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poor compression

dellatorre6708
Posts: 62
Joined: Fri Oct 27, 2006 3:00 am

poor compression

Post by dellatorre6708 »

1962 R69S.
I rebuilt motor, new pistons along with barrels sent to millennium tech for fitting. Heads done by Randy long. When I put the motor together I checked compression and only came up with 100 PSI on both sides. I figured the rings had to seat.
Started the machine and took it for a sixty mile run came home changed oil and next day retorqued heads and reset valves. Tried starting it and wouldn't kick start. Bumped it and it started right away. Took a short ride came home and pulled the plugs. Have a dry sooty build up on the plugs. The motor runs rich and the carbs wont adjust well at idle. I have a new coil gives a fat blue spark, timing spot on, carbs rebuilt. When the motor is running it runs well plenty of power will give a steady smooth 70 on the highway, no pinging, smoke, no oil burning but runs rich. I did a compression test on a warm motor and now I get 100 on one side and 80 on the other? I did a leak down test and the motor test excellent holds pressure for over ten minutes. How can the motor show good on leak down but have such low compression? Before I pull the barrels again, any thoughts?
Thanks. Frank.
NYC.
Frank,
NYC

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Micha
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Re: poor compression

Post by Micha »

AFAIK, 80 or 100 is excellent.
A motor will show different specs while running, because of the rings spreading inside the bore.
Michael Steinmann
R51/3 1952
Engine Nr. 529466

dellatorre6708
Posts: 62
Joined: Fri Oct 27, 2006 3:00 am

Re: poor compression

Post by dellatorre6708 »

Micha,
I'm a bit worried because my other machines show 150 PSI and better. This is what the motors should show in good condition.
R50-R50/2 120-140 psi
R60-R60/2 125-145 psi
R69S 150-185 psi.
Thanks. Frank.
NYC.
Frank,
NYC

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schrader7032
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Re: poor compression

Post by schrader7032 »

Yes, an R69S with 9.5:1 compression ratio should have 150+ psi. Probably should retest your compression. Note that you must either take off the carbs for the test or at least roll the throttle to wide open. Doing it warm is best. Duane explains things here:

https://w6rec.com/test-compression/
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

dellatorre6708
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Re: poor compression

Post by dellatorre6708 »

Kurt,
I did the test on a warm engine, carbs wide open. I thought my tester was bad so I used it on one of my later machines to verify it was accurate. This isn’t my first time doing this and I'm scratching my head trying to figure how the motor holds pressure on a leak down but gives a low compression reading? I guess I'll have to open it back up and see if something isn’t right inside? The weird thing is once running it goes down the road well, problem is it won’t kick start easily and it wont take air adjustments at idle.
Thanks. Frank.
NYC.
Frank,
NYC

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schrader7032
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Re: poor compression

Post by schrader7032 »

Have you verified the valve clearances again? Might be worth another leak down test. I find the leak down test a bit fiddly to do. You have to get the particular cylinder at TDC on the compression stroke and hold it there while you inject the air. The first few times I did that, the air forced the piston back down the bottom.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

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jwonder
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Re: poor compression

Post by jwonder »

Frank,

It looks like you did everything by the book and the tests are indeed not conclusive. I typically like the leak down test far better than the compression test because it is a finer test with clearer results in my mind, but the leak down test is a static test and not a dynamic one.

My thoughts are generally around the possibility (fact?) that the rings are not sealing at some part of the stroke on the cylinder and that is causing the compression test to be bad. Possible thoughts are:

1) The rings have not had time to seat properly! The compression test might not generate enough pressure to seal the rings, but the leak down would!

2) Are the end gaps correct? When running, air pressure from compression gets behind the rings and actually plays a major role in seating the rings which is why the ring end gaps are important.

3) What is the bore on the cylinder, is it 3rd or 4th oversize? Could be the cylinder has warped after initial startup.

There are several other possibilities but you get the idea. If it was mine I would:

1) Ride the crap out of it for several hundred miles and re-test. There are so many arguments on how to properly break motors in, or not, and if you have problems with rings seating the answer is typically to ride it hard for a while and get some heat cycles into it.

2) Do your leak-down test at different parts of the stroke. This is not easy but you may be able to hold the crank with an allen wrench and do this or use tie downs, rope etc to hold the kick starter in place. This might show leakage on the stroke, but you need to know where it is to get it right!

I really think the rings are not sealing and I would ride the heck out of it and then re-test.

James
James Wonder
Vice President, Vintage BMW Motorcycle Owners
2022 BMW Friend Of the Marque
Long Island, New York

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jwonder
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Re: poor compression

Post by jwonder »

Oh, you also said you had problems starting it. Some thoughts:

Do you have a lightened flywheel on it? Which one and how light? This can make starting hard.

How did you check static timing at start? I use a multimeter and set it to fire BEFORE the S mark, not after because there is latency in the spark and the timing light will always show a bit late. I just had an advance unit that I rebuilt that I set at firing just before S statically and at engine idle the S mark was no-where near the hole. I needed to move the metal plates out to increase spring tension (see the article in the latest magazine). If I had set the idle timing with my timing light at idle the starting (kicking) spark would have been greatly retarded.

Hope this helps.
James Wonder
Vice President, Vintage BMW Motorcycle Owners
2022 BMW Friend Of the Marque
Long Island, New York

dellatorre6708
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Re: poor compression

Post by dellatorre6708 »

James,
I'm thinking the same as you about the rings. These pistons are third over. I sent them & barrels to Millennium Tech, a top machine shop recommended by Benchmark works, for fitting. When I installed them the ring gaps were .30 MM. It's an assumption but if the cylinders were out of spec I think Millennium Tech would have advised me of such. Your advise about setting the timing a bit before the S mark is interesting and I'll try it. Strange thing about this machine is once running it runs great. The quest continues.
Thanks. Frank.
NYC.
Frank,
NYC

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schrader7032
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Re: poor compression

Post by schrader7032 »

Could the magneto positioning have changed? The magneto needs to be set right in order to kick start, but once the engine is running, the spark from the magneto gets stronger.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

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