Cracked Head or ??

[JDHMike]

Have just got my R42 running. Starts first kick. Seems to be missing on right cylinder also is percolating gas, it appears, at bottom of spark plug. Am going to do a compression test and was also thinking about running and warming up. Any thoughts, suggestions or assistance greatly appreciated. See attached photo.

Mike

[Darryl.Richman]

Congrats on getting it running!

There should be a steel insert in the head, made from steel or bronze, based on a look at my R52 and R12 heads. Possibly the insert is becoming loose in the head, or the aluminum is eroding around it, and allowing gas to leak out around it.

[JDHMike]

Thanks Darryl.
Suppose head must come off to confirm?

Mike

[Darryl.Richman]

Depending on how bad it is, you may not be able to see anything. You could do a leakdown test to try to pinpoint exactly where it's leaking. Just put some water around the leakdown plug and gradually bring up the pressure, you may see bubble before it sprays you. Unfortunately, probably none of the easy fixes using, for example, Loctite will work here because this area gets so hot.

Actually, looking at the photo again, it appears that the head has an adapter in it to bring the plug size down from 18mm, to take modern 14mm spark plugs. If you're lucky, it may be that the adapter is leaking, and it could be replaced. If you're really lucky, maybe it's just loose!

Otherwise, yes, the head has to come off to be repaired.

[JDHMike]

Thanks for your comments Darryl. The spark plug is a Champion UD16 and is burning very rich. I used a 22mm socket to remove and think it is an 18 mm plug. It measures 18 mm with my calipers. I should make an editorial comment here; I have wrenches and can read English manuals and have been an enthusiast for a number of years however do not profess to be a mechanic. I have worked on bikes for years and on numerous occasions have made repairs but have also screwed up on a number of occasions. This is something I have not seen before. I have the manuals however they are in German. I am sure I can get it apart however am a little nervous about putting it back together re torques and settings and other specifications not to mention availability of parts and gaskets, etc. Was going to do a leak down test however need to locate a 18mm adapter. I am fairly certain where it is leaking as I can see the gas bubbling at the lower edge where it screws into the head. Would like to get a second opinion before I start tearing into it. Maybe, just maybe, it is some simple little thing that someone has experienced previously.

Mike

[Darryl.Richman]

I'm right there with you Mike; I'm a bit pusher by trade and mess around with bikes as a hobby. You probably will need a new head gasket if you take this one apart. Look to Oldtimer Garage in Poland or Dreher Oldtimerteile in Germany for replacements. They aren't cheap, and you want the ones that are two part, with a fiber base and a copper cover that wraps around both sides of the fiber. There were no torque measurements at the time. Probably 20-25 lb.ft. is a good starting point.

[JDHMike]

Hey Darryl, These sites are good in that they help (me) think things through. The last thing I wanted to do was pull the head. At this point it looks as if I may have stumbled across the easy fix. It would appear that the spark plug was tightening but not seating. Replaced plugs and problem seems to be resolved. I will start and run a couple more times but at this point does not miss, although running rich, and no percolating.

[Darryl.Richman]

Great news! My experience with my R52 is that it does run rich, which is certainly better than running lean, especially as these motors tend to run pretty hot.

[Bruce Frey]

I can tell you are ready to join the select group of people with sidevalve tattoos (burn scars) from messing about with these old machines while wearing shorts;-)

Bruce

[Darryl.Richman]

You're right about that, Bruce. Also, you can easily test how strong your magneto is, by brushing your knee against those spark plug caps. I have the same ones...