'81 Maxim 650 - Popping, backfiring with gas + carb cleaner

Discussion in 'Motorbike Technical Discussion' started by donotemailme, May 23, 2007.

  1. donotemailme

    donotemailme Guest


    I imagine this is somewhat common, however I'm a new rider and I'm
    worried I've done some harm to my 1981 Yamaha Maxim 650.

    Background, with all my mishaps included:
    When I bought the bike a couple of weeks ago, it appeared to run great
    - started up every time, accelerated smoothly, no apparent issues.
    Then, I went out of town and didn't ride the bike for 6 days. I don't
    know how old the gas was that was in the tank - I'm assuming it was a
    couple of weeks old. Well, I left the petcock in the PRI position,
    not realizing that I have a different type of petcock than the bikes
    we trained on in the MSF class and I now know that the PRI position
    lets gas flow freely into the engine, which can cause big problems in
    rare circumstances but I'm sure is always a bad idea when you have old
    gas in the tank. When I tried to start it up the first time after my
    trip out of town, it wouldn't start. I tried for 10-15 minutes to get
    it going, but I then gave up. I returned two days later after doing a
    little research, sprayed a little bit of starter fluid into the air
    box, and the bike started up. I had to rev the engine a bit to keep
    it running, it would die if I let it idle with or without the
    choke. So, once I was confident that the bike would start, I
    drained the gas out of the tank, filled it up with fresh gas, and put
    about 4 ounces of B12 Chemtool Carb Cleaner in the tank. I then
    switched the petcock to 'On' (at this point I still didn't know what
    PRI meant), put a shot of starter fluid into the airbox, started it up
    and once it was warmed up it would idle fine. I then went for a ride
    around town, which consisted of mostly under-35 mph riding, just to
    get the carb cleaner working. It ran great for the first 45 minutes
    of the ride, but then suddenly performance went way down. It started
    stalling at stop lights unless I revved the engine, and usually when
    first accelerating from a stop it would not accelerate smoothly at
    all. There were frequent pops and the occasional backfire coming from
    the exhaust, and I was putt-putting along with the throttle cranked up
    pretty high. After about 50 feet of this putt-putting, something
    would catch, the RPMs would jump up to over 3K and it would start
    accelerating quickly. So, I put up with this the rest of the way
    home, which was about a 10 minute ride, and I then parked the bike and
    put the petcock back to the PRI position. The next day I read that
    PRI means prime, and it's bad to leave it in that position, so I set
    it to ON later that day, as soon as I could. After a few hours, I
    tried starting it up again to see if it would idle, but it wouldn't, I
    had to rev the engine to keep it running and there were still frequent
    pops and the occasional backfire.

    One other thing - I have receipts for quite a bit of work that the
    previous owner had done just a little over a year ago, including carb
    overhauling and new spark plugs.

    Are the symptoms I'm describing just indicating that the carb cleaner
    is doing what it needs to do or have I done some damage with my
    mistakes? I haven't checked if the plugs have been fouled, I plan to
    check that this weekend.

    I'm comfortable doing routine maintenance on my cars (fluid changes,
    spark plugs, brakes), so I think I can handle maintenance on the
    motorcycle, however big jobs like carb synchronization scares me. I
    have a manufacturer's maintenance manual for this bike.

    Thanks in advance for any assistance.
    donotemailme, May 23, 2007
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  2. donotemailme

    mk10 Guest

    No, you didn't do anything that would totally screw things up--things were
    already screwed up. You may have thought that everything was OK, before you
    worked on it, but you were wrong--things were not OK. As another poster
    pointed out: gunk, dirt, varnish, and your choke circuit were all
    compensating and making you think that everything was normal.
    If it were me I'd clean the carbs the proper way, not the "spray some stuff
    down into the carb throats" cheat method. You'll need to pull the carbs off
    the bike and take them apart if you really want to get them clean. All it
    takes is one small piece of crap in just the right spot inside to make the
    bike run rough. And, after you do this disassembly/reassembly job, you won't
    think that synchronizing carbs is such a big deal anymore.
    mk10, May 24, 2007
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  3. First thing is to determine how well it ran originally, and for how

    Easy starting? Smooth idle? Free revving? OK to redline? Running fine
    for a couple of hours? Had the bike been in regular use before you got
    it? If the answers to all these questions are yes, then it's not
    gummed carbs.

    It all sounds like carb trouble, but if the bike was running fine
    before, then the carbs won't magically have gummed up in just six
    days. No way. Could be a coil starting to go, for instance: I've had
    that happen twice on old Jap bikes, and the first time it fooled me
    into thinking it was a fuel problem, because the symptoms were so

    What makes you think the fuel was only a couple of weeks old? If it
    was, and the bike had been in regular trouble-free use, then it's not
    gummed carbs.

