Full power french import won't go over 60mph.
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This is a bit of a tee off from my power valve cable thread, however after confirming there is no big issue with my cables I thought it was better to continue the discussion in a more relevant thread of it's own.
So for people that didn't see my PV cable thread, I own what was and should be a full power 1993 french import DT125R. Back when I first got this bike it would happily sit with the clocks maxed out about 80/85mph. But I left it sitting under a cover for a while and unforunately it got a crankcase full of water and needed a rebuild.
So I pulled the engine. Fitted a Top Racing complete crankshaft with new crank bearings, upgraded small end bearing, Japanese Mitaka piston and a full Athena gasket and oil seal set. Then stripped the carb completely and made sure there was no corrosion or old full blocking anything, putting it back together with the original 240 main jet.
First test ride. Bike started nicely warmed up and seemed to ride okay at low rpm down my street, but as soon as I opened it up once it got to about 7k rpm it felt like I was hitting a hard rev limit. If it feathered the throttle down hill I could get it to rev past 7k, but under full throttle it just killed all power when I hit it. So took it home and pulled the plug which was looking very rich even though it was on the stock jets for an import. Decided to tryt a 220 main jet in it and this actually helped a little. Plug colour improved and it would now rev to 8k not 7k before doing the same. So took a guess the coil was breaking up under high rpm and fitted a new "racing" coil from square motorcycles, put a fresh plug in it and put it back on the 240 main.
Now the bike is much better. Pulls better, plug colour looks good and it revs all the way around to 11k. Well all the way to 11k in the first few gears anyway because it still won't go over 60mph.
Crank, Rod, Piston, Bearings, Seals = new
Carb clean and stock jetting
Coil, Plug = new
Power Vavle is set up and working correctly as per my previous thread.
Air filter is cleanRunning out of ideas, where would you guys recommend looking next?
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First things first, bin off that coil. Get yourself a genuine 3RM one.
https://www.fowlersparts.co.uk/parts/view/4RP8231026
I had a horrendous issue that took me months to diagnose, since it would run absolutely fine from cold-warm. But after a short while it just refused to rev. It took me literally forever to figure out, I swapped loads of bits out and considering my bike was a "fresh" rebuild I didn't think it would be the "Racing Coil" I bought. It hit me when my brother was having problems with his RD I noticed he had the same type of coil and the tuner said bin that immediately and all his problems went away!
So first things first, get yourself a genuine coil.
Later coils are a different part number:
https://www.fowlersparts.co.uk/parts/view/3KJ8231013So it might be that your coil isn't right for your bike/year. So just source the correct year/model coil for your bike and make sure it's a genuine one (Denso I believe makes them).
Secondly, I've seen this before whereby the exhaust got full of water after a DTR was stood outside for a long period of time and this had very similar behaviour. Running rich and lacking the ability to rev/pull hard.
So whip that exhaust off and give it a good clean, it was very odd and not something I had considered.
Later model DTs were fitted 210 main jet, but again this all depends on your carb that you're running and whether or not you're fitting genuine Mikuni Jets. Ideally you want to be fitting the right jet for the year/ignition etc.
