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1971 CB350

DSC_0063.JPG This is a project that I've been waiting to start for a little while. It was one of the bikes that I got from Martin's Collection, and it is a great candidate for restoration.

For starters, I wanted to take off the fairing and top box. There seems to be a pretty good market for these sorts of things on Ebay, so off they go. I did want to take some good pictures of the mounting points though, so that I can remember how they went on.

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Here it is back in a mostly stock CB arrangement, except of course for the luggage rack. These also sell very well, but I think they're too handy to get rid of. Already I can see some subtle differences between this 1971 model and my green 1972 version. For instance, the headlight bucket is metal and is painted to match the other red parts. The tank and side covers are a different shape, and the taillight lens is smaller. I'm sure there are several other differences, but these were some of the ones that seemed most obvious. I should mention that this one is titled as a 71, but the data plate has a date of manufacture of July 70.

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This one has been sitting with gas in the tank, so I'm sure the first order of business will be to clean the tank and carbs. I'll also check the battery to see if it is any good, but I'm not maintaining any high hopes.

The engine on this one has supposedly been fitted with 450 cylinders and pistons, and reportedly has a blown head gasket. I'll check the compression and see how it looks.

Now skip ahead a year or so. I removed the fairing and cleaned things up quite a bit. I drained the fuel tank and took the carbs and battery off of the green CB350 just to see if I could get it to run. I took it up to the house so that I could use the jumper cables from the car. It would turn over, but didn't fire. I checked the spark plug and didn't see any spark. I took the points cover off on the left side of the engine and started checking continuity with a multimeter. It turns out that there was some corrosion on the points, so I used the pointy end of an exacto knife to scrape off some green nasties. After a little bit of spray can contact cleaner everything was good again. I put it back together and attempted another start, and it fired right up. In fact it idled pretty well. I went inside to get my helmet and jacket, and when I got back it was still idling well. I was excited about having a working motorcycle again, but I still started on my usual test route. It's up hill all the way, so if I need to turn around I can coast most of the way home.

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Once I started riding up the hill I realized that it wasn't going to be so easy. Whenever the engine was under a load, it made lots of noise and didn't produce much power. I guess that this is the same symptom that caused Martin to suspect a head gasket. I haven't taken that engine apart yet, so I'm not sure what the problem was. I had another engine handy, so I worked on finishing it up and getting it ready to install. Meanwhile I put it away and waited to build up some motivation.

While I'm waiting for more motivation, I'll give you some other updates. The big blue c-clamp is part of my home-made valve spring compressor. I've been assembling two different CB350 engines, and this has been a necessary tool for sure. The second picture is of a broken cheap chinese tap holder. The third is of the SL175's engine almost ready to install. I've had lots of engine work to do lately. Now I'm feeling motivated, so I'll wheel the motorcycle back into the working area in picture 4 and start taking it apart.

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The first picture below is of the engine in the final stage of completion. The second picture is of the bad engine on the way out. Check out the third picture... I'm not sure what those seeds are for, but the mice must have made a nest behind the clutch cover. Picture 4 shows more.

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On the next row picture number 1 shows the motorcycle just after the engine came out. I took a break at that stage to roll it up to the house and spray it down with the pressure washer. There are several hard to reach areas that are easier to get to with the engine out. Even more importantly, it is really hard to push the whole thing up the hill with the engine still in it. Picture 2 skips ahead some to show the new engine installed. The oil drain plug was super over-tightened on the new engine, so I couldn't drain the old oil until I had the engine mounted again. It was a matter of getting enough leverage, since the motorcycle is a better lever than just the engine. I used a 24" breaker bar with two hands on the bike and two feet on the bar. Who on earth would tighten a plug so much?! The good news is that I was able to get the plug out without breaking anything. I drained the oil into a clean stainless steel bowl so that I could do an informal oil analysis. I was a little bit worried about some bits of gunk that I found in the drain plug, as are pictured in the second paper towel. I poured the rest of the oil through a screen and didn't find any other big chunks, so I considered it to be OK. I redid the top end of this engine, but did not redo the bottom end, so there is a small degree of uncertainty.

