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Observations upon the Honda 250-305cc Engine group Part 3.

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The biggest issue with most of the running 250-305 engines, now, is the incessant chatter of loose wrist pins in the small ends of the connecting rods. Normal wear, coupled with any piston seizures or even just insufficient oil changes can cause the wrist pin bores to wear out of round. New pin clearances in the connecting rods are specified as .0006-.0019”, with .0031” as the end of the out-of-specification range. Ideally, you can just push the pin through the piston pin bore and the small end of the connecting rod with firm finger pressure. When you can feel any “rocking” of the pin in the connecting rod bore, you will have some piston pin “clatter” during operation, usually at a part-throttle condition. Honda did offer some .01mm (.0039”) oversized wrist pins to help clean up worn rod ends, however the piston pin holes and the rod ends must be carefully and accurately honed to fit the o/s pins in order to make an effective repair. This is one of the most common problems with these engines and the hardest one to repair.
While the oil filter and drive chain/sprockets are all interchangeable, the clutch covers and outer oil filter cover plate come in two versions… Small Hole and Big Hole, so the clutch cover and filter covers must be a matching set.

While the port sizes on the twin-carb heads are all the same for either a 250cc or 305cc engine, the Bakelite insulators have different markings and ID holes. The 250cc bikes received 22mm carburetors and the 305s are fueled by 26mm mixers. The 29mm o-rings with part number 16173-260-004 should be used on both the carburetor flanges and the matching insulators for the 305cc carb installations. Unfortunately, some Honda parts listings are showing the 91302-PF0-003 O-RING (26.9X2.4) as appropriate for the 305s, when they are actually sized for the 250cc 22mm carburetors. Many Honda part numbers have been superseded to something newer that “almost fits” with some effort and in a few cases, the “interchange” attempt has gone wrong and often unnoticed except for people like me!
The carburetors are another story, all in their own. Pre-65 carburetors are known as “round bowl” and post-64 carbs were offered in the “square bowl” shapes. Most of the carburetor parts will interchange between the two other than the float bowls, float bowl gaskets and the floats, themselves.

The story starts simply with the Dream carburetors, which were sized at 22mm for both engine types. It is the same carburetor body, but with some small jetting calibration differences, depending upon either 250 or 305cc applications. The first of the twin-carb models, delivered on the 1961 CB72s were round bowl designs, however they were fitted with a “power jet” system, where a brass tube was positioned to draw fuel from the float bowl, up a passageway in the upper right rear carburetor body area, then fuel and air were mixed and drawn into the fuel stream at full-throttle settings. Having adjustable air and fuel correction jets made fine-tuning of the 250cc Hawks a very accurate exercise, but it was overly complicated for most mechanics.

In 1962, with the introduction of the CL72, which sported the same CB72 engine components (without the tach drive and starter motor components) the carburetor bodies used were the same as those of the CB72, however the whole “power jet” system was not employed. The fuel correction jet area was not machined for the 250cc Scramblers, but you could store a spare main jet under the little alloy cap for a change in altitude, if you were off-roading somewhere that might require a jetting change. The brass tube fitment and drilling for the rear facing air jet were all blanked off, unused. So, despite the use of the same carburetor bodies for these two applications, on identical engines, they continued to use two different styles of fuel metering on each model of the 250s.

You can see “power jet” possibilities in the casting of the CB77 carburetor bodies, however the “power jet” system was never used on the 305cc machines. Again, the carburetors came in early “round bowl” and later “square bowl” designs, but the rest of the jetting and slide configurations remained the same. The last of the CB77 carburetors delivered on the 1967 editions had all the excess casting features removed, so the carburetor bodies look clean and sleek in comparison to their forbearers.

Engine gasket sets are mostly all interchangeable, except for those targeted for 250s vs. 305s. The 250cc motors have 54mm bores while the 305s are carrying 60mm slugs. The fire rings on the head gaskets are sized for 1mm over pistons and not much more. Again, the 1960-61 engine cases/cylinders had a 8- hole gasket, instead of a 9-hole version, used on the 1962-on engines. OEM head gaskets are preferred, if possible to obtain. Cometic makes replacements now, as well. The newer D&K gaskets from Taiwan are much improved and recommended as a low-cost replacement set.

Oil pumps are all interchangeable. Originally, the CB/CA pumps were separate part numbered units from the CL72-77s, which have a large boss on the bottom to help support the engines on the Scrambler installations. Honda superseded the 259 code pumps to 273 editions, because the added material for the Scrambler installations is not an issue for the Dreams and Super Hawks, other than cosmetic. Internally, the pumps all use the same gears and gaskets. Early model engines had a different pump screen and an oil receiver, which was superseded with a screen which was modified for a higher bottom lip for the screen material to which it adheres. Early screens had a lowered screen edge, which allowed more debris to possibly pile up and restrict the oil flowing back to the pump.

