Migthy Mule Posted April 26, 2012 Share Posted April 26, 2012 Hi all, I posted this in the new members forum. but figured it should be here, sorry for the duplicate post. Ive decided that its time to resurrect my DT. Ive ridden bikes since I was tall enough to get on one anyhow a couple of years ago I decided id like to do some off roading and purchased a DT off ebay. its a 1980 model but has a 175 top end on it. with memories of my old 125 DT field bike romantically filling my head I had the generator rewound, replaced the wheel bearings and it passed its mot - great.... then I got about 11 miles from the mot centre and it blew a hole in the top of the piston. Unfortunately the restraints of family life meant its sat in my shed for a couple of years but Ive just taken the engine to some local guys for a rebuild. Next is to try and figure out why it blew up, Im not very knowledgeable on 2 strokes but I have been pointed in the direction of the carb and its jetting It has a Minuni carb fitted 2H501 Throttle slide cut away - 2.0 Needle Jet 2490 0-0 Main jet 160 Pilot Jet 25 ive looked through the forums, seems that a 2H500 carb (im assuming its the same as a 2H501) should have a 249 0-2 needle jet and a 130 main jet can anyone please tell me if this could have caused the piston destruction seems I also need to check the air box and its type can anyone help with identifying it? Is it possible that it has the 125 box fitted, I guess this would restrict the airflow. Thanks in advance, ive been lurking in the forum for a while as a guest, sorry my first post is a question. Link to comment Share on other sites More sharing options...
Moderator Cynic Posted April 27, 2012 Moderator Share Posted April 27, 2012 Where are those numbers coming from? The 2h500 carb does exist but not with that needle id, although it does have a 130 main. that would be for the earlier bike though. The 2H501 is an unknown for me? To be sure all you need is which side the oil tank is on, if its on the right of the bike( when your sitting on it) then you have the later airbox/cdi needing a 160 main, if its on the left you have earlier filter/cdi and the 130main will do. ............................ Early.... late type..................... vm24ss.... vm24ss id......................... 2h500.... 3u500 main jet............... 130.... 160 jet needle............ 4j13.... 4j6 clip pos frm top.... 3rd.... 3rd Needle jet............ 0-2.... 0-8 Thr cutaway........ 2.0.... 1.5 Pilot jet................ 25.... 20 Idle speed........... 13-1400.... 1500+/- 50 TBH so long as the main jet and the needle jet is right the rest is down to how fussy you are as you really won't notice all that much. The 1980 125mx is pretty much identical to the 175 save for obviously the cc, the carb and the clutch which is slightly bigger with an extra plate. And the fact the 125 needs spanking like a politition to get it going, wheras the 175 has a lot more plodability but doesent like revs. 8k is about as high as you want to go unless its for short blats. Link to comment Share on other sites More sharing options...
Migthy Mule Posted April 27, 2012 Author Share Posted April 27, 2012 Hi, thanks for the reply. The oil tank is no longer there but looks like it was on the left (the airbox is filling the right hand side) so looks like I have the earlier model, but with a 160 main jet does the higher number refer to a larger or smaller size? carb definately a 2H501 would having the 160 instead of the 130 main jet contribute to the engines self destruction? Im not too fussy about how well it runs at the moment I just want to get more than 11 miles from my rebuild. I can tinker after Ive had some time to get acquainted with it Link to comment Share on other sites More sharing options...
Moderator Cynic Posted April 27, 2012 Moderator Share Posted April 27, 2012 The airboxes changed with the oil tank and if its filling the right hand side then it will be the earlier 1979 type. The main jet being a 160 with that airbox shouldn't have blitzed the motor, its 30points too big it should run like a dog. There is info your not telling here. Has the bike got, Std exhaust and tailpipe? Effective std or equivalent filter? Gas tight inlet and exhaust joints? The biggy, properly sealed oil connection at the carb? How does it run generally, high tick over, bad starting? Gearbox oil level? The bike won't be 'just' lean, i doubt the carb settings are the problem. Is there a tag on the carb that gives you the 501 carb id. Link to comment Share on other sites More sharing options...
Migthy Mule Posted April 27, 2012 Author Share Posted April 27, 2012 cheers so the higher the jet number the bigger(figures) I have done my best to dig the bike out. and have taken photos, im going to have a look on the internet to see if I can tell what it should be. Not sure about the exhaust, is this any help? t It has an air filter and it looks clean but there are no markings on it ( strangely it only seems held in by the cover, should it clip on ?) the carb inlet seems to have been glued with clear silicone at the point it meets the airbox, with very little pressure i was able to expose a hole oil connection on carb seems ok if not very pretty The carb number just visible in this photo above oil inlet, cant find any other markings on it Sorry I cant remember how it ran, I only did 11 mile on it and that was around 2 years ago I remember it seemed to tick over ok and it felt peaky and a little bouncy but I was riding a cbr 900 daily at the time. Im going to see if I can compare exhausts visually on the net, thanks again for the help, the pictures show it in a bad light, been buried at the back of my shed too long. but ill get everything off it tomorrow and give it a good look over, and check the gearbox oil. Any suggestions/abuse greatly received Link to comment Share on other sites More sharing options...
