hoggyf Posted June 10, 2011 Report Share Posted June 10, 2011 Hi 1st off this isn't on a trials bike but I do compete on a GasGas and trials guys seem to be a bit more clued in on engines than the general biking fraternity so heres my problem. I've just replaced the engine on my sons pitbike with a brand new unit re-using the old carb and exhaust, engine is the same type but not same make so may have differnet spec cams, valves etc. The old engine ran fine with this carb and the carb is clean. When you blip the throttle wide open from tickover it bogs down and trys to stall but if you open the throttle a bit more gentley it'll rev quite nicely. Riding wise it pulls fine as you don't really go from zero to full open but the plug is a bit sooty, though thats just from tickover and reving in neutral (can't do a plug chop in the street really). It feels to me that it's getting too much air as though when you open the slide fully from tickover its just getting too much. Carb rubbers are new with the engine so should be ok. I'm happy to clean carbs out etc but when it gets into the "tuning" side of it i.e. changing jets, adjusting needles etc I just don't really know what I'm doing. any advice? Quote Link to comment Share on other sites More sharing options...
dombush Posted June 10, 2011 Report Share Posted June 10, 2011 Hoggy as you say it sounds like its a bit weak at pickup. I would:- adjust the idle screw slowly until the engine achieves the highest stable / consistent tickover. Turn the screw 1/4 of a turn richer to ensure that tickover is marginaly rich. See if this improves the overall pickup (if the ideal screw position is less than 1.5 turns or more than 4 turns from fully in, its likely that the pilot jet needs changing) If no improvement adjust the needle clip 1 slot down the needle (to raise the needle) see if this improves overall pickup re-adjust the ideal idle screw position as it is likely to have changed If no improvement (unlikely), try another slot further down the needle. However a change of 2 slots is quite a considerable change. After best pickup is achieved,find some open ground and move on to a plug chop at Wide Open Throttle. Try to hold wide open for at least 15-20 secs. Review the plug colour and adjust main jet as necessary sounds complicated written down.. its not Dom Quote Link to comment Share on other sites More sharing options...
copemech Posted June 11, 2011 Report Share Posted June 11, 2011 Hoggy, you state this is a pitbike, so I presume it one of them little honda copy motors of about 125-150cc? Pity about all this to do with a quick throttle opening bog, stalling and spitback on a 4T is basically a normal condition. A number of things come into play here, but larger carbs are one thing that seems to make this worse, it seems, even if the carb is clean and set. Quote Link to comment Share on other sites More sharing options...
hoggyf Posted June 11, 2011 Author Report Share Posted June 11, 2011 Yeh chinese copy of a C50, its a 125 and a cheap way of getting my lad into bikes and out riding (also known now as me going practicing while he bumbles around) I'll try the adjustments you suggest Dombike but doing 15-20sec full throttle's gonna be a problem anywhere I ride, unless its a sneaky dart along the back road but i don't fancy that much, knowing my luck I'd get lifted. Quote Link to comment Share on other sites More sharing options...
hoggyf Posted June 27, 2011 Author Report Share Posted June 27, 2011 Finally got time to take it and do a plug chop with this as the result Which I think looks a little rich? would you agree? If it is rich how would this effect the pick up off tickover, it also felt a little bit weak at the top of the rev range but that could just have been me over revving it a little?? Any advice greatly received! Quote Link to comment Share on other sites More sharing options...
cvgmmartin Posted November 18, 2011 Report Share Posted November 18, 2011 If it doesn't turn out to be a carburation problem then you may consider the following. I had a similar problem on a grass track JAP engine which would rev if the throttle were opened slowely,the problem would show up half way through a meeting when the engine was really hot. Turned out to be the valve clearances that were closing up as the engine got warm. Since you have put a new engine in perhaps it is not carburation. If the bike is missing badly then it may still show a sooty plug even if the carburation is O.K. regards Martin Quote Link to comment Share on other sites More sharing options...
kramit Posted November 19, 2011 Report Share Posted November 19, 2011 Hoggy as you say it sounds like its a bit weak at pickup. I would:- adjust the idle screw slowly until the engine achieves the highest stable / consistent tickover. Turn the screw 1/4 of a turn richer to ensure that tickover is marginaly rich. See if this improves the overall pickup (if the ideal screw position is less than 1.5 turns or more than 4 turns from fully in, its likely that the pilot jet needs changing) If no improvement adjust the needle clip 1 slot down the needle (to raise the needle) see if this improves overall pickup re-adjust the ideal idle screw position as it is likely to have changed If no improvement (unlikely), try another slot further down the needle. However a change of 2 slots is quite a considerable change. After best pickup is achieved,find some open ground and move on to a plug chop at Wide Open Throttle. Try to hold wide open for at least 15-20 secs. Review the plug colour and adjust main jet as necessary sounds complicated written down.. its not Dom Try a bigger pilot jet......goggle it........its an easy fix. PS...The plug looks great. Quote Link to comment Share on other sites More sharing options...
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