Tuesday, July 20, 2021

1974 CB750.. from New York!

I still don’t know who exactly “referred” a Honda CB750 customer to me from the beach area, but a nice young guy contacted me to see if he could get his head gasket replaced due to a huge oil leak. Both the owner and the bike came in from New York, where the elements are not kind to motorcycles left unattended for long periods of time. This one was pretty crusty, rusty and corroded and I couldn’t help but wonder why someone would want to put more money into it, but the request was to “Fix it, please”. I told him that I wasn’t about to tear the 175 lb engine out of a CB750 anymore without help, so he said he would give a hand, wanting to know more about the process and what it needed.

The bike was sent over on a flatbed tow truck and we gingerly rolled it into the driveway for a closer look. It was covered with grease, oil, rust, corrosion and looked to have had more than a few mods. Undaunted, we did a quick scrub on the bike, hosed it off, air dried it with a compressor hose and set about dismantling the bike to extricate the greasy lump from the chassis.

After 2 hours, the engine was on a dolly and up on the workbench for disassembly and inspection. Overall, the inside of the engine looked decent, but three valves had wear-through on the stem tips, so those went on the parts list. Obviously, all the valve stem seals needed replacing, but the cam and rockers all looked reusable. Parts were ordered and I hand-cleaned the top end parts as much as I could.

New intake manifolds were ordered, but the eBay seller shipped DOHC manifolds instead of SOHC versions, which were useless, as they are much larger than the early engine parts.

Correct parts were finally shipped from my friends at 4into1.com and reassembly continued over a period of a few days, as gaskets and seals arrived.

My customer/helper went out of town for a week, so I wrangled the engine onto a bike lift and wedged it back into the frame hydraulically and with minimum amounts of exertion. All the bolts and fittings needed to be cleaned off on the wire wheel just to make them easier to install. After another 2 hours, the engine was back in the chassis and ready for cleaned-up carburetors and electrical connections.

A sad finding during installation was the fact that JB Weld epoxy was all built up around the countershaft sprocket area of the engine cases. Apparently, it threw a chain and damaged the cases, so the shop just puttied it up and sent it back out again.

The bike had an aftermarket regulator/rectifier unit spliced into the system, plus there was another mystery module mounted up near the rest of the electrical components which I left alone. The bike came with a Li-Ion battery that weighs about 6 oz but wasn’t holding a charge in the past. I hooked it up to my automotive charger on 2 amps and AGM setting and it eventually reached the desired 14-15v charge rate. The electric starter cranked the engine over with the charged battery and it did fire up after the cleaned carbs were reinstalled and a petcock screen (which was missing) placed where it belonged.

The engine fired up and sounded fairly good. The old coils had fried wire ends where they screwed into the plug caps so they were trimmed back and new caps installed to give it the best chance of running properly. So far, so good… no leaks except for the oil filter housing where someone had cut the old bolt off and notched the hole where the bolt goes through. Luckily, I had a spare CB550 filter housing handy, so that solved that problem.

The front brake needed attention, but getting the caliper off the bike was another challenge. The two big 8mm Allen bolts that hold the two caliper halves together were seized solidly. The workaround was to remove the whole front fender, then the caliper mounting bracket and then take the caliper to the workbench, forcefully loosen the two bolts, put the caliper back on the hydraulic line so I could pump the piston out and then finally it was all disassembled. The caliper piston had a ring of rust pits all the way around so new parts were ordered, along with new pads.

Once the bike was back together, I took it out for a 15 minute test run, which went okay. I put the side covers back on the frame and went for lunch. When I came back to the bike, the power to the switch was gone! Nothing there. Checked the battery, which was down to 7.7 volts, but still lit up a test light. I put the battery back on the charger and dug into the fuse box connector which looked like it had been replaced or at least some of the wires had been redone. Pulling on the red wires in the connector yielded a poorly crimped wire end and one wire pulled right out of the spade connector. I replaced the connector lugs with new ones on fresh copper and crimped it tightly together. With the rejuvenated battery, the bike came back to life with power all around.

More problems surfaces as whoever had setup the bike with low bars used a damaged turn signal switch on the left and a headlight ON-OFF switch on the right side, but there was no way to select Hi-Low beams! Not only that, but the headlight was pulsing on-off. Putting 2 and 2 together, I realized that one of the modules in the wiring side of the bike was a headlight pulsing unit, which was for safety according to the maker.

In the process of checking over the lighting components, I noticed that the tail light was not coming on nor the brake light. I checked power into the light on the harness wires and 12v was present. Removing the taillight revealed that the light socket had fractured in half, so the ground wire was connected to the back half, which was separated from the tail and brake light hot leads. Back to eBay for a $20 used tail light assembly which cured that problem.

Eventually, I felt that the bike was safe and reliable enough to give back to the customer, who rode it back home about 15 miles on the freeway reporting that it was running great! I do want to try some slightly larger main jets in the carbs, due to the pod filters and a/m 4into2 exhaust pipes, but for now, the mostly dead CB750 has been given new life for now.

  Bill Silver 7/5/21

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