Moderator Cynic Posted April 27, 2012 Moderator Share Posted April 27, 2012 Yep 2h500 carb, looks like a gianelli pipe (common replacement for rotten stock pipe) with the generic tailpipe. The leaky airbox isn't good but not neccesarily the cause. All of it looks ok enough, unless you were spanking it at high rpm? Possible crank seal issue. Ig can also play a part but it doesent go off much on DT's. It needs to be absolutely cock on but if it hasn't been messed with it wont go awol. The people rebuilding the motor, are they doing a full build or just the top end? Link to comment Share on other sites More sharing options...
Migthy Mule Posted April 27, 2012 Author Share Posted April 27, 2012 Yeah I asked them to fully re build bottom, seals bearings etc, may be that i was just asking too much of it, not relly sure of its history. as I say an ebay buy I didnt even get to collect it as I was working away, he dropped it at my house. possibly not run in fully and I killed it. do you think i should fit the 130 jet, Im sure I read somewhere that if you are pre mixing then you should up the size by a few points. Thanks again Link to comment Share on other sites More sharing options...
Moderator Cynic Posted April 27, 2012 Moderator Share Posted April 27, 2012 I'd be inclined to get the overhaul kit for the carb and rebuild it to the spec for a 79 DT175 production code 2k4 with the jets to suit and then go from there, esp as your paying for the bottom end to be done. With that airbox TBH it shouldn't run with a 160. I have the later airbox on mine and that has a 160 main, i tried a 170 to allow for the pipe and a K+N and it just plain wouldn't run. It would start from stone cold and then just die out as it warmed up. Your talking about being over by far more. I would bet you were running with a bad air leak, maybe a blown crank seal and the massive over fueling compensated to get it running. These engines are tough and don't die easy, they don't take tuning well but they can take abuse in spades, believe me i've given mine plenty. Thats why i think your bike was well on the road to terminal anyway. As to the premix, personally i would fix the autolube system. It works incredibly well, is nowhere near as fragile as people think. They get disconnected because people have thrashed the bike to an inch of its life or run the tank out and its siezed (quel suprise) then blame the oil pump and remove/disconnect it (sound familiar). Chucking oil in the top of the tank, that just isn't as good. Link to comment Share on other sites More sharing options...
Migthy Mule Posted April 27, 2012 Author Share Posted April 27, 2012 Cheers you have been a great help, any idea where i can get the carb kit from? yambits seem to only have 74-77 kit and cant find anything on ebay and I dont have a local Yam dealer. Link to comment Share on other sites More sharing options...
Moderator Cynic Posted April 28, 2012 Moderator Share Posted April 28, 2012 Wemoto, here...http://www.wemoto.com/bikes/yamaha/dt_175_mx/78-79/picture/carburettor_complete_repair_kit/ That 'should' sort it, don't shoot me if it doesent though. Link to comment Share on other sites More sharing options...
Migthy Mule Posted April 28, 2012 Author Share Posted April 28, 2012 Brill, - you the man Have ordered the carb kit, fingers crossed. Spent most of the day digging out the bike from the back of my shed - 2 runs to the tip. kind of forgot what a pretty little bike it is someone has spent some time and money on it at some point, looks to me as though the frame has been re painted (proppery) and it seems to have a new swing arm and the wheels have been rebuilt, im guessing that that owner ran out of time or money though cause the rest is not quite as nicely finished. any idea how much its likely to set me back to get the auto lube set up, bearing in mind I need everything, I dont even seem to have the oil tank. Link to comment Share on other sites More sharing options...
Moderator Cynic Posted April 28, 2012 Moderator Share Posted April 28, 2012 You only really need the pump and the bottle, ebay should give you what you need. As i said the pumps are normally pretty good, you may well still have the pump in the cases so you just need the bottle and the plumbing. Check first. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted April 29, 2012 Moderator Share Posted April 29, 2012 Here's one to watch http://www.ebay.co.uk/itm/Yamaha-DT175-MX-oil-tank-/330724415840?pt=UK_Motorcycle_Parts&hash=item4d00b61960 Link to comment Share on other sites More sharing options...
Migthy Mule Posted April 29, 2012 Author Share Posted April 29, 2012 Here's one to watch http://www.ebay.co.u...=item4d00b61960 Ha that's quite funny I emailed this guy yesterday to ask if it would fit my bike, he wasn't sure and suggested i ask one of the forums. I was just about to post the link but you beat me to it. Cheers. Because my bike didn't have one I wasn't sure even what it should look like, Thanks again Link to comment Share on other sites More sharing options...
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