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Makotosun

1975 DT175 Misfire

  • kwzl1
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1975 DT175 Misfire was created by kwzl1

Haven't posted here for a while. Been working on my DT175 for 18 months and thought I had finally worked all the bugs out. At least it was running well.
Recently I went for a ride, and it suddenly started misfiring and losing power. I limped it home. Now it will barely start and won't run very long.

During the rebuild, I replaced/repaired nearly everything in the ignition system:  source coil, spark plug cap, plug, points, and condenser. Also bored cylinder, new piston and rings, replaced reeds. Rebuilt original carb. Used NOS Yamaha parts whenever possible.

I have since checked the points and timing, replaced the spark plug, tested the plug cap and the coil (primary and secondary) for correct resistance. I even disassembled the carb, and it looked fine.

Have spark when I kick it over.

What else should I check? Condenser? (I have already replaced it twice before when trying to get the ignition system working.)

I am at wits end. After this much time, I still can't ride it without issues. I hate to sell it, but if I can't get it running right, there's no point in having it.

Here is a link to a video of trying to start the bike. (Will only run briefly at full throttle). Appreciate any help!

www.youtube.com/shorts/af--1bI7Akk


 
Last edit: 30 Jul 2022 17:00 by MarkT.
30 Jul 2022 16:58 #1

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Replied by RT325 on topic 1975 DT175 Misfire

Ok, first of i thought it'd sucked a low flying duck into the aur cleaner it sort of started sounding so rich. But on sercond listen yes its ignition related. Not a crank seal popped out sympton i think. So go to the points & look through the window for a crazy wide gap for some unknown reason. Or the other way--invisible gap. If someone can keep it running for you look into the mag for spark jumping somewhere. Condenser Fell Off, or wire come off but sort of touching, that sort of thing. Flywheel key is usualy my first thought--so i'd go there first in hindsight. Has the earth electrode bit dropped off the plug. If i'm wrong on all counts then hass the main jet fallen out. I'm clutching at straws with that thought. Best of luck--go flywheel key. & welcome back of course.
Last edit: 30 Jul 2022 18:02 by RT325.
30 Jul 2022 18:01 #2

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  • Tom P
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Replied by Tom P on topic 1975 DT175 Misfire

When you were working on it did you replace the crankshaft seals? I don't see those on your list. Seems like a lot of smoke behind it, and it sounds just like mine did when the crankshaft seal went bad on the clutch side.
30 Jul 2022 20:08 #3

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Replied by MarkT on topic 1975 DT175 Misfire

Just curious if you used an original yamaha condenser?

It's such a tough call...  seems super rich which could be ignition but possibly other things like Tom P mentioned.   Is trans oil level dropping? 

Or even o-ring that seals float bowl to jet setter failed?  Modern fuels are hard on o-rings and while you can get the o-rings new from Yamaha a "NOS" o-ring might be a different material and fail with modern ethanol spiked fuel? 

I've had bad spark plugs run like hell like that too...  if you haven't put a known good plug in that's the first thing I would do.  Make sure it's gapped 0.020" to 0.024", no bigger.  Spark with plug out of head does not mean the plug is good.  I've seen that more than once. 

 
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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Last edit: 30 Jul 2022 20:35 by MarkT.
30 Jul 2022 20:33 #4

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Replied by RT325 on topic 1975 DT175 Misfire

Reading again it sounds like something happened on the moment, like something broke or fell off. I nearly alway say exhaust, like a ball of carbon jumped up the spout on the inside, but not this time.
30 Jul 2022 20:43 #5

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Replied by alnarv on topic 1975 DT175 Misfire

Another quick check to separate ignition problems from anything else is to put an inductive pickup timing light on it & observe the flashes. If it skips flashes in sync with the stumbles, you have an ignition problem. If it flashes steadily, look elsewhere.
alnarv
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31 Jul 2022 06:18 #6

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Replied by Bsimpson on topic 1975 DT175 Misfire

Sounds very similar to what my 400 does. Have you put another plug in it? Even though it sparks it could still be fouled. 
1977 DT400D Brick Red
1978 DT125E Leaves Green
31 Jul 2022 13:27 #7

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  • kwzl1
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Replied by kwzl1 on topic 1975 DT175 Misfire

I checked the gap on the points, and it looks good. Checked condenser wiring, and it is solid.
Flywheel key was one of my first thoughts as well, but it is fine.
Everything in the carb looks fine.
Thanks for the response.
01 Aug 2022 11:45 #8

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  • Tom P
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Replied by Tom P on topic 1975 DT175 Misfire

Did you check your trans oil level as MarkT suggested? If it's going down, that would suggest a leaking right side crank seal, which could cause your issues.
01 Aug 2022 11:55 #9

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Replied by kwzl1 on topic 1975 DT175 Misfire

Trans oil level hasn't dropped.
The o-ring seal to the float bowl is good.
I've tried several plugs, including a brand new one, gapped to .022", with no effect.
Thanks for the response.
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01 Aug 2022 11:55 #10

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