The dreaded blue smoke

Discussion in 'Airheads' started by motog, Dec 30, 2012.

  1. motog

    motog Been here awhile

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    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:

    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.
    #1
  2. RobboJ

    RobboJ Adventurer

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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
    #2
  3. carpetburn

    carpetburn Been here awhile

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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
    #3
  4. JonnyCash

    JonnyCash turd polisher

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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.
    #4
  5. disston

    disston ShadeTreeExpert

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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.
    #5
  6. Rob Farmer

    Rob Farmer Long timer

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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.
    #6
  7. supershaft

    supershaft because I can

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    You are ass backwards JC. Other than that you are spot on! :D

    Take it apart? The best thing is to do a leak down AND a compression test. It might just be lined up rings.
    #7
  8. JonnyCash

    JonnyCash turd polisher

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    What am I backwards about? You're saying that Nikasil jugs will go right into an R75 bottom end?
    #8
  9. supershaft

    supershaft because I can

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    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! :lol3
    #9
  10. disston

    disston ShadeTreeExpert

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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.
    #10
  11. supershaft

    supershaft because I can

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    I have never done that.
    #11
  12. Wirespokes

    Wirespokes Beemerholics Anonymous

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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?
    #12
  13. motog

    motog Been here awhile

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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.
    #13
  14. motog

    motog Been here awhile

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    Rob Farmer: I've seen the 750 - 1000cc upgrade kit now. It's for post- 1976 bikes so would need the spigot machined down too?
    #14
  15. Rucksta

    Rucksta SS Blowhard

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    Since the crankcase breather replacement is quick, cheap & easy would it not make sense to do it before you start pulling heads off?
    #15
  16. photomd

    photomd Been here awhile

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    IMHO, look for something objective. That's what the compression and leak down test is telling you. If they're off, use that info to investigate. As for the breather system, I'd guess you can pull the right spark plug and carb and see oil or carbon build up on the right side as compared to the left if the breather system is not working correctly. My guess is that the cold, thick oil is blowing by the breather system. As it warms up, the blow by decreases, but I'd want to find more info before replacing parts and fixing it. As Big Sid once said, the bike is telling you what's wrong, you just have to listen. In my garage, I always have to "listen" several times before I get it right. :evil
    #16
  17. disston

    disston ShadeTreeExpert

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    The crank case breather controls the pressure in the crank case. The original design on our bikes was a disc that was spring loaded, sits in front of air filter next to starter. A more modern one can be retro fitted. They work quieter, no Turkey Gobble sound, and are supposed to work better. The modern one is a reed valve.

    The breather allows the lower parts of the engine case that are not part of the cylinder space where the action is to maintain a constant pressure. In operation the engine is pumping air and fuel into the cylinder burning it and forcing the exhaust gases out. The pressure in the cylinder is constantly changing. It is being controlled by action of the valves. So we get what we want which is the four stroke gasoline engine. Fine and dandy, most of the time.

    While all this action is taking place in the cylinders the pistons are pumping air inside the engine case, but wait, that area is actually sealed. It is not easy, on a modern engine, for air to move in and out of the engine case. What happens is the air pressure inside the engine will increase. In the Old Days we ran a hose from the engine case to the outside air. Worked but it was messy. Exhaust gasses, blow by from the rings and noxious fumes were in the crank case. So the idea was to introduce these gasses to the intake manifold and have them burned in the engine. This is the modern engine PCV system and on our motorcycles we call it the engine breather.

    When this system gets clogged or the valve defective the exhaust gasses increase in pressure and can blow engine oil past the rings and into the cylinder. That is why we think you should check the engine breather. It is very likely the source of your problem.

    #2, I do not like the break in procedure you used for the new piston rings. You didn't say but do you remember, did you lube the cylinder walls and the piston rings with engine oil so they would not start up dry? (I'll warn you, this is a trick question)
    #17
  18. Stan_R80/7

    Stan_R80/7 Beastly Gnarly

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    Here is a link to the Siebenrock cylinder kits (in english):
    http://www.siebenrock.com/en/products/technic/

    The kits sold by Motobins (at somewhat of a discount) are Siebenrock kits. It looks like the kits fit /6 and newer engine cases (i.e. not /5 without modifications). While a leakdown and compression test are good ideas - the final proof is in inspecting the cylinder bore.

    Perhaps I am unique, but for me it takes about 45 minutes to get a head off my 1978 - but if you have not worked on many engines it would take longer. A new head gasket is all that is needed for reassembly provided the cylinder base is not disturbed. The Clymer and Haynes manuals describe the work in detail, but I found once the exhaust system is removed the job is very straight forward. Uniform loosening of the head bolts in steps is required. The left cylinder should be at BDC for inspecting the right bore, but otherwise it's not too tricky. Realistically, the total time is a few hours for removal, inspection, and reassembly. Take photo's along the way, keep everything clean, do not change the pushrods and valve train orientation, and remember anti-seize on exhaust nuts.

    Does this bike have 200000 miles? If the mileage is ~ 50000 then it is unlikely new rings are needed. Also, only the right cylinder smokes - where the crankcase breather vents - is suspect since that is a path for oil. But all this is speculation. The leak, compression, and visual inspection will determine if the cylinder needs new rings. Good luck!
    #18
  19. disston

    disston ShadeTreeExpert

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    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.
    #19
  20. motog

    motog Been here awhile

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    Disston: Thanks for that but the rings were not changed when the valves were done - so the rings should not need to be bedded in? is that correct?
    #20