gx31 honda 'sometimes' looses power

Discussion in '4-Stroke Engines' started by alexander, Jun 27, 2009.

  1. alexander

    alexander Member

    Hi folk, Alex from New Zealand here.
    I've got a problem with a honda gx31 engine that sometimes, especially when warm, looses power, sometimes to the point of just about having 'stuff all' left.
    But even when warm, sometimes it suddenly runs with full power again.
    That POWERLOSS goes together with a repeating "SWALLOWING SOUND", that sounds like the motor is not getting enough fuel or not enough air.
    Its not the sound that usually goes with a bad spark ('missing').
    It did that many times before, but usually settled back into running with full power again (even when hot) but now it happens more often.
    (The motor has never been overheated).
    Its not the airfilter.
    Its not the breather in the fuel cap.
    I took the carburator apart, cleaned it twice.
    I checked the sparkplug clearance and condition (i will next get a new plug to eliminate that possibility, just to be sure).
    I even checked the valve clearance.
    Could it be the MEMBRANE in the carburator?
    I looked at the MEMBRANE and the STEELDISK on it when it moves 'in and out' does so at an slight angle, not totally flat, which makes me think that it might sometimes open at that 'angle' (not getting the right amount of fuel/powerloss) and then suddenly go 'flat' (right amount of fuel).
    I will next order a new membrane (if i can still get one seperate).
    I also will try to run it without a FUEL FILTER in the tank just to see if the filter might be too dirty because the motor has done over 10000km/300 hours.
    That filter in the tank can't be cleaned because it is dense felt and has another superfine filter behind it, so after 300 hours i think it has done it's 'dash'. I will do that and get back to you regarding that.
    But i might be wrong with the fuel/air being the cause.
    Could it be the COIL or the distance of the MAGNETO to the coil .
    I moved that ones but hardly think that would cause such a problem, but i might be wrong, now that i say it out loud, i should realy change the distance and try again; .... does anybody know what distance the magneto should be from the coil; i adjusted it with minimal distance).
    I thought somebody might have had a similar problem and was able to solve it and can save me from 'sleepless' nights.
    Thanks a lot for your help. AL.
    Last edited: Jun 27, 2009

  2. Mountainman

    Mountainman Active Member

    thinking it's the membrane or ?? something gas related
    one cool thing about Honda
    they are easy to get hold of on the phone or e-mail
    they answer all questions and who knows better than them
    exactly what goes wrong with most
    most gas filters located in tanks can be blown out for cleanning
    Last edited: Jun 28, 2009
  3. fetor56

    fetor56 Guest

    Can't help u much Alex cos i've got a GX35 & their different engines...i've never had trouble with mine but it hasn't done nearly the K's/hrs as yours.
    You had a valid point about the fuel filter in the tank...at some stage it must reach the end of it's usefull life & thereafter start giving trouble.
    How do u mean u moved the coil in relation to the magneto...moved it how(removed & replaced)?
    I agree that it shouldn't make a noticeable difference(unless their way too far apart)
    The diaphram in the carb is also a known weakness(particularly over time) & different fuel additives(eg,ethanol) don't help.
  4. Esteban

    Esteban Active Member

    Are you using good, fresh gas in it ? I don't use any gas that is over a few months old.
  5. Old Bob

    Old Bob Member

    Per Honda service manual:

    Spark plug gap .024"-.026"

    Magneto air gap at flywheel .014"-.020"

    Clunk could be clogged with that many hours. If the power drops off and comes back thats a good sign the fuel flow is restricted.

    The metering diaphragm should be limp and no wrinkles.The pump diaphragm should also be limp and not stretched tight. You can get metering diaphragms seperate.
    (see attachment)

    Attached Files:

  6. alexander

    alexander Member

    honda gx 31

    Cheers mate, good idea, do you ring/e-mail honda in the usa or do they have a 'world
    0800 number'/e-mail, if the later, you wouln'd have the number handy? Thanks again, AL.
  7. Mountainman

    Mountainman Active Member

    alexander our motor biking friend

    you may wish to try this web site ??


