top end refresh and jetting question

[chris1971]

Hi chaps . was out last sunday with the hillbillys doing some woods enduro and a bit of bing bashing . bike was running really well but the last hour she started running lean . revs were hanging on a bit when i revved the bike . tried adjusting the air screw etc and noticed it turned very easily , so easy it felt it could vibrate out . any roads no matter how much i wound it in or out my crm was running lean . long story short i thought everything was ok so began my 4 mile run back to the van . by the time i got there she was rattling like a bastard . next day done a compresion test and it was very low so decided to pull the head and barrel of . plug was a powdery white colour and the barrel was scored really bad . had one of my mates who does re-bores etc have a look and he said it was bad but fixable . i have had to go up 1.75 mm oversize to make sure i have the perfect repair . i take it my worn o ring on the air screw is whats made her run lean . i run pre-mix at the correct mix but left my jetting standard with no problem in over a year of fairly hard use . my mate said i should really be thinking of going up a few sizes on my main jet thats 128 . any guys had this size of re-bore ?? and what jet size did you use .

[knackeredMk1]

All the signs of running lean. The question is why?

Has it been running fine until this trip? There has been a change somewhere in the intake or the exhaust.

The standard max bore size is 2.00mm so 1.75mm is fine. I did a rebore to 2.25mm but that was a special piston.

[chris1971]

hi knackered , yeah bikes been A1 . total rebuild a year and a half ago with new crank seals etc . checked the rubber inlet connector and thats fine . split the carb and both jets are clean and the carb bowl is clean . think its the mixture screw o ring . would a worn o ring cause such a change to the mixture

[pudzie83]

Mmmmm strange one if everything is set up right wrong set up would make it run poo tbh, and as for white plug that ain't healthy

[fallenmikethebike]

It might be worth checking the float level, and fuel tap filter, a lack of incoming fuel would weaken the mixture and allow air to be drawn in.
Mike

[gregorious77]

and check crank seals!
Generator side would cause lean condition. Clutch side would suck in oil or pressurise gearbox.

[chris1971]

I rebuilt the engine october 2014 and the bike wasnt really used until may 2015 . used honda seals as i am a bit OCD when it comes to stuff like this . i would hope i would get more life out of genuine honda parts , all the same you never can tell when any engine part might fail lol . got my barrel back last night from Paul at DMC and it looks lovely just waiting on base gasket , small end bearing and 4 shiney new cylinder base nuts coming so i can stick her back together . head gasket looks to be fine but just to be sure when i use the old gasket i give it a very light dusting of mat black stove paint to help things along .

[jompy]

I rebuilt the engine october 2014 and the bike wasnt really used until may 2015 . used honda seals as i am a bit OCD when it comes to stuff like this . i would hope i would get more life out of genuine honda parts , all the same you never can tell when any engine part might fail lol . got my barrel back last night from Paul at DMC and it looks lovely just waiting on base gasket , small end bearing and 4 shiney new cylinder base nuts coming so i can stick her back together . head gasket looks to be fine but just to be sure when i use the old gasket i give it a very light dusting of mat black stove paint to help things along .

OCD & your using the old head gasket !! buy a new one tight arse

[gregorious77]

fook that they are metal and good to go again init!

[jompy]

i was always lead to believe you could only use them once , the DT head gasket on the other hand was copper so you heated it cherry red & used it again

[gregorious77]

yeah probably only supposed to be a 1 shot but I have heard people use them more than once.
If you are just doing rings or piston you an just leave the head on and remove the barrel and head complete.

[chris1971]

FFs lads im scottish . head gasket is fine . had a few MX bikes and done this trick everytime with no problems lol

[knackeredMk1]

As long as they aren't rusty or bent then they generally are fine to use again.

[pudzie83]

Old ones are fine if in good condition i had one of knackereds ones on for weston
I have a new one fitted now that i did question fitting lol

[chris1971]

happy days . put the old girl back together today . spent most of the day doing it as i tend to get a bit hashy when i power on so took plenty of time . only problem was the power valve flap . the guy who done my rebore said he had it in place whilst doing the work and there were no issues of it fowling my rings . tried the piston in the bore before fitting it all together and moved it up thru the power valve flap feeling for any sort of resistance . found that when the valve was nearly closed position it was just catching the top ring slightly so attacked it with the drill and a big finishing flap grinder . spent about an hour cleaning everything up and blowing crud out the barrel ports etc . got the tank on about 6 tonight and gave her a couple of kicks . she fired up no problems and sat ticking over on the stand for about ten minutes . no rattles , no leaks so i took her out for a gentle ride . have to say i am amazed at the compresion she has now . the resistance on the compresion stroke thru the kickstart made me smile lol . hopefully tomorrow i can get a few hours running her in gently before giving it large .