First let me start with life expectancy. Were the proper steps taken when building the engine? Was rhe ring gap set to specs? If using a forged piston was the piston to cylinder clearance checked? The cylinder using a forged piston needs .002-.003 more clearance than a cast piston. Some kits come with a slightly domed piston. Was the valve to piston clearance checked? When replacing with a big bore or even a stock cylinder was the deck height checked? I have found as much as a 2mm differance in deck height from differant cylinders. The engine is not a puzzle which we slap together. It is a intricate piece of machinery which requires careful and precise assembly. Anytime you change parts from oem you are in a sence redesigning your engine. I know of several people who had enlarged the case in order to fit a 61mm or 62mm bore kit using a dremmel to grind it. They were very careful to put an wet or oiled rag to catch the grinding dust. I don't know of one which lasted any amount of time. Usually the pump was first to weaken dropping oil pressure which results in premature wear in the cylinder. The grinding dust is going to get in the crank. Because the casings are made of cast investment which is differant than the old fashioned way due to cheaper production costs this dust created by grinding is extremely fine finding its wat into the crank. As a kid I grew up in the days of true muscle cars and everyine of my friends had one. You could always tell those who knew what they were doing from those who didn't and who the smart ones were who didn't know what they were doing. The ones who knew nothing blew engines, the smart ones who did not know had sonmeone who did build their engine. The ones who didn't know and just bought parts and threw in may have had bad sounding cars but that was it and they were always working on the engine. My point is this, racing and or abusing an engine which most of us do will shorten an engines life stock or modded. But the person who just throws parts at their GY6, CN-250 even Honda 400EX whatever will not get any longevity out of their engine. The big bore kit ,cam,stroker crank if proper has nothing to do with premature engine failure. I can go on for ever with type and quality of parts yadayadayada, some time later.Now for the 50mph. My 150 will do 44mph in 300ft. I am working on 50mph in 300ft but have not got there yet. I am working on a new cam and than I'l work on the gearing some more. But currently my top speed is 47mph. I believe this is possible to hit 50mph but it takes some coin to do so, you will not find a bolt on kit that will do this. Right now I have a 62mm big bore with a 6mm stroked crank balanced, A9 cam, ported and polished head with lowered valve guides and new springs, 32mm keihn cvk carb, eastside exhaust, K&N air filter with the cold air intake. My sons kart will do 38mph in 300ft. and not much more on longer run.All speeds for both karts were checked with a calibrated radar gun and high end gps. His also has the 62mm bore with 4 valve head ported and polished, A-11 cam,3mm stroked crank, 30mm pumper carb, eastside exhaust, UNI filter with redneck cold air intake. We both run Dr Pully hit clutches and variators with his having 10grm. rollers and mine with 11grm sliders, 1500 torque spring. You can look your 150 and your buddies 250 as small block vs big block. On a long haul the 250 will labor less giving him the advantage for he has a 3mm advantage on stroke and 14mm on bore size. I nor my son have had any problems beating 250's in 300ft. drag. Ihad only been beaten once by inches and it was a Carter 250. We raced 3 times and I beat him twice and he got me once. And the outcome of all races I believe were due to the launch. I was running a basically stock engine with a 4 valve head, 32mm carb, east side exhaust, UNI, cold air intake, MRP heavy duty clutch, Koso variator with 9grm rollers. I would suggest a cam, 30mm carb, Eastside Exhaust, K&N or UNI with cold air intake, Large valve/port head ported and polished,or 4 valve head ported and polished
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