To refresh your memory, 2006 FJR with 90k miles, hot starting problem due to low compression in a cylinder or two. I never did get to speak to that mechanic to hear what he measured that was "below Yamaha specification" requiring an engine rebuild. What should have happened is the mechanic should have done a leakdown test to see if he had valve issues or piston ring issues. I mean, really, WE know an FJR engine is not worn out at 90k, puleeze!! And next, it would have been nice to stick a borescope camera down a spark plug hole or two... even dcarver knew how to do that, and showed us pics of nice clean piston tops. As you read, please do chime in if I'm missing something......
First up, a picture of the head, valve side. Exhaust valves are nice and clean, and on the back side, both the intake and exhaust valves had no residue buildup on them at all, which kinda surprised me given the ethanol fuel. My friend the owner (whose mechanical knowledge is non-existent) did use Yamaha Ring Free occasionally, he gave me a few of those little bottles once. But look at the intake valves, all crudded up on the combustion chamber side. That would be carbon, which apparently gets burned off on the hotter exhaust valves... I was not expecting them to be so clean.
Next up, we need to look at the cylinders and do some fancy measurements. Just how worn out is this rig? Well lookie here, a close-up of the bores shows the factory cross-hatching is pretty much pristinely intact! Impressive to say the least.
What you see also is a brownish band (combustion residue) at the top of the stroke where the piston does not touch the bore, but combustion does go down that bit until it gets to the next band which is the top compression ring. Next you have a space and the next band is the second compression ring. Next you have a space until the oil ring set. Engine fires at top of stroke and drives the piston down. What we do expect is a little wear on the cylinder wall and some build-up right at the top edge of the top compression ring, it's called ring ridge and usually you can feel it with your fingernail. I checked them all... no freaking ring ridge at all! Surprising!
Getting kinda obvious we don't have a wear problem, but a true forensic anal-ist would get out his fancy bore gauges and measure every which way to determine bore size, ovality, tapered bore.... FSM specs are Bore - 79.000-79.010 mm (3.1102-3.1106 in.), Taper/Out of Round 0.0020 mm. Now that's some pretty fine testicle measuring one would have to do and it would take quite a while. I have the bore gauges, but I used my top secret super precision measuring laser guided..... I mean..... it is easier just to measure all of them in one shot and divide by 4. In this case, this is way good enough!! Well, I know there are naysayers, but I went further, I only measured half of them and divided by 2. Looked in spec to me!!
Well, now that you're BORED, let's move on, stay with me here. Pistons are important, can't be too sloppy in the holes..... 78.905-78.980... OK, never mind, I got impatient too. These ones I did measure all of them at once, never even bothered with the math. Check 'em out!!
Lookie all the carbon...... and here's a coupla lessons. First lesson, figure shit out before you go tearing down an engine, you woulda caught this right away, and yes I am pissed how this went down. If your mechanic says I gotta tear it down, you say "Stop right there"... (Meatloaf - Paradise by the Dashboard Light, LOL). Get more answers..... FYB and I had this discussion when analyzing what went wrong with the '07. He figured it out without a teardown, and that bike is running great right now.
Lesson Two, very important...... You have a relatively high performance engine that develops max power around 7000 rpm. This basket case was all carboned up. Every time it left my shop there was this slow acceleration and upshifting before it got 200 yards up the small inclined road leaving here. I questioned my buddy further on how he rode generally on trips (I have never ridden with him). Basically, rides like Grandma, shifts at 3500 rpm, and you can guess cruising at highway speeds probably put it in the 4000 rpm range, passing the odd car might take him up to 4500. He told me it had not seen 6000, and he had never felt that great pull we know just starts to kick in around there. Fer chrissakes, take that engine to its happy place once in a while. You've heard "ride it like you stole it", well do it safely, you don't have to be an idiot, nor am I one to bounce the tach needle off the rev limiter, but get this baby into the power band!!! Ride the twisties in a lower gear, learn engine braking, whatever, but don't ride like Grandma. What's the use of having a high performance bike if you're not going to use it like it was designed to be used... I'll stop now.
Now I can't go putting dirty parts back together, so the next question is, how do I get all the carbon off these expensive little pieces of aluminum and titanium coated valves... the valves are easy enough to get out, but what dissolves carbon that won't eat the metal? So, I'm just gonna soak a piston in Ring Free and see what happens. If you got a magic potion, lemme know.... I'll try one with Simple Green as well, I heard it does magic sometimes.
If you have a running engine and want to clean the carbon out, there is an old old trick we used way back when. Engine running on fast idle plus, spray some water in each throttle body and watch the steam come out the exhaust. On the old 2 barrel carb cars, you could stall it out if you put too much water in, LOL. Carbon does not dissolve in water, so the steam action will flake it right off. There are commercial sprays for exactly this method, but they have other petrochemicals that could enhance the process.
When you're done, use one of these. This is a fancy one given to me as a birthday present, truthfully I've never used it, but it looks kool. You can buy cheapies from China that have a wee camera on the end of a USB cord, plug it into your laptop. I have one of those too, and have used it.
The hotrod urge was beginning to bloom -- since it doesn't need balancing or blueprinting can it be stroked or bored? I needed cams, are there any aftermarket cams with a hot grind? Will bigger valves fit? 
LET BE. JUST LET IT BE 
I have looked at the tranny gears, all look excellent, in fact, this bike had a second gear problem early in its life and parts got replaced. Looked at the balancers... not closely, but noticed they come apart and can get 180 out of rotation, so I just carefully left it alone and packed it away. The other one is still in the engine block, haven't looked at it for extra 'dots'. Middle gear has a shim. Briefly looked at the parts diagram, do not see alternate size shims available, so will have to look more...
