sheffy Posted December 25, 2012 Share Posted December 25, 2012 Hello, I recently purchaced a 1999 DT 125 R. I have had it serviced and have had fresh tires and a new expansion pipe. The old one had been eaten through by rust and so was blowing. This is where the problem lies. I left the garage and allowed it time to warm up properly and took it for a spin. It lost all its low and medium end power and hits a powerband at around 8k revs. It feels slower and is difficult to keep in the power. I went on the dual carriageway to see if it would pull through each gear and see what it tops out at. The top speed went from 81 with the derestricted stock pipe down to 71 with the new exhaust. When I change into 6th gear from 5th at 10k revs it 'bogs out', stops accelerating, and slows down until I have to change down. Could it be running lean as it wasnt upjetted from the previous exhaust and apparently you need to go one size bigger on the main jet? 240 to 250 is it? Thanks in advance. Sheffy. Link to comment Share on other sites More sharing options...
kenDAWG Posted December 25, 2012 Share Posted December 25, 2012 well 6th gear for me has hardly any excelleration also and i only use it to hold the speed i was doing in 5th just at lower rpm, when ever thing get changed on 2 strokes especially intake/exhaust it is a good idea to get it properly jetted to get the benifits. im lucky with mine it has been derestricted with full dep and snorkel removed and the spark plug is a nice light brown colour!! chuck in a new plug and run it for a few mile then have a look at the colour see how she is running, but the mixture is crucial to these 2 strokes you dont want her going wrong on you. merry christmas ken 1 Link to comment Share on other sites More sharing options...
sheffy Posted December 25, 2012 Author Share Posted December 25, 2012 Well is there a large chance of seizure? It will do 70 odd in 5th but literally decelerates in 6th, Not what I had hoped for. Well the bike has a full big one system and just isn't running well. Should I remove the snorkel or shall I order a few main jets slightly bigger? Thanks. Sheffy. Hope you all are having a good xmas! Link to comment Share on other sites More sharing options...
dt502001 Posted December 25, 2012 Share Posted December 25, 2012 Yes it could be running lean and that will cause seizeur,best to check the plug colour as suggested.If its white best to move the neddle up untill you get some new jets and take it easy.No need to fry a piston for for the sake of a few pennys to buy bigger jets. I'm not sure on what years but some years came with as low as a 210 main stock. Best to pop open the bowl and look but the 250-260 is the right set up from all the other threads I have read about re jetting for a pipe on that bike. Merry x mass 1 Link to comment Share on other sites More sharing options...
sheffy Posted December 25, 2012 Author Share Posted December 25, 2012 Would running lean cause a loss of power? It just doesn't rev out very quicly and revs take a bit longer than usual to return to idle. It is so much slower than previously. Cheers for your help guys. Keep it coming if possible. Sheffy. Link to comment Share on other sites More sharing options...
blackhat250 Posted December 25, 2012 Share Posted December 25, 2012 Well loss of low end and mid-range , points to the power valve stuck open i think , am just thinkin up the lines of the race bikes before P,valves, they had to slip cluch to reach high revs , and keep it on song, My opinion tho, merry Xmas,,,, Link to comment Share on other sites More sharing options...
Moderator Cynic Posted December 25, 2012 Moderator Share Posted December 25, 2012 Could actually be the otger way. You said the old pipe was de restricted. It may be rich. Does it need choke to start...? Link to comment Share on other sites More sharing options...
dt502001 Posted December 26, 2012 Share Posted December 26, 2012 Generaly speaking a lean mix will be more powerfull,as blackhat suggested the PV might be stuck open? have you pulled the plug to check the colour yet? Link to comment Share on other sites More sharing options...
sheffy Posted December 27, 2012 Author Share Posted December 27, 2012 To be honest, changing the exhaust would not cause the PV to stick open I haven't got a plug spanner to check the spark plug but to me it feels like under fueling when i hit high revs = main jet too small? Cheers. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted December 27, 2012 Moderator Share Posted December 27, 2012 To be honest, changing the exhaust would not cause the PV to stick open I haven't got a plug spanner to check the spark plug but to me it feels like under fueling when i hit high revs = main jet too small? Cheers. Well if thats the case prove it by briefly switching on the choke Link to comment Share on other sites More sharing options...
