Hello Unison
been following the thread for a while and think the direction your going is quite hard work and may end up being overtuned for its intended use
done quite a lot of stroker builds on minarelli and piaggio motors and i do cobra uk,s tuning and development work on there king 50 and just got the new 65 / 70/ 90 quad in to do.
if you aim to compete with a cobra then handling and machine weight are all important off road probably more so then power ,the power may get you 2 or 3 seconds back but not 1/2 or a full lap that you say your behind so put yourself on a par with handling and machine weight and youll be well on your way
for the engine you got 2 options for best results a dedicated stroker kit like 2 fast or simmonini or a stroker and mid race cylinder
the reason being that you dont want to exceed 190 degrees exhaust duration absolute max,this is more than enough to achieve the power you need to compete with the cobra.
if you use any of the high end cylinders with a standard crank your gonna be up at 195+ and with the stroker youll be through the roof at 205 degrees+ and find yourself taking on loads of work decking and playing with heads to get things civilised again,i know you had bad results with the mk but there normally good cylinders and the sp70 kit makes a great choice with a stroker crank the port layout is almost identical to the pro but timing is way low at approx 176 114 so when you put a stroker crank in you get great port timings and you have a recessed head that you can skim/play around with to obtain the best set up, from a long term point of view keep asking if its feasible to pay loads for a cylinder then put loads more work in to produce the end product for mass production, it seens ok first time round but when youve done 20 of them youll start asking yourself if there wasnt a better way.
the simmonini although ive not seen one /only on photo,s seems to have a great deal going for it.the price is incredible and has the sub boost ports that i prefer ,if it needs a little cleaning up then thats no big deal if it needs a replate then that wont blow the budget either,there cheap enough to afford this and if your planning a quantity of these then youll get a good price on purchase and plating.
before you start out with the italkit conversion you,ve been advised produce a port specification of how it comes (kartweb.com) do a great little program which will produce figures in seconds ive got tsr but often use the kartweb program instead then check on those calcs provided to see what they raise the spec to.The italkit lacks ex port area and its labour intensive adding subs and obtaining a good sized choke diameter without getting close or breaking through into the stud holes,do italkit not do a bridged port/sub version its a shame if not cos its a great cylinder with what i rate as 1 of the best plating jobs in the business.pwk 28 will be fine or tm 24 even. for pipe the r1200 would be a good starting point pvl digital or leonelli digital ignition/malossi or polini overrange the 2 g clutch will fail very quickly so strength is a key here seen quite a few break on quads the st6 looks good but not tried yet
for reeds stick with malossi block it s the only one that will give you good reed life and done back to back vf3 dyno tests that showed very little gain on the malossi over the operating rpm range that the variator holds