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Makotosun

1974 DT-125 with over speed issue

  • rbertrand
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1974 DT-125 with over speed issue was created by rbertrand

I have restored a 1974 DT125 that my neighbor was going to throw away. What, that's right I pulled it out of his trash pickup. I've replaced nearly everything on it cosmetically, seat cover, tach, etc. I want to let my boys learn how to ride on it but I still cannot trust the idol. Since first getting it to run it has had an over speed issue that without turning the bike off will rev out of control until boom! Now I'm new to two strokes but have been a mechanic all my life so I can turn a wrench, however, this has got me puzzled. I realized early on after reading a bit that air leaks will cause this catastrophic problem. So I have systematically done all the seals starting with the air cleaner all the way to the exhaust pipe. I have replaced the air filter being sure to grease both sides for a good seal. I connected it to the carburetor using a heat gun to reform the plenum making it fit proper including a new clamp. I have replaced all the gaskets leading up to and attaching the reed valve, verifying it was sealing correctly with no cracks or openings. I have replaced the piston, rings, and both the top and bottom cylinder gaskets torquing them to spec. and finally replacing the exhaust gasket and when it seemed this might not be receiving the correct pressure to seal I made a pair of shims to fit between the collar and outside of exhaust to increase the depth of the exhaust when torqued into place.

In addition I have done a compression test to verify it will pressurize and hold it. All the while the problem continued to improve. BTW the carburetor was replaced but seems to work better with the original I rebuilt in place. I think all the air leaks are plugged.
COMMENT IF YOU THINK I MISSED ANYTHING

Now if you made it through all that thank you!

Here's the million dollar question, is it the governor that is my problem now? I have set all the timing, point gap, plug gap, and all the carb settings and it will run, even idol at times but it still on occasion will go wide open with no throttle input. I would really like some input here for two reasons. First, I have no experience with this issue so nothing to go especially since I've never seen the bike operate correctly. Second, I found a governor after a national parts search, when I ordered it the dealership sat on the order for over a week and when they finally shipped it UPS LOST it! It is discontinued, part #444-81153-10-00, I cannot locate another one and now after 6 or 7 months of buying parts, and helmets for the boys, I am stuck right here.

Should I continue looking for the governor or is it something else?

If anyone has read this far bless you, you are a saint. If anyone can assist with info or locating this part bless you, you are my hero! I want to let the boys ride it instead of working on it all the time and it's soooo close!

Thank you!
06 Jul 2020 16:58 #1

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Replied by darinm on topic 1974 DT-125 with over speed issue

Wow nice work, it sounds like you have everything covered. I didn't even know they had a governor. For due diligence I'd imagine it has a new throttle cable on it? How is the condition to the splitter to the carb and oil pump? I've seen cracks in this cause weird issues.

I have a '74 DT125 engine in my garage I paid $50 bucks for, just for some clutch parts for a CT model so the electrics have not been touched. So if it's on there still (I'd assume it is) I'm sure we can work something out.
1972 Yamaha CT2 175
1972 Suzuki TS185 Sierra
2000 Suzuki RM100
2003 BMW F650GS
2009 Yamaha WR250R
2013 Yamaha XT250
06 Jul 2020 17:29 #2

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Replied by yamahoper on topic 1974 DT-125 with over speed issue

Just some basic questions... is your throttle cable routed correctly and not getting pulled from the carburetor. I know if my cable is routed to the right of the tach its good ,if it gets routed in the middle of the gauges it tightens up and when the bars turn left it will throttle the bike. Are you running standard size mikuni jets. Does your carb have an air screw and how far out are you?
06 Jul 2020 19:17 #3

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Replied by 2fishkev on topic 1974 DT-125 with over speed issue

A few more questions. Did you replace the crank seals and are the bearings ok? Does your slide move freely every time and go completely closed? Did you check the head for warpage? Have you retorqued the head? I have problems with the head nuts backing off.
06 Jul 2020 19:29 #4

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Replied by MarkT on topic 1974 DT-125 with over speed issue

"Governor" is not what the name implies. It's just an advance mechanism for easier starting. Timing should be fully advanced at around idle speed. Does not "govern" the engine speed in any way. Extremely unlikely to be the problem.

Everything mentioned is important... cable routing... if flange bolts on carb get over tightened the slide can stick...

I agree that crank seals are something you didn't mention and are most likely to be the problem if it just revs by itself without touching the throttle. Most likely the ignition side. Use a 1977-79 IT175 seal when replacing.
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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06 Jul 2020 20:05 #5

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Replied by Eazy-X on topic 1974 DT-125 with over speed issue


Aha! I wondered what this was on the flywheel :) I had a similar problem going over 4000 revs and changing the seal seemed to sort it (thanks again, Mark!)

The only other thing is the carb slide spring. On mine someone had cut the spring down (lighter throttle response I guess) but springs degrade over time like anything else. Before putting in a new spring I occasionally had runaways on idle.
07 Jul 2020 02:55 #6

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Replied by rbertrand on topic 1974 DT-125 with over speed issue

Thank you for your reply. Yes, it was quite a bit of trouble as I had to order the top cable and bottom splitter cable separate and they kept arriving and not fitting together at the barrel. I ordered a cable complete and the order was from china ahead of the virus but then was cancelled. So, eventually I had enough parts I put together my own fix by combining parts I received and the cutting to length the end at the throttle control. I have verified the cable is free with just enough play to not be an issue. Again, thank you for your reply
08 Jul 2020 10:02 #7

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Replied by Midlife Motor on topic 1974 DT-125 with over speed issue

Check your float level, it won't hurt. These bikes will increase RPMs just before running out of fuel in the bowl. I am thinking if your fuel level is too low in the bowl due to the float, it might be starving for fuel and too much air. Just a guess.
It's better to remain silent and be considered a fool than to speak and remove all doubt.
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08 Jul 2020 12:41 #8

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Replied by rbertrand on topic 1974 DT-125 with over speed issue

Thank you for your reply. Yes, I did initially have a routing issue but it has been corrected and I have verified it with rigorous testing. The jets are, as far as I know, stock. I cleaned and rebuilt the carb but as I am not the original owner I really don't know. I have it set at 1.5 turns out. It seems to run best around 1.5 to 2 turns out, but it seems completely random as far as the over-speed issue. I can replace the carb, but that is why I am wondering about the governor. I hate to replace it and find out it has no effect.
I should add, that one time it seemed to not over-speed I had put the cover on not realizing I had failed to remove the zip ties I used on the governor to advance the time for setting the timing. It started right off and did not over-speed. That's what made me think this was the problem. HOWEVER, as anxious as I am to fix the bike for use, the only source I found for the part after a national part search, shipped it with UPS and they LOST it! Anyways if anyone knows where I can get one that would answer the question for sure...
08 Jul 2020 12:50 #9

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Replied by rbertrand on topic 1974 DT-125 with over speed issue

Have not checked the main seals but I've not seen any oil leaking, something I have considered thank you. I have replaced all the gaskets and torqued the head bolt. So far they are staying put. Replacing the gaskets, cylinder top and bottom, reed valve etc. all have helped with enough improvement I think I'm good there but until it's fixed won't know for sure. Thank you for your reply
08 Jul 2020 12:54 #10

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