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Old 01-07-2015, 12:55 AM   #1
dunkee OP
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'77 DT250D: Should I patch, weld or replace badly-cracked head pipe?

I traded a very nice-but-gutless '78 Honda CM185 for this '77 Yamaha DT250 a couple months ago. It ran, but poorly, and the kickstart lever had been welded open/no longer articulated (pretty stupid, if you value the bones in your leg or that metal shaft). The frame and swingarm were given a sloppy blue spray paint job, and the odometer read 4000 miles before the cable disappeared.



I made things worse by running some old/too-rich mix through it. So I pulled the carb apart, cleaned everything, and replaced 3 components with new parts from a Keyster carb rebuild kit. Today I finally got it started again (with the help of a new spark plug/boot) and took it for a great 20-minute ride just before sunset. The mix I used today was 32:1, with Red Line synthetic 2T "Racing Oil"--seems okay.

One of the previous owners removed the Autolube tank, unfortunately, but had done nothing to disable the rest of the system (the A-lube oil hose and the double throttle cable were all still attached to the carb). So I cut the hose and plugged it up on both sides, and I cut the cable that went from the throttle to the A-lube pump. Seems like the best thing to do now is to replace the throttle cable assembly with a single cable that just goes straight to the carb (the other cables are rotten, too).



There's a huge crack in the head pipe (most of the left half), and I'm wondering if I should try to patch it with high-temp epoxy, have it welded, or look for another header. Does it have to be counter-balanced if welded/patched, so that it won't shake itself apart? I have some high-temp JB-weld that I'm tempted to try. Also, does a crack that close to the cylinder change conditions (back-pressure?) enough to damage the engine? (Can I ride it, or should I pull the pipe off tomorrow?)

Thanks!
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Old 01-07-2015, 01:26 AM   #2
dhallilama
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weld it. bring it to any welding shop (or trailer hitch shop) and it shouldn't cost more than a few bucks or some beer. Hell, where are you located? lots of guys here weld (myself included)

don't need to counter balance. just a simple weld. MIG, TIG, Oxy/Ace... any would do the job just fine. just pull the pipe, clean the paint off around the crack and wipe down the pipe so it's not a greasy mess. make it easy for the welder and it'd be done fast.

doubt JB would last long, at all.

i doubt any damage came of it, but couldn't say 100% for sure. i do know i've cracked pipes in that general area more than a few times when crashing, still rode back to the truck or even a few more laps.

edit: hell, come to think of it my '77 IT250, with a pipe that is pretty similar, cracked in the exact same spot. welded it a few years ago, never a problem since.
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Old 01-07-2015, 12:13 PM   #3
MATTY
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My old DT broke roughly the same place and i could not get one in the uk for love nor money at the time, i had no real option did it with gas (oxy/ accet) lasted from that day on until i sold the bike about 8 years latter.
Dont worry about the autolube either i ran mine about ten years on pre mix by choice not nessesaty.
Whats wrong exactly with the kick start can you grind off the weld ? It will have been warn and sloppy and at a weird angle so in their infinite wisdom they just welded it up, rather than take the time to repair it properly.

MATTY screwed with this post 01-07-2015 at 12:20 PM
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Old 01-07-2015, 01:37 PM   #4
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I've had a few DT/IT pipes welded up. Shouldn't cost much of anything and it's easy to do, assuming the rest of the pipe is good why replace it?

Now the question is, did it get bent when it cracked and will it go back on correctly?

I figure I would also suggest removing the autolube pump if you aren't going to use it and blocking it off with a little plate. Not sure why people stop using them in the first place... They are pretty dang reliable.
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Old 01-07-2015, 09:58 PM   #5
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People remove the oil pump because racing.

Don't just cut the cable and hoses. Remove the pump. Without oil it will seize. The block off plate is a standard Yamaha part (and is the same for many models). You will need to remove the side case to pull the pump and its gear out.
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Old 01-07-2015, 11:08 PM   #6
Gamequeazy
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Quote:
Originally Posted by ericm View Post
People remove the oil pump because racing.

Don't just cut the cable and hoses. Remove the pump. Without oil it will seize. The block off plate is a standard Yamaha part (and is the same for many models). You will need to remove the side case to pull the pump and its gear out.
True enough, didn't think of that.

Good call on the seizing of the pump, it runs via a gear off the crank and probably wouldn't be beneficial if it froze up.
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Old 01-08-2015, 02:52 AM   #7
MATTY
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Quote:
Originally Posted by ericm View Post
People remove the oil pump because racing.

Don't just cut the cable and hoses. Remove the pump. Without oil it will seize. The block off plate is a standard Yamaha part (and is the same for many models). You will need to remove the side case to pull the pump and its gear out.
This is correct i removed my oil pump because i was sceptical it was delivering the right amount of oil at the prolonged high RPM i was running my DT at.
I took the plunge and removed the pump and in this case i am certain it was the right thing to do.
In defence of the yamaha autolube system generally they do work very well and are reliable and practical to use, just pulling up at a gas station no mixing etc is a good thing. But i do think in some circumstances and with some models pre mix can be beneficial.
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Old 01-08-2015, 04:14 PM   #8
ericm
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The Yamaha oil injection system generally works better than premix, in that it varies the oil ratio with load. Cruising it's low, WFO at high rpm and it delivers a lot of oil.

The one situation where it is not as good as premix is extended overrun (throttle closed) like down a mountain. With the throttle closed the pump does not deliver much oil, where with premix if there's gas getting pulled through the jets, there is oil too.
Saab's solution for their two stroke cars was a freewheel, but a car is a lot heavier and will see a lot more closed throttle overrun. I never had problems with any of my RZ350s or RZ500 in this situation. Though I did seize a piston on the 500 due to a pinched oil line (my fault not Yamahas).

That said I did remove the oil pump from my RZ350 race bike... because I did not want to run out of oil. A couple track schools with the oil pump showed surprisingly heavy use.

For a DP bike I'd use the pump. And good synthetic oil like Redline. Even my leaf blower gets Redline.
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Old 01-10-2015, 03:46 PM   #9
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Thanks for all the advice. I didn't think it was necessary to remove the entire Autolube pump assembly, but if it is being run without lubrication I should.

I had time to pull the pipe off yesterday, and I started to clean it up for my local welder. It may be leaking from a tiny pinhole crack in addition to the huge Y-shaped crack that spans the right side of the pipe.




The new kickstart lever has a lot of play (a little more than half an inch, at the top of the lever), and I wonder if it's going to break the pin or base. Should I find a thicker circlip so that the lever fits the pin more snugly? The welded lever it had when I bought it was from different bike, I suppose.

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