It's nice to see a forum for 120 sled racing newbie dads who have reached there limits but their kids haven't. Thanks!
My question is in regard to the 2010 AC120 with the Yamaha 125 cc engine. I have two boys racing in six classes in ovals in Canada. We have a 2006 AC 120 (Improved Stock), a 2007 AC 120 (Super Stock), a 2010 AC 120 (Super Stock) and a 2009 AC 120 (Stock).
We moved the 2010 up to Super Stock and I'm now worried that that was probably the wrong decision. Rec-motors supplied us with their first adj. main jet for these 2010's but unfortunately, the main jet assembly is not fed the same way on the 2010 as previous models. Instead of the main jet threading down into the nozzle, the main jet is threaded into the side of the fuel chamber. Now, the Rec-motors adj. main jet assembly had to be modified (shortened) so that the main jet would not be interfered with as the assembly is threaded down the fuel chamber. With that now completed, I bored the main jet and reinstalled. I changed the pilot jet from a stock 37.5 to a 42.5 and reinstalled carb.
I couldn't get the sled to get full rpm (maybe 3500-4000) with governor by-passed. I ran out of time and will be attacking it today. I'm going to go back to the 37.5 pilot see if that helps. I've already removed air box to get maximum intake (have velocity stack). Obviously, I can't go back to the start since the main jet has now been bored. Any thoughts?
Post by Bob Vehring on Mar 3, 2011 14:06:44 GMT -5
Welcome to the forum I wish I hade a difinative answer for you. We've been waiting to get one of the new Yamis in and while we have had alot of Cats so far their all the old ones.
A couple things would help to get clear on this, it would help if you could post pictures of the carb and jet set up. Also let me know what exactly your SS rules allow for modifacation to the engine. Basicly any of these engines are set up to run at 3600, which is their governed speed. 4000 isn't that far above that so my guess is unless you have mods done that really increase the fuel demand, it shouldn't be off by a great amount.
In the stock classes, we really don't do alot with the piliot system, in racing its either throttle open, or clossed, compared to trail riding where mid range is everything and most driving is at part throttle posistion. The piolet only has to get it to transision to the main.
My first thought without seeing this is, if you shortened up the jet itself, it is now most likely metering off a different section of the needle further up. Depending on the angle of the tapor on the needle, that could change things