42
« Last post by Heckflosse on May 28, 2024, 02:52:57 »
Plugs are new, just a little sooty. They are NGK, couldn't get Bosch platinum. Stock heat range for the 250S. Properly gapped. I replaced the regular ignition with a 123 electonic set, new coil, and the plug wires came from a Mercedes vendor and have the steel covers; - not sure if they are carbon core, I'll check that. Ballast resistor is out of the equation, as per 123's instructions. Timing and TDC is verified, as is firing order. No slack on the chain, engine was not turned backwards either. Spark appears to be good and on time when I tested it. Amperage and voltage and everything is there.
It ran well enough on this stuff before, why all of a sudden it quit is the mystery.
Yes, the engine misses and backfires. Yes, it doesn't respond to tuning. Any tuning, at at all. No, I don't believe there are any vacuum leaks. It starts immediately, one pump of the gas. Just dies as the rpm drop. Idle set to 1000 plus, in order to get vacuum up from less than ten at that point. Throttle response is crisp, it revs right up, no stumble. Doesn't have any power. It misfires smoothly; has a fairly steady purr to it. It runs lousy very well; - I think I've optimized all the tuning parameters, it's got to be the most responsive, no idling, misfiring POS in the world right now, and that's why I'm leaning to a mechanical issue, tuning doesn't work. I've tuned hundreds of cars, and I've never come across something like this before except on a high mileage engine that was known to have a shot cam and was completely worn out.
Honestly gentlemen, if it was that simple I would have found it by now. I probably have 200 hours of troubleshooting in on this engine by this point. That's what makes this so frustrating. Check the carbs, check the fuel, check the ignition, check the timing. Open the carbs up, check something or an adjustment, replace them back on the car, set throttle shafts to idle, start the car, feather the gas pedal or hold it open so it runs, and then pop the linkages off and start adjusting throttles and mixtures. Nothing changes, nothing at all. Watch the vacuum gauge, which normally should register something, but in this case it does nothing but stays at 10 and that's only if I'm quick off the snap back with a touch of throttle. Otherwise it just does what it does best, wheezes and dies. I can do Zeniths in my sleep at this point, I know every part in them. I have 6 of the animals, and have replaced anything that appears suspect on the 2 that originally came off this engine and didn't have worn out throttle shafts.
(they weren't on the car when I got it, the set that was on there had worn out throttle shafts and I suspected and confirmed a vacuum leak)
All passages in the carb were measured and cleared, in some cases mechanically. One of the other things I do is build old pocket watches, so I'm very particular to detail and fine work, and I'm very patient. My patience with this engine is coming to an end; - I'll try a cam swap, and if that doesn't help, it's coming out and it's getting rebuilt. And then start all over again with good known mechanicals.
This engine is performing like it's completely exhausted and worn out, not just "tired" anymore. The horse has died and continually flogging it isn't helping anyway.
Recent Posts