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Question 1972 Yamaha 125 Wont Start, Popping Exhaust

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

might want to remove plug and put finger over hole while kicking to understand whether you have any compression
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02 Dec 2021 08:21 #11

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If you can feel a bump resistance pushing kick through by hand--or foot then ok it won't be 'that' stopping it running in my opinion although might be low on comp for good performance. Give it a push but be sure the throttle is closing & operating as it should. Don't want it lighting up with you hanging off the back like a flag. Saw that happen on an RM400 once & lucky no kids in the paddock go hit. In that case owner had had the throttle slide out & fitted it 'not lined up' with the guide pin.
If your starter is spinning it with a good car battery just play with throttle & choke--bit of choke on a closed throttle--then no choke but full open throttle. Eyeball that timing for a ball park setting. If its got a belly full of fuel--or twostoke oil from oil tank due to sitting then she'll take some spinning to get 'life' going.
02 Dec 2021 14:06 #12

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Based on the headlight bucket color, looks to be a '71? Brilliant red? If it has a screw on gas cap not the flip top then it's a '71.

Nevertheless...if the points/timing are set correct, the popping could be a kicked out crankshaft seal, probably the left side behind the alternator rotor. Stator & Rotor have to be removed to see the seal. Could also be the right side seal, but the whole right case cover needs to come off to see that. I would start on the left side. Need a bolt to drive the rotor off, there are plenty of threads on this site that describe removing the rotor. 
Allrighty Then
02 Dec 2021 20:38 #13

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Yes could sure be "as APEX says" above [which i completely forgot about]. i'd plug for the right side on those rather than left. Left side could be a bit of an issue to get at as need an armature removing special yamaha bolt although 'might' be able to see behind there with the field coil stator setup removed but think armature shrouds viewing. One things for sure--if you get it running with right side out it'll smoke worse than you can imagine but you'd have a job keeping it running, if at all. 24 & 25 shows a seal retainer in the link but i've seen them push the right side retainer away in an effort to escape.
www.partzilla.com/catalog/yamaha/motorcy...2/crankcase-cylinder
03 Dec 2021 01:49 #14

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

Thank you everyone for the help, I got the bike to run after replacing the positive terminal (it was a bit funky) now the bike will run, i can get it to idle somewhat but will die without choke, Have to tinker with the carb, I have a chinese carb on it right now and stock air filter
03 Dec 2021 18:30 #15

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Have a chinese carb on mine & plenty on the rich side. