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Thread: 76 R90/6, burning oil? carb misadjusted?

  1. #1
    Dee G flymymbz's Avatar
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    76 R90/6, burning oil? carb misadjusted?

    I took on a project for a friends brother this past winter. He has a 76 R90/6 that needs a 'bit' of TLC. I am beginning to think I'm in over my head. What I thought would be a simple tune up, new tires, clean up, etc, has turned into a full blown 'tear it down and fix this miserable thing'. Thing is, I've never done something this extensive before.

    There was so much oil caked on to the cylinder fins and under the push rod tubes, that I thought the tube seals needed to be replaced. I cleaned up the bike as much as I could, then rolled it back into the garage and started taking things apart. I pulled the exhaust off (its all rusted and pitted), and then pulled the heads and cylinders off. Now I'm treading into new territory here. I've never partially dismantled an engine before.

    I find that the tube seals are just fine, but there is no O ring between the case and the cylinder. hmmm.....

    These went to the local shop to be cleaned up:


    The pistons were caked with crud:




    I cleaned those up myself last week, today I pulled them out and ran them through the solvent tank.




    Because I'm new at this, I don't quite know why the pistons and such are like this. Needs rings? Carbs not adjusted properly???? I have read that the piston rings should have the gaps staggered, but on both sides, the larger of the three had the gap at the top (as in the picture), and the other two were almost lined up together at the bottom. Is that part of the problem, or ??? Any advice would be appreciated. I want to put this thing back together and get it out of my garage.
    Too damn many bikes to list

  2. #2
    Administrator 20774's Avatar
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    How many miles on the engine? What was the history of oil usage? Bike parked on sidestand a lot? I don't think there's that much unusual about the crud on the piston/head...that will happen over time and sure there's going to be bad gas, some oil usage, etc. The ring gaps are supposed to be staggered when first installed...the ring closest to the crank is the oil ring and should be at the top. I've heard people write that the rings rotate around anyway...not sure about that to be honest. To know if there was a ring problem, doing some compression/leakdown checks at first is generally what is done. After that, you need to measure the ring gaps at several places along the cylinder bore. Put a ring in the cylinder and use the piston to square it up. Then use a feeler gage to measure the gap on the ring. Push the ring down to the middle bore and remeasure. Etc. I don't recall the range of allowable gaps. You should also take the opportunity to measure the ovality of the bore using a bore gage. I found it difficult to use and get repeatable results...I ended up taking my cylinders to a engine machine shop and had them do it. My cylinders were out of spec and I chose not to bore them to the next size.

    The large o-ring at the base may or may not have been required for this engine...I can never keep that straight. IIRC there has to be a recess in the cylinder base to know for sure. Even still, I don't think it is a must...it can't hurt I suppose. But the cylinder base needs a thin layer of a good sealant - Hylomar, Permatex, Yamabond, Dreibond - there's a variety of choices out there. It has to be a thin layer to avoid plugging the oil return holes on the upper two head bolts.

    What about the two smaller o-rings for the two top head bolts? Those should be replaced if you went this far. I would also replace the pushrod tube seals, again if you're this far. They could be reused, but if they have any age on them, they'll only need to be replaced earlier rather than later.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    #4869 DennisDarrow's Avatar
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    +1 on what Kurt has to advise. The end gap measurement will tell you if you can just put those back in, bore it out, go first over, or just slap it back together.............Do you see hone marks in the cylinder? Kind of a scratched pattern that is circular and is visible all the way up and down all around the inside of the cylinder? For sure on the push rod tube seals and yes it is totally common for there to be NO "O" ring or ANYTHING at the base of the cylinder but a good sealant is really a good thing to do...............

    Sorry you got yourself into this mess..........LOTS of carbon can also be caused by a rich carb setting/jetting..........Slap that thing back together properly, get it running and tell the guy/owner that you did your best but you need to get on with life...........Hard to face up to that; but it all depends on how bad your wife needs the room in the garage.........God bless......Dennis

  4. #4
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    Back in the day: If I had the head off an engine and saw the edges of the piston(s) washed clean as in the picture, I would have said that there was oil getting past the rings and cleaning the carbon from the piston edges. After checking the cylinders for size and shape (diameter, taper, and out-of-round), I would have scuffed them with a deglazing hone and installed a new set of rings. Rings have been known to lose tension from over heat. If I was in that far, the price of a set of rings might be inexpensive peace of mind.

    Just a thought.

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