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1967 R502
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For a while I've been suspecting my 1967 R50/2 has had compression issues. I did a conventional compression test and it looked like the left cylinder was weak. I followed this up with a leak down test today and what I found confuses me.

1. Engine warm, Air filter off, Spark Plugs out, Throttle open. Fly wheel is at OT. Bike is in 1st gear and I stand on the brake. 100PSi Compressed air goes in, and comes right back out the intake tubes!.

Say what? Intake valve is supposed to be closed at OT, right?

2. Thinking that the flywheel has slipped, or that some doofus (me?) mounted it wrong in in the past. I turn the flywheel to "F". Noise from the intake tubes stops.

3. So I check to see if the engine is really at OTC, the old fashioned way. A straw goes into the spark plug whole and I rotate the crank until I can visibly see that the piston is at the top...and guess what? It matches OT.

So what is happening here?

schrader7032
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VBMWMO #7032
San Antonio, TX
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Only one side has the valves

Only one side has the valves closed...that's the side that is on the compression stroke...and you should be able to wiggle rocker arms. The other cylinder valves are not completely closed...that cylinder is on the exhaust stroke.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

1967 R502
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Thanks... however the air is

Thanks... however the air is coming out the intake valves not the exhaust valves. Which is quite puzzling

schrader7032
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VBMWMO #7032
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Well, I don't really know the

Well, I don't really know the state of exact valve positioning on the off-side cylinder. Could be intake is more open than exhaust??

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

1967 R502
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Here's the follow up.After a

Here's the follow up.

After a lot of frustration trying to get a reading at 90PSI (as recommend on ye olde youtube), i lowered the air pressure to 50 PSI and lo and behold I could get a sensible reading.

1. Right cylinder: 46-48 Psi @ 50PSi going in, so that cylinder holds pressure reasonably well.
2. Left Cylinder: 40 PSI @ 50PSI goin in. Could hear some wheezing in the case. So I am guessing the rings or cylinder need replacement or reboring. My guess is the cylinder. 20% drop off in pressure seems a lot.

What's your experience?

schrader7032
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VBMWMO #7032
San Antonio, TX
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My leakdown testing has a %

My leakdown testing has a % scale built it so I never knew what the leaking pressure value was. I've heard that up to say 5-7% leakage, it's essentially a good engine. Beyond 10% would tend towards getting things fixed.

As far as where the leak is going, I would listen in 3 places: 1) at the intake to the carb...that would be an intake valve leak; 2) in the muffler...that would be an exhaust valve leak; or 3) a leak out the dipstick hole...that would be a ring leak.

So has the engine run before? How did it perform? Are you trying to fix some performance issue? Just wondering... BTW what did your compression test tell you?

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

1967 R502
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The engine has run, and is in

The engine has run, and is in fact running now. Start up however has been a hassle for a while and I have long suspected that it needs a bore. Things like smoke coming out of the breather tube or a smoke ring around the timing hole.

There is no sound at the exhaust, valves or intake. And I can’t feel any thing coming out of the dipstick hole, but I can hear it.

Based on percentages, it would seem the left cylinder needs ring or a bore.

schrader7032
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VBMWMO #7032
San Antonio, TX
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Will you do both sides at the

Will you do both sides at the same time? Seems like that's the thing to do, especially if you have to increase the bore size on the left.

On my R69S, I think I was somewhat close to the maximum size. The mechanic just suggested that I put in a new set of rings and go from there. Just checked my notes...I had basically 10% leakage on one side, 11% on the other.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

1967 R502
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Of course if I rebore I will

Of course if I rebore I will do both. Otherwise the bike would be out of balance and ummm..interesting? Will find out later this evening I think. I’m gonna digg in and take off the heads.

1967 R502
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Strip down time:

Strip down time: Well the cylinder is not scored. Nor is the piston. Fair amount of carbon. However looking at the head it looks like gas might be getting out through the head gasket. May need to skim the head. Will check the cylinder for ovalness next.

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