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Not in the playbook… CL77 camshaft repairs in the chassis

Factory photo of the famous Honda 305 Scrambler
Factory photo of the famous Honda 305 Scrambler
AHMC

One of my customers from last year during the Scrambler onslaught of motor work had some residual issues with irregular idling speeds, which were noticed after the rebuild. What was noticed was some excessive radial/lateral movement in the points cam (around .006-007”) and slack in the camshaft sprocket advancer weights/springs. The point cam sideplay issue wasn’t quite as urgent as it normally would have been because we had installed an electronic ignition system on the bike. If the points system had been installed the point gap would have been varying that amount causing serious ignition timing problems which then translate to vacuum signal irregularities at the carburetor metering circuits.

Slop in the cam sprockets have been an ongoing issue due to the design of the flat tang inserted into a flat plate that is attached to the advancer weight system. There is always a bit of play in the tang/plate interface, then the advancer weights flail around against tiny return springs which are just a bit short of pulling the weights back fully into the retarded position.

A more serious condition exists when the sprocket assemblies have been spun around for about 15, 000 miles and begin to loosen up their rivets. When the assembly loosens up, the center camchain sprocket starts to cycle back and forth against the worn rivets, as the camshaft lobes push up against valve spring pressure and skid down the back side of the lobes. As the parts loosen up, there is an audible chattering noise which sets up and the ignition timing becomes unstable when viewed with a dynamic automotive timing light. When this occurs, you must remove the sprocket assembly and either replace it or tack weld the pieces together to prevent oscillation cycles.

My least favorite bike to work on would normally be the CL72-77 Scramblers, due to the placement of the exhaust system, covering up the left side cover, air filter/brackets, which must be removed to extract the carburetor for service work. However, the one saving grace for the Scrambler models is that the engine top cover can be removed with the engine still in the chassis.

Considering that my customer had to rent a trailer and drive the bike down from LA, I offered to do a “while you wait” camshaft service and see if that would remedy the problem. The bike arrived a little after 10AM on a Thursday and I dove right into it, stripping off the seat, fuel tank, top engine mount bracket and removing the spark plugs. The one thing you don’t want to have happen is for the cylinder base gasket to be disturbed when you are turning the motor over to work on the camshafts. After the top cover is removed, a couple of small ½” drive sockets were slipped over the ends of a couple of exposed cylinder studs and secured with acorn nuts. This clamps the whole cylinder head/cylinder package firmly to the engine cases, so no vertical movement occurs.

The next step is to loosen all of the valve adjuster screws, allowing the rocker arms to release pressure on the camshaft lobes in most positions. Normally, a “hook wrench” can be used to loosen the right side camshaft locking nut, but there isn’t enough room for the tool, so a dulled chisel/punch must be used with care to knock the nut loose and release the right side camshaft. With a long thin punch I was able to drive the right side camshaft out, free of the spline connection with the camshaft sprocket assembly. I rotated the engine around so that the master link came to the top and then packed rags around the camshaft sprocket while removing the link. I thread in foot long pieces of 18 ga. electrical wire with a knot tied in one end to secure the ends of the camchain and lower them down, out of the way of the camshaft sprocket.

The left side cylinder head cover hides the end of the camshaft retainer bolt, which expands the end of the left camshaft into the OD of the camshaft sprocket, securing it in place. The 10mm nut must be loosened to the end of the bolt threads, then using a hammer, a firm tap on the end of the bolt/nut will dislodge the through bolt from the inside of the camshaft sprocket splines. The left side camshaft can then be driven out carefully with the long punch, located against the flank of the cam lobes. Once the camshaft is clear of the sprocket splines the sprocket assembly can be wiggled out of the space in the head and available for repairs or replacement.

There were a number of used cam sprockets in my box of spares and trial fitting the points cams into various camshafts with matching splines, a perfect match was finally obtained. Particular attention is paid to the small spark advancer return springs. They can be gently bent on the ends to tighten up their tension on the weights so that they return fully to the retarded position with no loose slack in the spring ends. When the springs are set for a clean, firm snap return, then the spark timing will be much more accurate at idle. When there is slack in the weight system, the weights begin to swing out as soon as the engine is fired up and running. This will move the spark timing from the normal five degree before top dead center to ten degrees or more with the engine idling. This five-plus degree swing in spark timing will cause the engine speeds to rise immediately. The carburetor slides are barely lifted at idle speeds when the OEM settings are all in place. When engine speed is increased due to spark timing advance instead of carburetor slide lifting, the fuel metering system’s vacuum signals become altered drawing in more fuel that is necessary to keep the engine running. When that happens, the idle mixture air screws must be backed out to give more air to maintain air/fuel ratios that are normal at those speeds. The more the spark timing advances, the more the whole fuel ratio spectrum is affected, causing high idle speeds and irregular idle mixtures that often cause some misfiring at low speeds.

