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mikkopoika

DT 125 lc+Gianelli exhaust

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Hi
I'm new at the club but I've had very much help reading the old topics with my glorious battle with all the problems and faults with my old DT 125 LC. Thank you for that
Well I finally managed to det the old bugger up and running nicely. This ofcourse did not anymore satisfy my twisted need to fiddle about with the machine.
Since I got a bargain price on a Gianelli exhaust I decided to get a bit more fun from the bike and bought me one.
I was looking forward just to bolt it on and head for the fun. Did'nt quite go like that. The Gianelli was more of a weld on than bolt on. Had to shorten it from the rear to accept the original suppressor, weld a bracket to it and new nuts to hold the heatshield. I was thinking that this isnt for this type of bike, but anyway I had it fitted properly.
Then to the road and off from it. The sound was promising. It reved better and the best power was at about 9000 to 10000 rpm. With the original exhaust it was at some 7000.
I tested how fast it was and then it hit me. Disapointment. It took about 100km/h with 3rd gear and when i flipped the 4th in it just bogged. Like the main jet was too small. So I fitted mainjet two sizes bigger to 270. Still the same. Tried without the silencer, nothing but amazing sound.
So it lacks 20km/h from the top speed wit the original exhaust. Was 130km/h.
Is the mikuni vm26 just too small carb or is it something else?
I'm not somuch after high top speed, but this does not seem good as I might burn the piston with weak mixture on hard riding
The settings and mods in the bike are as follows:
Model dt125 lc mk3 -87
Carb. original mikuni vm26
mainjet 270
idlejet 30
airfilter foam pod
needle 3rd (middle)
fiber reeds
sprockets original
hmm.. what else.. rider some 65kg
almost fourty
still a kid =)
help needed
Thanks

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You need to do a plug chop....its the only way to gauge a 2stroke"s health,,, :yeah:

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How did you figure 2 sizes bigger was 270 and that it needed more fuel?. Is it a new pipe? Does it need decoking? What is the air filter like? Plug condition/age?

Thats just for starters 2strokes are a funny old beast.

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Short story long:

The bike was jetted with 170main and 25 idle jets. I took off the original airbox since it was broken. Fitted a foam pod type filter that is much more open than the airbox. After trial and error setteled to 250 main and 25 idle with the old exhaust, fiber reeds and ypvs at working condition.The exhaust has been burnt with a butane torch so it's not clogged, filter is clean ,plug is good and new. piston is new, compression is ok. With this setting the bike works like it should.

Then with the gianelli:

So the 270 is two sizes bigger than 250 (?). worked out the lack of fuel from when I hit 100km/h in third gear and change to fourth the bike behaves just like it did with too small mainjet. And since it works fine with 1/4to 3/8 throttle the lack of power refers to mainjet, or perhaps in this case , as the bigger jet did not help, to too small carb. Atleast this is what I'm suspecting.

A friend had a DTR that was fitted with 170cc cylinder and gianelli exhaust. He had a 32mm mikuni in it and it took the same jetting as the original exhaust. This kinda supports my theory on the carb size.

About the plug chop. It used to be a good method back in the day when we didn't have unleaded fuel with 5%alcohol to go with. My humble opinion in reading the plug is that it tells you if youre running very lean or very rich. The lead gave the colour to the plug. Anyway after a normal ride the plug looks to be okay, coffee brown that is. Havent tried the chop yet.

anyway, this is really strange, atleast to me.

Thanks for interest so far.

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Ok so everything you have said so far would make me think you have a sound knowlege, when jetting did you play with needle height settings before changing jet sizes? Many time I have been given a bike that someone has over jetted when a needle adjustment was all that was needed. A 270 main sounds too big to me unless you live in the mountians.

I would do a wide open top gear plug chop as suggested,2 stroke work best when run lean ,too lean and they blow up.Anyone who dosent belive this, just try driving with the throttle at one spot and shut your fuel off ,as the carb empties you will see the rpm go up with no throttle adjustment for a short time

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Sure I fiddled about with the needle. Sound suggests that (before the gianelli and the270 jet) with 250 jet it runs bit rich on about 1/4 throttle to cool down between rewing. On full throttle it sounds to be just right and idle should be on the spot. On full blast the needle is completely out from the jet, so it shouldn't have effect at all. I'm guessing that I should bother to give it the plug chop and before that try different needle positions. But thenagain, would bigger carb do the trick? Opinions are welcome.

Oh and about the mountains. No. I live in southern finland. No mountains here. Horrible summer still. temperature goes with in week from +5C to some +22C and the constant rain has some effect on jetting...

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Whats actually happening at full throttle, is it holding back,, if your lean, [ too small a jet] it will rev out clean , and seize if held there too long, if its holding back, [ bogging] your too big on M/J.

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It accelerates like a bat outta hell to 100km/h with full throttle but after that when changing to fourth gear it just does not have it to take the rpm up again, so yes holding back would probably be the term. But still, it did exactly the same when I burnt a hole in previous piston with too small mainjet. At that time it was definetly lean. After that I checked that there was no airleak, so I'm pretty sure that there isn't one now.

How about I get a thermometer to measure the exhaust gas? Should tell whats really going on when it hits the wall.

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Good call " i"v gat a spark plug temp gauge on my TS . uphill wide open,T, is the highest reading . sounds like float hieght,, as your being starved of fuel on big demand,,

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Sound advices. If it stops raining I'll try out atleast the plug chop and check the fuel level and not in that order. Finding a thermometer that can take the heat might be a problem still. I'll let you know if the problem solves.

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Yes! It did not rain this afternoon.

Checked the fuel level, it was bit low. I had forgotten to bring it to right level last time when I had electric failure wich symptoms were like flooding. So I just assumed the f.level was right but actually I had forgotten to bring it back. Demented me.

Did the plug run. Didn't bother to chop it since it seemen to be ok, milk coffee with wery little milk on it.

Fitted the gianelli back to bike, did another testrun. Again with cutting off the ignition. Plug had the same brown colour as before. Mainjet was at both times 250.

Still: Top speed remained the same. again at fourth gear the bike wont rev back to some 9500rpm. I'm beginning to wonder if it's a sprocketing thing because it does not seem to run lean and it's not overheating atleast according to bikes temp gauge.

As I mentioned before I dont so much need the high speed I mostly drive small dirtroads, pit and forest trails. So a bigger rear sprocket then? Is it possible still that with a bigger rear sprocket I could get greater highspeed if the bike then sort of has the power to get through the fourth and maybe fifth gears? I know that normally bigger rear s. decreases speed. But I think you get what I'm on about.

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Over geared" :thumb:

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Got a 14 teeth front sprocket. Rear is still 50. Took the bike for a run and now it was much more fun (rhymes :spin2: ) It now could handle fourth gear all the way to 12000rpm. Didn't accelerate on fifth thoug. Maybe a 53 teeth rear would make it even better. And I would get some use of the fifth gear then.

So it was overgeared.

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