bookem Posted March 10, 2011 Share Posted March 10, 2011 Having never torn a 2stroke apart I've got a question regarding the piston. When the reed valve is removed and you look down the intake port, how far should the piston descend past the port. Right now it appears that the piston doesn't go below the intake port at all. This would explain why I don't seem to be getting any fuel to the cylinder. I'm not sure how it could be possible for the piston to not descend far enough but it seems to be the case. If someone has a pic of the intake manifold with the piston fully descended that would be appreciated. Link to comment Share on other sites More sharing options...
nayruf Posted March 10, 2011 Share Posted March 10, 2011 Having never torn a 2stroke apart I've got a question regarding the piston. When the reed valve is removed and you look down the intake port, how far should the piston descend past the port. Right now it appears that the piston doesn't go below the intake port at all. This would explain why I don't seem to be getting any fuel to the cylinder. I'm not sure how it could be possible for the piston to not descend far enough but it seems to be the case. If someone has a pic of the intake manifold with the piston fully descended that would be appreciated. Hi bookem The piston does not need to descend below the intake port, the fuel air mix travels into the crank case via the ports cut into the side of the piston. When the piston ascends up the cylinder bore it closes off the intake path and a suction is created in the crank case this draws the fuel/air through the piston ports, at least this is how I understand it, http://www.whitedoglubes.com/2strokeengines.htm check out this web site it has good description of how it works. Link to comment Share on other sites More sharing options...
Moderator Cynic Posted March 10, 2011 Moderator Share Posted March 10, 2011 Having never torn a 2stroke apart I've got a question regarding the piston. When the reed valve is removed and you look down the intake port, how far should the piston descend past the port. Right now it appears that the piston doesn't go below the intake port at all. This would explain why I don't seem to be getting any fuel to the cylinder. I'm not sure how it could be possible for the piston to not descend far enough but it seems to be the case. If someone has a pic of the intake manifold with the piston fully descended that would be appreciated. Nah doesent matter at all, more modern 2 strokes like the DTR125 and my TDR250 for example, the carb feeds direct into the cases. 2 strokes are a masterpiece when it comes to how they work. Its all to do with pressure waves, thermal expansion, positive and negative pressure all working together. I love them. Boring (nerdy) info for you. A 125 cc 4 stroke engine takes in around 100cc of fuel/air mix each cycle which takes 2 crank rotations. With the fasinating way a 2 stroke works the 125 takes in getting on for 150cc (yep, more than should actually fit) of fuel air mix on each cycle that happens every rotation. Shows how 2 strokes are both more powerfull and use far more fuel/make far more power than a 4stroke. Why we can't develope them to be clean i'll never know. Link to comment Share on other sites More sharing options...
bookem Posted March 10, 2011 Author Share Posted March 10, 2011 Nah doesent matter at all, more modern 2 strokes like the DTR125 and my TDR250 for example, the carb feeds direct into the cases. 2 strokes are a masterpiece when it comes to how they work. Its all to do with pressure waves, thermal expansion, positive and negative pressure all working together. I love them. Boring (nerdy) info for you. A 125 cc 4 stroke engine takes in around 100cc of fuel/air mix each cycle which takes 2 crank rotations. With the fasinating way a 2 stroke works the 125 takes in getting on for 150cc (yep, more than should actually fit) of fuel air mix on each cycle that happens every rotation. Shows how 2 strokes are both more powerfull and use far more fuel/make far more power than a 4stroke. Why we can't develope them to be clean i'll never know. This has been very informative stuff. I never had 2 strokes to play with growing up and spent most of my time with 4 stroke. The 2 stroke seems like the greatest invention ever. 125 cc pulls my big butt around just great and I think the 175 will be perfect for my trike. 175 4 stroke would be way under powered for this. Once I get these 3 I've got working top notch I think I may try one of the Kawasaki 750's. Link to comment Share on other sites More sharing options...
