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'82 Seca 750 Refresh Project: Now Scooby's Bike

Discussion in 'XJ Technical Chat' started by Nuch, Aug 24, 2019.

  1. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    What you are seeing is the oil from under the shim squeezing out and changing the measurement. Being 0.01mm loose is not an issue at all. That will tighten up over time, so leave them in place as-is.
     
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  2. scoobydew

    scoobydew Member

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    All right! and just like that the engine is done. Minus starter motor and what not. Thanks to you guys. Still a long way to go but it's coming together. Also posted a pic of valve specs here in case for some reason I lose that sheet, the specs will always be here.
     

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  3. Franz

    Franz Well-Known Member

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    Did you find out what was causing the play on the rotor at the ignition pickups end of the crankshaft?
     
  4. scoobydew

    scoobydew Member

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    The play was caused by the chain slackening and tightening back up because the intake cam wouldn't turn past a certain point.

    Here's how she's looking so far. Left off a couple of things for cleaning off the rust. The fenders were very rusted up. Just finished the forks with some fresh fork oil.

    One question. For the U-joint. because there's little space between the engine and the swing arm under the gaiter boot. How am I supposed to torque the U-Joint bolts? I can't get a torque wrench in there. Only a S-shaped wrench which I used to remove them.
     

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  5. Dave in Ireland

    Dave in Ireland Well-Known Member

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    You can get an offset short spanner that will fit a torque wrench.
    Or you can just do it the same way 99% of others do and give them an adequate tightening.
    One grunt and NO fart.
     
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  6. scoobydew

    scoobydew Member

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    Hey guys,

    Has anyone bought the MAC Performance aftermarket exhaust for 1982 XJ750 Seca? The fitment said center stand retention. The pipes are a bit long and they take up the passenger pegs, which is fine by me. I don't take passengers.

    But I am upset that when I pull up the center stand it hits the exhaust mufflers. I'm pretty sure they're installed correctly but for all I know, I could have f'd it up. If anyone has any insight I'm all ears! I would love to keep the center stand. It's great for working on the bike.
     

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  7. Fuller56

    Fuller56 Well-Known Member

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    Here is a picture of a 750 Seca I recently traded off that had the MAC 4 into 2 system. If you can zoom in on the muffler mount you can see it was to the factory muffler mount hole and behind the foot peg mount. I don't think you have yours all the way forward yet. Your muffler has the bracket for the center stand to rest against when retracted, the muffler just is not positioned quite right yet. I did not install this system so I cannot give more info than what shows in the pic.
     

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  8. scoobydew

    scoobydew Member

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    Thanks for the help! I feel like a clown haha
     
  9. Minimutly

    Minimutly Well-Known Member

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    Like this - it's precisely the way I do it, I had a colortune years ago, threw it out...
     
  10. scoobydew

    scoobydew Member

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    Hey guys,

    Final stretch, hooked up all electrical today. Tomorrow gonna clean out the rust from the fuel tank and finally start it up for the first time if not this weekend then next.

    Got some questions though. Does the aux and headlight ONLY turn on when the motor is running? Because everything else is good, brake lights, turn signals etc but the high beams and headlights aren't on with just the key in the bike. Also, does the kill switch also shut off the starter motor? I know some bikes still crank the starter with the kill switched on. Just so I don't turn over the engine by mistake cause I haven't put any oil in the crankcase yet.
     
  11. Timbox

    Timbox Well-Known Member

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    If I recall, yes the bike has to be running and see power coming to the rectifier before the lights will turn on. Saves all the CCA for the starter to start the bike.
     
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  12. scoobydew

    scoobydew Member

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    Today was first start. Engine turns over. Spark plugs fire but the engine doesn't start. Since I'm still cleaning the xj750 fuel tank, I used an auxiliary tank. Primed it and all. Engine still wouldn't start.

    So I sprayed a bit of engine starting fluid and I got it to start for a bit then it died out. I hooked up the vacuum and fuel hoses. But I'm not sure if the cylinders are receiving any fuel from the carb floats. But they received the spray from the starter fluid.

    I'm gonna check compression and gonna pull the carbs. Anyone have an idea why the carbs aren't pulling fuel to the engine?
     
    Last edited: May 31, 2021
  13. scoobydew

    scoobydew Member

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    Well, bad news.

