wild foamy Posted February 11, 2009 Share Posted February 11, 2009 Hi everyone. Have fitted the big bore kit to my DT50 and this time added a bigger jet (#115, from the #98 that was originaly in there) and now it seems to be overly rich, starts on a light kick first time every time and idles very nicely, but in the low-to-mid range it feels very slow, if im in 5th gear at 30/35mph on a flat, it will barely pull me and only when i change down a gear does it accelerate. at first it was bad to the point where in the mid-range it would bog down alot, but i dropped the needle one position and it seems to have solved that. there is a large powerband high up the RPM, it can be physically felt when the engine reaches this as it really takes off but as soon as you change up a gear you loose allt he power and its back to square one. so whats the problem?, too rich?, too lean? i had a fiddle with the idle mixture screw a while ago to get it to idle nicely on the standard bore. regards Steve Link to comment Share on other sites More sharing options...
Moderator Airhead Posted February 11, 2009 Moderator Share Posted February 11, 2009 How come you did this Steve, have you decided to keep it now? Link to comment Share on other sites More sharing options...
wild foamy Posted February 11, 2009 Author Share Posted February 11, 2009 not really, just want to get it running right. ive had the DT 4 months now and im 17 this October, so thats only 8 months away, i dont think there really is much point in me getting another 50 if im just keeping it for 8 months. and after im 17 i will be taking my full test and playing with the Jawa instead, the DT will either be sold on or tucked up in the corner of the garage to be exhumed in a few decades. plus it is my day-to-day bike and needs to be running right, and apart from sending the first big bore kit into meltdown by not jetting it properly it has been reliable and i want to keep it that way Link to comment Share on other sites More sharing options...
Moderator Airhead Posted February 11, 2009 Moderator Share Posted February 11, 2009 , and apart from sending the first big bore kit into meltdown by not jetting it properly it has been reliable and i want to keep it that way If the jet was too small (lean running) and this causes engine to run too hot, characterised by pinking (pre ignition) and eventual hole burned in piston or at the very least a pock marked cyl head and piston crown...........so..........how much of this was true the last time it broke down on you with this big bore? and incidentally did you re bore it because you said you had score marks near the exhaust port? I'm not convinced it ran lean last time from what i've read so far Link to comment Share on other sites More sharing options...
wild foamy Posted February 11, 2009 Author Share Posted February 11, 2009 well with the first big bore kit i found deep scores all down the sides of the piston and the ports on the barrell, the rings were scratched and would not seal which resulted in lack of compression and the engine refused to start. for £30 i bought a whole new bore and barrell, at the time it seamed a much easier thing to do. what temperature (*C) should it be running?, i have an infra-red temperature gun and can take a reading running the current jet and a reading with the smaller jet and compare the two to see if the smaller one is making it run excessively hot. i dont particularly want to down-size the jet again and risk screwing up another big bore kit... the stock mikuni jet for the DT50 is an #88, and my one has a full BigOne exhaust and expansion pipe kit, a K&N filter and i think running a big bore would be too much for the #98 jet to handle. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted February 11, 2009 Moderator Share Posted February 11, 2009 But it didnt seize did it? you said it died, not a very descriptive term that! when a bike seizes it either a wont kick over or b locks up the back wheel and comes to a sudden stop what did yours do? Link to comment Share on other sites More sharing options...
wild foamy Posted February 11, 2009 Author Share Posted February 11, 2009 well it was there one minute (i was going uphill so had dropped a gear and was revving it higher than usual) and then completely cut out, no popping or other noises, just asif you had hit the killswitch and it was just winding down off the momentum from the back wheel, not seized up, just not firing spent several minutes kicking it and trying to bump it up and down the road but to no avail, after about a dozen kicks it would make a huge bang, scareing the sh*t out of me and the surrounding woodland inhabitants Link to comment Share on other sites More sharing options...
Moderator Airhead Posted February 11, 2009 Moderator Share Posted February 11, 2009 Still not convinced it was lean though, youre just going to have to do the old plug chop routine, wide open throttle up a steady incline after about 1/2 mile cut the engine and pull the clutch. Coast to a stop and examine your plug, you know the script youve heard it before and plug colours are in Haynes manual> Link to comment Share on other sites More sharing options...
wild foamy Posted February 11, 2009 Author Share Posted February 11, 2009 will do, need to drop into halfords tomorrow and get a 14mm 1/2'dr spark plug adaptor... my toolkit only came with a 10mm one Link to comment Share on other sites More sharing options...
