Observations....
Reading all published material about tuning the Mikuni carburetor, (the mikuni "bible manual", Group K's carb tuning guides, ect...) will give you a good insight for tuning stock/lightly modified motors. Using those guides and having some understanding of how they work, it's a piece of cake. BUT, once you modify (Big Bore, epoxy cases, install pipes...ect) your motor, the likelihood of making the engine have a continuous and seamless powerband goes out the window. Sure you can make them idle perfect, and have a mid to top end kick a$$, but the transition 1/8th to 1/3rd range will suffer...just like mentioned above. Will the modified motor in your boat allow it to "cruise" smoothly at 1/8 to 1/3 throttle without 4 stroking or loading up? Probably not.
The scoop, is the best...1?... 1 into 2?...or just 2?
61X motors are easier to tune with the single 44mm carb (they were designed for it). Going to a larger (46+mm) carb will give you a little better mid to top end power, but the low end will suffer. A 12 to 15 psi pop-off is where you will be with a modified 61x with a 46+mm, and a soft bottom end power. You will have to run a low pitch impeller to break through that "lean dead zone". You will consume more fuel than a good "tuned" motor. Bottom line, it will run better in that 1/8th to 1/3 area than the 62T single kit.
62T motors with the stock 38mm carbs (stock platform). I never messed with them. Of the 2 newer Superjets I ever rode, they seemed to have real smooth power. They need to pass strict emissions and the carbs have good all around metering. If you look at the tail pipe on that stock boat, there will be no BLACK, unburned spewmen dripping out.
62T motors with the single carb kits, need low pop off pressures, want to starve for fuel in the beginning (because of a poor inlet signal), then run OK up top. (because of the lower manifold pressures created by increased volume of the 2 into 1 manifold). Some of the 4 transition holes need to be bigger to let in more fuel when it is needed. There again, a 12+ psi (low) pop-off and large pilot jet is needed to make it not starve off the line. Then the metering becomes too rich?? WTF! Use a low pitch prop here again, but this time it's to get through the rich zone. Get your jet and spring kits out...Spend LOTS Of time tuning...you will become a expert removing that carb. You might get it to run OK...but, You will probably ditch the package like I did. Perhaps it would be cool in the stock platform. Bottom line, the weaker inlet signal will cause you to chase your tail.
62T performance motors that have 2, 46/48mm or larger carbs. These motors create very high manifold pressures (real good signal), more than what Mikuni had designed the low speed metering for. Those 4 little holes (not picking on which one), lets in too much fuel (in relation to the butterfly opening) and create a rich condition where you don't need it. Just the opposite of the other 2 problem conditions mentioned above. A 24 to 25 psi pop-off is what these setups seem to like. Bottom line here, Don't ride in that area of operation, idle then wide open. And be sure not to over prop it.
The finger gets pointed at....Those 4 little holes.
Basically, we have to deal with a low speed metering circut that is predetermined by Mikuni (their drilling). And it's all supposed to be "infinitely" adjusted by a pilot jet? (and a magic wand?) It's really impossible to do correctly. Those 4 little holes can't possibly provide precise metering to every different combination of motor that's out there. I think we need some kind of a update here.
Now, do you think that "off the shelf" performance carb/manifold package for $2000.00 bux is going to drop on your "motor" and run perfectly seamless like fuel injection? guess again, it most likely will run like crap, somewhere in that power range and you will spend lots of time TRYING to make things smooth out. And once you get that area right, there will be a problem growing somewhere else.
OK,I'm not saying anything about the motor builder(s) that prepare a tried and tested package for you that is dialed in and works. They spent countless hours refining this mess for you, so it works (disclaimer). Unfortunately, I had to learn all this the hard way....$$.......$..............
What we all need is a better carburetor with a adjustable transition circut (more precise)to solve this issue. Any IDEAS? And I'm not talking about the older round body 44mm with the 3 screws.
