Motorcycle Repair: honda 750 carberators, gas stabilizer, vacuum petcock

Question
My bike is an 1980 cb750 custom (honda). The engine is starting to miss at around 6000 (75-80 mph)rpm . It seems to me it is the carberators , but I have been putting gas stabilizer in it for the winter and stp gas treatment in it for a couple tanks before this started to happen. IF this is a carberator deposit problem is there a cleaner that I can put in the gas tank that cleans rather than prevents pluged outlets. Is there another problem I should be looking at? Thanks . Dave  

Answer
David, after 24 years they could be ready for an overhaul or at least a good cleanout.

I would drain the float bowls out into a cup so you can see what comes out and that will give you an idea of what you may find when you open the bowls up.

Yamaha makes a good carb cleaner that you can inject down the fuel lines to help clean out old deposits. You have to be careful with mixed in carb cleaners for long term storage as they tend to make the O-rings swell up and leak.

I would certainly check the petcock screen, but you will have to drain the tank to pull the petcock. Being that your bike is a 1980 version, it probably has a vacuum petcock. If it does, often the diaphragms get pinholes in them and either shut off or shut down the fuel available to the carbs.

The carbs have a diaphragm for the air cut (anti-backfire valve) and another one for the accelerator pump, so there are a number of possibilities for problems there.

I would also check the spark plug caps for matching resistance values, as they often fail. Be sure all spark plugs are good and the wires are tight in the caps and at the coils.

These bikes have two separate spark modules, one for each coil and they are know to go south on occasion, but are generally "pass-fail" in operation. They are mounted back by the battery.

Bill Silver