philhill Posted October 10, 2009 Share Posted October 10, 2009 Hi, help wanted. Have been a bit unlucky or un-careful recently as I have managed to hole two pistons on my bike. I had just finished rebuilding the DT as a 175 (previously 125 which holed a piston sparking off the 175 conversion) and it had been running fine except that when I checked the spark plug it was a bit sooty (dark brown) so I went one grove down on the needle, from 4 from the top down to 3. Took it down the bypass to see what it would do and holed the piston again this afternoon!! Oh BUGGER !!! The advice wanted is, do I need to strip the whole thing down again? Is it possible/safe to remove the barrel etc and turn the engine upside down and spray engine cleaner, WD40, high pressure water or whatever into the crankcase chamber to remove the shrapnel instead of a full strip down which I have just done previous on last hole? Then when I have rebuilt where do I start with the carb set up. The carb is a VM24SS 2H500 but set up like a 3U500 with a 1.5 Slide cutaway a 4J6 needle and 170 (instead of a 160) main jet. The 170 jet was to compensate for a Fresco exhaust and K&N type filter. Any help/commiserations welcome Link to comment Share on other sites More sharing options...
Moderator Cynic Posted October 11, 2009 Moderator Share Posted October 11, 2009 Hi, help wanted. Have been a bit unlucky or un-careful recently as I have managed to hole two pistons on my bike. I had just finished rebuilding the DT as a 175 (previously 125 which holed a piston sparking off the 175 conversion) and it had been running fine except that when I checked the spark plug it was a bit sooty (dark brown) so I went one grove down on the needle, from 4 from the top down to 3. Took it down the bypass to see what it would do and holed the piston again this afternoon!! Oh BUGGER !!! The advice wanted is, do I need to strip the whole thing down again? Is it possible/safe to remove the barrel etc and turn the engine upside down and spray engine cleaner, WD40, high pressure water or whatever into the crankcase chamber to remove the shrapnel instead of a full strip down which I have just done previous on last hole? Then when I have rebuilt where do I start with the carb set up. The carb is a VM24SS 2H500 but set up like a 3U500 with a 1.5 Slide cutaway a 4J6 needle and 170 (instead of a 160) main jet. The 170 jet was to compensate for a Fresco exhaust and K&N type filter. Any help/commiserations welcome Sorry to tell you this but dark brown with a little sotty stuff round the edge os spot on. If you want to do flat out runs down the bypass i would be going up on the mixture. Maybe going up 5 points on the main. Possibly more now the weather is closing in and getting colder. Although at all other time it will run like a sack of crap it will do better (as in last longer before it blows) on the bypass. Also are you sure the ig timing is right as that can cause the piston problems as well if its out. I think i've said this before but the DT175 isn't designed as a fast road bike. Its more a tickle through the trees, rag it up the hill, tickle through the trees kind of performance. If you look at the performance cousins of the DT like the IT and the YZ which are designed to run flat out for far longer you will see that the barrel and heads have massively more cooling area to handle the abuse they will get, also the IT stuff is a straight swap too (if you can find a good one). They (DT 175) can be tuned to go quite a bit quicker if you know what your at but it needs the barrel machining for port heights and piston timing etc. Not really a diy job unless you own or have access to a lathe. As far as it goes when the piston blew most of it will have gone straight out the pipe as it melted so the bottom will probably survive ok esp as you have just rebuilt it. Link to comment Share on other sites More sharing options...
philhill Posted October 11, 2009 Author Share Posted October 11, 2009 Thanks for the reply Cynic, I am wanting to use the DT as a road bike mainly to get to work over the winter, its great for town work but the bypass is part of my route as well and though there is a speed restriction of 50 it is tempting to stretch that out a little to see what the bike will do. You have warned in the past that the DT is not a bike for the bypass but in my teens I used to travel to and from Sheffield to Merseyside to see my parents whilst I was training, on a DT125 and about half that journey was on the motorway and didn't have any problem except for a blown exhaust once that was very painful on the ears! I would love to use the DT off road and there is a place called the 'Desert' just half a mile from me but it is illegal to use and has become too risky as police patrol and neighbours complain. If I could find a 'stealth' exhaust though I would have a go, shame no one makes one. With the previous carb set up ie none holing 4th from the top needle groove the engine made a loud clatter on a trailing throttle in low gears, low speeds, and loads of clatter and noise pulling away in 1st gear. This was reduced on the leaner mixture. If I go up 5 on the main but down to 3rd groove on the needle can I get the best of both? You seemed to suggest not? Would a cooler plug help and where can I find out more about the mods you describe? Thanks Phil Link to comment Share on other sites More sharing options...
Moderator Cynic Posted October 11, 2009 Moderator Share Posted October 11, 2009 Thanks for the reply Cynic, I am wanting to use the DT as a road bike mainly to get to work over the winter, its great for town work but the bypass is part of my route as well and though there is a speed restriction of 50 it is tempting to stretch that out a little to see what the bike will do. You have warned in the past that the DT is not a bike for the bypass but in my teens I used to travel to and from Sheffield to Merseyside to see my parents whilst I was training, on a DT125 and about half that journey was on the motorway and didn't have any problem except for a blown exhaust once that was very painful on the ears! I would love to use the DT off road and there is a place called the 'Desert' just half a mile from me but it is illegal to use and has become too risky as police patrol and neighbours complain. If I could find a 'stealth' exhaust though I would have a go, shame no one makes one. With the previous carb set up ie none holing 4th from the top needle groove the engine made a loud clatter on a trailing throttle in low gears, low speeds, and loads of clatter and noise pulling away in 1st gear. This was reduced on the leaner mixture. If I go up 5 on the main but down to 3rd groove on the needle can I get the best of both? You seemed to suggest not? Would a cooler plug help and where can I find out more about the mods you describe? Thanks Phil Yep i know what you are saying, but you were using 2star back then yes, wonderfull drop of lead slipped in there to neutralise the abrasive quality of petrol on valve seats, PISTONS, etc. And a better octane rating. As for the clatter i'd be checking the ig timing sounds like some pretty serious detonation going on there (causing slap). Link to comment Share on other sites More sharing options...
philhill Posted October 11, 2009 Author Share Posted October 11, 2009 Yep i know what you are saying, but you were using 2star back then yes, wonderfull drop of lead slipped in there to neutralise the abrasive quality of petrol on valve seats, PISTONS, etc. And a better octane rating. As for the clatter i'd be checking the ig timing sounds like some pretty serious detonation going on there (causing slap). Thanks Cynic, i will have a look at the timing when the rest is fixed but to be truthful i have no idea where to start with it as the timing is pre set with the CDI. The coil plate is fixed as previous with marks on the plate lined up with line on the crank case. There is a potential risk of the flywheel moving i suppose as I had probs tightening the flywheel onto the crank taper so will check this later. Thanks Phil Link to comment Share on other sites More sharing options...
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