® Ten things every first-gen DOHC owner should know

Honda did a pretty neat thing in 1979. They took the basic architecture of the single-cam CB750--including the cylinder stud spacing for example--and built around it a whole new engine. Honda used a lot of what they learned in their endurance roadracing FWS1000 racebike: four-valve "Pentroof" combustion chamber, dual cam chains, shim type valve train, etc., and distilled it down to a mass-production item. And it seems they could not help themselves but offer a roadracing conversion package as a bonus. The media ate it up, and so did the market. The CB750, CB900, CB1000C, CB1100F and CB1100R are really enjoyable machines. And not bad looking, either. Here are some tips for those new to this range of Honda's first-generation double overhead cam (DOHC) fours.

1 First, the fact is, all that highly developed engineering, as wonderfully as it works, has a crack in it. The problem is cylinder compression. And it's the bike's biggest shortcoming, its Achilles heel. Honda used soft valves in the DOHC models. Even very low-mile engines are already down to 130 psi and even less. This is roughly a thirty percent loss from the as-manufactured 170. Every machine. Bikes with only 7,000 miles on them suffer the effects, and valve wear can truly be extreme by just 15,000-miles. And all because of cheap valves. Fortunately, quality long-lasting aftermarket replacements are available and strongly recommended. Any way in which you think of this bike, any plans you make, and whatever is on your to-do list, it has to include addressing this problem.

2 Second in importance, the DOHC carburetors get a bad rap on forums, but they are actually a very good design, able to outperform any other carburetor you could use, in the context of the kind of riding ninety-percent of us do. However, they do have circuits that are really tiny and precise, thus they are prone to varnishing up easily. If the machine sits three to four weeks, that's enough to plug things up. You simply must keep Sta-Bil fuel stabilizer in them at all times, round the clock. Unless you think nothing of having your carbs expensively rebuilt each riding season. Sta-Bil really works. Use it. And by the way, contrary to what is said on the Internet, ethanol has nothing whatever to do with any of this.

3 Third, as with every vintage bike, the DOHC electrical connectors are of a design whose terminals are exposed to the ravages of the elements, thus they require frequent inspection. Expect corrosion and melting of the plugs. Repairs are inevitable, and the needed parts are available, even from Honda. Done right the repair of the three most at-risk connectors will never have to be done a second time. And it really needs doing.

4 Fourth, if you value your carburetors, stay away from jet kits. They are Bandaid fixes even on those machines for which they are halfway suitable. Actually, they make up for other issues on those machines that should themselves be addressed. But on the DOHCs, they have no redeeming value. They are all the bad and more. Don't ruin your carburetors with any carb rebuilding kit or jet kit, and especially a Dynojet kit. You'll regret it.

5 Fifth, another thing to avoid is non-OEM valve cover gaskets and bolt seals. Their inferior rubber material overcompresses and leads to oil leaks. The factory ones are still available.

6 Sixth, later DOHC models have a vacuum fuel shutoff valve which will not allow fuel to flow until the engine is cranking over during starting. So more time needs to be allowed when starting a machine that has sat long enough for the carbs to evaporate dry. You can reduce the strain on starter motor and battery as well as the potential for spark plug fouling by clamping the vacuum hose during the first few seconds of cranking (use a hemosat), then turning off the key and waiting while the fuel flows on its own into the float bowls. Give it at least two to three minutes. It takes a while to load up 200 milliliters of fuel.

7 Seventh, Dyna ignitions can be both good and bad. They're good as replacement coils, no question. But remember two things: they are no more powerful than the stock items, and avoid crimping either the plug wires or the battery supply wires. And unless you run an iPhone or GPS or anything with an LCD screen, it's best to ditch the resistive plug wires the coils come with. You can get real plug wires from Dynatech by requesting then at the time of purchase, or after.

8 Eighth, the transistor boxes ("spark units", "igniters") are going bad on these bikes, and Honda has long since run out of them. The real problem is the aftermarket igniters have the reputation of not lasting very long, not compared with the 30-40 year durability of the factory originals.

9 Ninth, it is well known that the charging systems on these bikes tend to fail their electromagnet rotors. This like all the parts on DOHCs has been discontinued from Honda, but fortunately aftermarket rotors are available. But they are aftermarket.

10 Tenth, fitting individual air filters is popular and combined with commensurate carburetor jetting and an expert top end engine rebuild, can produce a asubstantial power boost. That's nothing to knock. However, the filters by themselves will do little and yet will increase engine wear by at least a factor of three. Do what you want, but this is the reality.

I hope this has been instructive and useful.

 
Further reading:
Valve recession
Real issues

Last updated October 2022
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