    have a good day wished for you -- from -- Mountainman
  8. alexander

    alexander Member

    honda gx 31

    Hi mate, thanks for your reply, yeah eliminating the fuel filter issue should be easy: just using it without one should show if that is the problem.
    About the magneto/coil distance: I took the coil off without checking what the original distance was and put it back according to 'common sense' but as you see from the post below there is a definite distance tolerance that it should be within to function properly. Good to know, ehy.
    And that diaphram is a worry, i never liked diaghram carbys, even on my BMW. I should have a new one as a spare anyway to be able to replace when needed and to be able to use to eliminate the diaphram as a problem when something fuel/air related goes wrong....
    The fuel here in NZ is sh.t and that doesn't help at all, and some petrol station fuels here are better than others....
    'By the way', what is the different of the gx31 engine compared to gx35?
    And i totally agree with the weight issue and the ability to lift the bike over a gate, cause lots good tracks here have access trough locked gates and some bush tracks have trees fallen across them.
    On one 6 hour trip i had to lift the bike over logs so often that my arm hurt for days/weeks after....
    Thanks again. AL.
    Last edited: Jun 29, 2009
  9. alexander

    alexander Member

    Gas! good gas? no good gas!

    Sorry mate, no good gas where i am....but some is better than others....i think it would make a difference for sure but not as drastic a difference as what it is.
    But very good point.
    Cheers, AL.
    Last edited: Jun 29, 2009
  10. alexander

    alexander Member

    honda gx 31

    Thanks for that imortant information mate, a honda service manual would come in very handy, did you have to bye one or could you find one for free on the internet or from mates on the 'outernet' (and i will look on this forum to see if there is one somewhere....)?
    But your info should keep me going for now, especially since i am sure now that i should have a spare metering diaphram (and pump diaphram) anyway to replace when needed and to be able to use to help eliminate that as a potential problem.
    Your picture of the metering diaphram makes me also think that i can't remember that gasket that goes on top of the diaphram. Did i perhaps loose that when i pulled the carby apart the first time quite a wee while back, and didn't notice.... ('even monkey's fall out of trees' ha).
    That certainly could/would be the cause of 'the hard to find problem' with the fuel/air 'powerloss on/off'.... and an easy fix.
    Good on you, thanks for your help. Al.
  11. alexander

    alexander Member

    yeah man....

    Good site you gave me there, will keep me going for a while....got lots to check now, not the least looking for that mysterious gasket that goes with the metering membrane that i MIGHT have lost when i pulled the carby apart the first time long ago ('even monkey's fall out of trees'), that would be an 'easy fix' ....will need to check later....busy busy....hopefully less 'sleepless' nights to look forward to....
    Thanks again, have fun.... Al.
    Last edited: Jun 30, 2009
  12. pdxrhett

    pdxrhett Member

    Check your head gasket. I broke my gasket for my mitsu 49cc and it made that sound, and had near 0 power. It could be a loss of compression - i didnt read that you tested for it?
  13. alexander

    alexander Member


    Thanks mate, one should consider all possibilities....one of the good things about the gx 31 etc. is that it is 'forced air cooled' and doesn't depend on the air flowing past it trough the 'forward speed' like so many other motor bikes, cause that means even if you are going slow, like going uphill in 'summer', or stop/start in town (in summer).... it still gets enough cooling....
    my old (1970) bmw (motorbike) use to get so hot at times that i had to put an oil thermometer on it to make sure it wasn't getting too hot at times....
    I (gx31) go from sealevel to 400meters twice weekly and to over 800meters once every 2 weeks (with a 3 speed gearbox/hub) and i was worried the first time i did it in middle of summer in the middle of the day...., but the 'baby' just does it, again and again.
    i bet many normal 'aircooled' small motors would have given up long time ago....
    i have a compression tester and after 10000km wanted to know anyway what the compression is like and how much the motor is worn because the BIG QUESTION for me is still out there to be answered: how long does a 4stroke 30ccm motor last in a 'motored bike', especially when it gets the workout i give it.
    it needs to last at least 20000km otherwise i (and others) would have to replace it every year (no good). sofarsogood: no significant oil usage, no 'metal' ever in old oil, no obvious wear/pitting on visible valvegear, even the valve clearance is still within its tolerance etc. (having used only fully syntetic oil).
    BUT, you just never know...., even without a compression tester one could check a (cold) motor/headgasket with soapy water around the headgasket and look for bubbles forming....
    But, "you just never know" (did i say that before)....ones in middle of winter (long long time ago) my bmw crapped out, wouln't start and my mates and me spend the next hour or two (three) pulling the bike's electrics to pieces....and you know what it was....i (or somebody else) leaning on the bike had moved the (then usual) handlebar mounted (small and hardly used) 'killswitch' accidentally onto its off position....
    or the time dirt got IN BETWEEN the 'points' (no electronic ignition) and you look for everything else but THAT easy (but not likely) one....
    e.g.: a good way to look for (some) 'spark problems' (especially if it happens in rain) is to look at the coil/cable/magneto/sparkplugcap etc. in the DARK, then, any 'shortening' will be clearly visible....
    Hence asking MY MATES on this excellent forum...., because no matter how much you know or how many miles you have done on two wheels, THEY can give you just THE HELP you need, when you need it....
    (and i hope to be able to return it one day....)
    Thanks a lot everybody.... Have Fun.... AL.
    Last edited: Jun 30, 2009
  14. bamabikeguy