When spark timing is steady at idle speeds, correct idle mixture screw settings can be used and engine performance is even and steady. This was the initial goal of the camshaft/spark advancer replacement on this particular machine. Because of rising rebuild costs on this engine, the cylinder head was left pretty much unattended to, once the valves were checked for leaks using some carb spray down the ports when the valves were closed. Once the engine was installed and fired up, the idling issues became apparent and the initial diagnosis of point cam/cam sprocket slack was made. The owner understood the situation and nursed the bike through about 630 miles of break-in driving, but felt somewhat embarrassed when pulling up at a stop and having the blaring Scrambler pipes blasting out their tunes at over two thousand rpms. Sometimes, you can lug the engine down in gear at idle and the lowered rpms will cause the spark weights to come back into idle mode, but it is only a band-aid fix and doesn’t always work anyway.

Once the camshaft parts were all selected and repaired, reassembly was just the reverse of the disassembly with the sprocket wedged back into place and the left camshaft driven inwards to engage the master splines of the camshaft/sprocket interface. Once the sprocket was stabilized on the left, the right cam was driven into place noting the position of the points cam index mark as the tang of the advance shaft engaged the camshaft sprocket advancer drive plate slot. The camshafts and bearings must be driven inwards sufficiently to allow the edges of the cylinder head end covers to seat in, butted against the bearing edges. If the covers are not allowed to seat properly, the gaskets won’t seal and you get an immediate oil leak.

Once the end covers are initially set in with screws, the camshaft locking nut must be tightened to squeeze the sprocket ends against the right side camshaft splines. Once that part is secured, the left side cover must be removed again to allow the through bolt to be tightened up drawing the ends of the camshaft into the camshaft sprocket spines. It is an interesting design: squeeze down on one end and expand the opposite end into the spline junctions. Once secured, it definitely locks the camshafts and sprocket assembly into one cohesive unit for spinning upwards of five thousand rpms. You won’t find this procedure listed or recommended in the Honda Shop Manual books, but it is possible and it does work!

With the cams in place, electronic ignition rotor re-attached and the cylinder head top cover nuts torqued down to 15 ft.lbs. The rest of the bike was assembled, except for the seat, so access to the battery was available to connect the timing light. The bike fired back up on the second kick, but was a bit unstable and then stalled after a small backfire. There is a lot of variability in the ignition components and some backing plate position adjustments brought the ignition timing back to normal. Full advance was just at the first of the two (II) advancer marks, which is perfect for today’s fuels. Revving the engine a few times as it warmed up gave evidence of continued high idling symptoms, however. There was more than one problem, apparently! Lifting the back of the fuel tank a few inches allowed access to the throttle cables atop each carburetor body. Pulling up on one at a time showed that the right side carb slide was sticking in the carb body bore just a bit. I was SO HAPPY that it was the right side carburetor instead of the left one! With the side cover removed, a couple of air filter mounting bolts were taken off and the whole filter and tube removed from the carburetor inlet. Disconnecting the choke link and removing the two retainer nuts allowed removal of the carburetor in just a few minutes.

Placing the bare slide down the carburetor body and pushing it up and down a few times with your fingers/thumb will leave a wear mark in the soft carburetor body bore. I have used either some fine emery cloth or often just lightly scraped off the high spots with a Swiss Army knife blade. After working with it for a few minutes, it was moving more smoothly, but still hanging up a little bit. Searching the parts bins, a “late style” alloy carburetor slide was found and it dropped right down the bore without any signs of sticking whatsoever. The original slides are chromed brass and tend to hold heat more than the light alloy slides, which seem to be slightly smaller in OD perhaps and more friendly to the distorted carburetor body bores, which have gone through forty or fifty years of heat cycles, fuel exposure and over-tightening of the flanges.

This was the final fix for this particular repair. Dead accurate ignition timing coupled with free-moving carburetor slides yielded an engine that was running smoothly at idle and returning to idle from high rpm throttle blips easily and quickly. That brought big smiles on both of our faces, finally. The owner quickly loaded back up for his three hour return trip to the LA basin without even riding the bike again. An email follow-up, the following day, confirmed his satisfaction at the repairs made and his intention to ride the bike more often, now that it was tamed and behaving properly at stop signs and signal lights. The CL77 gets plenty of attention from other drivers, just as they are because of their styling and the sounds emanating from those long straight pipes. When heard at an even, loping idle the impact is even more compelling for observers and enthusiasts. It’s all part of the great experience of owning AND riding your vintage Honda motorcycle. Go ride yours today!

PS

Sorry for the lack of photos for this story, but I was on a tight timeline that didn't allow for photo-opts. Total repair time was about 3 hours.

Bill “MrHonda” Silver