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.”
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