. Get a bit of tubing that barely fits over the threaded portion which screws into the carb. Blow on the other end of that tubing. When the solenoid valve is open (car running or 12v positive lead hot and body of valve is grounded, you should be able to blow through the tubing. If not powered, or not grounded, shouldn't be able to blow through.

Also, solenoid valve on car, should hear and feel it click when power is applied. So key on, remove wire and tap it against the terminal. Should see a small spark and clearly hear it click.

Dieseling can also be related to point gap setting and ignition timing, so you have to set those first. That done, and solenoid valves verified working, next step is to verify idle mixture richness. Too rich will diesel.

Next up is idle rpm; too high idle rpm tends to diesel.

After that we start looking into the carbs. Over-rich mixture suggests to check the jets. Know that the primary and secondary mains interchange; make sure they are correctly located. See that the jets are screwed into their respective locations. Remove and check the float needle valve; 3.0 liter cars should have thicker shims than 2.8 liter cars. IIRC the spec for the shim is 1 mm for 3.0.

While you're checking the jets and needle valve, check that the floats are not leaking and partially filled with gas. A float that doesn't float would make the car really rich all the time, though, and you said yours goes away after a while.

On the outside of the carbs on my car is a heat-sensitive air bleed into the secondary circuit, feeding air just above (below?) the secondary throttle plate. It's supposed to be open at greater than 57 degrees F. There is a 14 ohm resistance heater in there, check with ohmeter for value. Note there was also another resistance value for some other setup but I don't have the blue books open just now.

Hope this helps, others may be able to suggest additional chances. If more come to me I'll post again. I have been in your situation and I'm not certain that I'm out of it yet.

Charlie
 
Thanks for the replies thus far.

As stated we do not have the thermo switches. The factory manual states they should open above 57 deg. Here in FL it is generally above that. By having them installed would it not make the car run a bit leaner during warm up or do I not understand their function? I do have a set of good carbs from a 72 3.0CS that has them. I notice the 3.0CS carbs have larger air jets than the 2800CS. Again could they be substituted into the 2800 to see if helps the extreme rich situation.

The soot is a real pain as have to put cardboard on the driveway as it spits and dribbles and does make a mess that has to be washed off.

I have tested the shutoff valve from the 3.0 carbs and will test install them on the 2800CS and see if they work.

Why the car does not have them (and the thermo switch) is a mystery. I suppose if is was a non-US car that might explain it, but the manuals (factory and Haynes) make no reference as such. It appears that all the carbs came with those items?

Thanks, jjs3800CS
 
zeniths

The Zeniths were used on fintail MBs, maybe that is why they are different. My dieseling was greatly reduced when the idle was upped to about 1000 rpm (and points replaced), at 650 it idled great but always dieseled. I was also told to blip the throttle and turn the key off when the rpm drops back down. It only occassionally diesels now. It still spits some gas out the tailpipe when cold.
 
Your car is not a US spec model, based on the VIN. The "chimney" top plate came on the 3.0 engine, but not originally on the 2.8 engine, so that's expected, but still, the carbs are odd. The lack of electric chokes is a mystery.

The dieseling is pretty much 100% the lack of the idle cut-off solenoids. The rich running sounds like a jetting issue, perhaps the idle/main swap as noted above. Your instinct about the electric chokes is correct; they would make the car run richer while cold.

Dave, your Weber issues are similar -- aftermarket Webers generally don't have idle cut-off solenoids, so they diesel. As to the soot/rich idle, if they are 32/36's, they're probably jetted too big in an attempt to match the performance of the original Zeniths. How's your fuel economy?
 
2800CS carb differences

In repsonse to the last reply, we do have electric chokes with the hot water supplement jackets, what we do not have are the "thermo start" valves. Since our original post we did install the fuel electric shutoff valves and that has indeed solved the dieseling problem. Also we re-adjust the carbs and the car does run a little better. But still during the warm up it spits back soot that is water and black carbon. There is no evidence of the head being cracked.

Thanks
jjs2800CS
 
OK, I must have been a couple of vodka and tonics past clarity when I wrote that about the electric chokes. :shock:

Zeniths have VACUUM chokes -- shame on you all for not making fun of me... :p

And I don't have any idea what a thermo start valve would be. I've never seen one on a carburetor, only on fuel injection.

The water you see is likely just condensation, it's common during warm up as the exhaust system is cool and so water vapor condenses as the exhaust flows to the exit. The soot -- well, sounds like it's still running rich. Did you check that the idle/main jets are not swapped?
 
2800CS carb differences

The item in question that is missing is really called a "thermostat valve" which is item #102 from the BMW factory manual page 13-00/6. I had called it a thermo start valve. Sorry.

[http://jjsandsms1.myjalbum.net/BMW_Carbs/slides/DCP_7250.html]

The photo in the link shows a carb from a 3.0CS. The "thermostat valve is at the top of the carb just to the right of the large vacuum can. The valve is what is not on my 2800 CS carbs.

Hope the photo link works.

Thanks,
jjs2800CS[/img]
 
No luck with your pic, but I think I'm with you now. In later years, Zenith added a heat-sensing circuit, the purpose of which I believe was to reduce emissions during cold start and warm up.

Here's link to the manual supplement in this stuff -- http://www.cscoupe.org/tech/zeniths/bmw2/bmw2.html

All that said, your 2800CS wouldn't have had that feature on the carbs, so it's not MIA on your car. I don't think this was implemented until at least 1974, and probably on US spec cars only.
 
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