Checked all measurements and here what I got.

Cylinder, measured front to back.
Top 82.98 mm
Bottom 82.87 mm

Piston, measured front to back.
82.35 mm

Top ring gap 25mm down from top of cylinder.
0.978 mm
Top ring gap 25mm up from bottom of cylinder.
1.041 mm
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Hopefully they'll show that something is out of spec, or someone can post up the figures from the manual so I can cross check them.
Any help and advice would be much appreciated.
Or any more info required, please let me know.

 

These are for a Bruin but the cylinder and head look very similar if not identical:-
Bore max dia 3.272"
Piston min dia at 5mm above skirt bottom 3.265" at 90 degrees to wrist pin
Top ring gap 0.025"
I got the Bruin book from 501 spares as a free download, they do have others.

 

Thanks so much for that.


I've done the conversions, looks like my bore should be o.k, piston will need remeasuring, and the ring gap is about 2-3 X what is recommended.
Fingers crossed it's just rings and a gasket set, not the end of the world if I have to get a piston too.
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I'll have a look on there for a manual for my engine.

 

Just a little update.
Ive got a piston and rings now so going to install them sometime this weekend hopefully.
WI'll post an update when I get it all back together and hopefully running.
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I've also found a service manual online for anyone else that may need it.
https://www.manualslib.com/manual/91...age=196#manual

 

A quick update.

Got it all back together with the new piston and rings, new head gasket and cylinder to block gasket, and everything set to spec.
​​​​​​Put fresh fuel in and tried starting, no joy. I am getting fuel though and a good spark.
Swapped the coil (Chinese one) for the old original one, tried staring, got a spark then it disappeared after a second every time I tried starting. So have put the Chinese coil back on and getting food spark again.
SWapped the Cd (original)i for a Chinese one that I got with the bike, no spark at all when cranking. Have put the original back on and getting a good spark again.

Going to do a compression test tomorrow to see what that's saying.

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Thank you for the link sir, I appreciate you.

 

So, I've finally got back to this quad again.
I've rechecked everything, it times up perfectly and valves are adjusted middle of the range. I managed to find a nos CDI unit with the correct numbers on it for a good price so got that to try, no change in anything, still the same as before. O bought a new coil, supposedly uprated, that has made no difference to anything.
I bought a leak down tester to investigate further, this has shown approximately 15% loss at 30psi on multiple tests. These were at tdc.
I will try to get a new compression tester as I wonder if my old one isn't working correctly.
Would it be worth getting a carb rebuild kit to see if that would help as it will catch and run for a few seconds when I squirt carb cleaner in to the carb with WOT so maybe it could be a fuel issue. I'm definitely getting fuel to the carb and into the engine, maybe too much, I'm not sure?
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I can't remember if this has been mentioned earlier in this thread but Yam inlet manifolds are notorious for cracking and leaking air in, so check that next.

Never used a leak down tester, my reasoning being, if compression is well down and it isn't valve clearance, you are going to have to remove the head anyway, so check everything when you have the head off and you will find the fault. However you should be able to find where that 15% is going by listening for the leak(s). If you have a good spark your problem isn't electrical, so stop buying electrical components. Personally, if compression is now over 90psi, I would be buying a cheap Chinese carb, they bring their own problems, but it should at least run, if your existing carb has been part of the problem.

 

Thanks for the reply merryman.

Inlet manifold was one of the first things I replaced on it.
heads been off to do piston and rings, no warpage visible on the head.
Going to have to get a new compression tester to check compression and see what that says, but I can't see it being terrible if the leak down test is showing such little leakage.
The leak is by the rings, I can hear the faintest hiss in the crankcase, but the figures I've found everywhere says anything less than 20% leakage is nothing to even think about, especially on new rings as they haven't glazed the bore for a better seal yet.
I'm totally with you regarding electrical stuff, the only reason I got the cdi was the price, think it was less than £50 posted and it's brand new (original Yamaha part still in the original bag). The coil was more to rule out the spark as the original gives no spark and my mate who I got the quad off bought the cheapest Chinese one he could find, so I wanted to get something a bit better to rule that out too.
I'm not normally one to throw parts at something until I can find the problem but this is the one thing I've had that everything says it should run and it doesn't, so I have to admit to buying a few bits unnecessarily to try and fix it or atleast help my diagnosis.
Regarding the Chinese carbs, I'd rather not get one if possible and rebuild the original as it's in fairly good condition, I just need new gaskets and stuff.
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Was cylinder bored or honed? was it egg shaped?