Horizontal Air Cooled Engine

[Roger B]

Are you sure it is firing at the correct TDC (or does this one fire every TDC)?

[Jasonb]

Even so you would expect the odd pop as residual fuel gets ignited at some point, one of the reasons I like to hand start as you can feel and hear what is going on when there is no drill whizzing away.

To totally rule out the spark shorting somewhere you could take the head off and check that the plug  is still sparking OK when screwed into the head rather than unscrewing it and trying away from the engine.

The other simple thing is make sure your battery if fully charged particularly at this colder time of year so it puts out as big a spark as possible under compression.

[Brian Rupnow]

So, here I am at the end of a 9 week build. To achieve this build in that time span required that I worked on some aspect of the engine every day of that 9 weeks. The engine looks good, I am satisfied with that. I am confused as to why the engine won't fire. There is nothing here that is different from all of my other running engines, other than the sparkplug. The sparkplugs are brand new, from a different supplier, and far be it for me to question  a brand new sparkplug. There is a possibility that the plug is shorting out internally, but if it was shorting out internally, then it would short out internally when laid out on the block. It doesn't. It gives a bright blue spark just like a good sparkplug should. My "need to machine something" has been satisfied (for now). Now I go into the detective phase of this build, and that is the part I don't really care for. I know I have fuel. I know I have compression. I'm not 100% sure I have spark. Even an engine with screwed up valve or ignition timing will fire when the engine is being turned by an electric drill. It may fire at the wrong time in the cycle, it may just give "pops and farts", but it will fire. You can hear it fire, and you can see smoke, and the cylinder will get warm. You can feel it kicking back against the drill when it fires. I'm undecided where this will take me now, but I will be sure to keep you informed.---Brian

[Jasonb]

There is a possibility that the plug is shorting out internally, but if it was shorting out internally, then it would short out internally when laid out on the block. It doesn't. It gives a bright blue spark just like a good sparkplug should.

Not so. the spark will take the easiest route with least resistance. I the open air that is likely to be from electrode to body as it should but once inside the combustion chamber under pressure that may not be the case and the spark could be going elsewhere.

You should just about be able to make up an adaptor 5/16 x 24 external and 1/4 x 40 internal and then try it with another plug, plus first doing the test I suggested with the head off the engine but plug in place

[RReid]

I think there was some question of the spark plug sealing properly in the head without a gasket? If air is leaking past the threads on the intake stroke, there may never be a combustible mixture in the vicinity of the spark.

[Brian Rupnow]

The sparkplug is not leaking around the threads. Someone on one of my forums didn't believe me when I said that the engine did have adequate compression, so they wanted a visual on it. Here it is. This doesn't get me anywhere closer to having the engine run, but it is kind of a fun video.
<a href="https://www.youtube.com/watch?v=jJEAny_Toh8" target="_blank">http://www.youtube.com/watch?v=jJEAny_Toh8</a>

[dwulfe]

Why would the spark arc occur anywhere but at the gap? Maybe try to ground the head directly with a ground wire. Just a shot in the dark

[Laurentic]

+1 on ground the head!  and +1 on Jason's idea to trying with the head off but plug in place.  All to do with eliminating the possibles, it leaves you with the probables.

[Brian Rupnow]

The engine just ran!!! God loves me after all. When all probable things have been tried, it is time to try the improbable. There was only one thing left to try that didn't involve machining something. I kept getting mixed signals about whether the sparkplug was really sealing against compression leakage or not.----So----Ten minutes ago I backed out the sparkplug, coated the threads with green 638 Loctite, then reinstalled the plug. Waited ten minutes and engine started up and ran like a trooper. I will get a video up tomorrow morning. I am so relieved.

[crueby]

Woohoo!   

[Kim]

Congratulations Brian!   

This one has been quite the battle!

So, was it just your compression leaking around the spark plug?  Or was there anything with the sparkplug getting shorted out and the green Loctite provided some insulation?

Kim

[Brian Rupnow]

All the Loctite did was to prevent compression from leaking around the sparkplug threads. 638 is my "go-to" choice for Loctite. It is an excellent sealant, but releases very easily with a bit of torque. Like I said, all of my engines run---eventually. Some run right away and surprise me. Some require much thought and fiddling with. Out of about 20 engines, I haven't seen this happen before. I keep thinking that if I build enough of these small engines, I will get to the point where I can diagnose whatever is wrong and fix it immediately.  The most difficult thing is knowing that everything is adjusted and set properly, and still the engine won't run. Thank you all for your suggestions and your interest.

[MJM460]

Well done, Brian.  That one certainly tried your persistence.

Looking forward to the video.

MJM460

[Jasonb]

Good to hear it is running and also know that the problem was one I raised as soon as I saw the photo of the new plugs.

Do be carful using 638 as a sealer, it should not release that easily being a retainer not a thread lock/sealer, don't want to strip the thread in the head trying to get  a plug out Maybe the heat of the head is weakening it.

[Brian Rupnow]

The fuel I used was new fuel. The battery is fully charged. There is nothing wrong with Steve Huck's sparkplugs. they work fine. I had Steve make two "special" plugs for me, and I knew there was very little shoulder on these plugs to snug down against the top of the cylinder. I didn't think they would leak around the plug threads, but I was wrong. We live, we learn.