The cylinder head top covers are all different for each of the models, plus there are variations within each version. The first 1960-61 Dreams and Super Hawks had the “rear breather” system mounted on the back of the upper crankcase, so had no fittings on the top cover for crankcase ventilation. When the CL72s were introduced in 1962, the other two models followed suit with a revised crankcase, top cover with labyrinth to separate the solids from the vapors and a baffle plate installed to help return the oil solids back to the engine cases.

-Dreams have two top motor mount bosses and the top of the cover is drilled for two holes which locate the condenser.
-Super Hawks have two motor mount bosses and the left edge is cutaway for clearance of the tachometer drive.
-Scrambler top covers came in two versions; The CL72 cover has a single-bolt mount boss with a solid, drilled mount hole, while the CL77s had a rubber-mount bushing inside the bolt hole, due to increased engine vibration factors.

In addition to the three model-type covers, there were variations in the breather tube fittings. Early models had screwed-in fittings, replaced with pressed-fit tubing fittings in the later years. On the 1967 CB77s, the pressed-in fitting was moved from the center of the cover to the extreme rear edge.

The kickstarter covers are all different in style and design, with “early” and “late” versions where the clutch cable joint was relocated more forward around 1965. The Dream and Scrambler k/s covers allow the direct connection of the kickstarter arm to the kickstarter shaft end via a “knuckle” attachment. The -Super Hawks have a forward-kick design, in order to clear the footpegs. This requires an intermediate gear which changes the forward, clockwise motion to rearward, counter-clockwise motion that the engine requires. There are three different designs of the clutch cable joints, to be used in each application.

-Dream k/s covers started out with a 45-degree angled cable joint that entered the cover at the extreme rear portion requiring a 4+ foot long clutch cable, which was routed inside the frame, out a slot in the right side cover and into the kickstarter cover. The next generation moved the cable joint up so the cable entered the cover at a 90 degree angle, just aft of the stator. Depending upon the cover series and cable angle one of three different cable joints were used on these machines.

-Scrambler k/s covers are leaner and have less mass at the rear portion, whereas the Dreams have a larger enclosed chain cover requiring more material to make everything match up around the countershaft sprocket area.

-Super Hawk k/s covers are noted for splitting down the middle, usually horizontally, but sometimes in other planes. Lack of regular lubrication, as well as any hard debris getting caught between the two k/s gears can cause jamming and stress on the covers, which will yield to heavy-footed kicking attempts. Attempts at kick-starting a seized engine will almost always be the death of any CB-series kickstarter cover. The cable joint location was changed in 1965, to the more forward location. The location change prompted an angled cable joint and different return spring, otherwise the k/s cover components were all the same.

There are several different, sheet-metal, transmission cover designs, as well. The 1961 CB72-77s, with the rear breather crankcase design, have a unique cover which wraps around the large round canister portion of the upper case. When the breather system changed to the top engine cover, the transmission cover became more symmetrical and rectangular in shape. Early engines routed the breather tube back along the frame, clipped into the engine harness wiring retainer band. The breather hoses were then routed down into a new notched opening in the front of the cover, where the hose was turned 90 degrees and routed below into a small drain hole. When an engine becomes worn and blowby vapors and solids begin to exit the breather tube, the oil drops will follow along the bottom of the transmission cover valley, exiting the drain hole and running down the side of the crankcase. This might appear to be an “oil leak” at one of the numerous seals located on various shafts protruding out of the left side of the engine, but sometimes it is just oil from the breather, instead.

There are five chains of various sizes and lengths which fit these engines. Those include: camchain, primary chain, oil filter chain, starter chain (electric starter models) and the drive chain to the rear wheel. Rear drive chain lengths and pitches vary, depending upon the model and year. Early 250 Scrambler used a 520 pitch chain, all Dreams use a 525 pitch chain and all Super Hawks use the fairly common 530 pitch chain. 250cc models are geared a little lower than the 305s, so you will find the 250 Scramblers running 40t rear sprockets, Dreams with 31 teeth and Hawks with 32 teeth on the back, vs. 37t, 29t and 30t sprocket sets for the 305s. Dreams run a 16t front sprocket, while the CB/CLs generally have 15t countershaft sprockets.

End of Part 3

Bill "MrHonda" Silver



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