I started this build thread over on www.hondatwins.net
, but I thought that I'd bring it over here too.
Let me start this log with some history of this bike. It started out as an all-original '73 CB350G, purple in color. It was given to my dad in the early '90s by a friend of his. It had been sitting in a garage for 10 years or so before that, and I think it was a case of the old ran-when-parked story. My dad is into antique American bikes, but the Honda was free and it was in good shape, so he stuck it in his garage and it remained there until I got out of the Marines at the end of '06. I'd grown up riding '70s Japanese dirt bikes and I still had my old '75 Honda XL125 although it needed some freshening up. I started working on it that winter and I was enjoying myself so much that I asked my dad if I could have the old purple 350 to wrench on. He was happy to get it out of his garage. He also showed me a junkyard '72 CB350 he had that was a wreck but had some good parts on it.
I'd like to take this opportunity to apologize and express a bit of remorse for hacking up a nice, all-original, CB350. Not that CB350s are rare bikes, but now I know better and as a rule I leave the nice, original bikes as they are and only cut up and heavily modify already messed up machines. If I could go back and do it differently I would, but at the time I was younger and less cultured than I am now. I was a good mechanic, but I was used to working on big, American, V-8s and was new to small Japanese motorcycles. I really liked Japanese bikes, but at the time I didn't view them as historically significant. So again, I repent. Now is now, and what's done is done, though, so I won't dwell on it and I intend to turn this poor machine into a really enjoyable bike again.
So back in the winter of '06 I sat on a milk crate in my unheated Illinois garage and got the 350 running. In true Honda form, it didn't take much. The hydraulic disc front brake was totally yucked up and non-functional, so I just took it off and made a spool hub. Hell yeah! (Remember, I know better now). I took it for a test ride and it ran well so I decided to plate it and made plans to make it my main rider. I quickly realized that riding a purple motorcycle was a thing that I found unacceptable, so I swapped the faded, gold tank from the '72 onto it and took the purple side covers off. I liked the look of the bare air boxes, and I would later take the minimalist approach to the extreme. I rode the bike around all winter and spring like this, no front brake and all. After a few minivan-caused Fred Flinstone-style panic stops I decided that I should invest in a front brake. Luckily, the DLS drum from the '72 swapped right on.
After that, I gradually modified things and took things off, and did some engine work. After a friend of mine had a catastrophic cam tensioner failure on his 350 I installed the KA Performance teflon slipper cam chain tensioner and the Camellia cam chain that goes with it. I was having trouble with the stock carbs and decided that I wanted some Mikunis. I listened to the wrong people and put 32mm units on it which were much too large for the stock motor. I managed to get it tuned well enough that it ran nicely and was fun to ride, but it was kind of finnicky about the throttle and I knew that it wasn't running at its potential. I rode it like this for a few years and countless thousands of miles. Here's what it looked like for the last few years I was riding it.
Then in the winter of '09 I had an engine seizure while riding the 350 for the first time in a few months. I had recently fixed up a Yamaha XS650 that had replaced the CB as my main bike, so the CB wasn't getting ridden as much. I was going down the road at 65 or so and the engine just tightened up and locked solid. I was bummed but I had the 650 so I just got the bike home and put it in the corner. I recently started to really miss the old 350, though. It was my first street bike and the bike that I really cut my teeth on mechanically. I decided to pull the motor out and see what happened.
It turns out that it had lost oil flow to the top end and the cam had welded itself into the bearing blocks. It wasn't caused by a lack of maintenance as I always changed the oil and cleaned out the centrifugal slinger regularly. I had all the extra parts on hand to rebuild it so I set about doing it. I'm happy to report that the KA Performance cam tensioner showed no wear whatsoever after thousands of hard miles on the street. One of the first things that I did when I first got this bike was to remove the electric starter. I really hate electric starters. I refer to them as "femstarters". I've found it to be a very divisive subject over the years so I won't elaborate unless you guys really want to hear it. Anyway, I thought that this would be a good opportunity to rebuild the motor into a set of later SL350 engine cases that didn't have the electric starter provision. I was going to have to split the cases anyway to get the new cam chain on. I had a nice set of SL cases that I bought a few years ago at a swap meet. I knew that the crankshaft was different since the SL crank didn't have the starter clutch so I got a good SL350 crank, too. I had done some research and the people "in the know" told me that everything (CB350 parts) else would swap right into the SL cases, but the motor mounts were in different locations so I'd have to do some fabricating to get the SL cases into the CB frame. In hindsight, these were probably the same guys that told me that VM32 carbs were the only way to go. If I would have researched it to the point of looking up part numbers I could have learned that nearly every part in the bottom end was different between the two models. Almost nothing interchanges. I think the main reason for this is that the SL uses a primary kickstart while the CB kickstart goes through the clutch, and this difference caused everything to be build differently. This makes sense, though. Since the CB is designed to have an electric starter as it's main starter and the kicker as just an emergency backup, I guess Honda found it acceptable to design the kickstart mechanism so that the transmission had to be in neutral with the clutch engaged for the kicker to turn the motor. More than once I found myself stalled in traffic, frantically trying to find neutral so that I could kick the bike back to life. No fun. Since the SL's only starter was the kicker and it was designed as an off-road bike, it really needed the primary kickstart design that would allow it to be started in any gear with the clutch disengaged, or engaged and in neutral. At the time when I discovered all of this I was bummed out because it looked like the SL cases were going to be totally impractical and I really had grown fond of the idea. However, I was able to locate a complete SL bottom end for a good price. Now that I have all the parts, I really like the idea of having a primary kickstarter.
