Recently, I had a call about a local CL72 owner who was seeking help for his 1965 model Scrambler, which was having numerous performance problems. He was located in N. San Diego County, so I referred him to my friend Randy Troy, who lived just a few blocks away. Randy has had years of experience in working on all kinds of Hondas and was trained as a Porsche mechanic and worked at a Datsun dealer back in the 1970s. He has a deep understanding about the “how’s and whys” of how things work, but had never been hands-on with a 250-305 Honda twin before.
He began with a basic tune-up to get it up and running, but encountered problems right away. The carburetors would fuel foul the spark plugs quite quickly, so the first thing he did was to remove the carburetors for inspection and cleaning/adjusting. If you have worked on any Honda Scrambler, you know what a PIA it is to get to that left side carburetor for any kind of service or removal. In the past few weeks, he’s become proficient in doing that job!
With the air filter tubes off, he was watching fuel spitting back into the inlets, even at idle, which is never a good sign. The compression readings were about 130 psi, which is about 20 psi low. This generally indicates that the cam timing is slightly retarded (a half tooth, which is remedied by changing a tooth on the crankshaft, not the camshaft). Sure enough the cam timing was off. Fortunately, only a CL72-77 allows for the top engine cover to be removed with the engine in the chassis, which allows for cam timing verification and adjustment.
With the top cover off and spark plugs removed, the first thing to do is to anchor the head/cylinder to the engine cases, by putting some short sockets on the studs secured with some nuts. This keeps the top end from lifting when you turn the engine over. First you have to locate the master link on the camchain, then carefully disassemble it and string some wire through the ends. Loosening the valve adjusters takes the camshaft lobe load off the cam, so it can be positioned in a way that the flats on the camsprocket are level with the top of the cylinder head.
Once the cam position is set, you have to jiggle the crankshaft around with the camchain ends held in such a way as to allow the crankshaft to bring the right piston up to TDC AND the ends of the camchain towards the top to reattach the master link once the timing is set. There is a lot of fiddling around to get this all to happen, but Randy did achieve the goal and the cam timing was back to where it belonged.
Once that was done, then the carbs got a long look. Randy noticed that there was wear and corrosion on the needle jets that might have caused some fuel spray problems. He was lucky to find a new set of needle jets at David Silver Spares, which arrived in a few days. Then he mentioned the other calibrations of the jets, which were not quite correct. He found #120 main jets and #40 idle jets in place of the normal #115 mains and #38 idle jets. On top of that he read out the needle codes, which turned out to belong to a CB72, not a CL72. The CB72s have a “power jet” fuel enrichening system which calls for a different needle taper. Stock OEM CL72 needles are NLA now so Randy had to order the oft-maligned Keyster carb repair kits, which come with new needles, float valve, gaskets, etc.
Keyster kits are made in Japan, but have been found to have incorrect metering needle tapers and poorly fitting float bowl gaskets in the past. When the kits came in the gasket issues apparently remained and the needles were of a different taper than the CB72 needles, so that might be a solution for the rich running. With new needle jets and Keyster needles, the bike ran too lean off the bottom end, but the fuel backflow problems at idle were solved. The needle clips were lowered to the bottom notches in order to richen up the transition from idle through mid-range, which improved fueling somewhat.
Randy also learned about the ignition timing vagaries associated with having the camsprocket controlling the spark advance curve. He set the initial ignition timing statically with a 12v test light, but discovered that running ignition timing often does not reflect the initial settings. In order to prevent over-advancing the spark timing, the idle timing had to be retarded back towards the T mark instead of the F (firing) mark.
Changing the ignition timing alters the vacuum signal to the carburetor metering systems, which causes the mechanic to have to alter carburetor mixture and idle speed settings. The ignition timing and carburetor settings are inextricably intertwined and there is not that much you can do about the ignition timing without tearing the cam shaft sprocket out for repairs or replacement. There is inherent slop and inaccuracies with this camshaft driven ignition system that cannot be completely overcome with a used engine.
With new carb parts, many of the performance issues began to subside, but Randy feels like the bike is still a little bit flat in the mid-range, so he bumped the main jets up to #125 to see how it worked; especially when jetting for today’s alcohol-blended fuels.
After all this intensive work to exorcise the demons of this bike, his test rides have highlighted a 2nd gear jumping out issue, which requires an engine removal and cases split to remedy the transmission woes. Carb jetting is still a bit off, mostly due to a lack of available OEM jet needles for a CL72. Sometimes you just do the best you can with what you have and then give it back to the customer highlighting the known remaining issues and an estimated cost to fix them properly. Randy probably got paid about half of what the job was really worth, time-wise, but he was appreciative of the chance to learn all about the mysteries of the 250-305 Honda twins.
To paraphrase Forrest Gump’s comments: “Vintage Hondas are like a box of chocolates; you just don’t know what you are going to get.”