    The rebuild project is fudged. Compression on cylinders was around 120 cold, #4 was around 90 until the engine warmed up and it jumped to 120. Sprayed a little oil and the compression for all cylinders shot up to around 150. Already a bad sign. Started her up and there's excessive white smoke and popping most likely meaning the piston rings aren't sealing correctly.

    I knew I should've re-bored the cylinders to the next size and gone with over-size pistons but I checked my specs, ring gap was good, cylinders were good, honed the cylinder and figured the stock size was good to go. I must've read piston to cylinder wall specs wrong.

    Not only is this a lesson for me but I hope it's a lesson for anyone rebuilding an engine, don't just double check or triple check but be 100 percent sure your readings are correct or you'll kick yourself in the ass later.

    I don't know when I'll tear down the engine again to re-bore the cylinder and change the pistons as this is quite a bummer and I'm left defeated. Especially due to the cost of rebuilding a bike. This was a nice quarantine project. But hopefully sometime in the near future I can get the engine running in perfect condition. Although it's no small job. All it most likely needs is a re-bore and a piston replacement.
     
  14. Minimutly

    Minimutly Well-Known Member

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    Hold on here a while now!
    There's probably nothing wrong with your engine - give it a little while before condemning it. Get it running on all four, starting and idling like it should, then warm it up with a few twists of the throttle to the red line, see the smoke stop. At any rate you need to give it some throttle to help the rings seal, if you can ride it, all the better.
    Do not despair yet!
     
  15. Franz

    Franz Well-Known Member

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    I was thinking the same, give it a chance don't pull it apart until you are sure. The gasses get between the piston grooves and the rings and push the rings onto the cylinder walls if am correct on the compression stroke. The smoke will be the oil burning off.
     
    Last edited: Jun 2, 2021
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  16. Minimutly

    Minimutly Well-Known Member

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    And also any oil used in the build of the top end goes into the exhaust and coats everything, slowly evaporating - or quickly depending on how hot and how quickly the exhaust gets.
    Please give it a chance.
    I re-read your build up - bad couldn't make out if you'd changed the damaged piston in the pic? You should have...
     
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  17. scoobydew

    scoobydew Member

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    Of course, all four pistons were destroyed which is probably why the previous owner had trouble with his idle and popping sounds. Though somehow his compression wasn't absolutely terrible. It wasn't good but not as low as I'd expect for pistons in that condition.

    I'll try it out again this weekend. From the short time I ran the bike since it flooded my backyard with white smoke, it held idle. But I heard some serious popping sounds which I'm not really too sure of the cause. I figured popping might be due to pistons from some blow by but reading up on it. It could be the carbs. I haven't touched the carbs since previous owner worked on them.

    I'll take a quick video on how the bike runs.
     
  18. Minimutly

    Minimutly Well-Known Member

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    Popping back into the airbox? This is weak fuel mix.
    Popping in the exhaust is a missfire, could be anything, but usually lack of fuel if the plugs and leads are good.
    Open the mixture screws half a turn each, before removing carbs. I'm not saying you shouldnt remove them, but I wouldn't without trying this.
     
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  19. scoobydew

    scoobydew Member

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    Tried starting the bike today but it didn't want to start up. Yesterday I was able to start it up no problem. Though I was using a different petcock. This cheap 8 dollar Chinese made petcock I found on Amazon to use to plug up the hole while I de-rusted the tank.

    Today I rebuilt the stock petcock and installed it to the tank instead of the Chinese one. When I started up the bike, it ran for about 5 seconds then shut off. Granted the tank was quite empty so I added a bit more fuel, turned the mixture screws out about half a turn each but it just didn't want to start. Despite rebuilding the petcock, I notice it leaks gas even though it's set to ON. I'm not sure if fuel is reaching the engine or not or if floats are flooded.

    I imagine the carbs are getting fuel as when I pull the fuel line, it's full of gasoline. Though since the petcock leaks, maybe the vacuum isn't pulling the amount of gasoline needed and floods the floats?

    Any ideas? Before I pull the carbs and make sure the floats aren't flooded with gas from start up attempts
     
  20. Franz

    Franz Well-Known Member

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    Do you have an auxillary tank for supplying fuel when you would be using vaccum gauges? Use that temporarily to see if it runs and keeps running? Missed the part about it keeping running with the Chinese petcock. Sort your rebuilt petcock.
     
    Last edited: Jun 6, 2021

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