Without a cool down you will not get consistant testing results that you can apply track to track, unless your letting the sled cool off to a certain temp each pass and even this will not provide the same results as you cant control the inside crankcase temp within reason. They sell little pyrometors that you just point at the object and it reads the temps. I guess people dont realize that a 2 stroke runs RICHER the warmer it is!!! So each pass you run the sled the richer it becomes, no matter how the jets are, each pass temp is added, the engine is richer then before, it will make less overall hp and drop more rapidly thru out the engine run time! The steel crankshaft/bearings/rods/steel bearing races in cases will hold heat long after the aluminum cylinders/heads have dissipated heat, and a warm crank heats up your inlet mixture, making it less dense,thus youll have a richer engine, LOWER on power in a SHORT accelaration run. This is precisely the reason most any drag sled will always make its quickest run the very first pass, the pipes get warmed up on the clean out but the crankshaft and rods have not gotten heat soaked so the air/fuel mixture is more cold/dense and makes more hp!
For a short accelaration run leaner mixtures makes more power faster then a richer mixture does, we are talking 6 seconds or less, not a lake race or 1/4 mile asphalt. It simply results in the pipe temp coming up faster resulting in a better return wave from the pipe. Exhaust gasses travel at the speed of sound but only at a certain temp for a given pipe, richer would slow down the return signals(because of a lower temp , increased time to reach the speed of sound, 1675 ft per sec.) time till later in the drag race, like 300ft down the track...too late.
Now, I will definitely say "yes" a motor will like fuel up top, no doubt about that, FUEL IS MPH!!! take away alot of fuel and youll instantly see a decrease in mph, but you will also see the short times pick up or gain,drop time from your short time. Take any 2 stroke engine either in the field or dyno and make the bsfc(dyno) at .450-.490 and watch the engine build hp very rapidly at a great gain per 1000 rpm, but then somewhere in the rpm range,usually near peak rpm it will taper off(if it doesnt seize) or rapidly begin to fall, this is where you need the fuel(again we are talking about 6 seconds or less here, the typical time of a 500ft grass drag!). most times when a engine is run on a dyno to make big hp it will be too lean to actually run in the field with the dyno jetting, and this all comes down to how the motor is actually loaded in the sled on the ground versus the dyno load applied and or the type of dyno pull made. I have seen motors make great hp on the pump only to fall right on thier face when in the field, put back in the richer jetting from the dyno jetting and the sled instantly comes back to life, the key is finding the jetting that will work for your clutching,suspension, load requirments of your sled, just get it close, thats all you need for backyard racing.
In grass drags with a srx, stock srx pipes, I used 45-47.5 pilot jets, needles dropped but shimmed slightly, and ran 135 mains, this worked good for 500ft, but at 520ft it would most likely nose over and/or seize the piston slightly, but what your after is rapid engine power, not long runs which do require alot more fuel. In 500ft your barely into the main jet, maybe only the last 125ft at the most, the rest of the run was on the pilot/needle circuit. You need the larger pilot jet because the main being smaller and needle not being over rich will cause the carb to flow a large amount of air and not fuel, going from idle or engagement speed to instantly wide open, the rush of incoming air needs the pilot to over fuel the air(richen) for a second to allow the engine to raise rpm and not cough or backfire thru the intake(which is a lean condition) into the next stage which is the needle, all carb circuits overlap each other so your never on the main till a good ways out from the start, remember me saying its tuff to blow em up in 500ft but not 501ft....LOL ...right theres why.
For backyard racing all you need to do is have the jetting "close", safe and close, getting every last hp wont make the differance between winning and losing. As the engine wont be run the same temp each pass nor will the track be groomed to what a professional starting line would be. The sled being just run for fun/grudge racing will gain the most from clutching and some suspension work. As long as the piston tops and plugs are not wet/black and or dry bare alum. looking youll be just fine. Just get her close on jets, work on getting a timer or at least use your speedo on a marked off distance and run the same routine every time, write stuff down, because youll make improvements then youll change more stuff and slow it down.........LOL!
Dont get too wrapped up in all the work needed to prepare for professional racing or youll soon see the fun factor leaving it very quickly.
last year i thought i had the viper dialed in pretty good, actually took a first in 700 mod and 3rd in 800 mod one race. But now i found out i'm running way to rich of jets, 153.5's opposed to the recommended 145's. Hopefully gain alittle ET there, we'll see..just takes lots of playing with. I actually have 4-5 sets of weights and turk suggested the 8ca's and they do work the best thats for sure. but i'll let you all know what happens each pass here in a couple weeks. I think the first grass drag around here starts the end of august.
