The past few weeks have brought a rather intense project to the fore, and one that has not had the resolution I hoped! For as long as I have had Beck running, the engine has run extremely rich, with stinky, eye-watering exhaust and various running problems. Even with the carburetors adjusted to their most lean setting, the rich mixture and bad running continued.
I was aware that Beck has internal engine problems also, with very low compression and high oil consumption, but I was hoping to get one more driving season out of the engine before rebuilding. I decided to rebuild the carbs to see if the rich mixture could be cured. And that has had a nice side benefit - a new friend from far away!
Mr. Tom Bryant of Wiscasset, Maine is an acknowledged master of rebuilding these carbs, and he is very generous in sharing his knowledge. His blog post on rebuilding the SU HS6 carb (Beck has two) has over 100 photos and lots of detail. You can find it at this link:
If you need your carbs rebuilt, I heartily recommend that you send them to Tom. By the time I bought the rebuild kit and the special reamers and other tools needed, I spent as much as a rebuild would have cost! But I like to learn new things, and Tom was very generous in donating his time and advice throughout the project.
I'm not going to duplicate his detailed procedure here, but just cover enough to prove I did it. I'll start with a discovery of another reason to get started - a prior mechanic had misinstalled the carb linkage on the front carb, with the linkage arm above the carb's throttle linkage instead of engaged with it. Sloppy work, but I don't think it was actually contributing to my problems.
Removing the carbs and disassembling everything was quite straightforward, but since I'm not exactly a carb expert, I took many pictures and pondered every step before taking it. Finally I had two carb bodies, and two piles of parts.
Cleanup was the first order of business, and I started with parts cleaner spray and wire brushes. That stuff is so noxious that I was wearing an organic vapor mask and taking frequent breaks. Then I read an article that changed everything. The article said to go to Harbor Freight and buy an ultrasonic parts cleaner, and fill it with water and vinegar with a bit of Simple Green. Since Harbor Freight has a store just a couple of miles from my house, I did it the next day. What a difference! I could just let the cleaner (which heats the solution to 140 degrees F. as well) buzz away. This float bowl, with decades of baked-on grease and fuel tars, cleaned up pretty nicely with three eight-minute sessions.
One area of the SU design that surprised me was the arrangement of the linkage to the throttle shaft. It was pinned on rather than bolted, and required quite a bit of force to get apart. UPDATE: I originally showed this picture to demonstrate starting the pin by supporting the linkage in a vise, with a small nut giving some clearance on the back side. That allowed me to get the pin started, and then I put the shaft in the vise to finish driving it out. Turns out this procedure is not the best!
Tom reviewed this after it was first posted, and noted that I should have driven the pin out the other direction because the pin is tapered. He included this photo to show his method for holding, along with the short and long punches he uses to drive the pin out.
Tom also noted that if he is particularly interested in removing the pin without bending the throttle shaft, he will "...drill a small (3/16"-ish) hole in a block of wood, then place the shaft and yoke with the pin above the hole and drive the pin out and into the hole in the block." END UPDATE
At the lathe, I made the Delrin bushings for the new throttle shaft. These bushings are the heart of Tom's process, and they make a great, low-friction seal that should last forever. I also turned a brass mandrel to use in installing the bushings.
Tom uses a drill press for the process, but since I have a great big milling machine, and also because I was terrified of ruining my carbs, I did everything on the mill, taking great care to work to the limits of precision that I'm currently capable of. First step was to use a 5/16" drill rod (metal that has been ground to be extremely straight) to align the carb body under the spindle of the milling machine.
You'll notice I have the rod in a collet instead of a drill chuck. That is slightly (say, 0.001") more accurate. I continued to use collets throughout the process, and taking it to perhaps ridiculous extremes, I did all operations by lifting the knee and table of the mill instead of using the downfeed. That's a lot more work, but maybe adds another 0.001" of accuracy.
Here, I'm milling a pocket to Tom's specification for the Delrin bushing:
After that, we could use the brass mandrel I made previously to press the bushing in place. The bit of plastic above the body of the carb was removed with a file later.
The same process was repeated for the bushing on the other side, and then it could be drilled and reamed through. Troy Nace was around for a lot of the process, and here he's taking his turn horsing that heavy knee up for drilling.
When I write these posts, I try to make a linear progression so you usually miss out on the mistakes, indecision, and general wandering in the wilderness. But I will mention a couple this time. After drilling and reaming the bushings, the throttle shaft was just tighter than I wanted it to be. I tried various things, even going so far as to remove the first set of bushings and make a fixture to check the placement of the pockets. Predictably, since I was working with exquisite care, it was all perfect.
I invited machinist Phil Oles over to review my work, and he agreed that all was good. So finally I re-read Tom's post, and realized that he specified an additional step. After machine reaming, he uses a hand reamer with a specific technique to guarantee alignment. I ordered the right hand reamer, and when it came I followed the instructions. Of course, it worked perfectly and then all was just as I wanted it. The moral: if a guy who has done a process 400 times tells you to do it "this way," then do it this way!
One more machining step had me worried. The new throttle shafts don't come drilled for the linkage. Instead, you have to drill through, hitting holes top and bottom. Tom said that careful alignment by eye was the only way, so I put the shafts in the carbs and aligned to his specification, and used green Loctite to bond the linkage to the shaft for drilling.
I then installed it in an ER32 collet block, and used the controls of the milling machine to precisely align the drill bit. Worked great!
I said that I would tell you about two mistakes, and the second one happened during that process. To align the linkage arm, you install the throttle plates. When I did that, there were clear gaps around the plates, like this:
"R" is the rear carb, and "F" the front one. I sent this photo to Tom and also to local friends, and in both cases they were very kind not to shout "DUMMY!" Instead, they just reminded me that the plates were beveled and I had installed them the wrong way around. In addition, these were new plates from the rebuild kit, and Tom pointed out that often those don't fit well. Sure enough, I compared them with the old ones, and they didn't quite match. I reused the old plates, and got a good fit.
After that, it was pretty straightforward to finish the carbs and get them back on the car. Troy was a big help there too.
I started the engine, and it settled into an even idle immediately. We were elated, but soon it became obvious that there was a problem. Nearly every time the throttle was opened, the engine backfired through the carbs. This video, taken well into the process of troubleshooting, shows what's going on. Email subscribers, use the link https://youtu.be/0uwFqn17gMA
I spent a ton of time trying different ignition settings, looking for vacuum leaks, changing oil viscosity in the dashpots, all to no avail. Backfires often are a sign of a lean mixture, so I kept trying a richer and richer mixture until.... when it was back to an eye-watering, bad-running mixture, the backfiring stopped. The working theory now is that the carb problems before were the only way the car could run!
UPDATE JUNE 4, 2019: After a process that consumed over three months and seven more blog posts, today I completed the upgrade to another engine. The working theory was correct! Those carbs run beautifully on the new engine. The problem wasn't with anything I did with the carbs, but rather with an engine that was just done. Here's a video of the engine starting and running - no backfires!
So.... Troy and I pulled the head to see what was going on internally. It looks sort of forlorn with half an engine.
Sadly, there wasn't much good news there. Look at the damage and pitting in cylinder number one - that will never seal.
The engine will come out this week, I hope, and then I'll go "down the rabbit hole" of deciding what to do in terms of rebuilding. I'm already in communication with Volvo guru Joe Lazenby about buying a running B18 engine to use in this summer's cruising while I'm building the next one. My hope is to build a "big-bore" replacement that will give performance to match Beck's race-car looks!
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