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DT 175 mx (1979) will not " rev out " ...

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:ph34r: I have recently aquired another DT which had a few amateur "restoration" issues >, engine oil in the 2 stroke tank ! , copy part barrel and head , seized suspension pin ect , anyway i have been through all the bike swapping some parts from my other DT which is undergoing a total restoration ( i have never run this bike yet ) , this has included a yamaha barrel rebored 1mm over with Mitaka piston , all new carb internals bar the floats plus rubber joints and yamaha filter and new cables all round , i have reset the clutch arm to cable so there is now some free play and checked the float height 3 times as someone had put a big fat copper washer underneath the needle valve (which i have now replaced with the proper part) also swapping the oil pump , the bike starts and idles and seems to be running well on the road , not smoking excessively and pulling well in the Low / Mid range) i have ran in the new bore for 500 miles and recently added one more tooth to the front sprocket but she still struggles to rev past 6000 / 6500 , any advice would be welcome , Pete / Lancs .

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If your talking about top gear and trying to get the old girl to rev past 6000 you would have wanted to put a 1 tooth smaller sprocket on Not bigger. If the lower gears will pull past 6500 she just dosen't have the steam to pull that tall gearing.

If its is all gears then u need to rejet have a look at the plug colour should be coffey double cream colour.The best way to check is to take a socket with you run the bike as hard as you can in top gear ,pull in the clutch and kill the eng. pull the plug out and check if white BAD needs more fuel, dark to rich.

Now the hard part only change 1 thing at a time OR YOU WILL CONFUSE YOURSELF depending on how rich or lean will determin how to correct it , main jet,air fuel screw or piolet jet or raise lower the needle.

Jets are easy bigger or smaller ,main mostly for big gains in fuel ,,piolt for fine tuning

needles come in so many different shape you could spend the rest of your life playing around with different combinations. So just make sure you have the right 1and someone didn't put in the wrong one. And the clip is in the right position normaly the middle, also for more fine tuning of the midle range of fuel delivery.

The air fuel screw I go 1/8 of a turn at a time Mark it as to where it is now and use that as your reference point , as in turning it in better or worse then if better than keep goin in that direction untill you get it right.But if its worse than you know you can go back to the orignal point and +1/8.

Everytime you change something do a plug check. This is a hard thing to get right so be paitent once you learn how to make all the small adjustments your bike will work better than most.

Now if I haven't total scared you away get at tuning your bike Cheers out Phil K

O PS. that fat washer may have been part of a "tuneing kit" which is a box of new parts jets needles and washers that all need to be there to get it to work right they were real popular so double check the # on you main and needle

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Just re read your post that copy head and barrel is probably a performance set up lots of differed companys were making that stuff back in the late 70 and early 80 as the orignal weren't that good for racing ,you might want to have a look into who made it DG is the only name that come to mind right now ,some realy good stuff different port hights/sizes/ shape,, different compression ratio with better flowing heads. You might be sitting on a rare gem and if thats the case your carb would have been modified to work with that set up ,,right down to over drilling some of the passage ways.

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thx Phil but you missread my question i think , the pattern barrel is in the skip , this was a recent copy and i did not want to run it so i pinched my proper Yammy barrel from the other bike i have , the bike will not rev past 6/ 6500 even with the standard sprockets 15 / 49 so gearing is not the issue . i will start by removing the inline fuel filter i fitted , possibly look at crank seal demise ? ( although she is not burning any engine oil ) , someone has had a look at the electric rotor side so i will investigate further in this area , i will find the problem eventually as there are only so many things it could be ... thanks for your time in replying / Pete .

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What cdi does it have, the 79 had a 2 coil set up one for low to medium rpm the other for high (6000+) rpm.

Look behind the flywheel if there are 3 visible coils then the cdi needs 7 wires to work.

If you mix the components up the bike will run but will do some strange things like capping the rpm.

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hello guys , well i took the inline fuel filter out of the equation and low and behold this was the problem ! ( i didn`t understand that fueling was so critical , the filter was choking the motor at higher revs ) , i will look at the front pipe when i have time , Lotherton Hall Show on Sunday 29th so we`ll see how she copes with a lengthy 70 mile " A road " run , bye for now ~ Pete / Lanc`s .

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* additional notes ~~~ bike has been running weird , low to medium throttle ( say 6000 ) is fine but a recent 200 mile trip showed she was not 100% at all , struggling to pull 35mph on full throttle in certain circumstances pfff , i could not stop the carb overflowing no matter what i tried , changing the float height 10 times or more even so there was not enough fuel ! so i changed the new needle valve for another new one = still same problem , i changed the floats from an identical carb = still same lol , changed the whole carb tranferring all the new valves and needle ect into the other carb body , then she holed a piston ! , i think this issue is timing related as it was the only thing i had not checked after purchase , so i`m trying to set the ignition at the moment after fitting another 1 mm over piston , i have carefully taken out the small tube in the header , sure she`ll be fine after i set timing , i hope ... happy biking !

/ Pete Lanc`s :P

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could be timing related if someone had set it wrong but there again and more likely than that is the carb vent or the pipe attached to it is blocked

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