Technical knowledge about the CB 750 Four
This page offers special information about the maintenance, the problems and the experiences. Especially for CB 750 drivers.
1. Getting the "optimum" or "the artificial respiration".
As everybody knows, there are two factors that have essential influence on the efficiency of a four-stroke engine: technical admission and end leaving side - or - in technical language: the "accelerate vent". The optimum is available and built-in standardly by the manufacturer but the intensivied legislation induced the engineers to rein it a bit. The first changes concerned the following components: the air-filter (and his box), the main-jets and the mufflers. It is relatively easy to take back the artificial respiration to the original condition again:
a) Air-filter box
It doesn't matter if K0, K2 or K6: if possible ALWAYS use the US type. It has the best chamber volume and further has the air admission of the original production series.
Only use K&N - because the original Honda applications (with the larger accelerate) are already not longer available. Insert K&Ns only ONCE and NEVER change them furthermore. This is true. This substitute is much more better than the original. Please take care, that no bikeshop "inadvertently" changes the filter during the inspection...
NOTE: K0 and K1 of applications have another size and part number like the then following K2 - K8 remarks. Caution with the tightening of the cover screws of the air filter system unit of the K0 version. The K&N filter are a little higher as the original paper filter. After the rubber lips have sat down, one doesn't recognize any difference any more. A desirable result : 15% more air-inlet as the "best" Honda filter of the first production.
The miracle filter doesn't have any influence on the slow jets. But the main-jets let the engine get much too hot. A result of the mixture which is too light. The sparking plugs are getting snow-white. Especially in the summertime an extreme overheating is reached fast. Then it's getting slowly critical. You must definitely change the main-jets before you want to start the engine with this air-filters and the bigger boxes! For the K0 and K1, you need 140s - K2 and following 130s main-jets. Quite important !!!!!!!! You can keep the slow jets unchanged. With this small exchange the "Four" is in best condition.
d) What's the best exhaust system?
Be sure: The best you can do is to order the exhaust system from USA or fix "export types". "HM 300" or the following, foreign tubes had a higher accelerate vent as all European "tries" and always showed the better results.
Don't heart the air filter box in any way! (No cuts or holes!). Instead of a performance profit you'll get a loss of power in company with a very strang cold start behaviour due to a missing harmonisation.
With these - relatively simple measures - you'll not recognize your CB 750 again, neither the sound (like 1969), the pace, nor the elasticity. This "big-bike" is something totally different now (no bigger "baby" anymore). I've tried it often on my own. This change yielded an increase of 7 - 8,5 PS on the performance test. Unbelievable! Now it's, like Soichiro said: "a dream".
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Now the very hot CB 750: Turbocharger. Turbocharging is basically just a variation of supercharging. Supercharging an engine involves installing a mechanically-driven device on the intake tract that forces more gas-air mixture into the engine than would be possible just using ambient air pressure. A supercharger is just a high-pressure air pump that overfills - supercharges - the combustion chamber of an engine. One of the big drawbacks with a supercharger, especially in motorcycle applications, is that it has to be driven mechanically. That means belts, gears, chains, or shafts must somehow be attached to the drive train of the engine. Those drive systems have to be lubricated, tensioned, and covered to keep out dirt and larger objects like feet. So somebody came up with the idea of driving the supercharger with the escaping exhaust gases by passing them back through a turbine wheel which would then drive the supercharging unit. Itís called turbocharging - his roots begin in the 30ís (!) in the Diesel-engines. Small, clean and without any works of service - you know - that was the time for motorcycle. He does a 750 Honda capable of doing 122-mph quarters, fourth-gear wheelies, 140 mph, and getting 65 miles per gallon sound incredible? It is.
Itís the smallest one available, and motorcycle need a small unit for two reasons: lack of mounting space and small engine, less exhaust gases and exhaust pressure are produced to drive the turbo. A supercharger or turbocharger can force a great deal more mixture into the combustion chamber than the same engine would get if it was breathing through a normal intake arrangement. What a super- or turbocharger engine does, then, is to increase volumetric efficiency. You get as much mixture to burn as if you had increased displacement, and, by increasing the amount of mixture in combustion chamber, you increase the amount that is compressed and burned. The efficiency of the turbo is determined mainly by how much escaping exhaust there is to drive it. Therefore, a less restrictive exhaust system or more displacement will do the most to increase the effect of a turbocharger. Also, since exhaust pressure is greatest at high rpm, the intake charge from the turbo is increased the most at higher engine speeds. The faster the escaping exhaust drives the turbine, the more air the impeller can draw in. At low rpm, the pistons draw in mixture past the impeller chamber almost like a normal engine. At high rpm, the impeller draws in the charge and forces it into the engine with greater pressure than the pistons could draw by themselves. This high-rpm charging is called "boost". Once the turbo begins to boost, power just spirals upward: more boost creates more combustion, which increases exhaust pressure, which is turn creates more boost. As long as the valves arenít floating and the points arenít bouncing so much that the spark occurs at the wrong part of the four-stroke cycle, the engine will continue to produce more horsepower: 97 PS (!).
The only one complete "Honda" OHC kit I tried out is made by American Turbo Pak (ATP), located at 2141 South Hataway in Santa Ana, California, USA - if this adress is actually - I donít know. Installing the ATP kit will take the average owner about a day. After this, you have a dragster. A turbocharger 750 isnít a mount to be recommended to the inexperienced rider. You going to leave (and I do mean leave) a lot of shocked people with Z1s, Kawas, 427 Cobras, and low-flying jets in his wake.
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Best wishes - and always and everywhere: Accident-free riding.......
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