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1975 DT175 Stumbles above 3500rpm

  • kwzl1
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1975 DT175 Stumbles above 3500rpm was created by kwzl1

Hi,
I bought a 1975 DT175 eight months ago, not running. I have been working on it ever since and have everything working finally. However, one problem has me stumped. The bike starts and idles fine. But when I try to accelerate, it starts to sputter about 3500 rpm, and won't rev beyond 4500rpm.
I have checked all components in the ignition system. Stator ignition coil tests good, as does the main coil. I have replaced the spark plug 3 times, the plug cap, the condenser 3 times, the points, and adjusted the timing both to spec and on either side of spec. Nothing makes any difference. I have also rebuilt the carb twice (once myself, and once again by a local shop in case I missed something). I have changed the main jet. The plug color looks fine. I also replaced the reeds as they were original, and not closing.
In desperation, I took the bike in to the shop. They changed the condenser, and after two weeks told me it wouldn't start and had no spark and that I needed to come get it. (Not sure what they did, but the entire electrical system was dead. I took everything apart, tested, and reassembled. Now it all works again, but I still have the stumble.)
I was hoping to enjoy this little bike this summer, but I am out of ideas of what else to check/adjust. Anyone have any ideas?
I have attached a video and the audio shows how it sounds. (Ignore the upside down video portion, as I was holding my cell phone with one hand.) 
Thanks, I appreciate any help.
Last edit: 24 Jul 2021 21:32 by kwzl1. Reason: Add video
24 Jul 2021 15:24 #1

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Replied by RT325 on topic 1975 DT175 Stumbles above 3500rpm

No video this end or am i just not seeing the link. Anyway--first thought is exhaust restriction in the tail section. Needs a burn out especially in the spark arrestor area deeper in the end.

But hold that thought above. Looking at parts pic i see it still has Oring3 which seals into the bowl to make sure the mainjet does its job otherwise it leaks past & drowns it up in the revs. 
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Last edit: 25 Jul 2021 02:00 by RT325.
24 Jul 2021 16:34 #2

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Replied by Bigdog302 on topic 1975 DT175 Stumbles above 3500rpm

Sounds  like a plugged up exhaust to me. time to burn out the pipe. burn out the pipe and burn out and clean the baffle.

I don't see the video or a link to it too.
2 1968 DT1s,1970 RT1M,1970 DT1C,1971 RT1B,1971 Honda Z50K2,1974 DT100A 1974DT250A,1974 Honda MT250,2 1975 DT400Bs,1975 Honda CR250M1,1978 DT175E,1979 Honda CR250R.
Dave
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Last edit: 24 Jul 2021 18:52 by Bigdog302.
24 Jul 2021 18:50 #3

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Replied by kwzl1 on topic 1975 DT175 Stumbles above 3500rpm

Thanks for the response. I did burn out the silencer (back half of the exhaust system-baffle?), but didn't get a lot of smoke out of it. didn't do the main exhaust pipe.
Video was there when I posted, but maybe wrong file type. Tried again with a zip file.
24 Jul 2021 21:43 #4

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Replied by kwzl1 on topic 1975 DT175 Stumbles above 3500rpm

did you mean oring 3? I replaced it as the old one was hard, but didn't help.
I tried adding the video again.
Thanks for your reponse.
24 Jul 2021 21:46 #5

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Replied by Gr8uncleal on topic 1975 DT175 Stumbles above 3500rpm

Air filter original, clean and not over oiled (if oil type)?
Last edit: 25 Jul 2021 02:21 by Gr8uncleal.
25 Jul 2021 01:27 #6

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Replied by RT325 on topic 1975 DT175 Stumbles above 3500rpm

Sorry yep Oring3 now i have my glasses on haha. You're onto though so all good. No video though. Hanging out for it to hear what its doing.. Think ya have to put it somewhere like youtube & link it over. I'm not good at that stuff. Might have to use Reply in place of quick reply.
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25 Jul 2021 02:05 #7

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Replied by asco on topic 1975 DT175 Stumbles above 3500rpm

You said that you changed the main jet but you didn’t tell us which way you went from what to what and what is stock?
Sometimes’ it’s difficult to get these bikes up to red-line’ but it has been done.

Any restriction from the air cleaner to the exhaust tip can work against you.

asco
YAMA-LAND RESTORATIONS, 818-521-2109 This email address is being protected from spambots. You need JavaScript enabled to view it. 1971 CT1-C (BRANDY) 1970 DT1-C (MONICA) 1973 100cc Hodaka Combat Rat (ABBY NORMAL) 1972 AT2M (ARNOLD ZIFFLE)
25 Jul 2021 08:53 #8

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Replied by Pete-RT1 on topic 1975 DT175 Stumbles above 3500rpm

To add a video, upload it to Youtube and then paste the link.
 
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25 Jul 2021 09:36 #9

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Replied by kwzl1 on topic 1975 DT175 Stumbles above 3500rpm

Yes, air filter is good.
25 Jul 2021 10:30 #10

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