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Thanks, that's what I've done, even so far as to get a vacuum manometer on them, here's what's funny; - if I blank off one of the carbs, the RPM picks up to 1500 or so. It will not run at all with the mixture screw turned in to 2 turns and the throttle plates level; - it will only run when on the power circuit.
Even with the valve lash set properly, there's a lot of spitting out the back end, and it sounds like multiple cylinders. And it will spit back out the rear carb at 1000 rpm every minute or so as the fuel builds up.
A little background; the car ran well enough when I got it, and so I put some new plugs in it, did an oil change, overhauled the carbs to try and take care of a hesitation issue, and then scoped it. Compression at that time was 10 lbs more across the board, vacuum at idle was low, about 12. Then it ran beautifully, and I thought I got the problem solved. Well, it didn't take 2 hours, and then it wouldn't accelerate, it would backfire out the exhaust violently and spit back through the carbs under load, and then it refused to idle. And that's where it's still at. I should note that the carbs had been overhauled (by me) 3 weeks before this occurred, so it wasn't the carbs going out of whack.
So here's what I'm thinking; - the cam is wiped, the duration is a lot longer than permissible and the valves aren't opening up enough, - so at low speeds, the engine doesn't seal up enough for good compression and dies off. Excess fuel either sits in the intake as a result of next to no vacuum, and the miss I hear out the back is because the exhaust valves stay open too long don't open enough either.
Does anyone have any other possible explanation for this, yes, I agree, it sounds like it should be simple enough, and I do keep getting this feeling I'm overlooking something really simple, but I've done all the preliminary "start with a dead engine" stuff. It's not that, I have plenty of spark, I have plenty of gas, both pump and return circuit are up to par, I've checked for vacuum leaks on the manifolds, carbs and bases, and the power brake booster, and that turns up fine. The carbs themselves are good clean Zeniths from the same engine, numbers and jetting match for the 250, and the throttle plates are in good shape and not loose. Needle and seats are checked and don't leak, accelerator pumps are new and put out a full shot, the fine passages are all clean (mechanically cleaned as well as chemically) and they were perfectly set to match when I put them on.
Spark is also good and strong; - it's an electronic ignition, can't remember off hand which company it came from, it was a Dutch outfit and the distributor curve is properly set up for this engine.
So yes, it appears I'm over thinking this thing, because it was basically running acceptably one minute and then like a toilet the next, but everything I've recently read about camshaft failure is pointing to this, the misses in the exhaust, poor idle, low vacuum, and the physical damage of the lobes actually being close to a full millimetre shorter and flatter looking than the other 2 cams I have as spares.
I guess if swapping the cam out doesn't work, then the whole thing is coming apart anyway, I don't know what else to do here. I just really didn't want to have to do that now. I know that engine is on borrowed time, it was when it ran.
And since I've never swapped a cam in a Mercedes before, I was wondering what precautions, or what tricks I have to do to swap it out without destroying anything else. If this was an American car it'd be a lot more forgiving with ham-fisted mechanics than this thing seems to be.
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