Yamaha dt50mx engine problems

[Harveybiker]

Hi all

I have purchased a 1990 dt50mx and having upgraded to a larger carburettor, air filter and full performance exhaust. The previous owner had said that he put a 60cc bore kit in it 2000 miles ago. However whilst I was testing it, it was lagging power and felt like it was being held back. Not long after that it stopped and I presume siezed. But not fully as I could still push it and the piston could still come up and down. 

I was woundering if I should put a 70cc big bore kit on it? 

I was also wondering what I should do with the oil pump which mixes the fuel with the 2 stroke mix. Should I keep it or just mix oil in the tank when I fill it up? As this could have been a reason for the sieze? 

Also could there have been any engine problems that would have caused this? The piston did not have a hole in it, but had loads of carbon and some small chips out of it.

I just need some advice and replies will be appreciated. I just want to get it back running.

 

 

[blackhat250]

Holy shit, that's a biggee,  , I take it you've    little compression now, , was there a  rattling noise as she seized ?,  it looks like lube starvation ,

.  member Wild foamy is an expert on these ,  hes done the big bore kits, .  I would add a temp sensor [ on plug]  and this could be saved before the bang, 

[Cynic]

You can't get a 70 kit for the MX, that's only the water pumpers. Google '4 corner seizure' as yours looks a likely candidate but we cannot see the rest of the piston, has it marked the barrel?

As long as the pump is set properly and your using good oil then standard setting for the oil pump will be fine, I had a DT50m back in the stone age good for 70mph, (lot of work, lot of money, bastard to keep in the power and, thinking back prob looked a twat with the full superman going on for terminal speed runs).

The point? used the standard yam pump with fully synth 2stroke oil on stock settings. Most of the oil is lubricating the bearings, so tweaking the pump does little to the actual oil doing any good just makes smoke. Oil in the fuel knackers the mixture as you have to increase all the carb jets to allow for the oil in the fuel.

Also, and its a marmite point on here, keep the pump.

 

[wild foamy]

From the photo, a combination of a significant lack of lube and lean running.

A lean DT50MX will run, and it will probably do so for weeks on a normal town-based riding pattern, until you are on a long open-throttle run somewhere between Hungerford and Newbury (by the Crown pub) and then it grinds to a halt.

iirc the stock 50 comes with an 88 size main jet, i was running a 98 with a 60cc bore kit  which was slightly rich but perfectly useable, you may want to decrease your sprocket ratios for your newly found power output, i was running a 48T rear and 13T front (numbers may be incorrect as this was 7 years ago, but smallest rear and second largest front), at 60kg body weight it suited me fine, adjust to accomodate.

 

with regards to lube, the oil pump in the 50MX is exactly the same as the one in the 80MX, it is just adjusted differently for the additional displacement, if you have a haynes manual it will tell you how to set it up, if you run the 80MX set up to accomodate the big bore you cant go wrong, i could scan the page and send it to you if needs be just let me know, i even have a 60cc bore kit that i never used which is in the for sale section.

 

If an engine is running out of parameters it wont necessarily "seize" solid, my DT50 was hacking along quite happily and then faded out and died and would kick over all day, occasionally backfire but would not run due to the piston rings being destroyed as a result of lean running.i "seized" it twice and not once was it stuck solid, it would spin over but had no compression and would just fart occasionally.

Harvey, have tried to PM you but i think here is a fault in the system (says my inbox is full... but it definitely isnt, as i have no friends and nobody talks to me), is your bike a G-reg or earlier?