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Makotosun

1972 Yamaha 125 Wont Start, Popping Exhaust

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Hello everyone, Jared here. I am relatively new to the motorcycle world, I recently picked up a 1972 yamaha 125 enduro, not running. Ive checked for spark, rebuilt carb, new battery is in. When I try to kick it over it will pop alot, from the back end of the exhaust and sometimes from the front end as well. I tried starting fluid with no luck either.  Here is a video example,
any help and criticism is appreciated! I really want to learn and improve. -Thank you Jared
01 Dec 2021 20:02 #1

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Replied by msavitt on topic 1972 Yamaha 125 Wont Start, Popping Exhaust

Cool bike, electric start very cool
starting fluid lights easily so do you have spark?  If yes then maybe low no compression, or a hole in piston?
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01 Dec 2021 20:12 #2

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Replied by DerveyZ on topic 1972 Yamaha 125 Wont Start, Popping Exhaust

I do have spark, I ordered a compression tester from amazon should be here Friday, I've never rebuilt a top end before so Im hopeful its not that, but the experience would be nice
01 Dec 2021 20:13 #3

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Replied by msavitt on topic 1972 Yamaha 125 Wont Start, Popping Exhaust

Is there a bump of resistance when you work the kick starter?
01 Dec 2021 20:17 #4

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Replied by DerveyZ on topic 1972 Yamaha 125 Wont Start, Popping Exhaust

Yes there is
01 Dec 2021 20:21 #5

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Is it an electric start model?, if so i'm just wondering why ya not using the starter. But if its magneto ignition i wonder about the crank flywheel key if someone's had the flywheel off. I only say that because you say it pops a lot from 'back & front' which can happen if the timing is 1/4 turn out through broken key--well--maybe not 1/4 out but sure miles from where it should be. Could do an eyeball check of the timing with something down the plug hole to find top dc the go back a fraction, 2mm max, & eyeball the points for starting to crack open at 'that' point. If its electric start it's slightly different but the same if you hold the advance mechanism open. Apart from that i don't suppose she's just flooded up in which case full throttle & kick till ya leg drops off. Then we'll come up with another guess. I have a magneto AT1 1970, similar model.
Ok, i can see the electric start wide left case now i look closer in the video. Has someone had the advancer points cam off & refitted it wrong--if that's possible. Just hold it open against the springs & 1.8-- 2mm or pretty much tdc on retard just to see if its in the ball park or something odd going on like--no crank key etc if its been taken off before you bought it.
Sorry, doing things back to front. Welcome from NZ.
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Last edit: 01 Dec 2021 23:03 by RT325.
01 Dec 2021 22:58 #6

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Replied by tennbamaman on topic 1972 Yamaha 125 Wont Start, Popping Exhaust

These folks will for sure get you going. Its a great site. I have a 1971 AT1C. Based upon video it looks like you are lacking strong fire and or fuel. When you said you used starting fluid where did you spray it. When i was first trying to get my bike to fire up, i took the plug out and squirted a little premixed gas into cylinder. Quickly put plug back in and tried to start. Started right up. It would run for just a brief moment confirming my carb was my problem. Sounds like you are well versed that the battery has to be charged .. if its like mine.
02 Dec 2021 06:37 #7

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Replied by Pedalcrazy on topic 1972 Yamaha 125 Wont Start, Popping Exhaust

Sounds to me like timing as RT325 has said with sheared flywheel key or as msavitt mentioned compression or hole in piston. I bought a DT100 not running and I kicked and kicked and it wanted to but wouldn’t start. Crack in top of piston. Ran great after piston was replaced.
1978 DT400E
1976 DT400C
1973 RT3
1971 RT1B
1968 DT1 (3)
02 Dec 2021 06:58 #8

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Are you using the electric start a couple times in video?  Really hard to hear most of it. 

If you are using electric start, I think you need to pull the head...  I don't think you have much if any compression.
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
02 Dec 2021 07:16 #9

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Replied by DerveyZ on topic 1972 Yamaha 125 Wont Start, Popping Exhaust

Yes i was using the electric start a few times, sorry my microphone on my phone is weird. Gonna check the compression as soon as I can, and if its lower then 100 I should take the top end off right? Ive never done anything engine related to be honest with you guys. If anyone has any pointers Id appreciate it!
02 Dec 2021 07:44 #10

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