wj:
Yes, I bought the balancer from JC Whitney. It is a good unit HOWEVER you still need to make an adapter. I made mine from a Tupperware bowl and it works fine. JC did not list any adaptors for the unit when I got it (approx 3 or so years ago). I forgot to mention if you do surface the top plate then you should also mill/file some material from the BOTTOM of the choke plate. After you surface the top plate it is possible that it will now set low enough due to material removal that the choke plate will catch on the primary venturi spray bar. Also, when you balance your carbs based on idle speed (I have an automatic so I set mine to 900 rpm) THEN go back and set the idle adjustment screws. Once done, you may or may not have to rebalance to speed again. Also, when you are balancing the carbs make sure you disconnect the carb-to-carb linkage rod. I didn't do this once (well, it was late on a Sunday) and couldn't figure out why I was just making matters worse. Your Haynes manual has the jet and air/corrector information you might want to check when you take the carb apart.

ONE ERROR in my earlier post was when I referenced the air corrector "4S", I should have said the Emulsion tube UNDER the air correctors - sorry. I believe yours should read "4S" for Primary and "4N" for Secondary.
You might also have a "5N" in the Secondary and that will not hurt anything. Check your jet sizes vs the Haynes manual when you have the carb apart.

You might want to pull a spark plug, actually numbers 1 and 6 and see if they are covered with soot. If they are, then you obviously have a super rich condition and that will point to other carb problems (high float, internal leak, stuck or improper setting on the vent valve(s) etc. Probably you don't have this as you would see a lot of black smoke.


Good Luck.

Dan

PS. (edit) Forgot to mention, before you go out and buy a wrench to cut in half, make sure of your carb mounting nut sizes. Some are 12mm and some are 13mm. Mine are 13mm. I think the originals were 12mm and replacements (as mine are) are 13mm and DONT OVERTIGHTEN them or you will crack the insulating blocks (don't ask about how I know that).

------------------
Dan Taylor/ Tulsa, OK MBCA '71 250C/'81 300TD-T

[This message has been edited by DANTRCAV (edited 02-17-2000).]

I just thought of something. There is a diaphragm that mounts on the carb cover, is triangular shaped, has a cap that has a raised cylindrical center, mounts with two of the cover bolts and a third, and may be part of the system we've been talking about. I'm not home to look at it yet, but I know that Haynes had a name for it on the carb diagram but no description of its function - could this be it? I wish I could remember the name or what passages connect with it. Any ideas?

WJ,

Very well could be it. The system will probably add fuel to the mixture somewhere before the mixture is dumped into the venturi or carb throat area. Probably not an additional hole in the venturi or throat area.

I can tell you've got the right attitude for solving your problem. You're thinking about it often and ready to get home and tear that sucker apart. I understand this, I just put a motor in my 240D (and after only 516K miles) and will probably get it started tonight. These projects are part of me until everything is worked out.

Don't turn loose of it's neck!
Larry

OK, I re-checked everything and found something really interesting. Thanks to Dan, I checked the stamps on the primary and secondary meter jets and, while they are correctly placed, they also say "Solex"! So my questions are: did Zenith ever use Solex parts, did someone probably make an incorrect parts swap (and does it mess anything up in doing so), or is the carb really a Solex and I've been trying to work a Zenith rebuild kit into it?! The caps on the secondary vacuum housings say Zenith, so that's what I've been going by, but I guess I really should check the numbers on the brass tab on the fron of the carb. Do either of you know where I may be able to reference these numbers to find out which carbs I have? Sherlock Holmes, eat your heart out...

There is one other possibility, and that is the advance weights inside the distributor. These weights spin with the distributor shaft and move outward by centrifugal force as engine speed increases. If the springs that control the movement of the weights are broken, or if the advance mechanism is frozen, the timing will not advance properly and the engine will be hesitant and sluggish when accelerating, with possible backfiring.

Also, the Zenith progressive carbs suck. They have vacuum dashpot-actuated secondary barrels which frequently fail to open properly, also causing the symptoms you describe. If the problem is narrowed down to the carbs, I would definitely switch to twin Weber 32/36 carbs, which have positive, mechanically-actuated secondaries. These carbs idle smoothly, are trouble-free, and will increase mileage and performance slightly over the Zeniths. I have a single Weber of this type on my BMW 2002, and it runs beautifully and reliably. They run about $225 apiece, and I think they will bolt right onto your stock manifold. Also, when you eliminate all of the vacuum plumbing associated with the Zeniths, your engine compartment will be a lot tidier, too.
Hope this helps
Colin

Thanks Colin. When there isn't a load on the engine (in park and idling) and I check the timing marks when I accelerate the engine, the advance appears to be working but I'm going to check it anyway. Can I check the function of the counterweights without dismantling the distributor and, if not, what are the tear-down steps (Haynes has a lousy diagram)? As far as the Webers go (and Dan, you'd probably have an accurate price quote for the whole job), I thought the swap would be quite a bit more expensive - I'll check into it! Thanks for your input on this!
Larry and Dan, I spoke with my local MB "parts guy" and he believes that depending on where the leak is, it could be the intake/exhaust manifold, even if the idle is smooth. She has absolutley no second thoughts about backfiring now - very bad. Regardless, I'd sure still like to solve the Solex/Zenith dilemma - I have the carb numbers and am going to search in these archives and the web for some info. Same parts guy said the carb numbers should start with "ZE" and they don't, so I'm not so sure these are Zeniths afterall.

