Honda's mid-range twins
Honda made its breakthrough in Denmark with the twins CB72 (247cc) and CB77 (305cc), which were in production from 1961-1968. For its time, these were fine and advanced motorcycles compared to anything else you could buy there first in the 60s. The engine in these small Honda motorcycles could rev and they were built to withstand it. Honda had made its breakthrough in motorcycling, and the street bikes showed that.
Healthy machine
The technology used was over-square engines with a short stroke and cylinder head with hemispherical combustion chambers and large valves and simple chain-driven overhead camshaft. These engines had 180 degree crank offset, which for a 4-stroke engine means out-of-stroke ignition and a slightly limp engine run. On the other hand, such an engine is in fine balance and is not affected to the same degree by the so-called second-order forces, as with 360 degree Twins where both pistons are at their peak at the same time. The successors to these machines were the CB 250 (249cc) and 350 (325cc). These machines were also identical except for engine size and gearing.
Big seller
These models were produced in over 600,000 copies, in the period from 1968-1974, which for a motorcycle is an incredibly large number. Excluding the CUB model, these models are Honda's most produced motorcycle model of all time. They were well-built machines like everything else Honda produced at the time. Particularly suitable for daily transport needs and a bit of fun at the weekend. The small engines had high performance for their time. Thanks to the fine vacuum-controlled carburettors, they had a nice smooth pulling force without power gaps and a reasonable fuel economy, as there were as many as 5 gears and a very fine gear change. They were therefore easy to ride and for many an excellent beginner motorcycle. Many at that time did not dare to start with larger machines than this, but many who bought them went on with one of the 4-cylinder Honda models that came out in those years.
Lacks classic status
Many of these machines still exist, but there is a long way between the well-maintained original machines. Many of them were used all year round and look accordingly. Although these models required only minimal maintenance, many owners forgot to provide it, and in particular the small oil centrifuge or sling filter was rarely cleaned as required. These engines did not have a regular oil filter, many also ended up with burned valves because the owner did not take care to adjust the valves, even though it was a job that could be done in a few minutes. Unfortunately, it is also very normal for spark plugs to tear the thread out of an aluminum top. If you strain too hard. A little GRAPHITE GREASE on the threads every time the spark plugs are changed is a good idea.
NOTE - The cam chain cannot be separated, the crankshaft must be removed to put a new chain in the engine.
A favorite Cafe Racer object
There are also many who have suffered a crazy fate as more or less successful Cafe Racer projects. Thanks to its distribution, the model is easy to obtain wear parts for, but things such as original exhausts are difficult to find. In typical Honda fashion, there are many small differences between the models from the K0 to the last K6 model. Many do not consider these small machines worthy of preservation, which is a big misunderstanding.
My experiences with the CB 250 and 350
In 2013 I found an original CB 250 with 33,000 km on the clock. It had been used as daily transport while its young owner was in training, he had put it away when he could afford a bigger machine. Fortunately coated in machine oil, so the rust had not taken hold. All original parts were in place. But the many years in the mothball had been hard on the engine, which had apparently never received any kind of regular service. It had to be fully serviced and the wheels had to be taken apart and cleaned as they were warped and the tires were dangerously old and stiff. Not a particularly exciting motorcycle, but surprisingly capable in town and on ordinary country roads. The engine hums nicely, and it is, so to speak, completely free of annoying vibrations. There is a surprisingly large difference between the 250 and 350 in the sense that the extra power in the 350 (36 hp) makes it somewhat more relaxing to drive.
Information from Carsten Mortensen, Virum - expert on this model
The large number produced of this model also means that many variants (1968 -1973) of this machine were made, for different countries.
For example, there were the Scrambler models CL, which were very popular in the USA.
I will stick here to the models that came to Denmark alone. The CB 250/350 had the following designations in Denmark: 1968 - 1969 was simply called CB 250/350, year 1970 was called CB 250/350 K2, year 1971 was called CB 250/350 K3 and finally the years 1972-1973 CB 250/350 K4.
Articles you must read:
Changes in the first cohort
Typically for Honda, there were a number of technical changes/improvements shortly after the appearance. At first the chain tensioner had both spring and oil pressure to tighten up, it was quickly replaced with stronger spring alone. The oil pump was made bigger, first editions cast iron pump housing with aluminum piston Ø 15 mm was replaced by aluminum pump housing with steel piston, now Ø 18 mm. The left main bearing (generator side) was first a roller bearing, now replaced with a regular track ball bearing. The carburettors got bigger float housings/changed floats, and then there was the fast camshaft.
The magnet (rotor) was cast together in aluminum as a unit (a cast lump), previously held together by magnets/steel plates/rivets. The starter relay was quickly replaced with one that worked. The seat bracket (hinge) was at the back on CB and K2, the side hinge on K3 and K4.
Difference between the 250 and 350 model
The differences on the CB 250/350 were small. Frame, fenders and tank side covers were the same. However, the first CB 250 were fitted with 1/4" smaller front and rear tires 2.75 front and 3.25 rear (CB 350 3.00 front and 3.50 rear).
Otherwise, the biggest differences between the 250 and 350 were in the engine. The cylinder/pistons are of course smaller in the 250 version, but there was also a difference in the connecting rods, which were thinner with smaller counterweights. There were 2 less rollers in both of the 2 middle main bearings (the pillar was, however, of the same dimension as the flywheels).
Then you want to make a 350 out of a 250, so find a 350 engine and the associated carburetors.
Many who restore these models have trouble getting them to run properly, often due to the carburetors. They must be known by the stamped codes on the page, e.g. K4 CB 250 = "725B" and CB 350 = "722A". You cannot uncritically swap around nozzles and needles. Unfortunately, they have often been replaced over time. The springs in the vacuum bell of the carburettors are also different, weaker in a CB 250.
It doesn't take much before the motors don't work properly and are difficult to adjust.
Hardly pumped engines
The first CB 250/350 that came to Denmark/Europe had a hot camshaft fitted. The CB 350 had 36 hp at 10,500 rpm and 2.55 kg/m at 9000 rpm. The CB 250 produced 30 hp at 10,500 rpm and 2.05 kg/m at 9500 rpm, and Honda found that with time a bad idea. They therefore took the camshaft from the CL model (not sold in Denmark) and put it in the CB models, and the effect decreased slightly for the same reason. CB 350 to 33 hp at 9000 rpm, 2.69 kg/m at 8000 rpm and CB 250 to 27 hp at 9500 rpm, 2.14 kg/m at 9000 rpm
Officially, all Danish "type approvals" are 30 and 36 hp, that was not changed. So in short, K 2, 3, 4, and part of the first (simply called CB) became slower in acceleration and top speed, but the sail line was strengthened and thus better for touring. Honda tuned its motorcycles to all they could handle (and sometimes a little more) to keep up with the 2-strokes.
A bit about tuning degree
A bit about tuning a 4-stroke engine such as the CB 350. The combustion rate in a small, fast-rotating engine like this is fierce. The flame front moves at a speed of 20 - 22 meters per second. This can only be done by faking the ignition. An internal combustion engine (petrol) typically ignites 30 - 45 degrees before peak, and combustion should preferably be completed 7 - 10 degrees after peak, to be economical.
A CB 350 that runs at 10,000 rpm. teeth approx. 42 degrees before peak, and combustion is only finished 70 degrees AFTER peak.
All combustion later than 15 degrees after peak is waste, which generates heat. Therefore, exhaust valves on these small Honda engines are exposed to a lot of heat, but it was necessary to be able to "keep up" with the 2-strokes. The working pressure/average pressure of 9.5 atm is quite a reasonable value, even on a modern engine today.