    bamabikeguy Active Member

    Name brand gas and, since they changed to 10% ethanol here, 89 octane gas DO make a difference...namely the Shell's and Amoco's have a consistency, while the discount types are mixed on site. (Too much water content would create a vapor lock effect.)

    I've noticed the difference right away when my old 87 octane started my engine a coughing, and talked to 3 chainsaw repair shops.

    Name brand blends and 89 octane is the recommendation they all gave me, it's what the professional loggers are doing nowadays. If your filters are good, that would take care of the "fuel" part of the equation.
  15. Esteban

    Esteban Active Member

    I like some of the older engines & as Bama said above, " There is a difference in gas." The old Ohlsson & Rice, Bike Bug types, & my K & S do not like cheap 87 octane. I use at least 89, & put in some " stabilzer ", that I also use for my boat engine. Runs better, & keeps the tiny carbs from getting clogged ! KEEP these little motors clean internally !!
  16. Old Bob

    Old Bob Member

    There is no head gasket on the Honda mini four strokes, the use one piece cylinder and head casting.

    Checking compression on a the mini four strokes won't tell you a thing with the compression release in place.It will always read like the engine is shot, thats how I bought a used GX31 for $17.50 shipped, the owner was told by an engine shop that the compression was low and the engine was shot.

    But you can perform a leak down test. A leak down test is better test of the cylinder condition anyway.
  17. loquin

    loquin Active Member

    Check to make sure that there's not something partially blocking the exhaust - these motors often have an anti-spark screen on the output of the muffler. Remove the two screws that hold the short chunk of exhaust pipe on the muffler output

    If you have loose piece of rust (for instance) that is periodically getting blown into (and blocking) the screen, it could cause your symptoms.
  18. alexander

    alexander Member

    Thanks for that, good info....with the fuelusage we have the price of fuel doen't make a difference so might as well use the best.
    Cheers, AL.
  19. alexander

    alexander Member

    "Leak Down Test"

    Cheers Bob, interesting to hear what you say about compression tests....i was wondering anyway because without being able to run the motor at a certain amount of revs (specified for compression test in service manual at 1200rpm) how can one do a C.test anyway.
    But what you say about a 'leak down test' sounds good.
    That sounds like 'seeing how much liquid' gets past the rings and cylinder....
    but could you perhaps explain more about that test and how it is done and how to evaluate the result. Cheers, AL.
  20. alexander

    alexander Member

    No sreen/spark arrester etc. on my 'baby', but now that you talk about the exhaust it does make me think....cause i do have a rattle in/on the exhaust that i thought came from the exhaust cover and backing plate behind the exhaust....should try running it without exhaust (or just without cover and plate to see where the rattle really comes from) just to see....but i need to run it for a distance....would bring some memories back from my 2 stroke days terrorising the neighbourhood with loud exhausts....have to dig my rad sunnies out for that one....need to make a custom exhaust anyway for my mark2 model, so now might be the time....
    Cheers for that info, AL.