2Smoke Posted November 21, 2010 Share Posted November 21, 2010 Hi, I've got an old 1979 175mx for a bit of off road/green lane fun. Its a bit ropey but good fun if you don't expect too much of it! It could do with a little bit more power though, I've fitted an expansion chamber with a DEP end can (takes me back..) and its going a lot better. Running with the top bit of the airbox removed, and with a 145 main jet seems to be a brink on the rich side but better safe than sorry, already holed 2 pistons while trying to lean it off last time! I have a friend with porting tools, and knows what he's doing but any tips for what to do with the ports? Don't want to end up with anything too peaky but I understand it'll always be a tradeoff trying to get too much out of the motors.. Do people fit bigger carbs? Further modify the airbox? Any other hints? Thanks! Paul Link to comment Share on other sites More sharing options...
blackhat250 Posted November 21, 2010 Share Posted November 21, 2010 Enter Card.B.Daves mechanic [ the Hatter] Welcome back 2smoke . 2004 - 7 posts , had a lean spell , Link to comment Share on other sites More sharing options...
Moderator Cynic Posted November 21, 2010 Moderator Share Posted November 21, 2010 Hi, I've got an old 1979 175mx for a bit of off road/green lane fun. Its a bit ropey but good fun if you don't expect too much of it! It could do with a little bit more power though, I've fitted an expansion chamber with a DEP end can (takes me back..) and its going a lot better. Running with the top bit of the airbox removed, and with a 145 main jet seems to be a brink on the rich side but better safe than sorry, already holed 2 pistons while trying to lean it off last time! I have a friend with porting tools, and knows what he's doing but any tips for what to do with the ports? Don't want to end up with anything too peaky but I understand it'll always be a tradeoff trying to get too much out of the motors.. Do people fit bigger carbs? Further modify the airbox? Any other hints? Thanks! Paul To be honest what you have done is pretty much it. Any more and the engine manners get nasty. Its more a case of getting power that is useable rather than some fabulous peak figue. As to porting stick with increacing the area of the transfer ports, don't raise them much if at all though. Get them as even as possible. When you look down the barrel with the piston in place it seems that the piston is masking the transfers and lifting the barrel will help. It makes bugger all worthwhile difference. The final nail in the tuning coffin is these engines suffer detonation as you have found already, this stopps a lot of what may be possible with the exhaust port and higher rpm. You could try the later 3U500 carb that runs a 160 main std as its designed for a different airbox and gives more high RPM power. Mine (an 81) with a pipe and some airbox tweaks is running a 170 and goes OK up to 9000rpm (briefly if you need it). Hope this helps. Link to comment Share on other sites More sharing options...
2Smoke Posted November 24, 2010 Author Share Posted November 24, 2010 Enter Card.B.Daves mechanic [ the Hatter] Welcome back 2smoke . 2004 - 7 posts , had a lean spell , I keep my head down! Thanks for the welcome To be honest what you have done is pretty much it. Any more and the engine manners get nasty. Its more a case of getting power that is useable rather than some fabulous peak figue. As to porting stick with increacing the area of the transfer ports, don't raise them much if at all though. Get them as even as possible. When you look down the barrel with the piston in place it seems that the piston is masking the transfers and lifting the barrel will help. It makes bugger all worthwhile difference. The final nail in the tuning coffin is these engines suffer detonation as you have found already, this stopps a lot of what may be possible with the exhaust port and higher rpm. You could try the later 3U500 carb that runs a 160 main std as its designed for a different airbox and gives more high RPM power. Mine (an 81) with a pipe and some airbox tweaks is running a 170 and goes OK up to 9000rpm (briefly if you need it). Hope this helps. Thanks for the tips, once I stop riding it so much I'll whip the top end off and leave my mate to it. Is it worth setting the squish? I guess from stock its probably 2mm or so. Any ideas what can be done to prevent the detonation going on? Seems to be a common issue with them, shame as pistons are becoming quite hard to find in certain sizes.. This is the manky little beastie in question.. Link to comment Share on other sites More sharing options...
2Smoke Posted November 29, 2010 Author Share Posted November 29, 2010 Went out yesterday, had some fun in the snow! Nothing beats old bikes in the snow Link to comment Share on other sites More sharing options...
Moderator Airhead Posted November 29, 2010 Moderator Share Posted November 29, 2010 Great pics Paul, where are they? Link to comment Share on other sites More sharing options...
2Smoke Posted November 29, 2010 Author Share Posted November 29, 2010 Thanks! We were up in a quarry near Caernarfon on the North Wales coast. It used to be my regular haunt when I lived there so thought it'd be good to have a look again, and it was certainly worth it.. I've got another 175 on the go for the mrs, once that's done I think we will be spending a lot more time up there, as there is so much space its amazing! Doh! This is a panoramic photo looking out towards Anglesey. Caernarfon is on the right and you can just about make Holyhead mountain out in the distance Link to comment Share on other sites More sharing options...
Recommended Posts