I have a 1984 dt50mx which after not running for 34 years ~I decided to revive. It was in a bad way as had been stored initially in garages and sheds, but the last 10 years under a tarpaulin. Long story short after a lot of TLC and replacing some parts I got it running and in August took it for MOT and it passed. Since then it started to lose power, then became more difficult to start. No it will not start. It has a good spark, timing is at 1.9 bedchamber, there is fuel. Compression seems okay. I have had the top off and replaced gaskets. The piston, rings and bore immaculate. It has only done 1k miles on the current barrel.  It has new crank seals and reeds plus new coil, rectifier, battery, condenser, points, plug cap.

 

Any ideas as I have now run out

Standard questions. 

Colour of the plug. 

Bright blue spark at the plug. 

Is the plug wet after repeated kicking. 

Plug wet and small amount oil.

I have also checked resistance on stator coil and ignition coil and all seems correct.  It ran yesterday on second kick, idling and revving for a couple of minutes nice and responsive. The first time of got it to start in two weeks then it cut out. My initial though it was fuel starvation. For no apparent reason it did not start after that. Today at best it runs for three seconds regardless of choke on or off or switching plugs

And yes bright blue spark. It was yellow before replacing coil

Have you checked for floaters in the fuel. 

They can randomly block pathways in the carb and then clear. Causing your symptoms. 

Hi Cynic

I have had the carb ultrasonic cleaned as it was basically full of resinous old petrol. The floats are sealed, needle good and jets replaced

Might be worth ruling out the petrol cap.... they are vented to stop a vacuum forming inside the tank, which can cause the engine to stop with fuel starvation. Had this happen on my 400/4.

Sometimes the seal sticks or the outlet tube on a trial is blocked....

Rule it out by seeing if it will run with the cap unscrewed/off.

....if it still won't start/run you know its not the cap.

it's a simple enough test with no cost involved....unless it's the cause!!

 

The seal acts like a one way valve, it lets air in but stops petrol leaking out.

Edited November 29, 20213 yr by NE0

The tank was stripped delisted and revealed with POR15 so no bits in it   New fuel tap too. The fuel flows well out of pipe with or without the cap on.  This has been cleaned and breaths well. But a good shout as i had this issue in a Kawasaki AR many years ago

Okay, heres another thing to rule out.....

What about the exhaust? if its choked it won't run.

Try disconnecting the silencer, it will be noisy....but unchoked.

2 strokes rely on a bit of back pressure to run successfully, so you won't be able to ride without it, but it would give you an idea if it's choked up too much.

if it doesn't start you know its not that.

I have caustic cleaned and washed out exhaust. I have also replaced packing in silencer. There was some old oil and some carbonisation but not bad. The bike had only done 4033 miles in its first year (1984) and then not run until this year. I really am struggling with this.  I have even bypassed wiring on stator direct to coil and makes no difference

if its only done so few miles it won't be a choked exhaust then.

However, you say the timing is 1.9.

is that for leaded 4 star fuel? which it was designed to run on back in 1984.

Now your using unleaded.

I'm wondering if you need to advance the ignition timing slightly to compensate.

 

 

Hi Neo

thats a good point. It actually has super in it as I was advised not to use the new E10 stuff now at the pumps. I will look to do this at the weekend. Thanks

Anything must be worth a try if you've tried everything.

But i just did a quick google search for timing with unleaded vs leaded, and apparently you retard the timing slightly.

One guy writes that for his classic bike engine he normally sets the timing where it "should" be and, providing he gets it going, goes for a ride up a hill, if it pinks he retards it a bit, if it doesn't he advances it until it does pink then winds it back very slightly. Remember it only takes a fraction of adjustment to alter the effect it has.

Like I said it might be worth altering it, you can always set it back to 'normal', if it doesn't work. it's only a screw adjustment afterall, but having said that, I'm not so sure I altered my DT for unleaded! .....I may have done , it's so long ago though!.......good luck Phil keep us posted.

Edited November 30, 20213 yr by NE0

When i first got it running in May it was old version unleaded and ran fairly okay, if a little underpowered at high rpm. Thinking about it it has got a lot worse since using the super so will mess with timing. I was advised not to use new e10 unleaded as not good for the carb etc

Next rime it cuts out ,spray easy start  up the carb,,,just to see if its fuel related

Well, I’ve retarded timing and now tries to start and backfires after a few kicks. Will have another go tomorrow 

Did you get it going?

 

Still not running. Will have another go tomorrow. Will pull flywheel and check coil, points and condenser connections. I expect them to still be fine

It seems spark is weak, not bright white/blue. Bypassing straight from stator to coil makes no difference. New coil, condenser, point HT lead and plug. The only thing not changed is the ignition coil on stator. Could this be the issue?

Wild Bison

the seals have been changed

On 1/15/2022 at 12:12 PM, Wild Bison said:

Your campaign is leaking gear oil from the box through the oil seal on the crankshaft ... this oil can still collect in the exhaust, with a bitter smell, and the candle should hang with black flakes of soot after you train, if everything is like that, then change the oil seal sooner or it will be worse. In your free time, check out https://www.sunsigns.org/bluebell-flower-symbolism-meanings/ and find out what awaits you.

This needs a ban...