View Full Version : OT - Small Engine Trouble Shooting

04-05-2019, 12:49 PM
About three weeks ago there was an extended discussion prompted by a mysterious problem with a chainsaw that wouldn't start when hot after some modifications.


The solution to the mystery turned out to be setting the plug gap to less than specs.
And the reasoning was that, while the plug appeared to have good spark when initially tested, the spark was weak or non-existenent inside the chamber when hot. Many here seemed familiar with that phenomenon, but it was a revelation to me.

My question is this: would a plug tester, such as I have, which has an in-line light, and can be inserted in series with the plug wire to flash with the spark, continue to flash even though the plug is not firing?

04-05-2019, 02:04 PM
Yelp--maybe, tester shows if electricity is going to plug, if plug is grounded it will flash. If plug has open circuit no flash.

04-05-2019, 02:15 PM
Yelp--maybe, tester shows if electricity is going to plug, if plug is grounded it will flash. If plug has open circuit no flash.

Yup as above.

If the tester is one that does not have a separate ground it will only flash if the electrical circuit is complete. A carbon fouled plug will allow the tester to flash because the circuit is complete. If the plug is incapable of firing due to worn electrode or a broken conductor for example, it will not flash.

04-05-2019, 02:51 PM
I use a spark tester that has a fixed gap that you insert into the cap and then ground. If the spark can jump the gap it is considered good. Remember that some small engine electronic ignitions require a pretty quick pull to work. A slow rollover will not give you a spark on those. I also use a induction style timing light to watch if the ignition is working while the engine is running. That is a good way to know if you have a intermittent spark or some other issue. Mike

04-05-2019, 04:07 PM
Sooo.. then it's possible that even though I'm getting a good flash in that tester, it is still a possibility that there is not a good spark across that plug gap, sufficient for fuel ignition. ...?
Too bad. Of course it at the least tells me I have current getting to the plug.

04-05-2019, 04:21 PM
True, at least it will verify that your ignition system is functional. Sometimes a plug that is weak will fire when placed on the head to see if it has spark yet the spark will fail to jump the gap under compression due to the electricity shorting to ground on a carbon fouled insulator.
Electricity taking the path of least resistance will sometimes find it easier to go from center electrode and then follow the carbon fouled insulator to ground rather than jump the gap under pressure.

04-05-2019, 08:32 PM
Willy mentioned an important fact above. Back in the 60s, service stations had a plug tester that was hooked up to the shop air and pressurized the chamber in which the plug was seated. You could see the spark (or not) through a window. A lot of plugs would spark in open air, but not at 100psi. I haven't seen one of those testers for decades, I guess the computerized engine analyzers made them obsolete. Too bad for the small engine guys.

A related memory: Years ago I flew a U3A, around 1955 version I think. Anyway, that airplane had an altitude limit for the magnetos because if you get into thin enough air the spark would go places it shouldn't! In other words, the lower the pressure, the easier it is for electricity to jump a gap.

04-05-2019, 08:52 PM
I flew a Turbo Arrow rated to 20,000 ft (but could go way higher...) , but it has pressurized magnetos for exactly that reason.