Hi all, i have a lovely 1975 DT 250. It struggles to rev past 3500 rpm? any suggestions on how to change this??
Also, the rear sprocket has 50 teeth and front has 15, the bike will climb up the side of my house but wont travel much faster than 45 mph?
Thanks
john in sunny UK

exhaust could be plugged up
couldn't find this model on parzilla to see if silencer is separate but if it is take silencer off and see if this helps. I like to BBQ my exhaust, that way I can cook dinner with same coals
once I am done

Suggest a return to the stock sprocket sizes, 14/44. This will put your top speed up to about 65mph. I agree with MSAVITT. BBQ your muffler and clean it out. After all it is 48 years old. It needs to get hot enough to burn all that internal goo into dust. Weigh it before and after to see if you have made any difference. Then run without silencer for a very short trip to see if that helps. There are many other things that could be your problem, this is just a place to start.

 from Dorset, John.

Briefly run/ride it without the airbox lid and maybe the air filter, just to see if this improves things.

As msavitt says, could also be a bunged up exhaust. Some of the US and Antipodean members put the exhaust on a bonfire or bbq to burn out the rubbish , but that's not so easy to do here in the UK! 

Wouldn't do any harm to clean out the carb and check the condition of the 'O' ring on the needle jet setter.

 

uncleal - can you not BBQ with charcoal in UK?

"uncleal - can you not BBQ with charcoal in UK?"

We can, but a) our neighbours tend to be in closer proximity than those in the US and b) our barbies are of a more modest size.

MACE, I think that we posted our initial replies at the same time, hence it looked like I was duplicating your post! 

First & only thought--exhaust in particular the removable end muffler. Put the complete tail muffler in the logfire in the house if you have one. Could remove end cap part first but put it 'all in' as the muffler has restricting spark arresting bits deeper down in the end & fixed in place & they block up with carbon.
Just had another thought [two in one night], if yours has the carb with the side mounted main jet then replace the tiny Oring on the jet setter that engages into the bowl where the slide needle goes down. #5 in the link
www.partzilla.com/catalog/yamaha/motorcy...0c/carburetor-250b-c
#4 in the second link might be clearer. "75 didn't show a DT250 although should be there & same as "74 & "76.
www.partzilla.com/catalog/yamaha/motorcy...74/dt250a/carburetor
If this Oring leaks the motor goes super rich like the main jet fell out.
Talking tail muffler again, when you've pulled it out, look down in where it fits & see if there as stepped ring of carbon where that pipe ends. If there's a step in there then poke it out or find a fitting steel tube & tap it through there. Maybe then give it a good rev to blow any carbon out before refitting the [now burnt out] tail muffler. Ideally remove the main exhaust & see what comes out shaking it. Better still burn the complete main exhaust out. But that isn't going to fit in the house fire--i tried haha & thought i might get some of it up the chimney then be able to close the door, was a fail. I hope its a simple fix & just the carb Oring.

Here's a link to pretty much exactly how i do it.
japan.webike.net/moto_news/webike-origin...oved-by-a-good-burn/

yamaha-enduros.com/index.php/forums/1972...aust-on-ls2-100-twin
& here's the header pipe on my 100 twin from years ago. Both pipes were much the same & she took of like a rocket after cleaning out. I wouldn't expect yours to be like it thou. Like i said--might be carb Oring.