No 1. Plug
Yep, a faulty plug will surely give you running problems. The condition of the plug when you remove it can tell you a lot about how your machine is burning fuel. If the plug has black soot on it it indicates that there is too much fuel whereas a light brown and white plug shows too much air. Check to see that there are no deposits shorting the electrode. If the plug looks OK then you are going to have to find out whether it’s getting any electricity going to it. It can be difficult to detect an electrical problem with the plug as they can often give a weak or intermittent spark that looks OK to the human eye. The easiest way to check for spark is to remove the plug and lay it on the cylinder block so that the electrode is touching the metal and then pull over the machine. (Remember, when installing spark plugs do not over tighten, just a quarter turn after feeling firm resistance is ample.)
It can be tricky to get the plug to stay in contact with the block, especially if the HT lead is short, and so a less awkward way is to use a stroboscope. These are basically an inline light that illuminates when there is a spark. To use them simply remove the HT lead from the plug and attach one lead to the spark plug (still in the machine) and the other lead pushes in to the HT lead. When you pull over the machine the light will flash if there is a spark from the electronic ignition. Personally, if I suspect the plug might be leaking charge or failing at high temperature I just install a new one. It rules out the plug itself.
No 2. Air leaks
Start by checking that the carb hasn’t come loose. May sound silly but I’ve had this quite a few times and an easy fix if you discover a wobbly carb. If the carb is on solid the next culprit can be the inlet manifold coming loose.
Take off the carb and give the inlet manifold screws a test to see if they are tight. If everything is solid go on to number three.
No 3. Fuel feed
Next inspect the fuel filter and pipe. Use a metal coat hanger to hook out the fuel filter from the fuel tank. Fuel filters are quite cheap so if the filter hasn’t been changed for a while I’d change the filter as a matter of course. Take a look at the fuel pipe. They tend to go hard over the winter and this can cause the fuel pipe to hold the filter above the fuel and not drop into it as it would do if soft and flexible.
Check for fuel pipe leaks above the tank. If the machine is still playing up then you are going to have to start going a bit deeper in to the fuel system mechanics. Remove the carb and throttle cable and take off the carb. You may well find that you are going to need a new set of gaskets. Remove the pump and split the carb. You will see a small circular piece of mesh. Remove this and check for dirt. Clean with carb cleaner and reinstall with new gaskets if you have damaged the originals during taking it apart.
No 4. Air filter
The air filter can often become clogged with oil and dirt. Take the foam filter out and clean with washing up liquid. Allow to dry and re install
No 5. Exhaust
Remove the exhaust and take a look at the outlet port. If there is coke around it carefully chip it off making sure that the residue does not drop in to the combustion chamber.
If you still have problems check for the following
- Stale fuel gumming the carb and filter
- water in the petrol or carb
- damaged HT lead or poor connection between the lead and the HT cap
- Intermittent electronic ignition
- rings gone or barrel scored
- split gaskets
- cracked inlet manifold
- spark arrestor