On my DT 125 MX (1981), whenever I go WOT and a reach higher revs in each gear the engine starts stuttering rapidly and won't rev/accelerate any higher. This is annoying because it happens just as I get into the powerband. If I change the spark plug, the bike runs pretty sweet and can rev much higher but then after a few rides it goes back to the stuttering problem. Any suggestions to fix this? Cheers

What colour are your spark plugs when you take them out?
Pictures would be good...

Sounds like its 4 Stroking ,,[ rich],, whats the plug colour look like,,,,  have the jets been tampered with ,?  Air filter  ?   you looked ?

Hi, thanks for the swift responses.

I am away for the weekend but will upload some photos next week of spark plug colours and other details etc next week.

last time I looked the spark plug was a brown colour so thought that was fine. Have changed air filter few months ago too. Im not sure whether the carb is the stock carb for the engine, also not sure if the jetting is definitely correct - it’s a mikuni 4J300? 
 

I lowered the needle jet by two notches, it didnt seem to make much difference.

it also is difficult to start - if its running rich is that the plugs being fouled? 
 

 

 

Rich =  dark  plug colour,  [ or black]  starting >  is it  with choke or better without,,? ,and if dropping the needle did nowt ,  it maybe the main jet too big,