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Yet another Carb issue...

Discussion in 'Central US and Canada' started by Dbennett, Jul 18, 2017.

  1. Dbennett

    Dbennett New Member

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    A little background… I have an 81’ Maxim 650. Last year I swapped the original air box for pods, but didn’t really notice much difference in the performance of the bike. The bike ran fine with plenty of power and throttle response. When I took the bike out of storage this season there were a couple of noticeable problems. The bike misfired until the engine was warm and there was a deceleration backfire. The misfire was tracked to a cracked coil and both coils were replaced along with new spark plugs and an oil change. After that the bike ran fine, good acceleration and plenty of power; except that pesky backfire. I think I tracked the backfire to badly cracked intake boots, so I replaced all four with new. Also, because I hadn’t cleaned the carbs in the last couple of years, I decided to give them a good cleaning. I did a complete carb breakdown with the exception of the butterfly settings as the sync seemed to need little adjustment. I cleaned every orifice thoroughly with cleaner and compressed air. Replaced the fuel/air needle, spring and O-ring. Checked the float needles and checked all the floats to make sure they were in spec and looked over all four diaphragms to make sure there were no holes or tears. After reassembly there is a noticeable difference in the sound from the exhaust, the engine stumbles when accelerating and bike has lost the pep that it had before. I set the fuel/air mixtures using a colortune plug but the bike can still be a little hard to start at times. Any suggestions on what I could be missing or something I may have overlooked?
     
  2. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    1) check valve clearances
    2) check fuel levels in the bowls
    3) synch the engine using a vacuum gauge set
     
  3. Dbennett

    Dbennett New Member

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    I have already set the fuel levels in the bowls to spec, so I don't think that is the issue. As for checking valve clearances and syncing the carbs, I opted to not do that until later as the bike was running good before the cleaning with the exception of the backfire. The bike had plenty of power, great throttle response and was overall easy to start before I cleaned everything. That's why I didn't alter the sync adjustment screws; I figured they might need a slight adjustment, but nothing major. If there was a major issue with the valve clearances, I would assume that it would have been noticeable before the cleaning. i.e the bikes problems would still be present after the cleaning. My original deceleration backfire is now gone; I assume because of the intake boot replacement and I am left with new issues. I feel that there is something out of place with the emulsion tube or needle that would cause my problems. But that's an assumption
     
  4. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    Out-of-synch condition will cause all sorts of odd issue that will appear to be due to some other cause.......
     
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  5. Dbennett

    Dbennett New Member

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    I can understand that. But if the bike was running "fine" before the cleaning, and the sync adjustments were not touched during the cleaning; could it not be assumed that out-of-sync carbs is less likely the cause of the issues? I'm not saying that I won't get them synced, I'm just trying to get as many possible ideas to look at when I get a chance to work on my bike again. Hopefully this weekend :)
     
  6. Stumplifter

    Stumplifter Well-Known Member

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    Dbennett- I mean no disrespect, but carbs are rather finicky, especially when you have 4 that need to work together. Your thought that they were good before and then you removed and cleaned them and nothing should have changed is wrong.

    Did you do a bench synch after cleaning the carbs? Did you break the rack when you cleaned the carbs? You are inferring that you did not because you didn't want to mess with the butterfly positions. That is a shortcut you should not have taken, now you should take it apart and put in new throttle shaft seals... cause those old ones at the end of the butterfly shaft are probably 30 some years old. If you broke the rack then chances are good that even though you didn't turn them, that the synch adjustment is not what it was before.

    It is time for Church, and then check your valve clearances before you put the rebuilt carbs on. Then do a running synch, colortune and one more running synch.

    Trust and believe the running synch (as Chacal mentioned) needs to be spot on. Ask me how I know...
     
  7. Dbennett

    Dbennett New Member

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    Stumplifter- No offense taken. I am not that familiar with motorcycle engines so I tend to look at as many different ideas of what the problem may be so that I don't miss anything. I'm not in that big of a hurry this late in the summer to get the bike on the road, more get it ready for next season. To answer your questions, I did not break the rack, but rather checked the butterflies as closely as I could to see if there was much of an issue. I used the "drill bit" method making sure both bits used was the same diameter using a caliper. Also, I examined all four using the port on the upped side of the carb chamber as a reference point. As I said, because the only problem I had was a deceleration backfire, I had not touched the sync adjusters and I bench checked the positions, I opted not to break the rack. At this point in the summer, and with starting a new job in two weeks; I might just wait and do a complete tear-down of the bike. I was thinking of starting a bobber project or maybe just get the bike repainted. I tried to get the bike synced this weekend, but I am having trouble finding someone with a gauge. I will keep looking though...
     
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  8. Stumplifter

    Stumplifter Well-Known Member

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    Great - sounds like you know what should be done and just need the time to do it (boy don't I know that scenario).

    You can make a home-made 2 bottle manometer for pretty cheap and it will work, just takes more time and patience for synching. Having something like the Morgan Carbtune makes life easier.
     
  9. cgutz

    cgutz Well-Known Member Premium Member

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    I built a simple and cheap 4 carb manometer which works very well.

    I posted about it: http://www.xjbikes.com/forums/threads/carb-synch-manometer.114770/#post-573818

    Follow Chacal's advice above. If you have done those 3 things and still have problems, proceed. Otherwise you might be misdiagnosing and you may end up creating other issues that aren't there. At least in my experience...:)
     
    Last edited: Aug 7, 2017
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