tankygsy Posted October 24, 2011 Report Share Posted October 24, 2011 (edited) someone please help, im trying to find out compression figures for a 2007 (or other years) 280cc txt pro gas gas trial bike. Mine refuses to start and i beleive its a low compression issue. Gasgas uk dont have any figures but surely someone would have taken figures - if only for maintenece monitoring! I know an accurate figure is impossible as all motors are different but a ballpart figure to withing 20-30 psi must be possible from someone here. Mine currently is at 120-125 psi. it refuses to start. it has fuel, it has spark but no bang.. Cheers. Glenn Edited October 24, 2011 by tankygsy Quote Link to comment Share on other sites More sharing options...
lowbrow Posted October 24, 2011 Report Share Posted October 24, 2011 Never checked the compression on a trials bike, but those pressures look pretty good to me and certainly not so low that the motor won't run at all. Ok, so you've got a spark, but are you sure it's at the right time? Check to make sure the woodruff key on the ignition flywheel hasn't sheared and allowed the timing to slip. Quote Link to comment Share on other sites More sharing options...
tankygsy Posted October 25, 2011 Author Report Share Posted October 25, 2011 Hi lowbrow, cheers for that mate. I've eyeballed the woodruff key and it seems all ok no sign of slippage. I'm returning from work today where I have been the last 5 days so will have another cack at it this week during my 5 days off- oh the joys of shift work A Freind of mine has a 280 pro same as mine so I'm going to do a compression test on his motor and compare results, his engine is fairly fresh so should get into the ballpark, just hope his engine still has stock internals. Quote Link to comment Share on other sites More sharing options...
tankygsy Posted October 25, 2011 Author Report Share Posted October 25, 2011 Ok did some compression tests on my mates motor (using my own gauge, last time my mate dud it as I was out of the island).. His motor read 155psi and mine is 140 psi. My bike should be firing up with this kind of figures. I have fitted my cdi and rectifier unit to his bike and it runs fine. as I said earlier it is sparking well and has plenty of fuel going through. Reeds are good and plug gets wet with fuel while kicking over. going back to look at the timing as per lowbrows comment above and see what I can find. Quote Link to comment Share on other sites More sharing options...
trialdude Posted October 25, 2011 Report Share Posted October 25, 2011 Have you checked all the regular problems, like; air filter fresh gas, right amount of 2 stroke oil choke stuck gas wire hung up idle set to high? maybe wheather conditions are different than when you had a working engine. Maybe screw idle jet in, for leaner mixture a bit, and start without choke? helping out with start gas, just to see if it bangs? Quote Link to comment Share on other sites More sharing options...
sting32 Posted October 26, 2011 Report Share Posted October 26, 2011 (edited) Ok did some compression tests on my mates motor (using my own gauge, last time my mate dud it as I was out of the island).. His motor read 155psi and mine is 140 psi. My bike should be firing up with this kind of figures. I have fitted my cdi and rectifier unit to his bike and it runs fine. as I said earlier it is sparking well and has plenty of fuel going through. Reeds are good and plug gets wet with fuel while kicking over. going back to look at the timing as per lowbrows comment above and see what I can find. you cant eyeball the key on flywheel. remove flywheel check key. when you have spark & air/fuel, no way a 2 stroke with even 80psi compression wouldn't run. I had a ty 80 that had less than that, it would run, just not hardly any power or throttle response... It has to be a timing thing or bad spark thing. easiest thing is woodruff key, then is it a really good crisp blues-park, with the gap at 12-15 thousandths of an inch? (very small gap, yes) how wet is the plug, I guess it could be flooded. petcock off, THROTTLE FULL OPEN, and take out sparkplug, then kick repeatedly (12 times or more) to work the flooded crankcase out of fuel, reinstall plug and treat bike as if it has been upside down, flooded. Edited October 26, 2011 by Sting32 Quote Link to comment Share on other sites More sharing options...
tankygsy Posted October 28, 2011 Author Report Share Posted October 28, 2011 Hi guys cheers for all suggestions but still no go. Pardon my terminology regarding eyeballing the woodruff key but I'm on iPhone keyboard. When I said eyeballing I meant I have had the flywheel off completely and checked the woodruff key thouroughly, no sign of slippage. The timing is fixed on these motors with the cdi unit advancing / retarding the spark timing so nothing mechanical there to check really. While doing some resistance tests on the magneto pickup (the readings were good around 96ohms on my meter) I did notice while probing the 2 terminals of the pickup and at the same time turning the flywheel to the spark point that I only had continuity through the puck up for a split second. Is this correct? I would have thought that provided the flywheel was at the correct degree of rotation the pickup would be a closed circuit. What do you guys think? Quote Link to comment Share on other sites More sharing options...
feetupfun Posted October 28, 2011 Report Share Posted October 28, 2011 My understanding is that the function of the pickup is to generate a pulse as the magnet passes. Maybe your resistance measurements across the pickup coil are being corrupted by that pulse Quote Link to comment Share on other sites More sharing options...
trialtrial Posted November 1, 2011 Report Share Posted November 1, 2011 Why not just pull the plug, leave it connected to the HT line, lay it on the side of the crankcase, dim the lights and look for a good spark. If you *really* want to have fun, pour some gas around it. Quote Link to comment Share on other sites More sharing options...
tankygsy Posted November 13, 2011 Author Report Share Posted November 13, 2011 yes yes very good Turned out the pickup was the culprit, not picking up very well at all. It appears to be due to water ingress and through getting bash during normal use. Running fine now anyway cheers for all suggestions and help. Never got any compression figures in the end, so0 just for ha ha's i replaced mine for future reference. In case anyone else needs to know; with new rings my gauge sees 163 psi, the motor is stock with a 0.8mm cylinder gasket and clean combustion chamber. A freind of mine has 155psi (on a reasonably fresh engine), and another mate sees 142psi. Readings taken on the same gauge. By the way bloody great site this, well done admin and very fiendly folks. Quote Link to comment Share on other sites More sharing options...
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