I begin by visually inspecting the machine, and I continue to do as as I perform various repairs and adjustments. A lifetime of familiarity with Honda's DOHC 450 has enabled me to visually make conclusions about the kind of care and attention the bike on my lift has received up to the present, and this directs my thinking on the systems I'll be optimizing as I proceed.
A compression test is the first tuning step. There is no point in investing time and money in a machine that has a seriously worn engine. Honda roadbikes of the 1970s are especially prone to having low cylinder compression due to the soft valves Honda put in them. I won't do a super tune on an engine that has low compression. So this needs to be determined right away.
The compression is usually appreciably low, so then I perform cylinder leakdown tests to pinpoint the area of the most wear. The leakdown tests are not needed if compression is adequate.
If compression is at or above a certain minimum after the above steps, the starter clutch is inspected. All 450 and 500 starter clutches are significantly underbuilt and therefore deteriorate quickly. Repairing this is a lot of work and the parts have been unavailable for a long time.
The carburetors are removed for rebuilding. The carburetors are completely disassembled down bare bodies (castings). After an ultrasonic bath, the carbs' circuits are individually hand-vetted. They are lovingly assembled and set up, and expensive factory float valves are fitted.
Then I adjust the valves. I use a 0.003" (0.08mm) clearance. This boosts the compression. The engine starts better, idles better, and has better throttle response. I also adjust the valves in the engine's firing order. It makes a difference. Every experienced career mechanic knows the advantage this offers. The cam chain is adjusted at the same time. Usuually, the cam chain tensioner must be removed and repaired as it tends to jam.
The ignition coils are inspected. The ignition coils, being part of a Kettering system, frequently overheat and need replacing. Just as frequently, the 450's coils weaken over time and simply need replacement even if they test out good. This is well known.
The ignition points are the biggest sticking point of this bike. Improper aftermarket parts and/or inexpert adjustment make this engine very unhappy. Consider the ignition the heart of this engine. It must be right or nothing else will be. This is much more important than the carburetors. Skillful tuning is done on the points using only factory parts.
The spark plug caps are next. They are always loose on their wires--indicating a lack of maintenance--and they are almost always degraded from their original 5K ohms to 10K or more. I nip the plug wires and install new caps. The original NGK caps are getting rare and quite expensive. The spark plugs may or not be replaced depending on their condition. These are also getting hard to find now.
When the engine is started a careful check for fuel and oil leaks is made. Once warmed up, the carburetors are painstakingly synchronized and an exhaust gas analyzer is used to confirm the pilot screw setting and if indicated, the pilot screws are adjusted to a reading of 3.0 percent CO.
The DOHC 450/500T super tune, which includes the carb rebuild, is worth $1800. Brake repair and electrical work, if desired, of course adds to that.
As part of the super tune I adjust the steering bearings--they're always loose--and test the battery charging system. The clutch is frequently improperly adjusted also. I am not talking about the lever clearance. That is not adjustment. Correct clutch adjustment is done at the pressure plate. I inspect and lube the clutch lifter mechanism and the cable at the same time. I also inspect the fuel tank and tank petcock, service the drive chain, inspect and air the tires, and complete my observations sheet. On the sheet I record the maintenance needed on the fork, tires, brakes, electrical, etc.
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On certain vintage Honda models I offer a service I call a "super tune". It consists of all the things that through long experience I have found are needed to get the bike back to the factory-intended state of running excellence after nearly fifty years of inadequate attention. Little of this service is concerned with things outside of the engine and its support systems, though I always create an observations document for the customer in which I list issues which are not part of the super tune. This includes such things as brake system overhaul and the inevitable electrical repairs. They're almost always needed, though not included in the super tune, but available at the same time if the customer so desires. I have been doing this for several years now. Even if your bike was "restored", it will need most or all of these things. A lifetime with these machines proves this. Following is an outline of what the 1970s 450 twin super tune consists of.