sezian Posted February 11, 2009 Share Posted February 11, 2009 I’m interested to know if anyone has experience of completely striping the DT engine and rebuilding it? What parts are needed and where they can be sourced? The engine (when I bought the bike) seemed to be running okay (rode home approx 15 miles with no problems) and I was told it had new main bearings and a re-bore only a couple of thousand miles ago. The bike has approx 24,000 miles on it and has been used on and off for the last 19 years. Should I leave it alone if it’s running okay or would there be some benefit in a full rebuild? From what I can gather there’s unlikely to be much (if any) performance increase but would there be some possible reliability benefit? Link to comment Share on other sites More sharing options...
taffny Posted February 16, 2009 Share Posted February 16, 2009 I'm thinking the same with my Resto, but have come to the conclusion that if it ain't broke why try and fix it. If you decide to go for it then Yambits and Wemoto on Ebay have various parts like Gasket kits that arn't too expensive. But if its running right I would save the hassle of a strip down until it was needed. Hope that helps, Link to comment Share on other sites More sharing options...
sezian Posted February 24, 2009 Author Share Posted February 24, 2009 Okay, good advice - It's not broken, however it did seem to lack a little power when I rode it home ??? I've got the engine on the bench with the head removed, the barrel off, piston off and rings removed. I've de-coked the head and now plan to de-coke the piston, it seems okay apart from the build-up of carbon. The rings look okay but I'm not sure what can go wrong with rings apart from breaking ? Any suggestions of what else to do whilst the top is off ??? I've toyed with the idea of polishing the ports, not making them any bigger just smoothing out the rough surfaces, although I'm not sure if this will make any difference ? I know when I was young and running around with mates on Fizzy’s & AP50's etc a good de-coke always made things feel faster (not sure if that was ever the case) and just to know you've actually been inside the engine and it still works was a great buzz. Now many years older and a little wiser I'm hoping to get some well gained experience from other 2 stroke fans on what does really make a difference and what’s worth doing. Many thanks in advance. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted February 25, 2009 Moderator Share Posted February 25, 2009 Im interested to know if anyone has experience of completely striping the DT engine and rebuilding it? What parts are needed and where they can be sourced? Good sources are Fowlers, Granby, any other that may advertise as vintage Yamaha spares specialists, the classified ads in Classic Motrcycle Mechanics magazine is a good place to look for suppliers, other sources are Wemeto, m&p, mps etc The engine (when I bought the bike) seemed to be running okay (rode home approx 15 miles with no problems) and I was told it had new main bearings and a re-bore only a couple of thousand miles ago. The bike has approx 24,000 miles on it and has been used on and off for the last 19 years. If your bike has had a recent rebore thats good and so you shouldnt have any problem there then, however there are routine checks thet can be done to check the condition of the bore, piston and piston ring wear, theses are very important checks especially on two stroke engines and should be done fairly regularly, I dont know if you are familiar with the wear specifications or testing methods but I will tell you anyhow in case you dont. To check for bore wear the easiest method is to measure the gap between the piston skirt and the bore in several places, invert the piston in the bore and measure the gap between the side of the piston and the cylinder bore. The specifications are 0.035mm - 0.040mm Another check, and probably the most important and most likely to be out of spec is to check the piston ring end gap (fitted). This gap will increases as the piston rings wear and so measuring the end gap is a great way of knowing the condition of the rings. Place the piston ring in the top of the bore and then push it down using the piston skirt to the bottom of the bore just above the cut outs for the transfer ports, use a feeler gauge to check the gap. The specifications are 0.3mm - 0.5mm. repeat for each ring and replace rings that are out of spec. The final check is to meaure the piston ring groove gap, the groove can wear in time. measure using a feeler gauge between each ring and its locating groove. The specifications are 1st ring 0.02mm - 0.06mm, 2nd ring 0.03 - 0.07mm Should I leave it alone if its running okay or would there be some benefit in a full rebuild? From what I can gather theres unlikely to be much (if any) performance increase but would there be some possible reliability benefit? Another thing to do is strip and clean the carb, use carb cleaner and compressed air to blow out the jets, pay attention to the air jet Okay, good advice - It's not broken, however it did seem to lack a little power when I rode it home ??? I've got the engine on the bench with the head removed, the barrel off, piston off and rings removed. I've de-coked the head and now plan to de-coke the piston, it seems okay apart from the build-up of carbon. The rings look okay but I'm not sure what can go wrong with rings apart from breaking ? Any suggestions of what else to do whilst the top is off ??? As mentioned previously rings will wear and its a good idea to check for that regularly. Another thing is while you are de-coking take note of the size of the piston that is fitted at present, it will be stamped on the top of the piston towards the front, for example 0.25, 0.50, 0.75 this info will be needed when its time to replace rings I've toyed with the idea of polishing the ports, not making them any bigger just smoothing out the rough surfaces, although I'm not sure if this will make any difference ? Many thanks in advance. Always a good idea to decoke and polish the exhaust port as it will aid gas flow and delay the build up of further carbon deposits Lastly the air filter should be cleaned by rinsing in petrol squeeze it out and re oil with two stroke oil. ...Paul Link to comment Share on other sites More sharing options...
