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4bb Engine to 82' 750 Seca frame

Discussion in 'XJ Modifications' started by Finnish Cafe, Feb 15, 2023.

  1. Finnish Cafe

    Finnish Cafe Member

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    Few weeks ago I bought this 750 Seca whit blown engine, started wondering what would my choices be for a engine, since I'm not too familiar with Yamahas besides outboard engines, speakers and pianos. Tried to collect as much information I could and ended up choosing latest 900 engine that is supposed to be bolt on for the frame, 4bb. I was hovering over 4km Diversion engine too but decided that it would have been too severe frame modification to make space for tilted cylinders and all other surprises so I took the easy(er) way. Ordered 65tkm 91' XJ900F engine from Germany through ebay and carbs from Baboon in Netherlands. Carbs came last friday and looked to be in reasonable condition, they were from 92' model XJ900F. Just had time today to clean them, I glass blasted them gently from outside and washed 2 times with ultrasonic cleaner before disassembling and then two times disassembled. They came out pretty nice. I blowed them dry with air and applied light coat of universal oil to every part, tomorrow I will get repair kits for them and assemble. Tomorrow will also be the big day, engine is arriving. Its gonna get busy now. IMG20230215180028.jpg
     
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  2. Finnish Cafe

    Finnish Cafe Member

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    Got the engine. It's in a bit of rustic condition, bike has probably been stored outside but nothing too crazy. Checked valve clearances first thing, they were all spot on in the middle of the values and everything i side looked clean and good as new. One of the front mounting bolts has seized in the eyelet of the engine and exhaust studs I need to change, little polishing / glass bead blasting for the casings, cylinders and head. Clutch, alternator and valve covers etc I have from 750 engine and they are in good condition. What about the exhaust primaries and collector, are they usable from 750? Tried to find their actual differences but nothing solid yet. Glass seems to be the way to go with engine casings, soda can't remove all the oxidization. IMG20230216181121.jpg
     
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  3. Finnish Cafe

    Finnish Cafe Member

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    Got some work done today. Blasted engine with 10kg of glass beads, it's still not show quality but it's for the road not the show anyway. I also realized today that the valve covers in 750 and 900 engines are different, so I had to clean that badly oxidized one that came with the engine. I didn't have time or patience to polish it to mirror finish but it's good enough, I still might paint it at some point. Carbs are also all done, it's not that far from the point of mounting in frame now. (Might even happen tomorrow.) IMG20230221204354.jpg
     
  4. Simmy

    Simmy Well-Known Member

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    the 900 cams have bearing supports at each end which makes the head wider.
     
  5. Finnish Cafe

    Finnish Cafe Member

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    Yeah, and 750 have wider space for cams. It's kinda weird that Yamaha has made so small design changes trough years, as the diversion era 4KM engine is visually still very similar even though it has more forward tilted cylinders and probably million other changes. Maybe it just is that when you build good engine once you don't need to do it again for 30 years or so
     
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  6. Finnish Cafe

    Finnish Cafe Member

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    So, engine is in the frame. Tight to squeeze it in but when it's in there it fits to every mounting point as it should. Looks big as it fills the space nicely, like it. When mounting these engines to frame it seems to be best to go in from right hand side as drive shaft is little less of a pain that way. C IMG20230223193418.jpg Changed few covers from old engine as well as stator which was in much better shape. IMG20230223184452.jpg 750 engines output shaft cover (?) Won't fit, ask me why I know. Breather case inside the cover has little extra space for bigger gear (I think). Other than that it seems to be the same. It's coming together nicely right now. 750 exhaust seems to fit 900 head, I think size of the primary pipes is same, or difference is about 1mm if any.
     
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  7. Finnish Cafe

    Finnish Cafe Member

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    Throttle cable is mounted differently in 900 engines Mikuni BS36 than 750 engines original Hitachis, so I had to build new mount from zero. I mounted piece of cut to form 20mm plastic under 2 mounting bolts of carbs 2 and 3 caps, then bolted a steel piece to that with cable mount. It's extremely tight spot, allmost everything needs to be made adjustable, here's a little video about that. https://youtube.com/shorts/PpzyYzqnH8w?feature=share

    Heres what she looks like now IMG20230225211611.jpg
     
  8. Finnish Cafe

    Finnish Cafe Member

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    And then there was life. Got her running last night, new engine sounds pretty good, no weird noises and responds to throttle nicely. Something wrong with petcock though, starves when I try to keep revs up, dries out filter. Plenty to do still, air box modification for bs36 carbs, gotta find some nice silencers, short conicals maybe. Then carb syncing, front end needs some attention too. But in Finland we have some waiting to do still before riding season, gotta keep busy.
     
  9. Roast644

    Roast644 Well-Known Member

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    Wow, great work on that bike! And an awfully short curve to go from "I don't know anything about Yamahas" to building something like that. The chopped frame is usually is not appealing to me, but I've seen a few in this scrambler style that look great. Very nice!
     
