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'81 XJ550 Seca Won't Idle

Discussion in 'XJ Technical Chat' started by Bob Beyer, Aug 29, 2015.

  1. Bob Beyer

    Bob Beyer New Member

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    I can't get my resurrected Seca 550 to idle...

    This summer my 16 year old son and I decided to resurrect my 1981 XJ550RH Seca. It had been sitting covered in the garage since 1991 with fuel in the tank. Horrible, bad idea I now know but that last time I rode it, I never thought it would be 24 years before I'd get back on it. Many repairs were needed. One the first thing we under took was to dismantle and clean the carbs. We left the carb bodies bolted to the frame rail, knowing that re-linking and re-syncing would be nigh unto impossible. We took apart everything else and kept all the parts for each carb carefully separated in labeled containers. We spent about 12 man hour carefully cleaning everything and reassembled them with great care. I made a point of cleaning but never adjusting the synchronizing screws. Many years had passed but I once had sync'd these myself with a set of carb stix and I thought that was as good a starting point as a bench sync. At this point we also had new plugs, fresh oil, and a new oil filter on board as well and had turned the engine crank by hand numerous times to be sure new oil was being pumped.

    When we had the carbs all back together, we put them back on the bike and reconnected the throttle and choke cables (newly lubricated). Everything worked smoothly. We fabricated a temporary fuel container with shutoff and connected it to the carbs with a new inline filter. After we filled the float bowls, I set the choke and turned the starter. It only took a few seconds for it fire up. A little rough at first but in less than a minute, it was running smoothly and would easily idle at about 1500 RPM. After it was warm, I adjusted the idle screw to about 1250 and it purred along at that setting for over 5 minutes. The throttle would respond crisply when I would blip it. It was like 24 years of neglect had been forgotten. Eventually, our little fuel rig ran low. We were satisfied that we were successful with the carbs, and that this bike would run and we could move on to other things like brakes. I shut off the fuel and let the engine idle until the carbs ran dry and it stalled.

    Several weeks later after I finished work on the brakes, some other issues, and a lengthy and thorough process to reclaim the fuel tank, I was ready to re-fit the tank and start the bike up again. I left the fuel filter in line for good measure. The tank has a new, after market, manual petcock that does not use the vacuum line. Therefore, I plugged the vacuum line from the No. 2 carb.

    I put about 2 gallons of brand new, high-test fuel in the tank. I set the choke, and turned it over. It started instantly without hesitation.

    Here's the trouble: I can't get it to stay running unless I keep the RPM above about 3,000 by setting the choke (which I know is also setting a fast idle circuit). Even after several minutes when the engine is well warmed up, if I try to back off the choke, it stalls. I've opened the gas cap to be sure it wasn't vacuum bound.

    On the advice of a friend, I pulled the brand new plugs and check to see if any had fouled. None had. I closed the petcock and completely drained all the float bowls to see if by some voodoo there was bad juice or schmutz in the carbs again. Clean as could be (so far as I could tell by catching the fuel with clean rags). I removed the fuel tank and carefully pulled the vacuum chambers off each carb again and removed the needle jets (main jets?) to inspect each one. Nothing found. Carefully reassembled everything. Still, same problem. It won't idle below ~3k RPM.

    What am I missing? Could this possibly be ignition timing and not fuel? I can't imagine what changed.
    I was hoping to get one ride in before the winter but fall will be here soon and the weekends are numbered.
     
  2. XJ550H

    XJ550H Well-Known Member Premium Member

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    did you set mixture screws to 2-1/2 turns out?
    when you drained the gas out of the bowls it probaly left enough fuel residue to clog up your pilot jets, mixture screw ports.
    the pilot jet circuit is what controls the idle, try recleaning that system.
    the enrichment circuit gets bike running when warm enough the pilot jets keep it running, main jets kick in around 2-3k rpm
    do not run bike out of fuel
     
  3. rocs82650

    rocs82650 Well-Known Member

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    +1 what Stump said. It's been 24yrs...imo also check the valve clearances, bench synch the carbs, check the float heights and use a unlit propane torch to test for vac leaks. When it's warm it should idle w/o the choke @ 2.5 turns from soft bottom on the mix screws. If it doesn't try turning them out another 1/8". At some point the carb shaft seals and wafer washers will need to be replaced. Hope this helps.

    Gary H.
     
  4. fiveofakind

    fiveofakind Well-Known Member

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    It seems you are a through bunch......

    Maybe you need to break the carb rack & replace throttle shaft seals & fuel o-rings.......after all it has been 30+ yrs.......why did you not break down the carbs all the way.........

    It is a must to go thru your carbs all the way 100%.....not 80% as you did ...........then bench-sync, adjust float levels, vacuum sync , & colortune plugs......period...no short cuts....

    Valve clearances must be within specs before syncing.....
     
  5. XJ550H

    XJ550H Well-Known Member Premium Member

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    you can only check the timing there is no adjustment for it.
    you may want to ohm out your ignition system just to make sure it is functioning properly.
    coils and pickup coils just to ensure it is firing on all cylinders.

    you do not need hi test for xj bikes.
     
  6. Bob Beyer

    Bob Beyer New Member

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    I appreciate all the advice and it's given me some things to consider.

    There are no mixture screws on these carbs. I was surprised by this when we serviced them. I was expecting there would be and told my son we'd need to keep careful track of the turns of any fast/slow mixture screws but there are none.
    To be sure, I just now pulled the bike out of the garage and looked again to see if somehow we missed something. We spent most of a day with the carbs apart, in our hands and laps. It if came apart with a wrench or a screw driver, we removed it. The only exception is that we left the bodies connected to the upper frame rail.