sezian Posted February 25, 2009 Author Share Posted February 25, 2009 Many thanks for the comprehensive info and very useful pictures, just the sort of detail I was after. I will certainly take your advice and measure everything prior to re-assembly and replace any parts that are out of tolerance. I know the piston is marked 0.50 which I believe is 2 sizes up from standard. You've mentioned its worth polishing the exhaust port which I will do but how about the inlet side ??? The casting looks quite rough and I'm thinking this would improve flow but not sure ? I have a new air filter installed so hopefully it should breath okay, I have the box section swinging arm model bike, although I'm not sure what the differences were to the air box ? Many thanks again for your assistance. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted February 25, 2009 Moderator Share Posted February 25, 2009 I know the piston is marked 0.50 which I believe is 2 sizes up from standard. Thats correct,plenty of room there for future re bores You've mentioned its worth polishing the exhaust port which I will do but how about the inlet side ??? The casting looks quite rough and I'm thinking this would improve flow but not sure ? Not sure myself either but wont do any harm so if you want to why not, also check yor reed petals are flat and that there isnt a big gap appearing between them and the reed cage, they tend to curve with age I have a new air filter installed so hopefully it should breath okay, I have the box section swinging arm model bike, although I'm not sure what the differences were to the air box ? Many thanks again for your assistance. I have read that your later version breathes better than the earlier one, I believe the two stroke oil tank is on the opposite side between the two models, and the route that the air takes through the later one is less complicated Link to comment Share on other sites More sharing options...
Moderator Cynic Posted February 27, 2009 Moderator Share Posted February 27, 2009 I’m interested to know if anyone has experience of completely striping the DT engine and rebuilding it? What parts are needed and where they can be sourced? The engine (when I bought the bike) seemed to be running okay (rode home approx 15 miles with no problems) and I was told it had new main bearings and a re-bore only a couple of thousand miles ago. The bike has approx 24,000 miles on it and has been used on and off for the last 19 years. Should I leave it alone if it’s running okay or would there be some benefit in a full rebuild? From what I can gather there’s unlikely to be much (if any) performance increase but would there be some possible reliability benefit? This is meant to reasure NOT scare..... Under my bench at the moment i have a 125MX engine that would start first kick and give useable performance yet when i took the barrel off the rings FELL out and i could measure the piston skirt clearance well over 1 mm although it is on its std bore. The engines in the DT's from the late seventies to the watercooled stuff are built to last, they will run and run, even when they are half dead they will give their all. With a recent bore and bearings (done properly) that engine will be great for another 20 thou. Link to comment Share on other sites More sharing options...
sezian Posted March 2, 2009 Author Share Posted March 2, 2009 Thanks Cynic, I guess the overall message here is leave it alone. I’ve now measured all the areas suggested and everything is well within tolerance so I’ve no worries with worn out parts. The only slight concern I have is there are some witness marks on the piston and in the barrel, it looks like the piston may have picked up something that has rubbed, hardly noticeable with a fingernail, in fact almost impossible to feel, is this a concern ??? Link to comment Share on other sites More sharing options...
Moderator Airhead Posted March 2, 2009 Moderator Share Posted March 2, 2009 Wouldnt bother me, sounds minor, oil the bore and piston on re assembly, when you first start the bike, hold the oil pump pulley wide open either by pulling the cable outer away from the adjuster tube in the case or by turning the pulley and hold it fully open until you are happy that the oil is primed into the carb spigot Link to comment Share on other sites More sharing options...
kiln Posted July 17, 2011 Share Posted July 17, 2011 Always a good idea to decoke and polish the exhaust port as it will aid gas flow and delay the build up of further carbon deposits Oldgitonabike:>Do you perhaps have more info on the exhaust porting bit, i.e. more pics on the procedure and more info on possible performance gains/losses after porting is done? Maybe a "Porting Post" please? Regards Kiln Link to comment Share on other sites More sharing options...
Moderator Airhead Posted July 17, 2011 Moderator Share Posted July 17, 2011 That barrel has been slightly ported...ie the top of the exhaust port has been moderately raised...I cannot comment further as I have never used it. It is really a job for someone with experience and the right tools, not for a novice. however rubbing the rough surface down to smooth with fine grades or wet'dry abrasive paper can be done by anyone. Link to comment Share on other sites More sharing options...
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