  10. Finnish Cafe

    Finnish Cafe Member

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    Thanks! I have some experience with bike building but it's mostly Triumph efi triple world where I feel at home. But building experience with anything always helps, I have been building crane trucks for last 12 years, that gives pretty solid backrest for solving building problems or at least courage to try to. We also have pretty nice workshop with crane, lathe, mill, press, mig, tig and stick welders, pipe bending possibilities, hydraulic hose crimping press, tire installing machine etc. So it's easier to work with hobby stuff if there is that rare spare moment to do so.
     
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  11. Finnish Cafe

    Finnish Cafe Member

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    Airbox situation. From 32mm to 36mm carbs not only the diameter increases but the spacing for the carbs too because 36mm carbs are larger every way. Carbs 2 and 3 are close enough to work without any modifications but 1 and 4 are too close to just connect with silicon hose and there's too little space between airbox and carbs to make any sort of bent pipe between so I needed to move holes in the airbox to the very edge of the box. IMG20230302175802.jpg I opened holes big enough to put 50mm silicon hose through it, and from inside the box I inserted 25mm piece of 50mm aluminium tube to the hose to lock it in place. IMG20230302175905.jpg
    To be sure I put a zip tie over the hose but it's really tight fit anyway.
    IMG20230302194251.jpg Openings for 1 and 4 I cut to the edge of the box, then inserted hoses through the flanges I made from 1.6mm steel plate, locked hose to flange with same 25mm alu tube ring and fitted them with polyurethane compound and 4 rivets each. IMG20230302201215.jpg I also applied some poly compound over the hoses where it goes through the box, even though it was so tight fit that it was 100% sealed fit before, overkill is the way. In last pic pipes are aligned with carbs pretty nicely, I still have to cut them to right length but that should be the easy part. Also some cleaning of the urethane is due and black paint after that, that thing doesn't look too good yet. Oh, got a 12Ah Exide AGM battery today too.
     
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  12. Finnish Cafe

    Finnish Cafe Member

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    That of course didn't work at all. There's not enough space for those pipes at all, I made the hoses longer and locked them in place with 10mm slices of that pipe outside the box so they won't back out when installing carbs. It wasn't easy but I got carbs in place and airbox mounted, don't really look forward doing that again anytime soon. ‍
     
  13. Finnish Cafe

    Finnish Cafe Member

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    It was carb syncing time today. I really don't know how much sync is altered with silencers as there is only 4-2 collector box in place now but I have to check it again after I get some pipes for her. I didn't have YICS plugged while syncing but eventually I got all carbs to run pretty close to the spec, 28.7-30kPa at idle.

    Also noticed that there's no charge at all, have to check that next. IMG20230305170012.jpg
     
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  14. Finnish Cafe

    Finnish Cafe Member

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    https://youtube.com/shorts/RNskq86Xr_o?feature=share She's charging now. Had to change alternator rotor, it was only thing charging related that I didn't change from old motor. Don't really know what could be wrong with it but now voltage is ok, lights on at idle 12.7V but jumps to 14.7V in about 2000rpm. Fitted headpipes and collector like they should be, except collector holding bolt in the middle. Ordered cheap Emgo Slashcuts for mufflers, if they are too loud I'll bite the bullet and order MAD exhaust's Scrambler mufflers, they would look great but cost over double. I still need to get oil cooler and rebuild front end but getting there. Petcock works now too.
     
  15. Finnish Cafe

    Finnish Cafe Member

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    IMG20230309185300.jpg Slip ons came today. Surprisingly they aren't stupidly loud and have pretty nice low tone and little distortion up high. Fixed oil pipe leak too and couple of oil coolers are on the way. (Needed one for Triumph too.)
     
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  16. Finnish Cafe

    Finnish Cafe Member

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    https://youtube.com/shorts/-kicwiEnq_I?feature=share took her for a spin. In over 10cm of fresh snow it wasn't long one but maiden voyage anyway. It feels like that could be really fun bike to ride when I get front suspension done and weather stops that constant nonsense with white stuff.
     
  17. Finnish Cafe

    Finnish Cafe Member

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    Piece of junk aluminium might find its place as a side panel, looks too heavy now but if I come up with something to lighten the look it might stay. IMG20230314191801.jpg
     
  18. tobar

    tobar Member

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    bike looks great..run it without the alu as in the previous pic
     
  19. Finnish Cafe

    Finnish Cafe Member

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    Thx.
    I would but there's reg/rec and fuse box etc. there, they need some protection from rain. I will cut that cover smaller so it leaves frame tubes visible and might soda blast edges to matte finish too, lwe'll see how it goes down. New oil cooler is now fitted, that in my opinion looks really nice for a chinese universal part, some hoses to do next. IMG20230321215828.jpg IMG20230321215802.jpg
     
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  20. Franz

    Franz Well-Known Member

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    Oil cooler looks great. Are you using a different adapter plate at the crankcase?
     

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