Moderator Cynic Posted March 10, 2011 Moderator Share Posted March 10, 2011 Thats their only flaw, big doesent mean better with 2 strokes. By the time you are up at those cc's the 4 stroke is catching up, sneaking in with driveability and fuel economy that the stroker can't match. Nothing and i mean nothing sounds or smells as sweet as a big smoker running old school expansion chambers though Link to comment Share on other sites More sharing options...
bookem Posted March 10, 2011 Author Share Posted March 10, 2011 Thats their only flaw, big doesent mean better with 2 strokes. By the time you are up at those cc's the 4 stroke is catching up, sneaking in with driveability and fuel economy that the stroker can't match. Nothing and i mean nothing sounds or smells as sweet as a big smoker running old school expansion chambers though Pulled it apart. The rings both have about a 1/4 inch gap so I think that may be the big problem. The con rod seems to have side play on the crank but no end play. I think I'll try new rings for now and see how it goes. They're cheap and if it isn't enough I think I'll get another motor. The bike isn't gonna get hard use and will only see pavement with a light hand. Hope the rings do it, with that much gap on the rings I can see where the compression was going Link to comment Share on other sites More sharing options...
Moderator Cynic Posted March 11, 2011 Moderator Share Posted March 11, 2011 Pulled it apart. The rings both have about a 1/4 inch gap so I think that may be the big problem. The con rod seems to have side play on the crank but no end play. I think I'll try new rings for now and see how it goes. They're cheap and if it isn't enough I think I'll get another motor. The bike isn't gonna get hard use and will only see pavement with a light hand. Hope the rings do it, with that much gap on the rings I can see where the compression was going Hang on champ, if you have never pulled a smoker down slow up. They ARE a different animal. The rod having side play but no vertical movement sounds fine, up to 2mm or .0787in is fine. The rings on a DT can have a gap of anything from 3mm .1181in to 8.5mm .3347in and be within spec depending on model. A 78 US spec 125 'should be' around 4mm .1770in for the top ring and 5.5 .2165in for the bottom ring. Its the installed gap thats more important. Thats more like .15 to .35mm or .0059-.0138in for both rings. By installed i mean compress the ring so it sits in the bore, then using the piston push it into its normall working area in the bore and check the gap. If there is a pronounced lip at the top of the bore you can feel with your finger it may be the bore is worn too far (normally they sieze first though ). A far simpler method for checking the rings, if the piston sides are as black well down past them they've had it. Link to comment Share on other sites More sharing options...
bookem Posted March 11, 2011 Author Share Posted March 11, 2011 Hang on champ, if you have never pulled a smoker down slow up. They ARE a different animal. The rod having side play but no vertical movement sounds fine, up to 2mm or .0787in is fine. The rings on a DT can have a gap of anything from 3mm .1181in to 8.5mm .3347in and be within spec depending on model. A 78 US spec 125 'should be' around 4mm .1770in for the top ring and 5.5 .2165in for the bottom ring. Its the installed gap thats more important. Thats more like .15 to .35mm or .0059-.0138in for both rings. By installed i mean compress the ring so it sits in the bore, then using the piston push it into its normall working area in the bore and check the gap. If there is a pronounced lip at the top of the bore you can feel with your finger it may be the bore is worn too far (normally they sieze first though ). A far simpler method for checking the rings, if the piston sides are as black well down past them they've had it. Ok good to know stuff like this. The 1/4 inch gap on the rings I mentioned is the installed gap. I put the rings in the cylinder and used my fingers to push them down to the working area as suggested in Clymers manual. I checked the bore with my fingers and found it smooth with no lip whatsoever. The underside of the piston showed brownish black though. I have read that is a sign of bad rings. Without a gauge I can't tell if the cylinder is perfect but it feels good and when I pulled the jug the piston seemed to be centered in the bore with no obvious out of round spots. Any more help will be appreciated and I'll keep you posted when parts arrrive, could be couple of weeks. Link to comment Share on other sites More sharing options...
Moderator Cynic Posted March 11, 2011 Moderator Share Posted March 11, 2011 1/4 inch installed is a bit much. The black under the piston if its excessive generally indicates that the piston crown is getting too hot, could be lean mixture, ignition out by a few degrees (one of the few things these engines are sensitive to). Knackerd rings won't help as you will as a result be running weaker as the compression getting past the rings reduces the draw from the cases. Link to comment Share on other sites More sharing options...
burton500 Posted March 14, 2011 Share Posted March 14, 2011 Nothing and i mean nothing sounds or smells as sweet as a big smoker running old school expansion chambers though I can vouch for that - I also run a RG500, and there is nothing quite like the sound of 4 spannies coming on song, and a blue haze out the back. Link to comment Share on other sites More sharing options...
