blackhat250 Posted January 11, 2015 Share Posted January 11, 2015 Well it a compression test now, did you change spark plug,,,and cap, trim 10mm off H/T lead too. Link to comment Share on other sites More sharing options...
jimbotipler Posted January 11, 2015 Author Share Posted January 11, 2015 New plug. I'll double check the right plug and gap. Yep, trimmed off some ht lead but haven't changed the cap. Looks ok though. Thanks folks. Will update once I've had it compression tested. Link to comment Share on other sites More sharing options...
jimbotipler Posted January 25, 2015 Author Share Posted January 25, 2015 Well, I ran compression test with my brand new compression gauge and Its reading about 6 bars. Manual says should be at least 11, ideally 12. So I squirted a bit of oil in and it went up to about 8. So I'm thinking piston. Got it apart now but don't really know how to proceed. Pistons out and the cylinders off and I'm thinking I'll take both to a local bike shop and get them to check for wear so I can get the right piston kit. Valves look in good nick but is it worth having them out and regrinding you think while I've got it in bits or best left alone? They don't look particularly coked up and the collets look ok. Anything else worth doing while I've got the top off? As always any advice is hugely appreciated. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted January 25, 2015 Moderator Share Posted January 25, 2015 Well as the head is off it might be wise to lap the valves in too? Link to comment Share on other sites More sharing options...
KevtheRev Posted January 26, 2015 Share Posted January 26, 2015 Check the valves by turning the head upside down and pouring a little petrol into it , if any fuel leaks through, the valves need attention . 2 Link to comment Share on other sites More sharing options...
jimbotipler Posted February 2, 2015 Author Share Posted February 2, 2015 Thanks folks. A quick update. A local garage checked the cylinder bore and that's ok However the exhaust valve was quite heavily coked - including the seat. Inlet is ok, so it looks like the exhaust valve could be the problem. Lapped them both in anyway and de-coked I'm putting in new standard size piston set. Any tips with regards installing the new piston? I'm trying to work out which ring should go top and which on the middle. It's supplied with a metal one and a black one which looks like carbon or plastic of some sort. I can't find any info in the manual about this. I'm guessing metal on the top as that will take the most beating. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted February 2, 2015 Moderator Share Posted February 2, 2015 Hmm don't guess jimbo ! 1: both rings the same thickness then? 2: Get a magnifying glass and look at the flat sides of each ring either side of the gap...any markings? Link to comment Share on other sites More sharing options...
jimbotipler Posted February 2, 2015 Author Share Posted February 2, 2015 They certainly look the same. I haven't got a micrometer. There are no markings on them. Link to comment Share on other sites More sharing options...
jimbotipler Posted February 2, 2015 Author Share Posted February 2, 2015 The edge of the top one in the old piston looks possibly slightly chamfered. And the brighter one in the new set looks slightly chamfered also? Link to comment Share on other sites More sharing options...
blackhat250 Posted February 2, 2015 Share Posted February 2, 2015 After regrinding them valves, clearance will be different, is it rocker or shims /bucket type,, Link to comment Share on other sites More sharing options...
jimbotipler Posted February 2, 2015 Author Share Posted February 2, 2015 Thanks mate. Rocker. Trying to work out which ring goes on top. Ive put the rings under a magnifying glass and there are no marks. Looking closely neither has a chamfered side - inner or outer. The only difference seems to be one has a bright outer. I've been trawling the internet and cannot find any info on this at all. The order out of the box was bright on top, then black, then the oil rings. I've put them in the piston in that order. What do we think? IMG_1829 by tiplerjim, on Flickr Link to comment Share on other sites More sharing options...
Moderator Airhead Posted February 7, 2015 Moderator Share Posted February 7, 2015 if you press each ring into the groove in the piston will they bottom out they should do, if one wont then swap them over and try again because one of them may be a keystone ring Link to comment Share on other sites More sharing options...
jimbotipler Posted February 9, 2015 Author Share Posted February 9, 2015 Thanks airhead. Had to take a bit of a flyer as I couldn't find much info. So I fitted them in what seemed the most likely order according to a site I found re chinese piston rings http://www.dansmc.com/rings.htm. Seems ok. Certainly runs well and is currently starting first push of the button. So to recap it looks like it was a coked up exhaust valve causing poor compression, enough to cause bad starting but not enough for it to not run. I changed the piston and rings as a Chinese set was about £30. Not sure if yamaha do a genuine replacement any more. So I reground the valves and decoked the head. Hopefully that's it sorted unless I've screwed up in the reassembly somewhere. Thanks again for all the helpful advice. 1 Link to comment Share on other sites More sharing options...
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