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Old 12-30-2012, 11:14 PM   #1
motog OP
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The dreaded blue smoke

Unfortunately it looks like time to do the full top end tear down on my 1972 R75/5 (valves and guides were done 2 years ago - should have bit the bullet - and the wallet - and done the lot at the same time). Motor is using too much oil and blowing blue smoke under acceleration. Full service has been done but before I lash out the big dollars I'll do a compression test and a leak down test. I'm also going to attach an oil pressure guage and check that the bottom end bearings are ok (shouldn't be the reason for the oil problem but should indicate if I have to overhaul the bottom end as well).

So I'm looking for some remote diagnostics if anyone is able to help:

  • motor still seems to have plenty of power
  • blue smoke comes from the right exhaust when the engine is revved
  • it gets less when the engine is hot - after an hour ride it disappears
  • previous work includes new valves, guides, seats, valve springs, top end gaskets, push rod boots
  • I've got no idea how many miles the bike has done - original speedo has long since gone
  • full service was recently done
  • carbs have recently been rebuilt with new gaskets, jets, etc (less than 500miles ago)
  • right plug has oily residue on it
  • left plug looks like it is running a bit rich (sooty black residue on it).
So part one of my question is this: Are there other things I should be checking for before I spend the big bucks on boring out the cylinders and replacing pistons? I've heard that the crankcase breather can spit oil into the right carb if faulty?

Part two of my question is: what oil pressure should I be looking for on this bike?

Last part of my question is: If I do have to rebore and replace cylinders I'll be doing most of the work myself (except for cylinder measuring and boring) and am just getting my prices together. I read this comment on the Boxerworks forum:

Quote:
If you are on a budget, check out http://www.motobins.co.uk/bmw-parts.php and look up 50110 pistons to fit an R80/7. They are 9.2/1 comp and the R75 cylinders can be bored to fit. They will be about $260ish for a pair. And you will pick up 3 or 4 hp.
So I look on the Motobins site and see that a pair of r80/7 pistons comes out at $254 AUD compared to $390 AUD for oversize R75/5 pistons. Both prices include rings, gudgeons and circlips.

Has anyone had any experience of doing this? If I go down this path am I going to find myself with some very thin walled cylinders?

Has anyone got any better ideas?

I've looked at several sites and the Motobin price is about the cheapest I can find for R75/5 and R80/7 pistons. Anyone have any leads to a cheaper source? Alternatively anyone got a better idea? I'm based in Australia.

Ive looked at the very nice pistons on the motor-israelen site but they're even more expensive than the Motobins ones.

Any response gratefully received.
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Old 12-31-2012, 03:47 AM   #2
RobboJ
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I'd buy some 800 nicasil barrels and pistons off Ebay for about 120. As long as they are not scratched they should be fine.
You may have to turn down the skirt by a mm or so to allow them to fit in your crankcase
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Old 12-31-2012, 04:25 AM   #3
carpetburn
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Have you checked your crank case breather? mine fires into right side of air cleaner and to right carb, if its blowing out oil that would explain the smoke only on the right cylinder
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Old 12-31-2012, 04:38 AM   #4
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Someone will straighten me out if I'm wrong, but /7 and up have different cylinder spigots, and so you can't put nikasil bore jugs on there. You could go R90 cylinders though, and just change out carb jets. You may be able to just go with new rings on your old pistons and bores. I just can't believe how expensive pistons are for these bikes.
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Old 12-31-2012, 02:53 PM   #5
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Quote:
Originally Posted by JonnyCash View Post
Someone will straighten me out if I'm wrong, but /7 and up have different cylinder spigots, and so you can't put nikasil bore jugs on there. You could go R90 cylinders though, and just change out carb jets. You may be able to just go with new rings on your old pistons and bores. I just can't believe how expensive pistons are for these bikes.
You are ass backwards JC. Other than that you are spot on!

Take it apart? The best thing is to do a leak down AND a compression test. It might just be lined up rings.
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Old 12-31-2012, 04:43 AM   #6
disston
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It sounds like rings, could be several other things. Do a compression test or a leak down test. Take it apart before you buy any parts. If the pistons are with in spec and the heads hold liquid gas when filled after turning upside down then you are in for new rings. It has to be determined if new rings are what you need before you buy them. If the heads are still serviceable OK. You say the heads were done two years ago? Any idea of mileage, usually considered a more relevant factor.

The next step if pistons are worn too much or the cylinders are worn too much is new pistons. I have heard that 800cc pistons fit. I think this is so but I have no personal experience. If you get over sized or larger pistons you will be going to the machine shop for boring.

