Boxerworks.com

Hello and thank you for allowing me to become a member. I am a retired AT&T systems admin but I work on old bikes, UJM or Universal Japanese Motorlcycles, as a hobby. Recently a friend brought me a 71 R50/5 to help him get running. He had it at a local well respected shop but after he got a quote of from $3000 to $4000 to rebuild his bike he had second thoughts. The story he told me was that the dealer was more interested in buying the bike, which he did not want to sell, than actually getting it back on the road. I don't know a lot about the history other than it appears to have been sitting for a few years, possibly decades, covered with a tarp and the PO would occasionally kick it over a few times to keep the engine free. The bike has 25k miles so very little is worn out that I can find. That dealer noted that the right side had low compression and removed the right cylinder and head plus both exhaust headers. When I got the bike I noted that the valves had very minor surface rust and the seats were the same. I also noticed that the second compression ring appeared to be frozen. I was able to lap the valves and the mating surfaces look like new now. The stuck ring freed itself up with a light application of PB Blaster and solvent cleaning.

So now I'm waiting for gaskets so I can reassemble the right side and newer carbs (originals are too far eroded/corroded to save and parts missing). Having no experience with BMW but I have a lot of experience with wrenching on bikes/cars/trucks. I also post frequently on other motorcycle forums, mainly vintage Japanese sites for Honda, Kawasaki, Suzuki, so I think I know the protocol for posting. I certainly don't want to offend anyone but I'm sure I'll have noob questions for the forums. Yes I do have the Clymer service manual. So to kick this off here are a couple questions right off the bat.
1. flywheel inspection hole. Is it supposed to have some sort of cover? I have boxes of parts just want to know what I should be looking for.
2. Advice on left side. Would you pull the head and lap valves there even though compression and leak down show no abnormal leakage? My feeling is "if it isn't broken don't fix it until it is".

Thanks in advance,
John

G'day John, and welcome aboard. After years of playing with Japanese bikes and their nasty cheesy screws you will love working on a BMW.

A useful place to find out about parts is the MaxBMW parts fiche:

https://www.maxbmwmotorcycles.com/fiche/PartsFiche.aspx



The timing plug is part number 12 in that diagram and Max will sell you one for $1.12

And if I had opened up the right jug and found issues I would be very curious to know what the left side is like. If there is nothing wrong you can put a tick on your list of things to check, and you'll have new gaskets and the head torqued properly.

John, as a former BMW dealer with probably the largest service department anywhere, I would have approached it differently. A compression test means nothing if the bike isn't hot and been run at least 100 miles. That would have cleaned off the valve and seat quickly. It is hard to say about the stuck ring, because we would never put one together that way to see if it would unstick itself, as that would be too risky.

However, that series of pistons are well designed and just don't fail, so I think it a very good possibility that the ring would unstick itself. I don't recall a single similar engine suffering a scored cylinder due to a stuck ring when we started one up after a long rest, rode it 100-200 miles and only then started checking things out. You played it safe and there is never anything wrong with that.

You also played it safe by buying new gaskets. Of course we had them in stock, so it was never an issue. However, in the early days before I had a "real shop" I many times reused both the head gaskets and cylinder base gaskets if they were still looking whole. The base gaskets can be reused many times. Sometimes the head gaskets come off in pieces, so we would just use a whole, but used gasket on our own bikes. I never had a failure with used head gaskets.

The rubber plug for the timing hole is just cosmetic. The original was white and maybe you can only get the later black ones today, but I don't know that for sure. It was true for a time.

Far more important is the timing. Just make sure that the spark plug sparks just as the mark on the flywheel shows the "S" mark. At least it can be run that way until you can check for proper advance. if using a strobe light for timing, shine it on the advance mechanism, as that is far more accurate than shining it on the flywheel for proper advance mechanism operation. The flywheel marks won't show smooth or sticky operation of the advance mechanism. The fly weights on the advance mechanism should show smooth operation with the strobe light.

The factory method for timing is just a starting point. The "S" is good for idle and it should reach the "F" mark by around 3k rpm. I wouldn't worry about optimizing the timing for power on a R50/5, as that is such a slow bike that you will think it is dead on your first test ride. Every 250 cc in the world will easily accelerate faster.

You probably already know that those bottom ends are all the same up to 1000 cc and that bike can become a full liter bike fairly easily. Those bottom ends are almost bullet proof and have little value because good used ones are laying around everywhere for cheap or free.

