QuestionI have a 1979 Honda cb750 kz dohc which fires on the button every time when the weather is hot, as in summertime. I live in England where the winters are mild and at this time of year it is about 12 degrees Celsius but too cold for my bike to start. It has been ok all through summer until the last few weeks until the temperature dropped. If I get it going, after about 5 minutes of trying and just before the battery runs dead, then let it warm up it will start every time after that with just a touch of the button. Once it goes cold again I am back to a none starter. I haven't changed anything since having this problem. Petrol is getting through because the plugs are wet. I think it is an ignition problem because I have heard that the coils are a little weak on these bikes and the original ones are on it which makes them 33 years old but I don't want to spend money on new ones if this isn't the problem. The plugs spark when they are out and grounded to the engine although they are not as big and blue as I would like. ignition timing is ok, carbs are synchronized also. But as I have mentioned the only thing that has changed is the clymate. I've heard this is a common problem but no one seems to have a solution, can you help? Thanks.
AnswerDavid, well this is an unusual question for me.
The DOHC engines used a transistorized ignition that has a couple of spark units that supply triggers to the coils. As you say, the bike is 33 years old and apparently all original components. There are known issues with the spark units, but I would start out with a good check of all electrical connectors to the ignition components, a check of the spark plug caps which should be about 5k ohms and ensure that they are firmly secured to the wires. Any dirt or corrosion can attract moisture which can cause spark arcing to grounded components nearby, so look carefully at the coil wires, and spark plugs too, on the outside. You can use a spray bottle with some water in it and spray around the plug wires at night, so you can more easily look for any arcing of the spark. Because the 1-4 and 2-3 coils fire at the same time, anything that affects one cylinder or either end of the coil will affect both cylinders.
I think there are aftermarket spark units on the market now, but not at a cheap price. The originals tend to have some solder joint issues and can often be repaired. There used to be a repair page on one of the Honda V-4 sites.
These bikes have a known issue with charging systems, so be sure that all is well with the battery charging function. I would certainly get a very good quality, fresh battery to spin it over. Fresh plugs wouldn't be a bad idea, either.
If there is any moisture in the fuel system it may be causing problems with the accelerator pump function and perhaps idle jets. The original carburetors came with limit caps on the idle mixture screws which are jetted lean to meet emission standards. Defeating the limit caps will allow you to richen up the idle circuits which may make for easier starting. Air cut valve diaphrams can fail and cause air leaks to the adjacent intake port.
Ensure that your choke cable is adjusted properly, so you get full choke function when the engine is cold. If the carb to cylinder head intake manifolds are all stock, they may have shrunken and/or cracked causing air leaks there, as well.
In the US we have alcohol added to the gasoline, which causes problems with cold starting and general running issues. That may or may not be an issue for you, over there.
DOHC Hondas are not my specialty, so you might be able to find other forums on-line who might have other solutions for this problem.
Bill Silver
