QuestionHi i have a problem with the spark plug sooting up on the left hand cylinder when the engine reaches 5000 rpm under load,runs ok below these revs and revs through the range when not under load,there is no smoke from the exhaust when running,can you help,regards harry.
AnswerHarry, I assume that this is a leftover CB500T based upon the earlier CB450 DOHC models which started out in 1965, here in the US. The last year for the regular production of the CB500T was 1976, as far as I know. Sometimes, the bikes were registered as later year models because they were sold after the production year was over. http://www.retrotours.com/1976-CB500T/
The left hand carburetor has an air-cut valve with diaphragm on it. When the diaphragm fails, it creates a big air leak in that side, which probably reduces the vacuum signals to the slide piston, throwing off the calibration to that side cylinder.
http://www.cmsnl.com/honda-cb500t-500-twin-dohc-1976-usa_model462/partslist/E++1
#9
Of course, before condemning the carburetors, which I would hope have been checked/cleaned and kitted up by now, always start with a valve adjustment, compression check (should be around 175 psi) to ensure that you have a sound foundation to work from. Low compression and/or leaking valves will cause the CV carburetors a lot of grief, as they rely upon strong vacuum signals.
Next would be a check of the spark advancer to verify that you are getting full spark timing advance to both cylinders, then an individual timing check of the L and R side cylinders, using a 12v test light to check when they open (F and LF marks, respectively). There were issues on early bikes about poorly grounded condensers, so check to see that the condenser is securely connected to clean and shiny metal.
A more subtle issue is to check your point gaps and see that they are over .012", preferably around the .014-.016" width with the point cam pushing them open to the maximum. Once the gap is set, then you can work with the timing plate to get them to open at the F(iring) marks. When gaps are too small, there is the possibility that both sets of points are closed at the same time, momentarily, which causes a voltage/current drop for both coils and poor performance follows.
Make sure that the ignition coil's primary windings are reading out about 4.5 ohms, with wires disconnected from the circuits. Check spark plug caps for about 5k ohm readings. Spark plug wires can age, harden and crack, allowing voltage loss to any surrounding metallic objects which are grounded to the chassis.
You should be using D8ES NGK spark plugs and stock carb jetting, unless you have modified intake or exhaust components. Check intake manifolds for cracking or bad o-rings/gaskets at the cylinder head.
Exhaust leaks at the exhaust ports/header pipe flanges can cause disruptions to vacuum signals to the carburetors, too.
Bill Silver