If the oil light goes out immediately when you start the bike and stays out while the bike is running you can leave the bottom end alone unless you just want to do more work than is needed at this time. Part of fitting new rings and/or pistons and such, which are called the top end in these motorcycles, is also replacing the big end bearings on the connecting rods. You will get a better idea of the condition of the lower end after you look at those.

Buying Nikasil cylinders and having them turned down is a nice modification. You need a shop with some experience in this mostly. Or at least they are willing to find out all the details of doing it properly. Since you have a 750 now why stop at 800? If you are going to buy Nikasil get the 1000. They will work with the cam in a 750.
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Old 12-31-2012, 05:12 AM   #7
Rob Farmer
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Swapping the oil breather for the later style one is a cheap fix and may buy you a little more time but to be honest it sounds as though you need a rebore and valve job. The only way to tell is to pull the heads and barrels and have a look at the condition of everything. Motobins do a power upgrade kit for the 750 that gives new nikasil barrels and high comp pistons for the same sort of money as new pistons and a rebore. I can't see any oversize 750 pistons on their site so you may not have the option of a rebore.
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Old 12-31-2012, 04:08 PM   #8
JonnyCash
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What am I backwards about? You're saying that Nikasil jugs will go right into an R75 bottom end?
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Old 12-31-2012, 04:57 PM   #9
supershaft
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Quote:
Originally Posted by JonnyCash View Post
What am I backwards about? You're saying that Nikasil jugs will go right into an R75 bottom end?
Oops! I was ass backwards. That last post alone reads like you were talking about /7's when you were talking about the the OP's bike. I didn't read through all the posts when I read yours. Sorry about that. Other than me not reading the entire thread, you were spot on!
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Old 12-31-2012, 06:30 PM   #10
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So SS, have you done any or many of the later 99mm jugs fitted to earlier 97mm blocks? I don't understand why it's not more common. Oh it gets done occasionally but not as often as it should be if it were easy to do.
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Old 12-31-2012, 06:34 PM   #11
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I have never done that.
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Old 12-31-2012, 08:27 PM   #12
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What doesn't make sense to me is the bike smoking till it's good and warm, and then it's fine. That suggests to me the oil rings are letting oil pass into the combustion chamber while sitting. Then it burns off during a good ride.

Don't go jumping to conclusions that everything needs to be re-done, that a whole new top end is in order! That's most likely not the case! Like SS said, the rings may be lined up, or they didn't seat on that side.

How was the engine broken in? What procedure was used? The left side is fine, and that's the side that normally smokes, so that's another indicator that the engine is basically fine. Does it make any odd noises? Do the pistons rattle?

It's easy enough to pull the right head and cylinder and take a look. Check the ring lands for wear.

Have these symptoms been there ever since the rebuild two years ago?
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Old 01-01-2013, 02:24 AM   #13
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Thanks all for the replies.

I'm hoping to get out of this without spending too much but recognise that it's got to be done right.

I think I'll go in this order until I find the problem:
  • Compression test
  • Leak down test
  • Right head teardown
  • Left head tear down
  • Crankcase breather replacement

I took it for a long ride today (100miles) and most of the blue smoke had disappeared by the end but there was still a little bit. I assume it decreases when the bike gets warmer because the various bits expand at different rates so things fit together better.

Wire spokes, I haven't noticed the blue smoke since the valves were done but I haven't been looking for it. I've probably done less than 3000 miles since I did the valves. But, now I think about it, oil consumption has still been higher than it should have been all that time. The valves were broken in pretty carefully - low revs for the first 500 miles, then running it through the rev range thereafter. No odd noises, no piston slap, no obvious bottom end noises..

Rob Farmer, I couldn't see the 750 upgrade kit on the Motobins site, could you point me to it?

Thanks again.
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Old 01-01-2013, 05:10 AM   #14
Rucksta
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Quote:
Originally Posted by motog View Post
I think I'll go in this order until I find the problem:
  • Compression test
  • Leak down test
  • Right head teardown
  • Left head tear down
  • Crankcase breather replacement.
Since the crankcase breather replacement is quick, cheap & easy would it not make sense to do it before you start pulling heads off?
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Old 01-01-2013, 09:03 AM   #15
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Quote:
Originally Posted by motog View Post
......But, now I think about it, oil consumption has still been higher than it should have been all that time. The valves were broken in pretty carefully - low revs for the first 500 miles, then running it through the rev range thereafter. No odd noises, no piston slap, no obvious bottom end noises.......
This is I think the heart of the current problem. The rings are not seated. They need higher revs and they were improperly broken in. Most builders today do not oil the cylinders or the rings on a rebuild. The idea is to wear the rings into the cylinders fast. Thus they will be seated and they will seal.

The excessive blow by is being dumped thru the breather into the right cylinder.
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