OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Hand and Power Tolls Power Tools GMC 750W Generator

Dear all,
I've just bought for $20 a broken GMC 2-stroke generator. I thought it would have been an easy fix but it was not.
The unit looks brand new but it won't start. The previous owner told me it died while it was running and it never started again.
I've already carried out the following tests:
- Compression: ok
- Spark: weak but there is spark ( spark plug has also been replaced)
- Fuel filters: all ok and petrol gets to the carby and to the spark plug
- Replaced old fuel
- Air filter: like new
- Checked carby (inlet needle, jets, float). The carby looks brand new. No gum, no nothing.
After these preliminary tests, I put a few drops of petrol into the cylinder and I tried to start the engine again. Nothing, just a very weak attempt to kick in and a tiny little bit of smog, but not a proper start.
Although there is a (weak) spark I have the feeling that there is something wrong with the coil.
Do you have any other suggestion? I don't really wont to trash an engine in near new condition.
Do you think it is worth trying to fix it?
All suggestions are welcome.

Seems to be ignition, its either coil or ignition module (in many cases these are combined in one unit)

Can you get a photo up of the coil & Wires, then we can work out if it a coil and chip, or just a transistorised coil.

Also, while you are in there to get the photo, carfully check all the wires are in good condition, and if you have a multimeter, check wthe wires have contiuity ( I Imagine they do if there is more than no spark) or shorting.

Often time can be saved on that kind of job by testing things in modules. The main advantage you have is that (apparently) it hasn't received any amateur attentions, so there is probably only one fault. We are being aimed in a particular direction by your comment that it has weak ignition. If you had not said that I'd be asking questions about compression: you said it is strong, but does it have strong crankcase compression as well as cylinder compression? A crankcase induction two stroke must have two compressions (as felt on the starter cord) per revolution.

If it has proper compression (two more or less equal, sharp resistance periods per revolution) the next issues to consider are fuel, spark, induction leaks, and exhaust blockages.

You said you had tried squirting a few drops of fuel into the spark plug hole, and it then kicked but didn't run. You need to put in a spoonful of fuel to do that test, or you may get the result that you got, from a perfectly healthy engine.

When you test spark it is essential to be testing the ignition system, not the spark plug. Hence if you do not see a steady stream of blue sparks across the plug, hold the plug lead about 3 mm from the cylinder and pull the cord. If the spark will not jump 3 mm it is pretty awful - a newish magneto should do better than 10 mm. I've had old Villiers engines with ignition about that weak that ran perfectly, but they were rather hard to start. Never condemn an ignition module without testing the direct spark - spark plugs, new or old, may be defective. I think I recall 4 instances here this year, where we went through long diagnostic procedures unnecessarily because a brand new spark plug would not spark.

One of the standard things to check in these cases is whether the muffler is blocked. This can happen in two main ways: wasp nests, and carbon build-up. The latter typically happens to two strokes that run for long periods, especially at light load or if the owner mixes the fuel with a heavy hand on the oil. It can also happen to four strokes with faulty piston rings.

In the end, I usually find the quickest and most effective way to check the entire induction system (but not the crankcase seals) is the fuel-through-the-plughole trick. If it runs for a few seconds on that spoonful, you have a running engine that has a fuel delivery or induction leak problem.

I have the same generator that gave the same problem, it ended up being a blocked exhaust. I still have it sitting around here.

Hi guys,
Thank you for all your comments and suggestions. Tonight I'll do a few more tests and I'll get back to you.

gmc generator hmmmmm lucky to run a 40 watt light bulb i have seen a few of these with dead coil steve,,,,

Hi guys,
Here is an update on my GMC generator-saga.
First of all I checked the exhaust hoping to find it chocked. Unfortunately it was nice and clean. Further to this, I checked from the exhaust port the cylinder and piston and both of them were like new.
In my first post I said that the spark was weak but yesterday I actually managed to get a good 5mm long spark when I tested the high voltage lead without spark plug.
Although this test would suggest that the ignition system is ok, when I poured a teaspoon of petrol into the cylinder I didn�t get any appreciable sign of life. After several pulls the engine actually tried to kick in making a bit of smog, but it was just for a fraction of a second while I was pulling.
Finally, after a few hours I decided to put everything together and bring the generator to the local mower shop. After being told that the servicing was $140 I decided to sell it to the shop for the same price I paid for it� I hate giving up, but there wasn�t anything else I could do with the standard tools I have. Hopefully my loss is their gain..I�ll check with the shop in a few weeks time what was wrong with it and I�ll let you know.

Assuming your new spark plug is OK and both compressions (cylinder and crankcase) are good, most likely it is a problem getting fuel to where it belongs. Could be tank, fuel ine, carburetor, intake air leak, or just that the choke or priming device isn't working. However I'll look forward to hearing what the mower shop tells you (if they are prepared to tell you anything).

Re-reading MaxGaras initial post, it seems he covered all the bases, the one thing that is intriguing is the fact that the original owner said it was running and just stopped and wouldn't start again...for the sake of this archive, it would be great to find out what was the prob.

Dear All,
Today I went to the mower shop where I sold my generator to buy some oil and I asked the manager if he managed to fix it. Guess what? He told me he had to trash it because he couldn�t work out what was wrong with it! This is probably why GMC went broken!
I'm afraid this case will remain unsolved..

Thanks for posting the outcome, MaxGara. This thread should be closed.

I wonder what was the prob....sad we'll never find out!
I shall now close it.