A bit of blue smoke on start left pipe - rebuilt engine

Paulrxs650

Paulrxs650

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I thought I would start a seperate thread on this.

I have rebuilt the engine and honed the bores lightly. the rings were in spec, so were reused and set with gaps at 120 degrees to each other. All seals and gaskets replaced.

Just noticed on cold start up that I am getting a little blue smoke for 30 seconds or so from the left pipe. Popped the exhaust off and carbs and wiped down the valve stems. Left it overnight, and there was no oil on the stems, so that made methink its not valve stem seals (they were replaced in any case). Popped the tank off and re-torqued the head. the bike has done 150 miles and the chrome nuts certainly needed tightening. Torqued to 26 ft/lb then to 30 ft/lb (as recommended by a few people on here). Started it up and still had a bit smoke on, then cleared away. Plugs are oily but then clear up straight away and look perfect after a run. Compression off the kickstart when warm is 135 - 140 each cylinder even.

I am suspecting either, rings not seated or head gasket issue - thoughts please?
 
When doing the work involved with a top end re-build, you should have installed new rings. Did you refer to the specs in the Yamaha Service Manual? Its important that the cylinder/piston clearances are in spec.

If the stud nuts needed considerable torque increase, then that sounds like they were much to low initially. Those nuts should have been torqued to 30 ft-lbs as part of the re-build. Its also advised to run a small amount of 3bond 1104 (or similar) around the camchain tunnel on the head gasket and the outer stud holes.

Plugs should not be oily after a proper re-build.

Engine could have been sucking oil across the head gasket when the torque was low, but now that you have torqued to 30 ft-lbs, it 'may' correct itself. Monitor the plugs and oil consumption as you put on some kms/miles.

If the old rings don't seat, you'll likely continue to have problems.
 
Thanks guys. When I checked, all tolerances were correct, hence the re-use of the rings - in hindsight probably not the brightest idea !
I think I'll have to put a few miles more on it and see what happens. If it worsens, I'll just have to pop the block back out and rebuild using new rings.
Out of curiosity, whilst searching the forum, I came across the "baking soda trick"? Anyone ever tried it?
 
When you say you "lightly honed", what type of hone did you use? If you used the 3 or 4 stone type, those aren't the best choice. You can easily enlarge the bore with them. The bottle brush or flex hone style is better for light honing for re-ringing. The stone hone job is rougher and it will take longer for the rings to seat. Maybe things will improve after a few more miles. Personally, I don't hone if re-using the old rings, only if new rings are going in.
 
Hi 5twins, thanks for the response. It was the flex hone type. Thanks for the advice on the rings. Next rebuild I'll know better :) yep I'll put a few more miles on as if running in. It has 180 on it as of this afternoon. I the glorious English weather allows I'll put on some more st the weekend and see how we fair. Thanks again
 
Read an old post by XSJohn and he mentions a level teaspoon on baking soda down the plug hole (not through the carbs) to assist in re-seating the rings. I'm tempted to give it a try just out of curiosity!

Thoughts please?
 
Hi, for the purposes of closing this thread, I thought I would post the results.
So, popped the engine out and stripped the head down. The head gasket seemed quite oily on its surfaces. I am not sure whether that was down to the dissasembly or the fact that oil had been passing over the top. Apart from that, the gasket was in very good shape. The bores were also in good condition with the expected vertical marks and the previous honing still showing clearly. Another well oiled, quick hone brought them back to perfect condition. Measuring the bores and piston clearances showed they were still within spec. I had purchased .50 over rings to be on the safe side, so they needed a little work to increase the end gap. I found a small water bed tile cutter was very useful for lightly grinding the end gaps while keeping the rings cool.
Lubed the barrels up well and popped the pistons back in. Some thin stiff nylon sheet and cable ties made useful ring compressors. Put the head back on, new gaskets etc set the timing and buttoned it up. Torqued in sequence to 25 ft lb, on the large nuts and 14ft lb on the 8mm ones. Left it overnight and the went up to 32 ft lb and 16 ft lb respectively.
Set the cam chain tension and valve clearances. Turned the engine over several times by hand and checked again.
Connected ancillaries back up and started on the second kick. Good news was there was no smoke. Went for a gentle ride of around 5 miles, up and down the box, alternating between moderate throttle and shutting off. Back in the shed, checked the head torque - no change. Re checked cam chain tension - OK. Went for a further 20 miles and repeated the same checks. Went for a further 30 miles and left it to cool overnight. Checked head torque again when cold - around a quarter turn on the large acorns back to 32 ft lb and similar on the 8 mm to their 16 ft lb. Restarted and noticed a very faint clonking noise. Shut the engine off and checked cam chain tension. The plunger was aroung 1mm in, so, several turns on the 10mm adjuster brought it back to flush. Restarted and all OK. Another 30 miles, left ot cool and head torque re-checked - all OK. Left overnight again and cold start - sounded OK and no smoke. Will re-check the head torque again, however, I am hopeful this has sorted the issue.

Many thanks to Retired Gentleman and gggGary amongst others for their input. I hope others find this thread useful :)
 
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