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I've been getting lots of work done lately, since the weather has been unusually cold. I'd rather be working on the airplane project, but the motorcycle work space is much easier to heat than the airplane work space. I was needing some lower brain function work one evening, so I spent some time cleaning up a shock. I had been intimidated about taking apart a shock, but it is actually pretty easy. I used the honda tool to set the preload to the lowest setting, then clamped the bottom end of the shock in a vice. I was able to use one hand to compress the spring and one hand to remove the two retaining pieces on the top. The shock is much easier to clean and polish with the parts separated, and the finished product turned out pretty well I think. Most of the rest of these pictures don't show the polished shock yet, simply because I haven't spent the time to swap them yet.

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Here's another picture of everything going together. In the third picture has the exhaust on. There was a small wiring problem under the seat, so I had to do some soldering.

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With the wiring fixed, I started working on the carbs. The floats that I had were actually damaged from rubbing on the bottom of the bowl. I need to check to be sure that the float height is correct. I've temporarily employed some JB Weld to seal the floats, with plans to replace them later.

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I added some new B8ES plugs and checked the compression. The left cylinder is picture 3, the right cylinder is in picture 4. I thought that 200 and 180 psi was exceptionally good, especially considering that I didn't redo the cylinders and rings.


With all of that done, I hooked up the portable jump start battery and tried to run the engine. It turned over well and would occasionally give a "poof" out of the exhaust pipe, but it wouldn't run. I checked for a spark, which was there. I spent some time reading dansmc.com to try to troubleshoot the problem. In the end, the problem was my error. I had timed the engine so that the spark was firing while the valves were open on each cylinder, basically 180 degrees off. I fixed that problem and it started right up. It runs fairly well with the choke on, but not really at all with the choke off. When I ride it on the street, I have to switch the choke off once I start to get above 5000 RPM or so. I started looking at the carb settings, and it turns out that there are a few different variations of carburetors for the CB engines. Since these carbs are off of the 72 model, I'm motivated to get the original carbs overhauled and ready to run. Hopefully that will fix the issue. While I have the carbs apart, I'll double check the rest of the settings and cleanliness too.

I spent some time polishing the chrome and cleaning everything, and it has come together well.

Skip ahead for a few months, and with some help from more experienced CB350 guys, I was able to isolate the engine problems. I went back to the original carbs and replaced the jet o-rings and the little black rubber plugs. I was surprised to learn that I could run the bike with the left cover removed (and not spray oil everywhere) so I purchased a timing light and used it to time the engine. Wow, that's a really awesome tool! Those changes helped. I switched to the most solid set of exhaust pipes that I had, and adjusted the idle mixture screws and idle stop screws, and now it's running pretty well. About as well as I could expect really! I started venturing out a little bit further from home, even occasionally down hill.

In June I took it to the AMA Vintage Motorcycle Days at the Mid-Ohio and used it as an attention-getter while I sold off most of my 350 parts. It was a very successful weekend, and I had a great time. I didn't sell the bike, but with the price at $1500, that's no surprise. I took it to Oshkosh for the airplane show and rode it some there. The cool thing about riding old motorcycles is that people always want to stop and talk about them. On the drive up to Ohio I had someone talk to me at just about every gas station and fast food restaurant. Since Honda made so many of these, just about everyone who was involved in motorcycles in the early 1970s either had one or knew someone who did.

Riding on the Parkway
Tabitha and I loaded up this motorcyle and the 175 in the truck and drove to the parkway in August. We rode together for about 20 miles down to Linville, and even got off the parkway for some riding on the regular curvy roads. On that ride I started having an intermittent cylinder, which I first blamed on fuel starvation. But when I switched to the reserve fuel, it didn't get any better. When the engine was under a load it would miss, but as long as I kept the BMEP low it ran on both cylinders. This was pretty discouraging, and I took both carbs apart before I realized that the problem was actually a cracked spark plug cap. The insulator was missing in one spot, and this was actually enough of a problem to cause the spark plug to short to the case at times.
Cracked Spark Plug Cap
I replaced the cap and the problem was gone!

In October I rode it all the way to Gastonia, then to Maiden and back. That was about 120 miles ( took the long way), including some time on a 65 MPH limited-access highway. Since then I've been riding with Danny on his new Shadow, and running a few occasional errands around town. It's amazing how an otherwise frustrating trip across town becomes an adventure on a 40 year-old motorcycle. Lately I've been enjoying having a motorcyle that I can just start and ride. It isn't perfect, but it's getting to be much more reliable. Having so much power compared to the 175 is nice too!
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This page last modified 10/04/14