Hello. I've had this bike since it was new and its run great...until recently. It starts, idles well and gets going up to around 35 mph and then starts bogging down on exceleration at about 4000 rpm's. I've changed the jets, new plug, fresh gas and played with the needle height adjustment. On occasion it will run perfectly for several miles, other times I can't go 1/4 mile before it starts acting up. Any hints or suggestions on what's going on? Thanks.

Assuming the carb's okay and doing its job the next thing might be fuel supply. Have you an inline filter that's clogged? Maybe the petcock has a screen filter full of crud?

Hi & welcome if you're new. On the face of it without actually hearing it run i'd say the tail pipe muffler has got either a build up of carbon in the spark arresting thing down in the tail section that looks like a jet engine, or a stray few lumps of carbon have bounced around & got lodged in there with same result. It's not that easy to see in there properly as think the vane bits are angled. To prove a point--if you're far enough away from neighbors--is run it minus the tail pipe & give it a [damn good thrashing]. Also with that complete end muffler out just look down in where it plugs in as carbon can build up where 'that' pipe stops inside. That's all guesswork of course & might be something blocking the air filter box intake [low flying duck] or air filter crumbled with old age [like me]. We love video's with good audio. Just thinking again, by memory the "75 DT250 still ran points ignition so therefore need around 15 thou points gap. Don't worry about timing at this stage as it'll be close at a nipping 15thou & you've had the bike from new so not bee played with by "experts". If the points fiber heal has worn down over the years & points have closed to almost invisible gap then the timing gets very retarded & so does the performance. All a very big guess though, but if you find it's blocked or semi blocked in the pipe the revert to original jetting when you've burnt out [cleaned somehow] the tail pipe. Cheers from 'down under' in NZ.

Random video i found of one persons idea on how to burn it out--worked for him. With similar mufflers to your tail piece muffler i've just put mine in the house log fire & left it over night.

First, let me apologize for mis-spelling "acceleration". Yes I have the magneto ignition and its a bear to get to the points but was able to give them a few strokes with a point file. (thanks rt325). I then took the exhaust baffle out and cleaned it up abit. Oily, but not carboned up.
Started it up and ran like new. Assume it had to be the points. Hope it lasts. Maybe time to a pro tune up. Thanx for your replies.

Glad she is running well again!

Can't be that many genuine one owner bikes around!