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I'm having a hard time

Posted: Fri May 01, 2020 2:52 pm
by schrader7032
I'm having a hard time understanding how the left side could keep firing without the cap off...are you sure it was running? How long did you pull it off? Did the RPM slow at all? Whenever you pull the plug cap like that, the magneto should redirect the spark energy to the safety gap on the magneto body. Did it do that?

When check continuity across the points, you can't just put the probe in there and get the right reading...what will happen is that the meter will find continuity by an alternate route that you can't really see. There is a small wire that needs to be removed in order to do the continuity test. In the picture, see the small black wire that connects from the left to the top of the orange insulators. I find that I have to remove that in order to monitor the gap. I believe Vech sells a "buzz box" which does all that for you without having to remove the wire.

I'm not familiar with

Posted: Fri May 01, 2020 3:34 pm
by schrader7032
I'm not familiar with Aircraft Spruce's buzz box...same as Aircraft Spruce the source of Vech's tool?


Posted: Fri May 01, 2020 3:35 pm
by westeagle
So while both sides were running, I pulled the left spark plug cap and the RPM did not change. I incredulously checked the exhaust pipe and there was still some smoke coming out of it. The 11mm safety gap was sparking as well for the left side. I let it run like that for about 15 seconds.

After I turned the engine off, on re-start, whilst keeping the plug on the right side only, the phenomenon did not happen: it ran on the right side only, as expected; then I re-connected the left spark cap and it re-ignited that left cylinder (with both sides running now at higher rpm -as expected); then I took the left cap off again, and both sides kept firing (no rpm change)! NOT expected! -Ive never seen this before. Is this a case of preignition?

In regards to the points I do have the buzz box from Aircraft Spruce and I did use it to do a static timing check, but then I was probing with my regular multimeter looking for any electrical anomalies, and thought it odd that there is continuity between those points even while gap is open.

R50 running rough -electrical questions

Posted: Fri May 01, 2020 3:48 pm
by westeagle
Hello friends

I recently re-assembled my 1957 R50 engine after seizing the right piston (not completely "seizing" but severely scuffing, causing piston slap) likely due to too tight of piston ring end gap, coupled with oil starvation (lack of oil hole on R side), then possibly exacerbated by running it a bit too hard during break-in. I had the cylinder honed, and drilled a new oil hole to the right side of the block, then replaced the piston with a new one of the same size, but this time I had the rings adjusted to the proper end gaps, as suggested by DA.

After doing the slingers and replacing bearings and seals within the past year, this is the second time I've had the engine apart completely. I had to do a second full-engine tear down, because the new oil-hole drilling project requires it.

On this recent reassembly, the bike has been running, but very poorly, and to my surprise, when I pulled off the spark plug cap on the left side -the cylinder kept firing!!

Also, with the bike off, while checking with my multimeter, I get continuity across the points gap when gap is open -is this normal?

It has been a one-kick starter before this, so I am grasping here. I did a static timing setup to the S-mark and everything seems like it should be a go...

Any ideas?

Buzz Box

Posted: Fri May 01, 2020 3:59 pm
by westeagle
I think so. The device is more than just a continuity meter. I believe it measures capacitance. It is mentioned in the Betjemann book.
-So I guess I just answered my own query: the reason the buzz box is used is that a regular continuity tester shows no difference whether points are open or closed, when the normal wiring of the points remains intact.

Are you sure the left side is

Posted: Fri May 01, 2020 4:58 pm
by davidmcctan
Are you sure the left side is running ... I had a situation like that , but my on right side.. I had to use a temp gun to figure out it wasn't running at all (But it was hard to tell by the way the bike ran) . I got the temp reading and the right side was 100 degrees cooler than the left.

You can also just switch the spark plug wires at the coil . The bike fires at the same time for left and right . It the problem goes to the other side , that may help you isolate


Posted: Sat May 02, 2020 8:25 am
by Daves79x
Sorry you had to go through all that a second time. I suspect your main issue was not ring end gap, but too tight cylinder/piston clearance. That, plus running pretty hard (I think I commented on that elsewhere) during break-in and no right-side oiling hole, lead to disaster. But that's water under the bridge now.

Are you sure your normal idle with both cylinders running isn't too high. If it's too high, the flywheel effect will keep the engine running fairly normally even with a plug wire pulled. It clearly can't 'fire' with the plug cap completely removed. Are you sure you have the cap completely away from the plug top? Spark will jump a long way.