Guest Mozzer Posted March 18, 2011 Share Posted March 18, 2011 did you try cleaning the carb out, it may be blocked if its been standing for a long period?, also you could of ckecked the piston and rings before stripping by puttin a little bit of oil strait onto the cylinder head which blocks the gaps if the rings or bore has gone?, let us know how you get on moz Link to comment Share on other sites More sharing options...
bookem Posted March 19, 2011 Author Share Posted March 19, 2011 did you try cleaning the carb out, it may be blocked if its been standing for a long period?, also you could of ckecked the piston and rings before stripping by puttin a little bit of oil strait onto the cylinder head which blocks the gaps if the rings or bore has gone?, let us know how you get on moz the carb I put on was brand new. I tried it on another bike aftre and it ran good. I wondered the same thing at the time which is why I tried it on the other bike. Tried the oil thing too and and when it dripped out the exhaust port I knew I had a problem Got some new rings on the way and will be honing the cylinder as well. The bore seemed round and no lip or taper so I hope this will fix it. Next week we should know if that is what it needed. Link to comment Share on other sites More sharing options...
Guest Mozzer Posted March 22, 2011 Share Posted March 22, 2011 ahh right well atleats u have done the main checks lol, keep us updated mox Link to comment Share on other sites More sharing options...
bookem Posted April 11, 2011 Author Share Posted April 11, 2011 Took awhile to get parts but I have the motor back together with new rings and head gasket. Have checked timing as best as possible. Don't have a dial indicator that clymer wanted but found tdc with a small screwdriver and then checked points. they are opening just before tdc. I installed carb, added gas and spun flywheel with drill. It sucked in the gas/oil and blew it out the spark plug hole. Installed a good plug but no go. not even a little puff. Compression teastor is loaned out but seems harder to kick and does pop my thumb off the spark plug hole. At this point I'm thinkin maybe condensor bad or maybe 6 month old fuel no good. Any odd things you guys have run into would be great. Thanks. Link to comment Share on other sites More sharing options...
Moderator Cynic Posted April 11, 2011 Moderator Share Posted April 11, 2011 Ig timing is the one thing these engines need to be EXACT, i did mine with a piece of bar and some nuts with the head off. Tighten the bar till just touching at tdc then mark the timing using a feeler gague to get the distance BTDC. Not as gracefull as a dial gague but it works and is really accurate. Link to comment Share on other sites More sharing options...
bookem Posted April 13, 2011 Author Share Posted April 13, 2011 The brand new bloody coil was bad. I decided to swap in a running DT125 and when I did she didn't run anymore. The only thing different was the coil so I swapped it out. Started on second kick. Swapped the DT175 back in with the coil from the DT125 and same thing started on second kick. First time I've had a brand new coil be bad. Really threw me off by giving such a good spark when holding plug against head. Link to comment Share on other sites More sharing options...
burton500 Posted April 13, 2011 Share Posted April 13, 2011 The brand new bloody coil was bad. I decided to swap in a running DT125 and when I did she didn't run anymore. The only thing different was the coil so I swapped it out. Started on second kick. Swapped the DT175 back in with the coil from the DT125 and same thing started on second kick. First time I've had a brand new coil be bad. Really threw me off by giving such a good spark when holding plug against head. Was this a genuine Yamaha coil, or a pattern? Would be handy to know what to avoid. Link to comment Share on other sites More sharing options...
bookem Posted April 15, 2011 Author Share Posted April 15, 2011 Was this a genuine Yamaha coil, or a pattern? Would be handy to know what to avoid. The coil was a re production by an ebay company. Just a bad one I guess. I have run across a new wrinkle though. This should test the 2 stroke gurus. The motor starts on 1 kick and run great with out the expansion pipe hooked up. As soon as I bolt the exhaust to the cylinder it wont start for anything. All I'm hooking up is the pipe that runs from the cylinder. I'm not connecting anything after the expansion chamber so it is not a plugged spark arrester. I've blowen air through the pipe and there doesn't seem to be any blockages. Really curious as to what is causing this. Link to comment Share on other sites More sharing options...
Guest Mozzer Posted October 26, 2011 Share Posted October 26, 2011 maybe the fuel air ratio of your carb isnt quite right, with it being a new carb it wont have been set up to your bike from new, but from what you are saying it is really restricting the exhaust gasses but all i can think of is down jetting the carb. Is the wadding in the back pipe ok?, I have a similar problem with my race pitbike, its been tuned and i put it on the road and took the race pipe off and put that pipe on my new race bike, and now it dooesnt run very well at all because the exhaust gasses are restricting the flow due to the smaller pipe. Also does it run if you hold the throttle open a little bit and try taking your air filter off with the fill exhaust system on ? mozzer Link to comment Share on other sites More sharing options...
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