Beck TD has been running poorly lately. Quite a disappointment, because it had been running beautifully, and my recent work installing a new radiator has also worked out beautifully, with cool running no matter what the weather. The problem was a consistent "miss" that marred the driving experience in both normal traffic and when urging it along. Previously, the engine was willing to run all the way to the 6000 RPM redline, and now it was giving up the ghost at around 3500! Here's a video - listen to the exhaust note for the consistent miss.
There's a old adage that states, "95% of all carburetor problems are actually ignition related." That prompted me to focus there first, and I did all the normal stuff - plugs, setting the timing, adjusting the valves, adjusting the carbs, checking the compression, new plug wires, testing distributor cap and rotor, swapping in a spare ignition coil... nothing helped! I turned to the experts on the Volvo Powered MGs group, and they offered other suggestions, none of which helped.
The one suggestion I dragged my feet on was to try a different distributor, because that's a lot of work. I had converted Beck to electronic ignition way back in 2018 (Post 36), and it had worked extremely well ever since. But I finally gave in, since I had two other distributors sitting on the shelf. I installed the one that looked the nicest.
Oops - I forgot to check it out first, and after installing I found that the body of the distributor was frozen in the mounting sleeve, and I couldn't adjust the timing. I worked on that thing for hours before giving up - it was stuck, and wouldn't move. I switched to the other distributor. It was rusty, so I spent some time cleaning and sanding everything. I eventually got to the bottom layer, and had another surprise. See the circled spring in the photo below? There should be another one on the other side.
SO, I stripped the frozen distributor down, and stole its springs. They were quite different:
Figuring that different springs were better than no springs, I put them in and gave it a try. It ran, but missed in exactly the same way. The problem was not the distributor! I put my electronic one back in, which works much better.
Just a point of interest... I had four ignition rotors from various sources on hand, and three of them matched the expectation of having a built-in resistor of around 2.2 kohms. But the fourth clearly was manufactured with no resistance at all. Either an older design, or maybe even one designed for racing. I tried it, of course - made no difference.
Finally, in conversations with my expert friend, Jake, we concluded that I must be one of the 5% that actually has a carburetor problem. I had been balancing my carbs with a procedure that uses some wire pointers to show when they are balanced. Jake suggested an older procedure, that requires removal of the air cleaners. It involves slightly lifting the piston that dampens quick throttle movements. You can see that piston in the excellent diagram below, which was provided by the site howacarworks.com.
The theory is that if you slightly (1/32") lift a piston and the RPM drops, the mixture is too lean. If the RPM rises and stays there, it's too rich. If the RPM slightly rises and then settles back down, you're in the sweet spot. Using that test, I could tell that my front carb was rich, and the rear carb was lean. Carefully resetting until both were at or near the sweet spot settled things down, and suddenly Beck was running well again!
A quick aside: There were two tools (one homemade) that were a big help. First, you need to set the throttle to 1500 RPM for both ignition timing and carb testing. The simple wooden wedge below, made on a belt sander, allowed me to precisely set the RPM by inserting it in between the throttle linkage and stop. Infinite adjustment!
The other cool tool was this tiny custom wrench, made especially for adjusting the mixture on SU carbs. Thankfully, it is really cheap from Moss Motors.
After this sorry episode, I decided that tool will live permanently on Beck's key ring.
So, how did this mess happen? Clearly, this was a self-induced wound, because carbs don't just readjust themselves. My best guess is that I started down the wrong path when I fabricated new air filters in Part 83. I redid the carb balancing procedure during that, and probably got it a little wrong. Subsequently, I "messed around" with the mixture without doing the full procedure, and made it worse instead of better. As it got worse, I was more aggressive in messing around, and clearly made some wrong decisions there as well.
I also realize that maybe I don't quite understand the procedure I was using to balance carbs. It involves two wire pointers that should match, and I had made a fixture that supposedly helped. But when I rebalanced the carbs at the start of the explorations reported above, I convinced myself it was right, when it really, really wasn't.
Jake told me of a special tool that uses dial indicators to very precisely do that balancing act. After hearing my story, he just bought a set! I told him to try them right away. If they work as well as we hope, I think there will be some new tools showing up at the Grant St. Garage...
This whole episode of troubleshooting took around 12 hours over three consecutive days. I'm not really upset about that. After all, my hobby is working on cars. I learned things I did not know previously about distributors and SU carburetors. My car is running well again. It's all good!
No comments:
Post a Comment