TOPIC:

Hey folks I'm back on here after laying low for a while after losing my license for speeding. I guess I'm probably not the first guy on here to do that! Anyway I have another 4 months up my sleeve to try to get my 305 running right. I'm planning to do some tests etc with my local shop to confirm if new pistons and rings will be the most likely solution before I pull the top end off to confirm. I was re reading some old posts, someone suggested kz650 pistons could work as a swap (new pistons and rings for 305's are unobtainium) with a 1mm over bore. 305 =61mm, 650=62mm. It's unlikely anyone would know this for sure but there just happens to be 1 KZ 305 piston for sale on eBay right now that I could buy and send to one of you 650 guys to check for me. Would anyone be up for that? In case you don't know I'm in New Zealand just to complicate things further for me. Thanks

Damn, I took that fellas info on board without checking. According to the specs in my manual the 305 bore is 55mm

Hmmm, I just had a look through the Wiseco catalog. The std bore x stroke of the KZ 305 is 55x52.4. The Wiseco catalog has what appears to be a big bore piston for a KZ/gpz550, it says std bore x stroke is 58x52.4 and it says the bore needs to be 61mm. Would a 6mm over-bore be too much? Would this work? How would I find out if this could work? Maybe if I sent that piston to Wiseco they could check, or am I dreaming?

Just a guess here but that it too much to over bore the cylinder to fit a 58 mm piston. Drop by a local engine builder, preferably a race engine builder, and ask.

I have to ask. Why do you think the pistons are shot without first taking it apart? Or do you just want a bigger displacement engine?

I think it ends up as a 61mm bore. I guess the piston replacement is a worst case scenario. I have had this smokey condition for a long time and it's never gone away. I've had new valve guide seals installed, checked & re checked the valve clearances. Had the compression checked (although they told me it was fine). My plan is to do a full diagnostic at my local shop then if that gets us nowhere I'll pull it apart and have a look see in there.

I'm just doing some background homework I guess...

ok since compression is good but still smokes you most likely have a stuck or broken oil ring( actually rare). Stuck more that likely. I mixture of 1/4 Marvel Mystery Oil and 3/4 normal engine oil at next oil change might un-stick it. What is your oil consumption? Rarely do pistons go bad. Something might poke a hole in one but they just don't wear out if there is oil in the engine. My suggestion is to see about availability of new piston rings. Standard and oversize. Had the bike sat for a long time, neglected, prior to you buying it?

While you may actually find a piston solution, please be aware that there is much more to consider than just the diameter. The wrist pin diameter, dome height, valve cut outs, distance between the wrist pin and piston top, and the skirt to crank clearance must be compatible.

That's right. Running mismatched pistons can make the engine grenade.
Steve

Sorry, this has been a bit of an on going issue for me, there is quite a history to this in different post on this site and do the ton. You could be right regarding the rings. They are actually brand new. I got the last set on the planet and there are no more. It's possible that the mechanic messed up or that some how the new rings didn't take. I guess that's where I was starting to think about new pistons because if I could get the parts maybe a 0.5 re-bore would be my answer.

TexasKZ wrote: While you may actually find a piston solution, please be aware that there is much more to consider than just the diameter. The wrist pin diameter, dome height, valve cut outs, distance between the wrist pin and piston top, and the skirt to crank clearance must be compatible.

Ahh yes, that's the sort of info I was after. So it would be VERY unlikely that another type of piston would fit.

So to get rid of that smokey ass where do I go next?
I have a new oil filter on the way.
I'll put new Shell Rimula 15-40 oil in it. This was recommended for my 750 and I've used it on the 305. The old oil produced a disgusting creamy snot on the inside of the oil filler and oil window & crank case breather and god knows where else. The Rimula cleaned that right up so a new filter will probably help.
Valve clearances are all within spec.
I have a new timing advancer but the timing may need to be checked & re-set but I'm not sure if this would cause the smoke.
Valve guide seals were replaced last year.
Carbs are clean & float levels check & adjusted.

Should I just go ahead and pull the head off and take a look? What should I look for?

If the rings didn't seat, a coarse hone might do it.
Steve