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1982 XJ750 Maxim Tach woes...

Discussion in 'XJ Technical Chat' started by Bilfknmury, Aug 20, 2018.

  1. Bilfknmury

    Bilfknmury New Member

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    Good afternoon, I recently bought this bike from my dad (bought it new) and am down to the last gremlin that I need (want) to take care of.

    I have been searching through the forums looking for a solution to my problem but coming up short. My 82 750 is having tach issues. After startup and riding around town the tach will hover around 3500 RPM and when I get on it it will jump to about 4500 RPM. After shutdown the tach will slowly lower back to 0 RPM.

    Everything I have found so far has been checking for corrosion on the wire leads in the back of the tach. This doesn't really fit the symptoms but I checked anyway. I cleaned the terminals with scotchbrite and put some anti-corrosion spray on them so I know there is a good connection.

    I'm kind of hesitant to tear apart the gauge cluster and tach so I figured I would tap the resources from this page to figure out what to look at next. I would hate to tear into anything that doesn't need it.

    Thanks! 20180818_155147.jpg
     
  2. Toomanybikes

    Toomanybikes Well-Known Member

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    Dust and moisture along with spider webs will cause that. Taking it apart is not a big deal, just take your time. Q-tips and computer air blaster and go at it.
    Ps very nice bike
     
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  3. Bilfknmury

    Bilfknmury New Member

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    Thank you! I'll give it a whirl and report back.
     
  4. XJ550H

    XJ550H Well-Known Member Premium Member

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    Nice Helmet too
     
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  5. Bilfknmury

    Bilfknmury New Member

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    I was able to get it apart today. It looked super clean. I fully removed the tach and the needle moved fairly smooth. I put some lube on the shaft towards the back of the tach and actuated the needle a few times. I put everything back together and started the bike with no movement. After the bike warmed up I revved the engine and the tach vibrated a bit to just under 1k RPM. I tapped the side of the cluster and the tach bounced to about 1200 RPM. After it got over the 1k hump it seemed to be acting normally. After shutdown it took a while for the needle to go from 1k to 0. I'm not sure why it would be sticking at this point.

    I guess if it works above 1k I can live with that.
     

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  6. Jetfixer

    Jetfixer Well-Known Member

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    One other thing to check is remove the fuel tank and check the connections to both coils and check the grounds as well .
     
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  7. Bilfknmury

    Bilfknmury New Member

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    Thanks! I think that will be a winter job for me.
     
  8. Rooster53

    Rooster53 Well-Known Member Premium Member

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    That sounds like a mechanical defect in the meter itself, and not the control electronics mounted to the meter. Did you notice that occurring when you had it apart and were exercising the meter movement?

    Not easy for everyone to do, but if you have access to a variable power supply you can disconnect the meter from the circuit card assembly. Connect the power supply to the red and black wire and the scaling should be close to 210 mv / 1000 rpm. Even if the scaling is different (not sure about models and differences) just starting from 0 volts and slowly increasing the meter movement can be observed and verified to move smoothly as the voltage is increased.
     
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  9. Bilfknmury

    Bilfknmury New Member

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    While it was out it had a slightly harder movement when activating the needle. I'm not sure if that was just in my mind though because it's very easy to move.

    Unfortunately I only have access to a variable 28v DC power supply.

    The past few days I have been riding the bike to work and it is a 60 mile trip each way. Only on startup after it has been sitting do I have to give it the love tap to get it to move above the 1k rpm. After that the tach works normally.

    The only other little thing that I just noticed yesterday evening when I pulled into my driveway. I had the bike in neutral and I noticed the green light surging from a little dimmer than normal to full brightness. After I noticed this I looked at the headlight and I could notice it slightly changing brightness as well. It stayed full bright when I kept it at a higher rpm. I dont know what these bikes have as far as alternator or equivelant. Doesn't the tach get its power directly from that? Could it not be producing enough power at idle for the initial startup or is that just normal with an older bike?
     
  10. Rooster53

    Rooster53 Well-Known Member Premium Member

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    It is normal for the lights to increase in brightness as the engine is reved and the alternator voltage increases. It is actually a reassuring circumstance indicating that the charging system is functioning.

    Yea it takes so little pressure to move so you are not likely to feel any defect. However, if you move it up to 5K or so and let go it should move back to zero smoothly without stopping or slowing.
     
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  11. Bilfknmury

    Bilfknmury New Member

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    That's good to know about the charging system!

    I'm pretty sure there was a distinct "catch" about the 1k mark if the needle was allowed to drop. If that is the case the only way to fix that is to replace the tach?
     
  12. Bilfknmury

    Bilfknmury New Member

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    I just checked, they are pretty proud of the tachs on ebay...
     
  13. fiveofakind

    fiveofakind Well-Known Member

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    Nice bike...I have one exactly like it......I am the original owner like your father........it appears he took very good care of it......

    Enjoy it....
     
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  14. fiveofakind

    fiveofakind Well-Known Member

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    when these bikes are dialed in, they are screamers down the road.......they will put a smile on your face......harley riders cut their teeth on bikes like these....

    Just one thing to add......if those are the original brake lines......you may want to think about changing them over to steel braided lines.....you dont want to trust your life to rubber braided brake lines...just sayin......
     
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  15. Bilfknmury

    Bilfknmury New Member

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    That is on the list of things to do. I don't think they are original but I want the steel braided lines just for the worry free factor.

    So I've put over 500 miles on the bike in the past week and a half. After the love tap to get the tach going on the first start of the day it will work after a brief shutdown for gas or whatever.

    There is one more thing that is worrisome. After I got the bike back from the shop it started up and ran like a champ from the get go. about 5 days in it starts right up with a little choke and I let it warm up for about 5 min. It takes a little bit of throttle work to get it going for the first couple of miles and I definitely don't have full power. After a few miles it starts running like nothing happened and it's fresh from the factory. it sounds like something would be clogged but if that was the case why would it go away when the bike gets to operating temp?
     
  16. fiveofakind

    fiveofakind Well-Known Member

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    have you had your bike's carbs synchronized & plugs colortune tuned ???
     
  17. wgul

    wgul Active Member

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    Yes colortune did wonders for me. It used to take 5 or so minutes to warm up in the summer. The po had the carbs too lean. Now it warms up much faster.
     
  18. Bilfknmury

    Bilfknmury New Member

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    Once I bought the bike I had one of the local shops make it running since it had been sitting for the past 3 years. I'd hope that they would have synchronized the carbs after cleaning them. I'll have to search the forum when I get a chance as I'm not familiar with the colortune. I'd have to guess they didn't do that as nothing was mentioned on the work order.
     
  19. Bilfknmury

    Bilfknmury New Member

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    *Update*

    I took the bike out of the garage yesterday morning and when I went to start it nothing happened. Ends up being a bad battery. (My dad bought it new 3 years ago, used it for a couple months, then put it on a tender. Age just kills batteries.)

    I threw the new battery in the bike and it started like a champ. It took care of the difficult idle when cold and it just seems to run better now.

    Anyway, just wanted to update the fix since I like resolutions.
     

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