Revived VTR - Trying to sort out 4k vibration

[Cedardrew]

Righto, stuck into the rear tonight. Rechecked the compression this time on a cold engine and got 160 front and 138 rear, so still a reasonable variance. Pulled the valve cover and checked the timing and clearances first, all spot on.

Next was pop the head off. Noticed some lettering, pen markings on the casing in behind the cams. Hmm maybe someone's done some work here before. Head came off and I was greeted with quite a clean piston, definitely cleaner than the front. Instantly notice of the intake valves looks a bit dirtier than the other three. Is this the problem child? Squirt some WD40 down the intake and exhaust ports and leave it 10 minutes and notice barely noticeable seepage around the suss intake valve, but more noticeable seepage around one of the exhaust valves. Most of my research said it's usually the exhaust valves that start to crust up and cause loss of compression so maybe this is it.

I guess from here my best bet is to pop the valves out (all four), take a look at the valves and guides, clean the valves up, re-lap, new valve seals, new head gasket, whack it back together and hope I gain at least 10psi, hopefully more, and my vibes magically vanish.

Right after pulling it out.


Piston, unusually clean after 40,000km?


After wiping with brake cleaner then shooting WD40 up above and waiting 10 minutes.

[tony.mon]

The pen markings are production marks. The only ones of interest are inside the crankcases near the crank webs telling you what big end and main bearing shells that particular engine needs.

Just make sure that you keep each head's camshaft bearing caps with that head, and you can ignore any other engine graffiti.
The caps are engraved with a number to match each head, so hard to get mixed up.

Even harder when you only have one head off at a time!

[sirch345]

That was quick work

As you say, worth removing the valves to see if they need re-lapping in now you have the cylinder head off and for checking the valve guides and seals hopefully something shows up once the valves are removed.
Seeing as you have the head off it might be worth checking for any cracks between the valves in the head itself at the bridges between the two valves (the two narrower bridges more so than the wider two), I doubt you'll find any, but for peace of mind worth a look. If any there they should be easiest to spot at the valve seats while the valves are removed,

Chris.

[Cedardrew]

*** UPDATE ***

Was a bit of a delay while waiting for parts from Japan but in the mean time I popped the valves and took a look. Both exhaust valves had slightly leaking seals, though one worse than the other. A bit of freeplay in the guides but nothing alarming. Intake valves were clean and tight in the guides. Only some minor pitting on the valves and seats, mostly the exhaust valves, which was cleaned up with some lapping compound. I also gave the rest of the head a good clean and decarbon. Once the base gasket, exhaust valve seals and new water pump o-ring arrived I whacked it all back together and ran another compression test.

No change, at all Front still about 20psi higher than the rear on a cold engine. Did a wet test on the rear with another teaspoon of oil and no change, maybe 2-3psi. Based on this, my conclusions were:
- Rear valves probably not leaking as they were not that bad in the first place and are even better now having just been cleaned and lapped.
- Rear rings probably fine as well. A 145psi compression test isn't concerning and adding a bit of oil down the spark plug does nothing.
- Therefore, maybe the discrepency is actually the front reading too high due to excessive carbon build up, since it's giving a higher reading than the spec in the service manual.

Wouldn't have a chance to test ride until the next day so I did a bit more research. When I redid the front, I intentionally didn't remove all the carbon as I never intended to open up the rear so didn't want to throw out the balance. However, the front plug was noticably darker than the rear and the front piston definitely wasn't as clean as the rear piston in the photo above. So maybe at least some of the compression discrepency could be from additional carbon build up in the front cylinder accumulated over the last 40,000km. Plenty of stuff on the net about people having this happen with cars where one cylinder reads much higher than the others.

Based on this, before even riding the bike (since I still had the tank and fairings off anyway), I grabbed a can of Nulon Upper Engine Cleaner, fired the bike up, then shot some down the intake and then the the vacuum port. Not much happening, just dropping rpm. Then I remove the spark plug and basically filled the combustion champer with the stuff (it comes out like a foam) and let it sit for an hour. Then I whacked the plug back in and fired it up again. Ran on the rear cylinder for about 20 seconds before the front cleared up enough to fire up and then heavy smoke bellowed from the exhaust for a good 2-3 minutes before finally clearing up. Then I shut it off and put the bike back together for the Monday commute the next day.

So, rode the bike today. Didn't expect to have solved anything but guess what, the 4k vibration is completely and utterly gone. Bike now pulls smoothly straight through the early 4,000s until the higher rpm vibes gradually comes in at 5k and beyond which from reports is pretty normal for these bikes. There is definitely no longer any distinct kick in vibration right on 4,200 which it had before.

Unfortunately, I can't pinpoint what solved it. But it was either cleaning up the rear cylinder or decarbonising the front. It's a shame I didn't get a chance to test ride between the two as I'd have like to know exactly what cured it but on the upside, the problem is gone and I can just enjoy the bike now. Out of curiosity I'll probably recheck the front compression to if it actually changed at all.

[alanfjones1411]

Well done 10 out of 10 for perseverance and a job well done.

[tony.mon]

Good result, perseverance pays off.

With the upper cylinder cleaner, as some of it is likely to have leaked past the rings, do you now need to do an oil change?

[fabiostar]

Happy days thats a result

[sirch345]

That is really good to hear, well done

Chris.

[Cedardrew]

With the upper cylinder cleaner, as some of it is likely to have leaked past the rings, do you now need to do an oil change?

I'd say if you followed the advice on the can and just let it sit two minutes before purging it you'd be fine, but I let it soak for about an hour so I'm guessing some would have gotten past the rings. In saying that it's probably easily dilluted by the large quantity of oil. I only have cheap stuff in there now though so I think I might do one more cleaning cycle and then drop the oil anyway for some better quality stuff.

After a couple more test rides I'm happy with the outcome. There's still a vibration that gradually comes on from 4.5k onwards and follows into the higher rpm but I think that's just the nature of the engine. The clear and pronounced vibration that was hitting right after you cross 4k on the tacho, which was not at all gradual, is gone.