I have a stock CB400f, and decided it is time for a complete engine rebuild.
The facts :
I bought this CB400f 3 years ago from the first owner, he bought it new in 1975 (I have the original bill of sale from the shop where he bought it new).
The owner putted 73.000 km (45360 miles) on it, always maintained the service intervals, and always used Castrol (RS) mineral oil. When he bought it he had to chose between a second hand BMW or a new CB400F.
His wife advised (forced) him to go for new, he always rode second hand motorcycles and had to wrench a lot, so you guess what he ended up buying. The BMW and the CB400f couldn't be of a bigger contrast, and in fact he drove the CB400F as-if it was a BMW. He told me he only once did rev it on the German highway above the 7000 rpm, but not higher than 8000 rpm, but only once. He never exceeded the 8000 rpm, doing that, he never feld what the CB400f was capable off. Afterwards he was better of buying a BMW, it was fitting more to his riding style.
Four years ago I met him on a classic event, and I noticed his bike, it was in showroom condition. He told me that he was suffering from balance disorder, and he was afraid of driving it (and his wife too). He was then 73 years old. Being afraid of driving it, I asked him if he wanted to sell. He said yes, and it took him a year to emotionally part from it. He actually cried at the post office during the paperwork, because then he realized that it not only meant that he sold his bike, but also that his motorcycle riding days where over. He rode motorcycle his whole life.
I drove it home and I fell in love with this little sohc four. It is my first four, but I have other classic Honda´s as well, but all twins (a few CB72's, early dreams, and a Kawasaki KR1-s 250 twin stroker).
Like I said, it is in showroom condition, and everything was original, he cared for it like as if it was his child.
The original drive chain and sprockets where on there too. I got the cb400f with a can of grease, a stove and a few other things. He bought an extra chain with the bike, and he cleaned and lubricated one chain, while the other one was on the bike and changed it every 1000 km.
I did change a few things directly, other chain and sprockets, tyres, brake fluid, and front fork oil.
Because an original rear sprocket is unobtainable, I made one from aluminium and I latched the special piece from the old sprocket to the new one.
I also changed to Castrol full synthetic oil (10W50), after cleaning the sump pan (wasn't that much sludge in it).
The engine was very, very lazy. I went to a friend of my with a dyno and measured the curves as I got it, and I measured the curve after pulling it a few times to 10.000 rpm.
The first run was very difficult for the engine, it hardly pulled to the 9000 rpm (took ages) and showed no more then 26 Hp on the rear wheel. I pulled it to 10.000 a few times, and found 3 or 4 extra horses in the second run.
If the readers of this thread are interested in the dyno runs I will scan these and put these here on this thread.
We also synchronised the carbs (that was really necessary) and the engine became much more silent (it made a lot of mechanical noise before this).
We checked the compression, and 1,2,4 where good, but number 3 was lower (but not below specifications). The logical reason is that the screw for the tacho cable was never released while doing valve adjustment, so he never took out the tacho cable, and that makes it hard to adjust properly.
He advised me to ride it for a few thousand of km, and then see what the engine did afterwards, so I did.
In the beginning it was doing only 110 / 120 km an hour in fifth gear and wasn't strong enough to pull in sixth gear, besides that, the acceleration was poor. But after a while, the engine had to get used to me, it pulled to 150 in fifth gear, but still not able to pull in sixth gear, and the acceleration got a little better.
But when hitting 7000 rpm, the clutch started to slip, so I decide to get some decent clutch plates and stronger clutch springs. After that, it didn't slip any more, and every time I hit the 7000 rpm, the engine appreciated it more and more.
I took the CB several times to Italy for my work, did some long distance rides to customers, and now, after 3 years of riding it, the tacho reads 2.000 km, so it actually did 102.000 km.
At 90.000 km the cam chain started rattling, and couldn't be adjusted any more. The primary chain also rattled a lot, and the compression of the 3th cylinder was getting better, but always stayed behind compared to the others. I figured out that it was necessary to start thinking of an engine rebuild.
So I started to collect the necessary parts from ebay and other sources. I soon found out that it is more difficult to get parts for this CB400F then for the older twins I have. But when you have patience, you can achieve a lot with little money.
It took me two years to I find and buy two new cylinders blocks (I couldn't resist the price, and it was cheaper then boring / honing), std pistons, std. piston rings, valves, valve springs, main shaft, silent rubbers, packings, oil seals, kick starter shaft, secondary shaft, clutch outer, clutch inner, primary gear, etc.
Three weeks ago I started with the engine, and will document everything in this thread.
Jensen
