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Jerrod's 82 Yamaha XJ-750 Maxim

Discussion in 'XJ Technical Chat' started by Jerrod Robinson, Sep 22, 2020.

  1. Jerrod Robinson

    Jerrod Robinson Member

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    You may be right, didn't pay anything for the XJ, so I have no problem fixing it. And the CB isent stock, has a cam, and a few other things goin on. Not saying it's some Superbike lol, but it outran a 104c.i Harley heads up, which I believe is 1700cc. It does ok. But yes the XJ is ment to be a bit of a step up in power.
     
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  2. Jerrod Robinson

    Jerrod Robinson Member

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    It has no trouble starting or idleing, no smoke of anything that would indicate the rings are gone, besides acceleration being kinda low. Was gonna see how it respond's to a functioning exhaust and go from there.
     
  3. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    As suggested above, a compression test is the best place to start with any old bike you aren't familiar with. Personally, for me it's a pre-purchase prerequisite due to the high unavailability of things like pistons and rings for a lot of old bikes. I don't want to buy into a mechanical dead-end.
     
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  4. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    You sure the inner cable isn't broken in the middle somewhere, and only "catching" itself enough to spin backwards? The other possibility is that the tabs on the drive are mashed and not catching except when you rotate the wheel backwards. There's really no way to put the speedo drive together backwards, as long as it's on the left side of the wheel its innards should be turning the proper direction. The Virago unit should fit fine, it has the same part number.
     
  5. a100man

    a100man Well-Known Member

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    Compression test will help diagnose low power. If the results are poor you then need to see if it's bores (worn or stuck rings) or valves (burnt, bent or poor clearance adjustment). Have you checked air filter?
     
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  6. Jerrod Robinson

    Jerrod Robinson Member

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    Yeah, I didn't think reverse assembly was even possible, just didn't make sense. It is on the left side, and the cable is intact. The drive on the Virago seems to be the exact same part, unless the axels are different sizes it should line up. Gonna give it a shot I guess.
     
  7. Jerrod Robinson

    Jerrod Robinson Member

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    At the end of the day, I have a speedometer app on my phone that works well. I would just like to have a fully functioning Bike.
     
  8. Jerrod Robinson

    Jerrod Robinson Member

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    Yeah, someone has been screwing around with this speedo drive for sure. The screw holding the cable in place is completely stripped. Looks like I'm gonna have to take an easyout to it.
     
  9. k-moe

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

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    Soak it really well in PB blaster first. It's exposed to a lot of wet and grime.
     
  10. Jerrod Robinson

    Jerrod Robinson Member

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    Will do.
     
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  11. Jerrod Robinson

    Jerrod Robinson Member

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    Just bought an 82 750 Maxim, it runs decent but is down on top end power. Doesn't have much trouble getting up to speed, or maintaining it. But feels very restricted in the top of the RPM range. Feels normal at around a quarter throttle and under, But just feels restricted above that, like it's not getting enough of something. Original thought was high speed jets were dirty. But now something else has started. After around 20 minutes of riding, the Bike starts backfireing hard, and then stalls. Was able to choke it, and get a little further up the road. But I made the mistake of stopping, and it would not start again for a half hour. But then started and ran fine, like nothing had happened. I cleaned the floats and jets, and the 2nd carb from the left was much worse then the others, it was black in the intake side where the air filter clamps. Im assuming this is the cylinder that is backfireing. Has anyone ever had something like this happening?
     
  12. Timbox

    Timbox Well-Known Member

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    Sounds to me like you have to go through the fuel system. If is was not running well at first and then as you were riding got worse, sounds to me like junk in fuel. Top right of every page is a icon, click it and start to read up about these bikes.

    https://www.xjbikes.com/forums/threads/tech-topic-the-real-costs-of-maintenance.14581/

    Always making sure the gas tank is clean, you don't want to clean the carbs all up and then have them get full of crud again from the gas tank. Good luck and keep us posted.
     
  13. Jerrod Robinson

    Jerrod Robinson Member

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    No, the tanks clean, Not that simple. It's more Systematic then that. Starts after the same amount of run time, everytime. And it always takes the same amount of time to start running again. I would say the Coils, but the previous owner installed a brand new set a week before I bought it. I don't believe the lack of power, and the stalling are connected. My lack of top end power, is either the fuel line I ran is too small, or my air intake is clogged up. And since the plugs claim it's running rich. I'm gonna go with the intake being clogged.
     
  14. Jerrod Robinson

    Jerrod Robinson Member

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    I also considered the fuel could be boiling, becuse the petcock is very close to the head. But I have a clear fuel line, and it is not. Also the prong for the prime tube on the petcock has been broken. I'm not sure what kind of issues that could cause?
     
  15. Jerrod Robinson

    Jerrod Robinson Member

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    Yeah, I'm on a non running period right now. And all it does is crank and backfire, so I know it's getting spark. And after the correct amount of time pass's it will fire up like there was never a problem, and run fine for another 20 minutes, then do it all over again.
     
  16. Jerrod Robinson

    Jerrod Robinson Member

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    And thank you for the link, I am new to the XJ. But not new to classic bikes. My first ike when I was 15 was a 78 Yamaha 650 Special. Owned a CBR-600 for a few years, and became a Dad so I decided to slow down a bit. This past summer I did a frame off restore on a 74 CB-750/4 that involved completely rebuilding the top end. It's my daily now. But I've wanted an XJ since I had my XS, and that leads me here. I did get the Bike for dirt cheap, but I have good cash on deck to get it going again. Rebuilding old motorcycle's has become a big part of my life I suppose. I enjoy it. This bike is a runner, it is Machanically sound, something weird and unpresadented is going on with it though, the last guy couldn't get it figured out. But Im going to.
     
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  17. Jerrod Robinson

    Jerrod Robinson Member

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    And if someone has experience that I do not, I am very interested in that knowledge.
     
  18. lostboy

    lostboy Well-Known Member

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    I had a pick up coil fail only when hot. When it first failed it had a big backfire just as I pulled in to the drive way. I was lucky that it failed then. After it cooled down it would start then after a bit of time it would stall. At that time the pickup coil would not show continuity.
     
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  19. tabaka45

    tabaka45 Well-Known Member

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    Sounds like it is starving for fuel after a few minutes. Are you sure the gas tank cap is venting properly? I assume the valves have been adjusted. I had a situation where my bike would sometime shut off when I hit the brakes and wouldn't start for 10 minutes or so. Turned out to be bad alternator brushes. That reduced the voltage so much that at idle, or very low rpm, there wasn't enough voltage to fire the plugs and lights at the same time. You might check your voltage. Should be at least 12.5 at idle and close to 14 at 2500-3000 rpm.
     
  20. Jerrod Robinson

    Jerrod Robinson Member

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    See and that was my original thought as well. But previous owner said he just replaced the coil, showed me the Receipt and the coils look brand new. I suppose he could have bought a bad set, I think it's unlikely. But possible I suppose.
     

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