# Problem starting Homlite St155 Trimmer



## CAPTRICK (May 18, 2005)

Having starting problems, I have relpaced all the diaphrams in the Zama carb and cleanded the unit; I have 92psi compression but can't seem get a spark, have checked and gaped to about 13 thou on the coil and get about 25 volts AC that fluckuates up to 50 volts AC from the plug wire to ground when I use drill to turn the flywheel. Does anyone know what voltage the coil should give out and should it be constant? Does the gas bowl have to be full when you prime the unit? there is gas getting into the chamber but the unit won't turn over. Any help and advice will be greatly welcome.


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## hankster (Jan 1, 1998)

Use a spark checker that you can find at any automotive store... pretty much fool proof. Stick on the spark checker, turn it over and if you have spark it is good. If no spark and your wiring is OK then the ign. module is bad. It is not uncommon for ign. modules to go bad. 

Not sure what you mean by "gas bowl" but you do need fuel in the tank when priming.


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## CAPTRICK (May 18, 2005)

The little primer plastic butten bowl at the bottom of the carb. Is the voltage at the plug AC or DC? is there a rectifier in the coil assembly? I believe that the magneto cicuit use to deliver AC which had to be changed to DC by the capacitor in the old circuit, do the electronic units convert the power to DC?


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## Exporter (May 21, 2005)

My trusty Homelite finally decided to play dead two weeks ago. She would not hit at all. In the past,if I was careful NOT to overprime it it started on 4 to 5 pulls.

I found the problem was a filthy switch. The switch you hold in to shut down. I used carburator cleaner, 5 to 6 good shots to clean the contacts. She wont start unless the contacts are making a good connection. The carb cleaner did it. It starts good now.


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## hankster (Jan 1, 1998)

I really have no idea if it is AC or DC. There is no external rectifier. Really, what the voltage is is more information then what you need. Get a spark checker and see if it sparks.


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## hobe (Mar 29, 2008)

hmm my st 155 just grounds to case when i push the kill button?
i have 95 lbs compression nice blue spark and cleaned the walbro and have fuel at the plug but it doesnt pop. fuel in the cylinder
no fire either.
Consider my self pretty good tinkerer but this has me puzzled. tell me more about the contacts you speak of please.
hobe


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## pyro_maniac69 (Aug 12, 2007)

95 is kind've low, is your muffler clogged? are there any scratches inside the cylinder?


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## 30yearTech (Sep 14, 2006)

These models have a knack for getting loose mounting screws everywhere. Make sure all the hardware is tight, especially the cylinder, carburetor / insulator, and crankcase cover. An air leak on a two cycle engine will keep them from even trying to start.


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## crazyhobby (May 4, 2008)

Hey Hobe I'm having the same prob with mine. If you find the fix drop me a line and I'll return the fav


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## dawgpile (Apr 28, 2008)

Captrick...just an fyi. DON'T TRY AND MEASURE THE PLUG VOLTAGE! The spark plug voltage is neither AC nor DC in the traditional sense. It is a very high voltage, variable frequency, narrow pulse-width signal. In other words, it goes from 0 volts to a very high voltage and back to 0 very quickly. This is dangerous both to you and your instrument. Most meters, particularly the newer digital ones have an upper measurement limit of approximately 1000V AC or DC. Cheaper ones are in the 600V range. Check your meter as an fyi. It's usually imprinted right on the face where you put the probes.

Keep in mind that spark plugs are running off of several THOUSAND volts! That's why the spark is able to jump a quarter inch plus gap in a spark tester and the plug at the same time! That kind of pulse coming into the average modern DVM can 'smoke' the input protection circuits. Analog meters are much more forgiving! You need a special high voltage probe to properly measure this type of signal. However, even if you had the proper instrumentation, it wouldn't matter. It's not spec'd anywhere in any service manual. No need for sophisticated and potentially dangerous test set-ups!

As Hankster said, the use of a gap type tester is always the preferred and safest method to check for spark. It is all you need to determine that your ignition is working properly! Keep your meter away from the high voltage side of your ignition system at all times!!! (You can certainly use it to troubleshoot the grounding circuit which is used to shut off the ignition to stop the engine) Your meter and your heart will thank you!

Finally, relative to the problem you're having, if you've determined that 1) your spark is good, 2) plug is wet(ie it's getting fuel), and 3) compression is good(this covers a lot of ground including all fasteners tight, cylinder not scored, etc. etc. and is more than likely where the problem lies!) and 4) muffler spark arrestor not clogged then I'd pull the flywheel and make sure the key isn't sheared.

Keep us posted!


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