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1979 Yamaha DT 250.....Need help

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I have a pretty decent condition 79 DT250. It has about 8,000 miles on it and it is currently registered and insured. A few years ago I played around the gearing. I believe stock gearing is 14/47. I have a 13/51 now and it seems to be topping out at about 50 mph. Going against the wind is painful; maybe about 45 mph. I put a new piston in it the same time I did the gearing but then it sat for a few years. I'd like to get it going again and sell it because I don't need it.

Also, I played around with the carb and I found that the carb has two adjustments on it. I'm not sure which one is which, or where they need to be adjusted. Any help would be appreciated!! I don't have a manual either.... Thanks

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Are you talking about the split cable setup for your throttle cable? If your talking about on the carb itself. The screw you can turn by hand is your idle set screw, and the one you need a flat head screw driver is your idle mixture screw. The gearing is probably what is killing your top speed. I used to own one of these and currently own a 78 dt 400 (same bike pretty much). Even with stock gearing the top speed is only around 70.

Edit-

I just did the calculations and it should still be able to hit around 60 with that gearing. Did you clean your carb out, and give it a general tune up since its been taken out of storage.

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I bet the top speed you may have now of 60 is with the engine screaming.

I think you should put that 14 tooth back on the countershaft. Hopefully, just a chain adjustment can handle that extra tooth. I have a DT250 with the stock gearing and I'm not that happy with the top speed....

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maybe they came with different gearing in the u.s but in the uk stock gearing for a 79 dt was 47-16,which is what ive currently got on mine,top speed a little over 70,which is more than enough with mx tyres and drum brakes,sometimes run with a 15 gearbox sprocket,would do about 70mph like this

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Okay so lets do the math

Stock gearing us spec is 14/47=3.36

current gearing is-13/51=3.92

Let's do some math...

Top speed is listed at 67mph 67.7 actually but I'll round down to be concervative.

Simple cross multiplication or whatever this is called, sorry this isn't showing up how I want it.

67.7mph/ x mph/

3.92 * 3.36

Anyways...

67*3.36/3.92= 57mph

Generally speedo's read about 5-10% higher than actual speed, this depends on the tire of course.

So I conclude its more than just your gearing. Unless its really windy, it shouldn't affect your top speed so much especially at only 50 mph. I think your engine is down on power for some reason.

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So I finally had the time to get it apart and discovered my gearing is actually 15/46. Which, doing the calculations got me a top speed of 73 mph. I put brand new tires on it; also a new battery. I pulled the plug and it looks ok. So, I pulled the carb off and found it was contaminated with old fuel and such. I'm going to clean it up with parts cleaner and compressed air. I will set my air/fuel screw to 2 turns out. However, this carb also has an air screw. I do not know where to adjust this. Does anyone have any idea?? The bike starts up perfectly and idles fine. It's slow on the quick turn of the throttle but with an even motion, revs up fine. When I put it in gear and take off, i can only twist the throttle maybe 30% before it cuts out on power. This seems to change when I play with the air screw. I've never changed jetting so I'm not sure where to go. Any help will be appreciated. Thanks!!!

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That series DT250 was notorious for the silencer chambers clogging with carbon and seriously restricting power. I had limited success with heating mine with a dorch and hammering it to get the carbon out, but the only really effective solution I ever came up with was to cut the stock silencer off of the tail pipe and weld a YZ250 can onto it. It was only a little louder than stock, but ran better.

It's easy to test to see if this is the problem. Take the silencer off (leave the side panel off or you'll melt it) and take it for a test spin. Without the silencer, it should hit HARD at around 4000 rpm and pull well past the 7000 rpm redline.

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Did you check the emissions garbage on that bike? It was only on the 79. Mine gave me some problems I cleaned it and it worked for awhile, then cleaned it when it started acting up, the next time I just blocked it off. Do you have any pictures. My carb had an extra hole in the carb where an air screw would be, but this was plugged and that's how it stayed.

So did you put it back on the bike since you cleaned it? I'm confused around halfway through the post.

From what I understand you took the carb off the bike but have yet to put the clean carb back on. Let us know how it runs with the clean carb.

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So I finally had the time to get it apart and discovered my gearing is actually 15/46. Which, doing the calculations got me a top speed of 73 mph. I put brand new tires on it; also a new battery. I pulled the plug and it looks ok. So, I pulled the carb off and found it was contaminated with old fuel and such. I'm going to clean it up with parts cleaner and compressed air. I will set my air/fuel screw to 2 turns out. However, this carb also has an air screw. I do not know where to adjust this. Does anyone have any idea?? The bike starts up perfectly and idles fine. It's slow on the quick turn of the throttle but with an even motion, revs up fine. When I put it in gear and take off, i can only twist the throttle maybe 30% before it cuts out on power. This seems to change when I play with the air screw. I've never changed jetting so I'm not sure where to go. Any help will be appreciated. Thanks!!!

the screw at the front is a fuel screw the one at the back is an air screw, you go in for lean and out for rich with the fuel screw, and its the opposite for the air screw baseline settings should be 1.5 turns out on both screws you may or may not to rejet, my dad has the same bike with stock jetting which is 150 main and 50 pilot it needs to be changed to a 160 main jet and a 60 pilot it sounds like your main jet is clogged up tho clean it out and try it with those settings adjust from there, if you go in too far with the air screw or out too far with the fuel screw than you need to richen up your pilot jet if its the opposite you need to lean it out

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