QuestionI've just bought a 1980 Suzuki 450 it starts and runs ok, but once it gets up to higher Rpms it starts backfire and loosing power, I think it's timing is set wrong but not compleatly sure.
AnswerI have owned '78 & '83 GS400s (the 425 & 450 are basically bored out 400s), so I know that somewhere between '78 & '83 they went from points to electronic ignition. I don't know off hand which yours has.
If if has points, the timing can be checked by connecting a test light between the points and ground and turning the engine over manually to the appropriate timing mark while watching for the light to come on. This should be considered part of routine maintenance. It is covered in your shop manual better than I can describe.
The procedure for timing the electronic ignition basically involves turning the engine manually to a timing mark and setting the bump on the timing advance unit so that it lines up with the pulse coil. Since there is no mechanical contact there is no wear so they almost never go out of adjustment and usually only need to be set if you have had them apart.
If the engine runs well at lower RPM I would be more inclined to think the Automatic Timing advance Unit (ATU) might be the cause. They are supposed to be inspected and lubricated regularly. If the ATU has been neglected it can hang up and not advance the timing as much as it should for higher RPM. The remedy is usually just cleaning and lubricating it.
I also recommend checking the valve viming & adjusting it if necessary. Earlier models with 2 valves per cylinder have bucket & shim system that is very reliable (in over 70,000 KM my '78 needed 2 shims changed one time) I don't know if they still do, but most Suzuki dealers used to have shims in stock and would swap the needed ones for your old ones for a nominal charge.
Later models with more valves have screw adjusters that need more frequent adjustment. Again, it is in the shop manual.
A word about shop manuals: If you don't have one you should get one before you pick up nother tool. Your bike is an antique and you will have a hard time finding a shop that will work on it. The sad fact is that a lot of the mechanics working now are too young to remember bikes this old and aren't interested in learning about them so that they can do a proper job since yours may be the only one they see this year. This means that you will have to do most of your own work, and working on a bike without a good shop manual is just not worth the headaches. (My bikes are from the same era - that's why I fix them myself.)
I recommend that you get two manuals - a Factory Shop Manual and a Haynes or Clymer. That way when you don't understand the explanation in one you will probably figure it out when you have read the other too. If you spend some time on Google you might even find a factory manual available for free download.
BTW: Is your bike really backfiring or is it afterfiring? Backfiring is when fuel air mixture is ignitted in the carburetor or intake tract and fires back through the carb & airbox. It can cause fires in the airbox and carb damage and is very dangerous. Afterfiring is when fuel that managed to get through the engine without burning ignites in the exhaust system (after it has passed through the engine). It can loosen baffles in the muffler but is generaly more annoying than dangerous. Either is a sign that something is not right.