JUST ME! Posted February 12, 2009 Share Posted February 12, 2009 Hi everyone. Have fitted the big bore kit to my DT50 and this time added a bigger jet (#115, from the #98 that was originaly in there) and now it seems to be overly rich, starts on a light kick first time every time and idles very nicely, but in the low-to-mid range it feels very slow, if im in 5th gear at 30/35mph on a flat, it will barely pull me and only when i change down a gear does it accelerate. at first it was bad to the point where in the mid-range it would bog down alot, but i dropped the needle one position and it seems to have solved that. there is a large powerband high up the RPM, it can be physically felt when the engine reaches this as it really takes off but as soon as you change up a gear you loose allt he power and its back to square one. so whats the problem?, too rich?, too lean? i had a fiddle with the idle mixture screw a while ago to get it to idle nicely on the standard bore. regards Steve Geday mate. This time around you are approaching it the correct way. Far better to start way too rich and work back. Initially concentrate on the upper rev range, main jetting, and work down. So long as the plug is sooty, you now have to gradually decrease the main jet size until the thing starts to perform properly hence why it is good to come to some arrangement with your bike shop re borrowing and replacing jets. Without witnessing your previous drama first hand,on the information you put forward, I feel sure last time you leaned it out. If it was mine I would be premixing the fuel at least until you have the jetting correct. Reduce the main jet size in small increments. Becoming impatient will cause another blowup! Best of luck! Paul. Oh yeah, don't ride it expecting a mind blowing difference in performance. A 10cc increase is only going to make a small difference. Starting to wonder what effect this kit is having on the port timing. Is it a good quality setup? I frigged around with wiseco oversized stuff on DT250s years ago and found their pistons at the time to be complete crap! Anyway, soldier on and see what you achieve. Link to comment Share on other sites More sharing options...
wild foamy Posted February 12, 2009 Author Share Posted February 12, 2009 the top end is fine, as soon as it is past the midrange it really takes off and you can feel it. dont really want to go downsizing jets incase i make the top end too lean... Link to comment Share on other sites More sharing options...
Moderator Airhead Posted February 12, 2009 Moderator Share Posted February 12, 2009 Thats fine but you need evidence that its fine with the plug chop, hence the wide open throttle. This has to be done before any further adjustment, get the main jet size established. Then when this is done you plug chop again at 3/4 throttle and use needle height adjustment to correct here Link to comment Share on other sites More sharing options...
mckenzie Posted February 12, 2009 Share Posted February 12, 2009 well it was there one minute (i was going uphill so had dropped a gear and was revving it higher than usual) and then completely cut out, no popping or other noises, just asif you had hit the killswitch and it was just winding down off the momentum from the back wheel, not seized up, just not firing spent several minutes kicking it and trying to bump it up and down the road but to no avail, after about a dozen kicks it would make a huge bang, scareing the sh*t out of me and the surrounding woodland inhabitants sounds to me like the rings have nipped up, this happened to me with my Italjet formula and my dt. did you notice if there was less compression? Link to comment Share on other sites More sharing options...
wild foamy Posted February 12, 2009 Author Share Posted February 12, 2009 compression was pretty much gone and no amount of bumping it or kicking it would get it to run, only the occasional loud Bang. Link to comment Share on other sites More sharing options...
mckenzie Posted February 12, 2009 Share Posted February 12, 2009 yep, as i suspected, the worst case senario is a fresh piston kit and hone the bore, although if it has bit in to the barrel it will need a rebore, maybe .25 oversize to clean it up. and you may need to split the cases if there are bits of rings and piston missing. have you instpected the barrel yet? my guess is there will be a few pickup marks on the exhaust side of the barrel, as to the cause it could be your carbuation, ignition problems ie detonation or pre-ignition, or not enough oil getting into the engine. have a look at your barrel and piston and post a few photos up of the damage Link to comment Share on other sites More sharing options...
wild foamy Posted February 12, 2009 Author Share Posted February 12, 2009 erm, the big bore i had on it is gone, the bore was fine albeit a little scratched but the piston had very deep scratches and the rings were ruined. for the price i was quoted for a re-bore and new piston kit i picked up a second brand new kit off ebay and that has not missed a beat since ive had it. did a plug chop and the original plug is a chocolatey brown colour, also put a new plug in there but needs to run it in before doing a plug chop as it doesnt seem to be showing any particualr colour well. if i turn the fuel tap off it does seem to run better through the mid-range before cutting out. Link to comment Share on other sites More sharing options...
mckenzie Posted February 12, 2009 Share Posted February 12, 2009 erm, the big bore i had on it is gone, the bore was fine albeit a little scratched but the piston had very deep scratches and the rings were ruined. for the price i was quoted for a re-bore and new piston kit i picked up a second brand new kit off ebay and that has not missed a beat since ive had it. did a plug chop and the original plug is a chocolatey brown colour, also put a new plug in there but needs to run it in before doing a plug chop as it doesnt seem to be showing any particualr colour well. if i turn the fuel tap off it does seem to run better through the mid-range before cutting out. could be overfuelling, your jetting may be wrong although the plug colour suggests its burning ok, have you checked the position of the needle in the carb?? i see someone has already mentioned that but i guess this is the problem Link to comment Share on other sites More sharing options...
wild foamy Posted February 12, 2009 Author Share Posted February 12, 2009 have you checked the position of the needle in the carb? yeh, when i first put it on it was bogging in the mid-range to the point where it would not rev higher and sounded like it was 4-stroking a lot, i dropped the needle one position and this helped a bit Link to comment Share on other sites More sharing options...
mckenzie Posted February 12, 2009 Share Posted February 12, 2009 yeh, when i first put it on it was bogging in the mid-range to the point where it would not rev higher and sounded like it was 4-stroking a lot, i dropped the needle one position and this helped a bit well you seem to be on the right track, how is it running now? Link to comment Share on other sites More sharing options...
wild foamy Posted February 12, 2009 Author Share Posted February 12, 2009 see first post Link to comment Share on other sites More sharing options...
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