Assuming the car has points ignition, the advance weights are beneath the circular plate on which the points mount, so the distributor does have to be dismantled to some extent. The cam that opens the points slips over an inner shaft on which it rotates as the weights move in and out, thus advancing and/or retarding timing as needed. The weights are attached to a flange that rotates with the points cam. My 2002 had a frozen advance mechanism, and it caused symptoms similar to those you relate, though not as severe. Hope this helps, but I'll bet this is a carb problem.
A twin Weber kit for a six-cylinder BMW is about $550 complete, so the price should be comparable for your Benz. Once these carbs are installed and adjusted, you can forget about them; they'll need no further attention for a long time.
Colin

wj:

I have the "official" MB Zenith carb tuneup book so when I get home (yea, had to work on a Saturday - software upgrade stuff) I will post the numbers you should have. I will look at mine (I still have them) and see if they say Solex anywhere. BTW, I think the part you are talking about (triangle shaped item on top of the carb) is used to apply a vacuum to the automatic choke unload piston. I will check that also. It has been a while since I looked at the zeniths. My Weber conversion was $550.00 total and I got it from a place I would not recommend (and I won't name it here). One thing I didn't like with the setup I got was they used plastic ball-ends on the linkages. After I broke one and had one fall off, I replaced them with std. MB metal ball-ends (at about $1.50 each - cheap). I suggest you contact JAM Engineering on the West Coast (jam@redshift.com). One problem I had with my Webers was that they were not jetted for the MB and it took me quite a while to get it sorted out. John at JAM will see that the carbs are set for your application. The 32/36 Webers are a fit for many applications and each carb is jetted for a SINGLE carb application on 4 or 6 cyl engines. When you put on two carbs it requires ALL jetting to be redone. The carbs come with a conversion kit that will allow a direct bolt on to the stock manifold and also will allow the use of the stock aircleaner. Webers have a rectangular top so there are two adaptor plates. Kit also includes new linkage and fittings and new insulator blocks. JAM gives a 25% discount to MBCA members. I did replace the stock aircleaner (last year) with a pair of K&N's. I think the Webers are worth the investment. As CMcon98 says, the Webers are mechanical and they really are fool proof. The automatic chokes are also much easier to adjust since they face outboard to the fender and not aft towards the firewall. I use slightly different choke settings for summer and winter.

DT


------------------
Dan Taylor/ Tulsa, OK MBCA '71 250C/'81 300TD-T

[This message has been edited by DANTRCAV (edited 02-19-2000).]

Thanks guys. Pulled the exhaust/intake manifold off this morning (pain in the rear) and found something unexpected. Inside the exhaust manifolds, there is a plate valve that is attached to a spring and some type of counter-weight (spring and counter-weight on the outside of the exhaust housing, flap on the inside). One of the flaps is stuck shut, blocking the exhaust passage, and the other doesn't move very easily and has a broken spring. This setup looks like an intake pre-heater since this section of the exhaust manifold is bolted to the intake manifold. I'm going to clean it up and see if I can get it back in working order. The gasket is in better shape than I thought with no burn-throughs. One concern though - there is a significant amount of oil residue in the intake passages. Would this be normal considering the amount of backfiring through the carbs, or is there something else going on?!

What you found was, indeed, an intake preheater or "heat riser". It directs exhaust through a channel in the intake manifold (not into the intake tract) to heat the choke pulloff elements, which reduce the amount of enrichment as the engine warms up.

One thing that occurred to me is that you could have a chipped or burned valve in the engine. I'd perform a compression check, just to be sure. If compression is very low on one or more cylinders, check the valve adjustment. If it's correct, there is likely an internal engine problem. If compression is OK, and it still runs poorly after you get it back together, I'd get a Weber kit if you can shell out the cash.
Colin

WJ,

Hang in there.

Larry

wj:
Yes, you have Zenith carbs. I wanted to wait until I checked mine (and my box of spares) to be sure. Some (not all) of my jets are also stamped "Solex". Here are the numbers that the standard carbs had when new:

Main metering jets: x115
Secondaries: x125
Air Corrector (the brass tube sticking up)
Main: 90
Secondary: 110

I agree, if you can stand the cash bite go with the Webers.

Hang in there - let me know if you need anything else.

Dan





------------------
Dan Taylor/ Tulsa, OK MBCA '71 250C/'81 300TD-T

Well, she's running much better with the new manifold gasket, but still not without problems. Going to check the valves next. I'm also going to check all of the vacuum connections that sprout from the intake manifold - looks like one goes down to tranny, another to the brake vacuum assist, and a third to the "peripheral" vacuum system (i.e. the blue lines). I haven't heard any hissing within the cabin, but I'm going to check all of the connections anyway. I don't think a minor leak in this system would cause noticeable problems, but you never know. Does the tranny shift from vacuum or mechanical linkage? A final thought - what are the chances of a partially clogged exhaust system contributing to this problem?

WJ,

Yes a partially clogged could be contributing, but I don't think it would show up they way you describe. Is the ole' darlin' still hesitant to accelerate? Does it stumble and maybe "pop" when you snap the throttle open? I still think that if you have vacuam leak somewhere, it would effect alot of things, but it would effect idle more than anything.

Keep on pluggin' and Good Luck,
Larry

Bad news I think. I talked to my parts guy and he too suggested that I check the valves (should have listened to you long ago Colin). As a quick check, he told me to hold a napkin up to the exhaust and see if it ever gets pulled against the outlet and, sure enough, the paper pulses back-and-forth. Very upsetting. I want to use a compression meter to make a definite diagnosis, but how do I know if compression in a cylinder is low that it's the valve and not a bad ring? If it is the valves, is there a way to check if it's just sticking, out of adjustment (I haven't read that far in Haynes yet - does it give a good procedure?), or completely shot? I'll save my dreaded questions about estimated cost of a valve job for later (after I've bought some Scotch)...