upon tearing down my grandsons cr80 I found it had a 105cc
big bore kit in it. however the carb jetting was stock, are there
any source's for correct jetting for this modification?
Does the bike run OK? Does it show any signs of not being jetted correctly?
When I put a 240 kit into an old KDX200, it ran best with the stock 200cc jetting (I tried much richer at first). Keep in mind the bigger engine will suck in a greater quantity of air and fuel with each intake stroke than the smaller engine, even if the jets are the same. I've been told KTM 300's run smaller main jets than 200's with similar carbs.
In some old books of Maico's(250-490) I have, it states that the smaller engines have bigger main jets with the same carb size. Someone told me that this was also due to the fact that a smaller engine is usually revved higher and longer to get the most out of it and therefore needs more gas and thus lubrication.
Just a thought!
thanks for the imput.Guess we'll start with stock and go from there. I remember we had Noleen do their mods on a wr500 which required rejetting
but they supplied spec's for that. I ordered a rebuild kit for the 105 from LA
Sleeve but it came with no specs. Before the teardown my other grandson's yz 80 ate this cr alive so I was thinking improper carb. However it was easy starting and had good bottem end. just went flat as the yz was coming on
My old big bore KDX also would not rev out like a stocker. Even after jetting it right, peak HP seemed down from stock. Gaining bottom in exchange for top end is acceptable for a woods bike, but not so good for minicycle MX.
My guess is to get the most out of the big bore you may need a porting expert to work some magic.
As a less expensive alternative, the rider could adapt his style to the bike's good low end.
Common for big bore bikes to not rev as hard as a stocker and jetting often has to be leaner as the others say. To get the best out of the CR change the sprockets. Drop a couple on the rear and the bike will then more effectively use the torque of the bigger bore and it should hook up better out of the corners etc.
'In some old books of Maico's(250-490) I have, it states that the smaller engines have bigger main jets with the same carb size. '
Smaller engines pump a smaller volume of air which means less vacuum in the venturi, thus you need a larger jet
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