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  1. #1

    780 to 750 Carburetor Swap Questions

    Hi guys,
    I recently put a new top end (.50mm over) on my 1995 SLT750. I had to purchase several parts to reconnect the oil injection as it had a block off installed. (The block off is somewhere in my back yard about 100 feet away from where I was standing when I threw it ). I have had it in my head about swapping the carbs to the newer 780 SBN's (Black ones) as the oil is injected directly into the fuel chambers rather than the check valves into the airbox/FA base. I got a set from a 1996 SL780, and swapped the linkage rod bracket to the 3rd carb so as to install them traditionally and with my existing throttle & choke cables. (Quick tech tip, when reinstalling the individual carbs to the metal bracket that holds the three together, bolt them to an intake manifold to get them all perfectly lined up THEN tighten the screws to the bracket. They will be out of sync if not perfectly lined up.) Anyways, here's the question. Since I deleted the oil circuit and am now pumping gas AND oil through the fuel circuits in the carbs I think the larger jetting should actually be close enough to dial it in with the needles. I realize the cases were ported in the 780 as well as additional exhaust ports. But, doing some quick math, my stroke is 65mm and my bore is now 70.25mm multiplied times 3 gives me a displacement of 760 cubic centimeters. The 780 doing the same math with a 71.30mm stroke equates to, well, 780cc. So with a difference of 20cc divided by 3, I have a variance of 6.66666 cc's per cylinder. Not very much at all, but the 780 is ported more aggressively. It is my belief though that Polaris installed the much larger jets in the 780's to accomodate the oil injection being routed through the carbs rather than the affore mentioned differences. Has anyone ever done this swap, and if so where were you at jetting-wise?

    Tracy
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  2. #2
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    You should be fine with the 750 jetting. The 780s were jetted different because its a bigger motor that needs more fuel.

  3. #3
    I'm leaving the 780 jets alone for now and I'll see where I'm at when I'm breaking in the engine. Pilots are the same at #75 but the mains are #110 rather than #90 as found in the 750. I believe this was done to incorporate both the gas and oil due to the fact that the oil is injected directly into these carbs rather than through a seperate oil circuit into the intakes as found on the 750.

    Say we have three parts of liquid going into a perfectly tuned engine at WOT, and for simplicity we'll make all the parts an even 1cc. One part (1cc) from the primary fuel circuit, one part (1cc) from the main fuel circuit, and one part (1cc) from the oil injection circuit. For the engine to perform to the perfectly tuned settings, the engine requirements of 3cc's per minute must be met. If I delete the seperate oil circuit, (the check valves as found on the 750's), and run the oil AND fuel through the carburetors (premix) I must compensate by allowing more liquid through my remaining circuits (Primary and Main) to achieve my sum total of 3cc's. Otherwise, I'm getting less fuel to my engine than it requires and too much oil. This also explains why a machine with a block-off installed can be spotted from a distance. The area around the exhaust is predictably black with soot as the machine was not rejetted to allow for the proper ratio. Anyways, looks like I'm plowing new ground on this so I'll update with results as soon as the weather allows here in Indiana.

    Tracy

  4. #4
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    The amount of oil being injected in the carbs is so small it makes almost no difference. Maybe go just 1/8 turon out on the high speed needles. The reason you can notice a premix ski is because mixing oil and gas at 40 to 1 or 50 to 1 uses a lot more oil and creates a lot more smoke and soot than with the injection system. Typically with oil injection you use about half the amount of oil you would use if you were running premix. I think you will be way too rich using the 780 jetting.

  5. #5
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    One my 94 SLT750 with 780 cylinders the jetting I found to work the best is 107.5 mains & 77.5 pilots, this is with the 780 flame arrestor & retaining the original oil injection which is not variable rate & is injected into the intake manifolds. If you are using the original flame arrestor, the type with opening on one end, then you will be putting too much fuel in the motor, & should drop main jet 95, if you are using the 780 flame arrestor you will need a little more fuel, 102.5 should be plenty & safe if you have to use fuel with ethanol. But either way 110 mains will be too much fuel for a 750.

  6. #6
    Hey thanks for the replies! Sadly it's just too cold to put it in the river under load. It sounds great on the trailer. It idles smoother and is more responsive than either my SL750 (with rebuilt carbs) or even the SL900. Since the top-end is fresh I'm going to leave well enough alone for now and break it in with the 780 jetting. It's kinda tough to dial in jetting with both premix (50/1) and injection hooked up and limited to 1/3 to 1/2 throttle! I'm really pleased with this swap so far though as it looks great with the open filters and I've eliminated the potentially problematic injector nipples.

    Tracy

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