Heya TT!  I recentally bought a 77 DT 250, I'm not very familiar with these line of bikes. The bike was running great about a week or so ago and then a weird problem happened. It seems when I drop it into gear and take off, it feels as if there is a massive power starvation. Also, it will die while idleing after about 6 secs. I have checked many times of the important 3. Gas, Fire, Air. I am recieving all three. I cleaned the carby atleast 4 times. Can't possibly thing what that might be, someone suggested a wheel bearing may have broke and causing a lot of strain on the engine. I checked the wheels, it rolls freely and its not loose. Also,

Does anybody know if this bike has enough pep for the Colorado Rockies? Im kind of bigger guy, 195 lbs 6'1 if that helps any.

Is it normal for this line of bikes to " eat " spark plugs. It seems I go through one about every 1 - 2 weeks.

As I said above, the bike is in rough condition, but I bought her for the mountains in hopes of trail riding. Is she worth restoring to the point she doesn't look like a beater bike anymore? Or am I in over my head. haha

:: Additional info, I appologize if this in the wrong forum section.

Thanks very much for the help!

Well I have had a similar model dt and have a dt 400 which is almost the same bike. I would have to say your jetting is off because neither of my bikes foul plugs, I do change them about every year but have never had one foul on me. You might need to lean out your jets. Do you have pics of your spark plugs? Unless you can get a title I wouldn't bother restoring it to perfection. The mono shock DT's aren't all that collectable or valuable. Have any pictures of it? The power isn't the greatest, I know my 250 would struggle to keep speed revved out in 5th gear WOT on uphill roads and it was geared down significantly. They are reliable bikes though as long as you take decent care of them. I've never had my 250 or 400 leave me sit anywhere. If you want more power they are pretty easy to port and jugs are pretty cheap if you mess up. Checkout yamahaenduro.com for oodles of info and helpful people.

Here's what my 79 DT 250 looked like after about a year of riding after I painted it. I would try not to paint it if I were you.


Oh if your curious about the engine I'd probably take the top end apart just to see what kind of shape it's in and decoke it. It's an easy job to do and you can reuse the head gasket, you'd just need a base gasket because you will rip that when you take the top end apart. And an easy way to tell if you need a bigger or smaller jet if your bike is running poor is to turn the choke on. If it runs worse than before when it's on you need a leaner/smaller jet and if it runs better you need a richer/larger jet.

Have you checked the air filter out and check to make sure the spark arrestor isn't blocked with carbon?

Here's my 78 DT400. I would just make it so the bike doesn't look like complete trash, I'm still on the road to semi restoring mine. I think I might get the frame powdercoated and paint the engine and case if I find another bike to ride around in the mean time. I ride mine pretty hard on some very rocky trails and I'm pretty impressed with it for what it is, a 280lb tank. I wouldn't dump a whole lot of money into the bike unless you want to keep it because you'll never get it back. Your looking at a 500$ bike below, mainly because I don't have a title and no one wants a 30yo dirtbike.

Edited by 79yamdt, 13 March 2012 - 10:57 AM.

Heya!

Thanks for all of the very helpful info 79yamdt. Well, from my last post. I got her running, and she will idle now. The only problem I can't figure out, " but I do have a clue ^^ " is why it really feels as if there is a massive power lose while in 1st - 3rd gear. Mainly during acceleration I can tell. But that clue I have is that I think I may have to replace my crankseals. 1st, " correct me if Im wrong and none of this adds up. " I pour a TSP of 2T oil into the cylinder and instead of my regular 90 PSI Compression test. Im reading 100 - 120... 2nd. Im leaking alot... of what appears to be transmission oil under the engine of my bike. If that sounds right to sombody please point it out to me, cause im still not 100% sure haha.

I have considered taking off the head gasket to look at my top end. But theres a big barrier thats holding me back. I have 2 bolts that are practically welded on there with rust. Unfortunatally twisted one of the head off of one..  

Keep the replys coming please! Thanks again to all!

YamahaRider21, on 13 March 2012 - 11:51 AM, said:

Well 90psi isn't the best as far as a compression test goes but it's not to horribly bad either, I've seen 70's and the engine still ran okay. As long as it starts fine I wouldn't be too worried about it. It is probably due for a rebuild whenever you get around to it. This low compression could also be the reason why you are down on power. Maybe you rode it a little too hard since your kinda a bigger guy and the thin mountain air probably isn't too kind to the old bike. A compression test isn't to test if a crank seal is bad it's mainly to test what kind of shape the piston/jug is in and if it needs replacing/reboring. Does it only leak if its running? Have you checked to see if your crank breather hose is clogged? I know mine would push oil out the seals before I found out the crank breather was clogged and packed full of crud. Well if you twisted the head stud off maybe it's time for a new to you top end from ebay? You should at least replace that stud or have it replaced because I would think that may cause the head to warp since one section would not be secured by a bolt and free to expand and pull away from the jug. Or did you just round the nut?

Crank seals wouldn't be a bad idea, but I think that's not the main source of your problem.

Edited by 79yamdt, 13 March 2012 - 02:59 PM.

Do you have the correct spark plug installed? A clogged exhaust will also cause a performance loss. Your bike should not be eating plugs. Start with the simple stuff first.

/

He's correct look at the simple stuff first. The plug should be a B8ES if you run NGK almost all yamaha 2 strokes from the 70's used this plug. What oil mix are you running?

I took your guys advice and looked at all the simple stuff, and none of them seemed to be the problem. I figured it out yesterday when it just happened to come to me. When I purchased the bike, " which was only about a month ago " I havn't givin her an Oil Change. So, I figured that the oil in there might be pretty yucky. Popped the plug out, the oil was of course, very nasty looking. Put some good old new 10 w 30 in there, gave her a test drive, everything was smooth as silk. Wanted to reply on 78yamdt, your right on the Jetting. I believe I do need to go leaner. Im working on getting that fixed. But for the while, she runs great again, and am yet to take to the mountains.  The oil mix that I am running is Polaris 2 stroke Performace through the auto luber. Thanks again for everyones help.

It's good the autolube system is still intact, it makes life so much easier. Watch out on what oil you use though, i'm not sure if your aware, but most engine oil thinner than 10w-40 has friction modifiers in it that may cause the clutch to slip. If it has an epa energy conserving star/decal thing on the back of the bottle it contains friction modifiers. I run rotella T-5 in my bike which is a semisenthetic diesel truck oil but is the correct weight (10w-30) and does not have friction modifiers in it. It's a lot cheaper than motorcycle specific oil since it works out to be about 4bucks a quart when you buy a gallon of it.

What makes you think it's running rich? Did you do the test I mentioned? Or are their other signs of a rich running bike?

" And an easy way to tell if you need a bigger or smaller jet if your bike is running poor is to turn the choke on. If it runs worse than before when it's on you need a leaner/smaller jet and if it runs better you need a richer/larger jet."

What makes me think Im running a tad bit rich is that I took what you said about the choke, and when I turn on the choke while its up to temp. The bike begins to bog down and die. And yes, I am running the correct oil.

That guidance is mainly if your bike is running like complete crap, you said your bike was running a lot better. My bike will do that too with correct jetting. The fouling plugs somewhat often may be a hint that it's a little too rich. There's a bunch of jetting info on 2-strokes in they yamaha 2-stroke forum. I'd give that a few reads and then go about jetting if you think you need to. Are your plugs black when you replace them? Mine are always a brown/dark tan of some sort, which they should be.

http://www.thumperta...rich-read-here/

Edited by 79yamdt, 16 March 2012 - 03:57 AM.