Crankcase recirc
Powersports vehicles did not come under emissions regulations until more than twenty years after cars did. For motorcycles this was during the 1978 model year. Motorcycles did however get some emissions-anticipating specifications as early as the mid 1970s, one of which was crankcase recirculation. Powersports vehicle crankcase fume reingestion from the crankcase to the air intake in most cases has pretty benign effects on performance and maintenance, while it of course lowers evaporative hydrocarbon emissions.
But there are problems with this system on certain motorcycles. For one, on all powersports machines having this crankcase rebreathing, it means that over-filling of the crankcase--whether through incorrect oil change technique, or due to either fuel or water dilution of the engine oil artificially raising its level--can affect the engine's intake system in some not-so-good ways, there being a more or less straight path from crankcase to intake. Even properly-maintained crankcases can result in fouled air cleaners, the accumulation of grime inside throttle bodies and carburetors, and accelerated carbon formation on intake valves. Not so good.
Constant velocity Honda carburetors such as the venerable metal labyrinth style Keihins found on the DOHC 450/500 and the SOHC 400/450 twin are especially vulnerable. The recirculation system’s distilled crankcase hydrocarbons form a greasy black slime on these carbs' moving parts, inhibiting proper function, and when built up around the throttle butterfly’s circumference, can affect proper engine idle. Hondas and Kawasakis having metal piston CVs form this brown or black deposit most notably on the tops of their slides, a sign that the crankcase is having undue influence on these machines' intake systems, and ultimately requiring cleaning of these parts to recover lost performance. At the very least it is imperative on metal piston CV models that the airbox "dump" hose (on the 1978 and later bikes) be inspected periodically (unplugged and/or squeezed) to eliminate the buildup of distilled crankcase vapors. Consequently, knowing that most folks like me are lazy, I routinely encourage altogether defeating the crankcase recirculation plumbing on bikes having metal piston CVs. It's easy to do, reaps maintenance dividends and, well, I simply hate seeing grunge being fed into just-cleaned carburetors!
Draining carburetors
It seems a traditional expedient in the motorcycle consumer world to drain the carburetor float bowls when the bike is not going to be used for a period. The intention is to thwart the accumulation of resin in the carburetor circuits. However, this is not as useful as we would like to think. Due to a number of factors, draining is only partly effective. First, many Honda carburetors still retain fuel in the bottoms of the float bowls even after they are drained. The CBX1000 with its angled carburetor mounting is an example. Second, and more importantly, there is often also fuel remaining in the carburetor’s many passages--the larger ones because carb castings are intricate, the smaller ones because of capillary action, i.e. fluid viscosity. Even running the engine with the fuel valve off until it stalls may not get all of that out. Fuel will remain, and if that fuel is not treated with a perservative, it will gel. So the proper technique is to treat your gas with Sta-Bil, ride the bike at least two miles to get it into the carbs, then drain the carbs.
Supposed matched CV carb tops
This comes up on user forums often, the idea that Keihin’s metal labyrinth type carb tops are matched to their slides and thus should not be transposed to other carb bodies. It’s an Internet myth. This misunderstanding probably originated in someone observing the fact that Honda did not sell the tops and their slides separately but together. “Aha”, someone thought: “they must be matched parts!” However, a career in powersports would have educated this person differently. When a manufacturer prepares to introduce a new model, part of that of course includes planning their spare parts strategy. Contracts are written with the parts vendors and negotiations in the end determine how much and in what way these parts will be made available. The classic example is two OEMs who both used Keihin carbs, and therefore used the exact same float valve needle. Honda sells that needle only with the seat, while Kawasaki--a much smaller company and one who presumably made a more practical arrangement--sells the exact same part separately. Decisions made at the manufacturer level are why certain replacement parts are grouped together. At the cost the OEM is able to negotiate, it makes sense to limit the number of separate part numbers to reduce ancillary costs of which inventorying is only one of many. There are in fact many assemblies--parts grouped in collections under one part number--in motorcycle parts books for this reason. You can’t for example get a carburetor o-ring from Honda without buying a complete set of them. You can’t buy a needle jet without it coming with the jet needle. You can’t buy the wire cable to the points without buying a points assembly. You can’t buy just one piston ring. Nor can you buy a universal joint for a drive shaft equipped Honda without buying the whole driveshaft. No one would reasonably regard any of these parts as “matched”. It’s commerce, not engineering, at play here.
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