I actually had a quick look at this bike over a year and a half ago, when I was dropping off an auction bike for the owner. He was riding it and then it started making a noise and then BANG. I checked for compression with my finger and it didn’t seem like it had any. The exhaust valve was tight, so I loosened it and tried to start it up… BANG!
The bike owner and I live 120 miles apart, so the bike has been sitting since then waiting for one of us to connect and get/deliver the bike. After it was sent to someone nearby, the diagnosis was incomplete, so the bike finally was brought down to me for a deep dive. The bike came here as a whole machine, so we both worked together to tear off the accessory bits and lift the motor out of the chassis and onto the workbench for a look.
Once the hardware was removed and the head lifted, the horrors of the situation developed rapidly.
The whole cylinder sleeve flange was broken off and lying atop the cylinder block, floating loose. The piston was all torn up with broken sections between the top ring and crown. There was shrapnel floating around inside the cylinder on top of the piston, but little damage to the combustion chamber. At first it was thought that the engine had burned a piston, but in consideration of the appearances of all of the parts, it seems like the cylinder sleeve just failed at the top and dropped down far enough to catch the top rings in the gap, which lead to catastrophic failures of the piston.
With the cylinder block removed, the cylinder walls were actually okay, apart from a groove in the damaged right side sleeve. At first, I thought that the wrist pin clip was left out or had jumped out of the pin bore, but closer inspection showed the clip still in place. Apparently, a piece of the piston debris got caught between the piston and wall causing damage. Interestingly enough the bottom of the sleeve had odd grinding marks that couldn’t be matched to anything in the region. That one is still a mystery.
Well, the pistons were the high compression versions, even though the engine was a 1965 which had lower compression pistons. The cylinders had been bored to .75 for these pistons and they didn’t show any signs of seizures or other overheating, so apparently, the cylinder wall just had a failure in the casting. I have had one other engine do that to me after a rebuild, but the cylinders had been bored to 3.00mm oversize and it failed in about 5 minutes of run time. This engine had about 1,000 miles run in on it so it was an unexpected and unforeseen failure.
Digging deeper into the cylinder head, I noticed that the spark advance shaft had more than the usual amount of free play, advancing the shaft easily with my fingertips. When the head was torn down to check for valve damage, the sprocket advance weights were missing a return spring on one side! Not sure where the spring might be, but removing the oil pump and screen didn’t bring it to the surface.
I removed the oil filter from the clutch cover housing, as it was the late-style Big Hole version, which allows some careful removal and reinstallation without pulling the whole clutch cover off. I was surprised to find a thick layer of debris inside the filter itself, but also a lack of oil present inside. Usually when the filter shaft is removed, the oil inside the housing drools out, but that wasn’t the case here. The rest of the inside of the engine appeared to be getting well lubricated, but I noticed that the engine cases and the clutch cover had been sealed with black RTV, which can be a death knell for small Honda engines when the sealant plugs off vital engine oiling channels.
So, the cylinder head came apart to check valve conditions and all had cupped seats and faces, plus one exhaust valve on the troubled right side had a nick in the edge. One other valve had the tip face getting chewed up so all four valves were replaced after I cut new seats in the head. I replaced the missing advancer weight spring and then put the puzzle pieces all back together again.
A search for a set of cylinders or a 250cc sleeve only turned up a possible new set of cylinders from David Silver Spares, described as rusty. Photos will reveal how bad it really is. Once the cylinder question is answered, then we can chase down some new pistons. I have a half dozen 250 pistons and have found that they come in three different compression heights.. 10.0:1, 9.5:1 and 8.5:1. The early ones are marked CB72 inside the skirt while the later ones have CB72 on the outside next to the pin bore.
One must choose the piston setup carefully, as Honda lowered the compression in order to reduce the number of piston seizures that occurred due to overheating. Lugging the little 250cc engine on a 300 lb motorcycle with low torque values causes the pistons to work very hard in a space of only about .001” to .002” clearances. With the known issues of erratic spark timing and today’s questionable fuel qualities that didn’t exist when the bikes were built, these engines can and will seize their little cast pistons to the cylinder walls. Usually, the seizures are momentary, but when they occur the damage has been done, leading to lowered compression readings, smoke, and oil consumption which causes oil-fouled spark plugs or sometimes even more detonation because of oil burning.
Today’s unleaded, alcohol-infused gasoline causes the engines to run lean, which adds to the heat equation, so some careful re-jetting must be considered and tested before disaster strikes. As always, the spark timing must never exceed the II hash marks on the rotor when the engine is revved beyond about 2500 rpms. Excessive spark timing leads to piston seizures even when everything else is spot-on.
Back together again…
The customer tore down a spare “rebuilt” engine from his stock and it had .50 fresh pistons installed with a little bit of water damage along one side of a cylinder but it was old rust pockets and out of the way of critical piston ring travel. After cleaning up the old gasket material, re-honing the cylinders and checking gasket surfaces for being flat, the reassembly resumed in earnest. I did replace the center guide roller with one of the reproduction items from CMSNL. Once the top end was reassembled, we lifted it back off the workbench and rolled it on a dolly back to the naked chassis. It has been very helpful to have an extra set of hands available to hoist the engines back into the frames, as loading the Scrambler engines are a bit more challenging because they load into the right side instead of a vertical lift like that of the CB and CA models.
I put a floor jack under the oil pump to help position the engine as the mounting bolts are installed. From that point, it is just nuts and bolts going back together again. After about an hour, the bike looked like its original self once more. We installed a new points plate and did a quick static timing check before starting the engine. Initially, it was acting odd, only running on nearly full choke. The old petcock and carbs had some leftover old fuel residues in the bottoms of the bowls, which may have caused some fuel feed issues. I rechecked the float levels and main jets (120s in this case) and restarted the engine. It was still coughing a bit which was caused by some late timing on the right side points. Once the timing was set to the F marks, it came fully back to life and sounded healthy.
A brief run around the block and through the gears revealed a nice shifting transmission, even running qualities, and a front brake that needed a lot of adjusting. Beyond that, it was “mission accomplished” after some 10 hours of labor, including help from the owner. I’m still not sure what caused the liner to crack and break like that, but it may have just been a flaw in the sleeve casting. Anyway, all’s well that ends well, so another CL72 is back into circulation again.
Bill Silver aka MrHonda