    The idea that I could have clogged the slow speed jets by running the carbs dry hadn't occurred to me but although that seems unlikely, I'll admit it's possible. The only way to access these again will be to pull the carbs back off the bike and remove the float bowls again. :-( Something I was hoping to avoid. I was probably being too cautious about leaving fuel in the system. In the back of my mind I had the thought that if we didn't finish this up before it gets cold, it could be the better part of a year before we'd return to it and given that leaving fuel in it last time didn't work so well, I was probably over compensating.

    With respect to breaking the carbs from the frame rail into separate units, there is very little advantage to this in terms of what can be cleaned on the carbs, only the mechanical linkages between them would be more accessible. These were completely cleaned and are working smoothly. The service manual sternly warns never to separate the carbs and I was heeding this advice. All other components were removed and cleaned including the choke rod linkage. I even removed the heat shield and the lower frame bar which also holds the main idle screw (it would be impossible to remove the float bowls otherwise). I completely cleaned the master idle screw as well. I backed it off exactly 5 turns, cleaned, turned it in exactly 10 turns, cleaned, backed it off exactly 5 turns. I left the bodies attached to the top, rear frame rail as advised by the Yamaha service manual. I would say I went through the carbs >99%. The spring loaded mechanism that links the throttles together looks like a mouse trap that could be difficult re-assemble if I were to break the carbs apart and as I said, the book strongly advised against doing so. I was able to completely clean all of this linkage anyway with it in place.

    Remember, post assembly, and re-installation, everything ran as smooth as can be, idle and all.

    As far as the valves are concerned, my secondary plan was to go through the valves again after it was running. I have done this twice, a couple decades ago. The first time required a few shims to be replaced, the second turned out to be an inspection as everything was still in spec. Although the miles that were accumulated since could have changed this, I don't thinking sitting still, no matter how long, would have a dramatic change on valve clearances. Even if I'm wrong, it wouldn't mean that the idle needs to be set at 3,000 RPM.

    I'm not familiar with the unlit propane torch techniques to test for vacuum leaks. Something about venting propane near heat and spark gives me some pause. However, I'm always open to new things so if someone can explain, I'll give it a try.

    If there are some mixture screws I have missed, I'll be happy to consider them but I'm not seeing them.

    With respect to ignition, I guess I was wondering of it was possible that some electronic component/circuit wasn't advancing/retarding the ignition for idle. Just imagining.

    Thanks again for the ideas.

    https://www.flickr.com/photos/bbeyer/20827343639/

    [​IMG]
     
  7. fiveofakind

    fiveofakind Well-Known Member

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    These are the Mikuni carbs right ???
     
  8. k-moe

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

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    There are indeed mixture screws, they are covered by brass plugs that have to be carefully drilled out and removed.

    http://www.xjbikes.com/forums/posts/23725/

    http://www.xjbikes.com/forums/threads/in-the-church-of-clean.14692/

    http://www.xjbikes.com/forums/threads/the-secret-life-of-carburetors.14751/

    You need to remove the pilot screws to fully clean the pilot circuit.
    You need to fully break the rack to replace the very, very old (and likely leaky) throttle shaft seals. Leaky throttle shaft seals will allow the bike to run beautifully sometimes, and like crap (or not at all) other times.
     
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  9. fiveofakind

    fiveofakind Well-Known Member

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    Bob,

    Pilot mixture screws from Hitachi 32 Carbs....you have Mikuni but you have pilot mixture screws as well

    [​IMG]


    Pilot mixture screws are next to Enrichment Plungers ( Cold Start )........yours probably have the plugs covering them still there....probably have never been removed.....

    [​IMG]
     
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  10. Bob Beyer

    Bob Beyer New Member

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    OK. So I stand officially corrected. There ARE mixture screws and I was oblivious to them.

    K-moe, thanks for the links. I took a quick look at all of them and it looks like some excellent reading. I will read all three threads before I go any further.

    Fiveofakind, yes, they are Mikuni, and thanks for the excellent pictures. There must still be brass plugs next to the enrichment plungers as I saw nothing that looked like screws there.

    Clearly I will need to pull the carbs back off and revisit them. Not what I was hoping for but if that's what it will take, then I have no choice. Given the amount of ugly crap that was on everything else, it's hard to imagine that the mixture screws don't also need attention.

    The valve cover gasket is leaking more than I'd like (which would be zero). I should probably replace that and since I'll need to remove the valve cover, I might as well gauge the valves again. Somewhere in my took box, I do have the special little Yamaha tool for holding the valve down in order to get the shim out.
     
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  11. rocs82650

    rocs82650 Well-Known Member

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    +1 Bob. You're a man of intellectual thinking.

    Gary H.
     
    Last edited: Aug 31, 2015
  12. fiveofakind

    fiveofakind Well-Known Member

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    As they say in the defense industry, nothing short of right, is right......

    and

    I quote Chacal,


    "The Gospel of Clean:

    But all of the above is actually the good news! Because now you've isolated the performance problems down to one specific component.

    But that's also the bad news. Because a proper rebuild and re-tuning of these carbs, while not mechanically difficult, can hold quite a few surprises for the un-experienced, the un-wary, and especially for the lazy or sloppy rebuilder.


    Our basic scripture in regards to carb rebuilding (or any other project, for that matter) is this:

    Do it once, completely, thoroughly, and correctly.

    Because: that's how you save yourself money, time, frustration, and aggravation."
     

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