Hey all Last night a mate of mine dropped over a 1997 GPX250 that wouldn't start, He has been riding it a bit then one day it wouldn't start. First job was pull the carbs down, what we found was the pilot screws were set at 4 1/2 turns out , I lowered them to 2 1/4 , the manual says 2 , but the bike has a few kms on it. We cleaned the carbs as best we could. I started the bike and it idled nicely, I then the revved it and it went to up 6000 rpm and very slowly came back down to 2000, Mark said it did that all the time. There is a diaphragm on the side of the carbs, so I pull that off, cleaned it and reassembled, it was better. I then used my gauges to balance the carbs. We ran it up and it ran heaps better, almost to the point that it was right, I noticed that the output from the lh exhaust was warm, but the rh was much cooler. I pulled the plugs and one was perfect, the other was black ( he has tried start ya bastard in the past to get it going). We also did the oil and filter while we were at it. I had an old GPX motor on the shelf so we pulled a plug out of there and fitted it to the bike, it idled perfectly, revved and dropped back to idle nicely. I rebalanced the carbs and then set about for a road test. When I took off the bike lost power, I mean just died, I had to rev it to 5000 + rpms to get a launch , anything under under 5000rpm was shite, over 5000 rpms and she responded. Im a little perplexed, I am going to get some plugs today so see if that helps and maybe change the pilots to 2 1/2 , I have never seen a bike do this before.... any other ideas, Im slowly running out of ideas. Very weird. Chris
Have you checked the valves at all? Perhaps the clearance is too tight or something like that, so it runs fine at first, then as the engine warms up and things expand, the valves start wanting to stay open too long and it will only run well at high rpm, kind of like how when you put a high performance aftermarket sort of camshaft in a car, they can hardly hold idle but run well at higher rpm, just a thought though How would you describe the black end of the spark plug? I've heard before that carbon fouled plugs (dry, sooty, black on the end) can also sometimes be caused by not enough power being sent to the plug from the coil
If valves are ok for clearances and compression good, air filter clean, then I would suggest you have a look at spark power. Battery voltage? Coils giving correct ohm readings? Plug wires/caps with correct resistance? Voltage at coils? It may need 5000 rpm for the alternator to give enough voltage for the plugs to fire properly under load.
Cheers lads, the owner just texted me and said he bought a spare bike for $300 so we have some aspires to play with . The blackened plugs are due to him using start ya bastard
fitted another set of carbies after giving them the threebond treatment, he rode it home, all good !!!