I got the motor all back together the other day. It's the complete SL bottom end with the complete CB top end. It has a new Camellia cam chain and all the damaged parts from the seizure have been replaced. Since I had it apart I got the cylinders honed and put new Honda factory rings on the pistons.
I had been thinking about what direction I wanted to take the bike in this time around. I had a few ideas, but I knew that I wanted to address some of the things that were holding the bike back now that I'm older and smarter and a better mechanic and tuner. First on my list was to change out the too-big carbs. I don't like the design of the stock Keihins at all, so I planned to just put some smaller Mikuni VMs on. Next was to get some decent exhaust on it. The little shorty pipes that I had were too loud and were such a poor design that I'm sure they were hurting performance over the stock mufflers. While gazing at the bike in the garage I decided the direction that I wanted to go. I'd always admired the SL350s, both the early ones and the later ones that were more dirt-oriented. I liked the look of the high front fender and the abbreviated seat in particular. I also really liked the high pipes that came on Honda CLs. So a high-fendered street scrambler with CL pipes and a single (or at least shorter) seat it would be.
I was able to address both big improvements that I wanted to make to the bike in one fell swoop when I realized that I had a set of really nice Mikuni VM28 carbs from a '72 Yamaha R5 that will work great with a bit of rejetting. Furthermore, I talked to a friend that's building a CB350 race bike and wanted some VM32s and happened to have a set of nice '71 CL350 pipes hanging in his shop. Bartering is great.
It turns out the the motor mount locations are in the same position after all. I had to clearance the frame on the right side of the bike where the rear of the motor mounts to clear a part of the engine case casting, but it was very slight.
The other thing that was necessary was to add a 6mm spacer on each side between the front motor mounts on the frame and the case. I used a stack of 3 washers on each side that worked out perfectly. Crude but effective. Here's the hybrid motor all bolted up in my CB frame.
I was having trouble finding a seat that I liked the look of. I tried a bunch of different seats on and hey were either too short so the rear fender and the loop of the frame over the fender looked awkward, or too long so they almost touched the tail light, or too wide so that they hung out of the frame rails and looked stupid, etc. I thought that I'd have to end up getting an early SL seat, but I wasn't able to find one for sale anywhere. Being a one (or maybe two) year part on a bike that I don't think many were sold, coupled with the abuse that seats get didn't have me feeling too optimistic about finding one. Well yesterday while I was looking around in a different building I came across an old Suzuki TS90 chassis with a good looking seat on it. I took it over to the 350 and it turns out that it looks good to my eye. The only problem is that it doesn't meet the fuel tank just right and looks a bit unprofessional there, but I can live with it. I really like the proportions and shape of the seat on the bike. It will take a bit of fabricating brackets and mounts, but it's actually a really close fit already. It will be getting a fresh seat cover at some point, but the foam and pan are in great shape.
Today I addressed the mounting of the high front fender. It would have been easy to use a universal plastic fender, but I really wanted to keep this bike looking late-'60s period correct. I have a '70 SL70 with a simple metal high fender that's painted the same color as the tank and I love the look of it. I took the fender bracket from the triple clamp of the same TS90 that I robbed the seat from and welded it to the bottom of the CB350 clamp.
I'll get it painted up and reinstalled soon. For the fender itself I'm using a stock front fender from a CB350 that I removed all the bracing from, shortened, and drilled to match the fender bracket. I mocked it up on the bike and I like the way it looks. I hope to blast the fender and get it painted tomorrow. I'll put up some more pictures when I get it on.
My heart is really set on having period correct on/off road tires on this bike when it's done, but I can't find any of the old universal trials tires for sale. They're the ones with the closely spaced, tall, square tread blocks that used to be on every dirt bike and are usually still on every dirt bike that's been neglected for a few decades. Cheng Shin used to make them and I liked them a lot, but I guess they don't make them anymore. Does anyone here know a source for these tires?