Dear Ole Mechanic;
I have a six-year-old gas-powered string trimmer. It is one of the less expensive brands but has done a decent job for my needs. I used it back in the spring of this year and it ran OK, but it seemed to be a little low on power. When I dug it out yesterday, it wouldn’t even fire, even after a lot of yanking on the pull-start rope. Thinking that the gas had gone bad, I mixed up a fresh batch. Still nothing, so I got a new spark plug, checked the gap and was ready to install it when I remembered your troubleshooting advice--“Fuel, Spark and Compression.” Before I installed the spark plug, I put the wire on it, grounded the plug against a metal part of the head and pulled the rope again. I could see a spark--at least I could until the plug slipped off and touched my hand. BOY, did it ever have spark! After I got calmed down, I put the spark plug back in, pushed the primer bulb a number of times, choked it and proceeded to pull on the starter rope untill I was blue in the face. I can tell that it still has compression, fresh fuel and a lot of spark. Now what?
Shocked the #@&* out of me
First off, I apologize. I've been around this stuff for so long that there are some details that I take for granted and forget to mention. I have a home-made jumper-wire that I clamp on the threads of the spark plug and attach the other end to a metal part of the engine. That lets me keep my hands well clear of the plug and thewire. Even a small engine like the trimmer must generate enough electricity to jump the air gap between the two electrodes of the plug. What you experienced was about 15,000 volts. Fortunately, the amperage is measured in millionths of an amp. It won’t kill you unless it messes up your pacemaker. I should also tell you that even if the wire is on the plug, the wire can leak electricity through the insulation--especially if the wire is an older one. Just touching the outside of the wire can be a shocking experience. Sorry bout that.
Now to your "blankety blankety" weed trimmer. After priming, choking and pulling with no start or even a pop, the spark plug should be pulled out again. If gas is getting into the engine, the electrodes will be wet with gas. I'll bet that you'll find your plug dry, which indicates that fuel is not getting into the engine. There are several commonly-found things that will keep the fuel from getting into the engine. First, the fuel filter in the gas tank. When the tank is empty. the filter is what rattles if the trimmer is shaken. To get the filter out of the tank, I use a piece of wire with a hook formed on one end. I push the hook end into the gas tank filler hole and hook it around the fuel feed hose which has the filter on the end of it. It may take several tries to snag it. Once hooked, I gently pull the hose out through the filler hole till the filter comes out. The filter is not expensive but what usually happens is that the old, brittle hose breaks and the trimmer must be turned untill the filter can be seen thru the filler opening. Then it can be fished out with a pair of needle-nose pliers.
That brings us to the next common cause of no fuel in the engine. Old, brittle, broken or cracked fuel lines will allow air into the fuel-feed hose. An air leak will cause air to be drawn into the carburetor instead of fuel. No fuel in the engine results in no start, of course. This is a common problem in older trimmers and chain saws. It is such a common problem that I buy fuel line in four to six foot lengths. A trimmer usually takes less than six inches of fuel line. The problem with buying fuel line is that there are at least four different sizes used by trimmer and saw manufacturers. If you buy fuel line, be sure to take a sample of the old lines with you. There are usually two different sizes on a string trimmer. Also, the fuel-feed line is a real pain to install. Getting it on the carburetor is not too bad after you get some of the engine covers off and out of the way. The problem is getting it installed in the gas tank. The hole in the tank is smaller than the outside of the fuel line--just enough smaller that once the line is pulled through the hole, it fits tight enough to make a fuel-tight seal. That way, fuel cannot leak out but the hose will still carry fuel from the inside of the tank to the carburetor. In other words, the fuel line must be pulled inside the tank to allow the fuel filter to be installed and to move around so it will pick up fuel from the bottom of the tank. Another way for the carburetor to get air instead of fuel is if the primer bulb has a crack in it. Air will enter the crack and be pulled into the carburetor instead of fuel. The primer bulb is replaceable and can be found at better auto-parts stores and other places where trimmers and chain saws are sold. Again, take the old one with you as there are different sizes of bulbs.
The air that was pulled into the carburetor has caused any fuel in the carburetor to dry up, leaving a gummy residue. A gummed-up carburetor will keep fuel from getting into the engine. The only way to clean the gum out is to remove, disassemble and clean the carburetor. There are a number of small parts in the carburetor--including at least one very small, fine spring that is nearly impossible to find once it flies out in that direction. Which direction is that? Well it's very hard to tell because the spring is that small and that fine.
Has it ever occurred to you why the less-expensive trimmers are priced that way? If you divide the original cost by the number of years of use that you got out of it, it gets really cheap. They are intended to be a throw-away item. It is just hard-headed old German descendents like myself that are too cheap to go buy a new one. It is also true that I get satisfaction out of "MJW" (Making Junk Work).
Herr Professor Nuzanbolts