Re: Burned Pistons No. 5&6
what stands out is that you have a pair of identical rebuilt motors and only one failed.<br /><br />On the engine with the 2 burnt pistons, because the other engine I'm assuming is completely fine, tells me maybe, and I stress maybe, that it may have had an improper valve setting. And if that were the case, I would expect to see evidence of valve damage as well on those two pistons. If all the other cylinders look good, and have some carbon buildup, then I think that would further indicate the valvetrain was the culprit and damage was limited to just those 2 cylinders on that motor.<br /><br />With only 34 hours on it, I'm guessing they saw minimal high rpms? Motors I hope turned up over 4200 rpms easily, you have correctly pitched props? If not, then you're overloading the motors and burned pistons are usually the consequence and I would expect similar damage to the other engine as well.<br /><br />As for the carbs, buy a book on the quad's if you're going to continue to use them.
http://www.bob2000.com/bookscarb.html <br /><br />yes, if you rebore and/or re-cam then you'll will need to verify you're carb is sized right and jetted right. But from what I know about carburetion, if a stock 454 with the correct oem 750cfm carb is then rebored to 496, you will need to at least put in bigger main jets (and other parts depending on type of carb) to account for the extra air (along with fuel) the motor will call fall. If not, then it will definitely run lean, and burn pistons. Were all the spark plugs burnt? Maybe those two burnt cylinders, because of motor design would run leanest, hence they were just the first to go, and the rest should show signs of running lean. If that's the case, then you should have the same problem on the other engine as well, I'm sorry to say.