By far the most important book to have is the parts book. Nearly all of the printed manuals are very poorly done. You will find excellent websites that tell you 98% of anything that you need to know. Do you know of mine? It is w6rec.com and keep in mind that Google is your very good friend.

Come back often with questions. Better to ask and be conservative instead of ruining something. Contrary to most motorcycles, a BMW can more easily be damaged by over tightening than under tightening. At least 90% of the failures that we found due to improper torque, were over tight, not under tight.

We won't leave you high and dry, if we don't know an answer, we will just make one up for you.

I am now visiting the Czech Republic and jet lag has me up in the middle of the night, unable to sleep, so I am typing away.

Wow thanks guys! My hesitation to pull the left side apart was fear of stripping a fastener. I personally didn't think the valves or seats looked that bad but it was already apart so why not lap them and insure a good seal. As for the stuck ring. I could see that it was not bright and shiny like the first compression ring and it was level with the skirt of the piston. Since it's still on the con rod I just sprayed some PB Blaster on the ring and turned around to get a shop tower. When I looked back it had freed itself! Couldn't have been 5 to 10 seconds. Cylinder bore looks like new no pitting or scoring. Yeah if they had fired it up it would have unstuck before the engine got up to temp. Maybe.. Now I know it's free. I did buy both head and base gaskets and all 4 valve tube seals plus both valve cover gaskets. It's been sitting for a number of years so I'm going through it front to back. The carbs were toast. So heavily corroded that there were holes in the pilot jet tower. Got a new set of carbs coming and hope they are in better shape. I'm still going to take them apart and check/clean it all up. I got the impression these were pulled off a bike and unknown if the bike was running or not. I already have new float bowl gaskets. So I am hoping all I have to do is disassemble and clean them up.

As for timing I've seen some videos but it looks like a simple single point setup. I also have 2 pro quality powered strobe lights. I am guessing that if I see the advance wings all the way out prior to 2500 to 3000 RPM the springs are worn?

Getting too long winded here. I'll post problems as they come up or if I have questions.

Thanks again.

Airbear wrote:G'day John, and welcome aboard. After years of playing with Japanese bikes and their nasty cheesy screws you will love working on a BMW.

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FYI those Cheesy screws are not Phillips head screws. Common mistake because they look like a Phillips head. They are JIS or Japanese Industrial Standard screw heads. You have to use a JIS screwdriver on them and they get a lot less cheesy... The biggest problem I have is where the previous owner(s) have tried, and failed, using a Phillips head screwdriver and really messed up the head. Sometimes I can remove the screws using a JIS driver but more often than not I'm using my reverse drill and extractor. The good news is that you can run down to your local ACE hardware store and buy a replacement JIS head metric screw for about 25 cents.

Now another piece of good news is that almost every screwdriver, even the cheap give aways, at Harbor Freight are JIS screwdrivers even though they don't specifically state JIS. They are all made in China and that is the standard. And JIS screwdrivers work great on Phillips head screws.

Yeah if you are working on a UJM (Universal Japanese Motorcycle) you have to have JIS screwdrivers, an Impact screwdriver, reverse drill bits and extractor set. Oh and plenty of PB Blaster. Never try to loosen a fastener without first soaking it down with PB Blaster. Tip of the day!

tip of the day is to use a bit of grinding compound to greatly increase the "bite" and reduce the tendency to jump up and out of the slot, or whatever. I always carried it in my tool box, still do. We used it often in my shop.

I have a set of those JIS screwdrivers and love them.
Especially on my '75 Honda CB400F.
Also on my Japanese cars.

Duane Ausherman wrote:tip of the day is to use a bit of grinding compound to greatly increase the "bite" and reduce the tendency to jump up and out of the slot, or whatever. I always carried it in my tool box, still do. We used it often in my shop.

I love it when you talk dirty, Duane.

Just in case anyone was interested in seeing the bike I'm going to post a few pics. I'm not sure of the protocol so I'll just post thumbs:

When I got the bike delivered to me:




After I put the right side together with the help of some youtube videos and set the valve lash both sides. The left side, previously untouched, was pretty tight. I got them set and .10 and .20 mm per the Clymer manual.



Now just waiting on carbs which should be here Friday or Saturday. I'll test fire it first then on to the clean up. I have new plugs and wires, points and condenser and brand new AGM battery. This thing needs an oil change bad and I do have the oil and filter here but I think I'll test run it first to get the oil warm then drain while still warm/hot. I know there are differing opinions on that and welcome any suggestions.

It is so easy to do, I would drain is as is, then drop the sump for a good cleaning. No need to recirculate whatever particulate may have settled out.