The above trimmer is suddenly running rich after two uses this season. The plug is wet with fuel, 4 strokes throughout the rev range, smokes excessively and has a rough idle. It does start about the same as before - 3 pulls. Last season I adjusted the high low needles on the carb and it ran perfectly. Now the engine will barely respond to changes in the needle settings.

It has fresh fuel, new plug and clean air filter. The choke is not stuck. I swapped out the new plug with the old and it ran the same. I don't see any leaks. It has about 40 total hrs on it.

Below are things I thought to check. Am I missing anything particular to this model?:

Clogged muffler screen
Reed valve (can this been seen through the carb and is there an area they usually crack or chip?)
Spark/Ignition coil ( test/resistance specs on the ignition coil?)
Flywheel key
Carb - stuck diaphragm needle

 

Sounds like the carb needs work or replacement. Yes, you can see the reed valve and there are no places that they normally crack or chip. A bad reed valve probably wouldn't cause this sort of problem though.

 

After other items are checked, is it worth pressure testing the carb fuel pump and metering diaphragms for leaks or just rebuild/replace? I have a theory that maybe the fuel pump diaphragm is cracked and drawing excess fuel through the impulse line. I would think it is a good time to replace the fuel lines at this time too.

 

Just open it up and see what's wrong. No sense pressure testing anything until you're done, if you wish, because it obviously has a problem. Your theory may be correct, also the needle may not be closing fully.

 

Well, looks like it was the ignition module/coil. It went out partially with a misfire and then later went out completely. During misfire it didn't burn all the fuel and the plug showed rich.

A new ignition module is $50-60 . So one question is how much life is left/reliability of this trimmer vs getting a new next generation of the trimmer model 79440 (316.79440) for about $100-150. Do you know about the repair history or quality info either of these models? Or will the low end trimmers of the higher end brands such as Echo, Husky, Stihl etc. give better performance and reliability. This might be good subject for a new post, but I would like to read your and other's input.

 

You said you checked the coil... is it not sparking? They do fail occasionally. It's hard to know how much life is left in the trimmer, not knowing how many hours are on it and how it's been maintained. These don't last long anyway... I think they are built to run something like 19 hours? Low end trimmers usually last roughly the same. I'm not a fan of them. Echo and Stihl definitely give better performance and reliability. There are lots of 20 year old stihl and Echo trimmers out there still being used today. Mine included.

 

Yes, I checked the coil again after cool down while on the bench and no spark. The kill switch and wiring checks out OK. Must have died completely after the last run.

The only other remote possibility is the center coil looks gap is not even because it vibrated out of position. It is not hand loose though. It is .012 at the closest end, but looks like .040 on the wide end. I cannot see the outer leg gap where the mounting screw is , since I haven't had a chance to remove the main recoil cover. As you probably know the clutch drum needs to be removed to get the main housing off and every over cover removed to get full access to the ignition coil.

 

Just an update: I decided to check the ignition one last time before I bought a new trimmer. I found and fixed an intermittent in the kill switch circuit and got spark. I tore down the Zama carb and found a distorted metering diaphragm and bad sealer on the welch plug. I cleaned and rebuilt the carb with the full kit, resealed the welch plug ( see my other post) , tuned the needles and it runs great.