    If it had been laid up for some years immediately before you got it,
    and you only ride it for five minutes then maybe it is gummed carbs,
    or quite possibly crap from the tank in the carbs (though I'd have
    thought the fuel filter in the tap would catch it)..... And you've
    been given two reasonable answers as to what to do next.
    chateau.murray, May 24, 2007
  4. donotemailme

    donotemailme Guest

    It's true there's a lot I don't know about how this bike was treated
    in recent years. I do know that the guy I bought it from quit riding
    because his knees went out on him. Jan 06 he put about $1000 worth of
    work into the bike - including carb reconditioning, tune up, battery,
    new fork seals, front brakes, new float bowl gaskets, new air
    filter. In April he got the carb needles replaced. I believe he
    rode it at least part of last season after that. I'm checking with
    the seller now to see what more he can tell me.

    I want to burn at least through this tank of gas with the carb cleaner
    in it, then if it isn't any better I'll look into the suggestions that
    have been made. Problem is, the engine consistently stalls when
    idling, and then it won't restart without some starter fluid. I'm
    thinking I don't want to ride around with starter fluid strapped to my
    bike, so since I've started having trouble I've only ridden the bike
    around the neighborhood. It's become quite a pain to go through this
    tank of gas. Is there anything I can do to keep the bike from
    stalling other than holding on the throttle whenever it's supposed to
    be idling?

    I think
    donotemailme, May 25, 2007
  5. donotemailme

    ottguit Guest

    Why not just drain the Gas out of the tank, either thru the petcock,
    or take the tank off and empty it? Does sound like it might be Coil
    ottguit, May 28, 2007
  6. donotemailme

    lugnut Guest

    I believe your bike has a pionts and condensor ignition.
    The symptoms you desvribe are classic for a failed/weak
    condensor. I would start there instead of screwing around
    with the carbs since it started the problem in a relatively
    short period of time and running.

    lugnut, May 28, 2007
  7. Wrong.
    The Older Gentleman, May 28, 2007
  8. donotemailme

    lugnut Guest

    Now I feel right at home. My wife must say that at least a
    dozen times a day. Then, she usually proceeds to tell me
    how stupid I am, why, and exactly how to do it next time!

    lugnut, May 30, 2007
  9. donotemailme

    donotemailme Guest

    I'm still having some trouble getting my Maxim 650 to run, and I've
    gathered some more data that I'd like to share. I hope to be able to
    devote a good number of hours to the bike this weekend.

    I adjusted the idle screw (my service manual calls it a 'throttle
    screw' - item #7 at http://www.bikebandit.com/partsbandit/showschematic.asp?dept_id=650201)
    so that it would idle without holding on the throttle. That worked
    pretty well for a few days, I let it idle for 15-20 minutes a few
    times just to see if it could do that without stalling, and I also
    went on a very short ride. In my adjustments of the screw, sometimes
    the idle would go quickly up to 3000+ RPMS, and I'd cut the engine and
    close the throttle screw a bit more.

    Then this past Friday I went to fire it up to take it for a ride, and
    it started up fine. I held on the throttle a bit to get it warmed up,
    around 3000 RPMs. It started sputtering a bit, then there was two
    very loud backfires - like gun shots. Then, it stalled. I then tried
    to start it again, and it wouldn't start. I sprayed a little starting
    fluid on the air filter, tried starting it again, and it backfired
    again. This time I saw fire coming from the air inlet, it appeared
    that the starting fluid had caught fire. No big deal really, it
    burned up and the fire went out. The air filter was not damaged,

    So I let it sit until today, and I tinkered with it a bit. I made
    sure that the battery was fully charged (I've got a battery charger
    that I left attached to the battery while I was cranking the engine)
    and adjusted the starter cable so that it was functioning properly.
    Figuring that the only thing I really did was fiddle with the throttle
    screw, I turned it all the way counter clockwise and then started
    turning it back clockwise slowly until it would start. Finally, it
    starts now, if I hold the throttle while pushing the start button.
    Sometimes, although not regularly, when I'm trying to start it it will
    backfire very loudly. Currently, I have to hold the throttle to keep
    it from stalling. That makes sense to me, since I don't have the
    throttle screw adjusted so that it will idle, only so that it will

    One thing that happens consistently is that once it's running and I'm
    holding on the throttle at about 2000 RPMs, after running at that
    engine speed for 1 to 1-1/2 minutes it will just stall. No backfire
    or anything, it just simply dies on me. That has happened regardless
    of whether or not I have the choke on.

    So tell me, I have to clean the carbs this weekend, don't I? What
    else should I check?
    donotemailme, Jun 7, 2007
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