kenDAWG Posted December 27, 2012 Share Posted December 27, 2012 get your hands on a plug spanner its the easyest way to tell how its running and there like £5 along with a new spark plug (iridium is my choice) sounds like you need to go up a main jet but if you go up a main jet you wont know if its correct unless you checkyour plug afterwards i cant emphasise how important the mixture is on a 2t you get it wrong it will cost you more than a plug spanner and a couple of plugs. cheers ken 1 Link to comment Share on other sites More sharing options...
sheffy Posted December 27, 2012 Author Share Posted December 27, 2012 Well if thats the case prove it by briefly switching on the choke Well if thats the case prove it by briefly switching on the choke I'm sorry but I don't understand what turning on the choke will prove.... Thats the first thing i did and it just bogged down too much to notice a difference in the higher revs. Link to comment Share on other sites More sharing options...
sheffy Posted December 27, 2012 Author Share Posted December 27, 2012 Could actually be the otger way. You said the old pipe was de restricted. It may be rich. Does it need choke to start...? Yes the old pipe was derestricted, gutted and welded back together. But this new pipe is an aftermarket pipe that is basically the same but just lighter. Its a big one ( ex fresco). Link to comment Share on other sites More sharing options...
sheffy Posted December 27, 2012 Author Share Posted December 27, 2012 Thanks ken, I will try tomorrow. I spoke to a local bike mechanic who said "Check the main jet size, if its less than 240... don't ride it" By that i presume he means that anything less that 240 will fry my piston ? does anyone know what size i need to buy, 5mm or 6mm etc. Link to comment Share on other sites More sharing options...
robertdtr Posted December 27, 2012 Share Posted December 27, 2012 I've heard this is pretty common with the big one exhausts and remember reading somewhere(90% posiive it was the dt forum) that adjuting th powervalve slightly to the right solved this problem. Link to comment Share on other sites More sharing options...
kenDAWG Posted December 27, 2012 Share Posted December 27, 2012 I'm sorry but I don't understand what turning on the choke will prove.... Thats the first thing i did and it just bogged down too much to notice a difference in the higher revs. turning the choke on reduces the airflow so if the bike is running lean then this would infact make the bike run better its just used to check the running... im not sure on the jet sizes mine was setup by a garage, if the mixture is wrong you should never run your bike especially a 2t you will risk ruining your topend and maybe your bottom end. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted December 27, 2012 Moderator Share Posted December 27, 2012 Thanks ken, I will try tomorrow. I spoke to a local bike mechanic who said "Check the main jet size, if its less than 240... don't ride it" By that i presume he means that anything less that 240 will fry my piston ? does anyone know what size i need to buy, 5mm or 6mm etc. its a mikuni large hexagon type putting the choke on will richen the mixture of course,... so if it pulled better even briefly it would back up your suspicions of running lean Link to comment Share on other sites More sharing options...
sheffy Posted December 29, 2012 Author Share Posted December 29, 2012 its a mikuni large hexagon type putting the choke on will richen the mixture of course,... so if it pulled better even briefly it would back up your suspicions of running lean Well i'm not sure of this because... yes it works as a simple diagnosis in some cases as restricting the air flow richens the mixture causing a 'better' running condition. However I think in my case, its under fueling at the top end not lean (in a sense it is lean lol) Any how it didn't make a positive difference, just bogged severely. From that I presume its running well untill higher revs. I'm about to order some more jets, as someone said its worth it for a few quid to prevent seizure or worse Stock is 240, so shall I order 3 or 4 or more higher? Thanks guys, you've been great! Link to comment Share on other sites More sharing options...
Moderator Airhead Posted December 29, 2012 Moderator Share Posted December 29, 2012 it wont need more than a 250 if indeed it needs that but by all means try a 250 However at this stage I doubt it will give you much of a performance difference, it could be that the new pipe simply isnt as good as the stock one Link to comment Share on other sites More sharing options...
sheffy Posted December 29, 2012 Author Share Posted December 29, 2012 it wont need more than a 250 if indeed it needs that but by all means try a 250 However at this stage I doubt it will give you much of a performance difference, it could be that the new pipe simply isnt as good as the stock one Not to sound offensive, but you don't recommend re jetting for a performance exhaust, where as almost everyone i've spoken to (online and in garages....even the shop who sold it me) says it may need rejetting. Not a dig by the way! Cheers for the help! Link to comment Share on other sites More sharing options...
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