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Thread: 650-780 jugs?

  1. #1

    650-780 jugs?

    OK guys i have read in a few spots on here about being able to put a set of jugs off a 750 or a 780? is this true?


  2. #2
    lxman88's Avatar
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    yea but the 780 requires another style header pipe then the 650 750

  3. #3
    so is it the same header pipe as the normal 780? just bolt on and go?

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    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Converting 650 or 750 Fuji engine to 780cc version

    OK, here's the skinny on the 650 or 750 conversion to a 780:

    650 = 65mm bore X 65mm stroke - 68 HP (43 MPH)
    750 = 69.72mm bore X 65mm stroke - 80 HP (48 MPH)
    780 = 71.3mm bore X 65mm stroke - 92 HP (52 MPH)

    NOTE: 1992 and 1993 650 and 750 use a different cooling system and exhaust from the 94 and newer models. So they will require more modifications because of it.

    The 650/750/780 all use the same cases bascially. The 780 are slightly different because they have an internal channel or "port" cut into them that the 650 or 750 doesn't. You can still use your cases.

    The 780 came with reed spacers between the reeds and engine case. They were installed to help prolong the life of the reed petals (moving them out of the way of the turbulance created by the spinning crank shaft) and also add a little more mid-range punch. Reed spacers can be added to any 650 or 750. But you will need 3/8" longer bolts and 6 more intake gaskets. (2 per spacer)

    The crankshafts are identical.

    The cyl base gaskets are identical.

    The 780 cyls differ from the 650/750 in the form of having 2 more exhaust ports added, and slightly larger spaced head studs. What that means is you cannot install 650 or 750 heads on a 780 cylinder. And vice versa without modifying them. (and that is not recommended)

    The 780 cyls also use an entirely different bolt pattern for the exhaust manifold. So you MUST have a 780 exh manifold, 780 gaskets, and matching bolts as the 650/750 will NOT work.

    Some 750 and 780 pistons can be interchanged. The 750 has a 69.72mm bore and the 780 has a 71.3mm bore. So a 780 piston can be installed to a 750 cylinder that's been bored oversize, or say a 2.25mm oversize 750 piston can be installed into a 780 cylinder that's been bored for it. NOTE: if you bore a 750 cyl for a 780 piston, you will most likely need to have the head machined for the larger piston as squish clearance and possible piston to head contact might become an issue.

    Most 780's use 28* of timing adavnce (some SLT 780 used 24* of timing due to heavier loads placed on it because of 3 passanger weight) while the 650 and 750 used 18* and 24* depending on model and year. That's why you'd want a 780 CDI to get the most out of your conversion.

    All 650 CDI and 1993 750 use an analog signal CDI and electrical stator or magneto. They are not compatable with a 780 CDI. 94 and 95 750 use a digital signal and CAN be used with a 780 CDI.

    While on the subject of ignition, the 650 and the 1993 750 flywheel also CANNOT be used with a 780 conversion. You MUST use a 1994 or 95 750 flywheel or any year 780 flywheel.

    All 650/750/780 carbs are Mikuni Super BN 38mm. Only difference is the 780 have a built in return flow restrictor, have the oil injection fittings mounted into the carbs, and the 780 are plumbed for a triple outlet fuel pump right from the factory. You CAN use your 650 or 750 carbs. You will just need to rejet them for the conversion.

    The 780 came with a high output (65L per hr) triple outlet fuel pump from the factory. I HIGHLY recommend using this same fuel pump for any conversion. The round 35L per hr fuel pump MAY be adequate, but I'd go with the HO for safety sake, not enough of a $$$ difference between the 2 IMO to risk a piston problem.

    The 780 uses the same exhaust pipe as the 1994 and newer 650 and 750. The only difference is the cooling orifice for the 780 is 1mm, while the 650/750 use a 2.3mm orifice. You can easily swap the fitting or possibly solder and the drill the fitting on your existing one.

    Finally, to take adavntage of the added HP, you're going to want to install an impeller rated for the 780 HP. Or else you're going to be hitting the rev limiter and not getting full benefit of the 780 conversion and your new found power. Some 780's shared the same impeller as a 750 (PN ending in 079). Others came with a Skat trak swirl 15/20.

    Sorry for the long post, but that basically sums up the 650 or 750 to 780 conversion.
    Last edited by K447; 06-09-2012 at 11:49 PM.

  5. #5
    very good post!!!!! that is alot of info. Looks like i need to do alittle more reaserch.

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    ok now I am unsure of the timing I need. I bought a 1993 750 that has been sitting for a few years because it came with a CDI from a 780, F8T33074 and I need to use 780 flywheel, which I now have. Clayton G. said to advance the timing by 6 degrees but the above info makes me unsure.
    "Most 780's use 28* of timing adavnce (some SLT 780 used 24* of timing due to heavier loads placed on it because of 3 passanger weight) while the 650 and 750 used 18* and 24* depending on model and year. That's why you'd want a 780 CDI to get the most out of your conversion. all 650 and 1993 750 use an analog signal CDI and elec stator or magneto. They are not compatable with a 780 CDI. 94 and 95 750 use a digital signal and CAN be used with a 780 CDI.
    "
    Any help on what my timing should be?

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    BlueFishCrisis's Avatar
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    Quote Originally Posted by Frozen View Post
    ok now I am unsure of the timing I need. I bought a 1993 750 that has been sitting for a few years because it came with a CDI from a 780, F8T33074 and I need to use 780 flywheel, which I now have. Clayton G. said to advance the timing by 6 degrees but the above info makes me unsure.
    "Most 780's use 28* of timing adavnce (some SLT 780 used 24* of timing due to heavier loads placed on it because of 3 passanger weight) while the 650 and 750 used 18* and 24* depending on model and year. That's why you'd want a 780 CDI to get the most out of your conversion. all 650 and 1993 750 use an analog signal CDI and elec stator or magneto. They are not compatable with a 780 CDI. 94 and 95 750 use a digital signal and CAN be used with a 780 CDI.
    "
    Any help on what my timing should be?
    Timing is based on motor and use. More advanced timing for one up riding, less advancement for riding with more people and / or pulling tubers. The timing is built into the CDI. You select the timing by changing out your CDI. You will need to find out how much your 780 CDI has based on the part number.....

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    RLACEMAN's Avatar
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    When I first did my 750-780 conversion I tried the 780 CDI (with 24* advance) the difference from the 750 CDI was very slight in the acceleration dept & I mean VERY slight, top speed rpms was only 10-20 higher, I didn't think it was worth the risk of more advanced timing, this way I can safely use 89 octane fuel too.

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    so the original timing is 16 degrees + or - 1.5 so 24 degress should be fine?

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    BlueFishCrisis's Avatar
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    Quote Originally Posted by RLACEMAN View Post
    When I first did my 750-780 conversion I tried the 780 CDI (with 24* advance) the difference from the 750 CDI was very slight in the acceleration dept & I mean VERY slight, top speed rpms was only 10-20 higher, I didn't think it was worth the risk of more advanced timing, this way I can safely use 89 octane fuel too.
    94 SLT750 came stock with 25* advance.......?? Might see more of a difference from 16 to 24*...

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