QuestionHi, please excuse my ignorance, i'm on a learning curve here
After having a my cylinder rebored, a new piston, cam chain, guide and tensioner my 73 Honda SL125 fired up and sounded the best it has ever been.
The piston is now 56.5mm over the original 56mm and is a piston from a CB/CG/XL 125 with the slightly domed top which increases the compression from 8.0:1 to 9.0:1.
The cam chain felt much tighter when I put it on that the one that was one. (The tensioner is on the slackest that is goes) Is this normal with a new chain?
After doing a few heat cycles (warming it up, just in the garden, and then letting it cool) I took it for a little ride. (Only around 1-2 miles I saw that the plug looked like it was getting very hot. There was also a tapping noise under acceleration, which sounds like the valves. I checked the clearance and adjusted them to 0.05mm when I was assembling.
The stock carb setting pilot screw is 1 1/2 to 1 3/4 turns out from closed and to get a brown plug it has to be 1 turn out. Gives you an idea of how hot it is.
Taking it easy I went out again today and its still the same. 2 times there was a horrible noise and a loss in power that felt like it was seizing up but can't be sure. I pulled the clutch in immediately and let is sit for a while to cool down. It started up fine both times and carried on fine.
What could be causing this kind of heat and tapping? Could they be linked?
Thanks very much.
John
AnswerJohn, how much clearance did you put on the piston? Should have been 1.5 to 2 thousandths of an inch.
Tapping/rattling noise under load is probably detonation, due to too much compression, too much spark advance, too lean jetting.
I am finding that here in the US, the main jets have to be increased one size to compensate for the diluted fuels we have now. ALWAYS use premium fuel grades in your motorcycle.
The idle screw is just that.. for idle mixtures. Once the throttle has lifted about 1/8 open, the idle circuits are out of the mix.
Try raising the needle, by lowering the clip and adding in more main jet. Try setting the spark timing back at T mark for awhile.
If the camchain went on, then it should be the correct one. There were different lengths for the later model engines with the 2-piece cylinder heads. The stack height of the base gasket, cylinder, head gasket will affect the center to center length of the camshaft to crankshaft. On these engines you have to straighten out the tensioner by screwing the adjustment down. You don't want the camchain too tight, as it will cause excess friction internally and the engine grows as it warms up and makes the situation even worse.
Will assume that you did put the camshaft in correctly with the punch mark at the top of the cylinder head opening, with piston at TDC. Compression readings should be about 175psi or more with a domed piston. 200psi isn't out of the question.
It is quite likely, given a new piston/bore and detonation/pinging conditions that you did seize the piston. If that is the case, the rings won't be able to seal against the cylinder walls properly, and you will have a loss of compression and some oil burning that won't go away even with proper break-in mileage.
If you have altered the air filter or exhaust, you will have to rejet the carb appropriately.
Bill Silver
