CB750F Project: Part IX

Somehow it happened, that I spent the better part of this weekend working in the garage. In the end, however, I discovered some new issues, which leave me quite puzzled.

With the new brake piston and a rebuild kit, I was able to complete the resurrection of the rear brake. Everything went back together smooth, and the new piston fit in perfectly. I replaced the old, rusted piston on the master cylinder, changed all seals, and put it all back together. On the caliper, I got away with a thorough cleaning, replacing the old seal with a new one, and lubricating the sliders. I then bled the brake on the bench, which was quite easy, and prevented me from spilling brake fluid all over my newly painted frame and swingarm.
Fitting the brake assembly on the bike was somewhat challenging, and I had to remove a few pieces off the bike to get it in there right, but in the end it worked out. I did not take any pictures of the process, but what I had to do was to remove the aluminum passenger footpeg mount and assemble everything off the bike. Then it went back on as one piece. To mount the brake reservoir, I also had to remove the lower rear fender. For the brake lever pivot, all it took was to brush off the old, dried grease, polish the inside of the bushing, and apply clean, new grease. It now operates like a dream.

While I was at it, I installed the rear wheel. Of course, I took it apart first, cleaned all the bushings, spacers, and bearings, and packed them with fresh grease. Sometime in the past, someone busted the threads on the rear axle nut by hammering on it. Well, when I got the bike, the rear axle was frozen in solid. A few squirts of PB Blaster, and a good whack with a rubber mallet loosened it up enough to take it out. After removing the old, dried grease and cleaning the axle with 600 grit paper, it all worked smoothly. I cleaned up the threads on the axle with a ¾ file, then got a temporary nut at a hardware store. It is quite difficult to find a new 18mm castle nut, but I have one on order now. In the mean time, a plain 18mm nut holds things where they belong.

With the rear wheel done, I moved on to the front wheel. Same story here, but before installing it, I mounted the front fender and brake hose brackets. Everything was then tightened to the correct torque specs.

 Next on the list was to install the wiring back on the bike. This was somewhat of a pain, as the harness is pretty stiff up front, but a few quietly uttered cuss words later it was where it belongs. Tidying everything up inside the headlight bucket was more of a challenge, and time consuming.  I took a Q-tip and some dielectric grease, and lubed all connectors as well. Since I’m going with lower handlebars, I have an extra length of wiring that has to be tucked away in there as well. I’m running out of room, but I hope it will work in the end.

 Up to this point, I was making some good progress. Feeling motivated, I decided to tackle the airbox and carburetors. Oh my, oh my, oh my! Between not having enough room to get a finger anywhere, and not wanting to scratch the newly painted frame, it took me over an hour to get it in there. Despite lubing the boots, the carburetor assembly would not go into the cylinder boots, and when it finally did, the airbox boots were all over the place. I tried sliding the airbox over, but for the life of me I could not get all four boots to seat properly. There was always one that ended up crooked. I ended up taking the carburetors off the cylinders again, attaching the airbox boots, and then sliding the whole thing forward. Some cussing and heavy breathing later, it was finally in. What an awful design!

I also installed a new chain. This one has a masterlink, so no more taking the swingarm off to work on it.


After installing the exhaust, I decided I wanted to hear the engine. Well, the truth is, I wanted to rebuild the front brakes, only to discover that the master cylinder cover is beyond repair. I’ll have to order a new one. So, with the brakes being a no-go, I decided to hear the engine.

I installed the tank, connected the fuel line, and poured in a little bit of gas. And here the issues begun.

Number 2 carb started leaking from the drain nipple. Hmm…I cleaned and reassembled the carburetors before taking the bike apart, and I remember testing them on the bike. At that time, there were no leaks.
So, after making sure the drain screw was all the way in, I took the crew out again, and polished the cone-shaped tip. A little oil on the O-ring, and it went back together. No leak since – let’s hope it stays that way.

Then, I started the engine. Cylinders 2,3, and 4 ran fine, while cylinder 1 only came on with RPMs above 2k. I checked gas delivery and spark, and could not find anything wrong. I also know that the carburetors are clean, and there are no vacuum leaks in the boots. Well, this is one puzzle I’ll have to investigate later.