Fuel/air Problem?

Discussion in 'Technical' started by simonm, Jul 16, 2017.

  1. Hi, I'm looking for some advice.

    I was off road a few days ago and my engine stalled. It fired up again and idled fine but whenever I opened the throttle (in gear) it spluttered and died. I managed to get home slow and steady at 15-20km/h with no problems but if I tried to push it any more, the engine would stall.

    I expected it was a carburetor blockage preventing enough flow of petrol or air so took off the carb and cleaned it out.

    Having put it back on, the problem remains unchanged. Engine fires up fine, revs fine in neutral and will drive at 10-20kmh but cuts out under any real load.

    I've checked there is no blockage in the fuel line - it flows fine to the carb. I've cleaned the air filter and run the engine with it off and also changed the spark plug just for good measure but nothing has made any difference.

    Even if there is no fuel reaching the carb from the petrol tank, the fuel in the carb bowl is enough to drive 300-400m at normal driving speed (I know this having once left the fuel tap turned off after doing some repair work) so I'm struggling to work out why the engine stalls immediately if I rev it when in gear.

    Does anyone have any suggestions?
  2. Sounds like the main jet.
  3. Thanks. I did take that out and clean it when I cleaned the carb but I will look again to be double sure.
  4. possible vacuum leak...check all your fuel hoses and check everything is tight on the carb...also check your fuel filter...also check your crankcase breather tube for blockage or kinks.

    also when asking for mech help it's always a good idea to explain what type and year of bike your riding.
  5. It would indeed help if we knew what bike it is, how old and if modified or not.
    "revs fine neutral".... sustained high-ish revs?
  6. #6 simonm, Jul 18, 2017
    Last edited: Jul 18, 2017
    Thanks for the tips. It's been raining almost constantly since Saturday so I haven't had the opportunity to do any further work. I'd already checked the fuel filter and flow from the tank to the carb. As I said, even with the fuel disconnected it would normally run a couple of hundred m at normal speed on just the fuel in the carb bowl.

    During a brief rain stop I checked the fuel pipe snugness and found the rubber was possibly slightly hardened and loose. When the rain stops I'll replace that first as it's a quick and easy job (and looks like it may need doing anyway) and also check the crankcase breather.

    If that doesn't help, the next step will be to take out the carb again and check/clean it a second time.

    P.S. It's actually a dirt buggy (Chinese) but it's made using bike parts. The engine is a 260cc Yamaha replica (as used in a Yamaha Majesty but bored out an extra 10cc) and the carb is also for a bike. (Yes, I know these things have low build quality but it's what was available in the country I live in and it's just a toy.)

    Thanks again for the advice. It is much appreciated.
  7. Right... now we know.... it's got 4 wheels.......

    ....and so fatally flawed I'm afraid...... hahahahA...
  8. Hahaha. I bought it in the knowledge that it would be low quality and high maintenance (but good fun). Actually, since I've had the buggy it's given me fewer problems than either my dirt bike or road bike, both of them Suzuki.
  9. I replaced the fuel lines and checked the crankcase breather and the problem remains.

    I'll take the carb off again and have another look but it's still raining at the moment so can't do it now.

    In neutral it idles fine and revs fine at medium level. In gear it idles fine but cuts as soon as I put my foot down and try to go above a few km/h. I noticed that after cut out I often have to wait 20-30 secs before it will restart.
  10. You have looked at the fuel. Do not overlook the ignition.
  11. #11 Lee, Jul 19, 2017
    Last edited: Jul 19, 2017
    I forgot to mention the rubber boot that connects the carb to the cylinder head...the chinese use the cheapest materials possible so the rubber is often crap and hardens after a few heat cycles, so check all the rubber for cracks, cause it definitely sounds like a vacuum leak...also check around the cylinder head as it could be a head gasket leak as well, you're looking for any oil leak around the head gasket.

    I lived in China for 8 years and had a bike shop there for a while...I sold those 110cc and 150cc Chinese made "dirt bikes" the engine in yours is of the same quality...and they are shit...

    case in point...during assembly one time I was tightening an engine mount bolt and snapped it in half...the main bolt that holds the engine in the frame...I'm not a particularly big guy or strong for that matter and it snapped like it was made of glass.
    good luck.

    Edit: one way to eliminate the suspicion of vacuum leak as a source of the problem...

    if you can get hold of one of those propane torches... the portable ones like for camping and such...start the engine and while it's idling, turn the torches gas on but don't ignite it...pass it over and around the carb at all the connection points for hoses, the boot etc...also pass it around the cylinder head gasket...if the idle increases at any point then you've got yourself a vacuum leak...if not...it's time to move on.
  12. They call it Chineseium... weakest metal known to man...

    I too was thinking ignition not fuel. If you increase the throttle in neutral, does it stall out???
  13. Thanks again for all the useful advice.

    The rain stopped for a couple of hours this morning so I had a chance to do some work. I wasn't sure the ignition/spark was the problem as the engine would run at higher revs in neutral but would not run at the same speed in gear. I decided to take the carb out and give it a really thorough clean.

    The jets seemed fine but I gave them a good cleaning anyway. The only thing I found that was suspect was one of the air in/outlets inside the area circled. (I'm not actually sure what this part is - comments welcome). Anyway, I cleaned everything thoroughly whether it seemed to need it or not.

    After reassembling and refitting the carb I fired up the buggy and put it in gear as the rain started to fall again. I only managed a quick 2 min test drive around the block but it seemed to be running OK. I only got up to about 40km/h so I'll have to wait until the rain stops again to check it runs flat out without problems but the signs are promising.

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  14. I believe the part you circled is the vacuum operated fuel pump...that would indeed be your problem if it wasn't working or got stuck somehow.

  15. Aahhhh, that would make sense. When I stripped it down I blew into the breather inlet/outlet and it seemed to be blocked. When I opened it up it had a narrow capillary about the diameter of one of the holes in the side of the jets that appeared to be blocked. (leading from the brass nozzle in the circled area to the internal area where the small diaphragm is). I cleaned it with a fine piece of wire and squirted in some WD40 to flush it through and that seems to have done the trick.

    Thanks a lot for the info.
  16. Full marks to Lee.

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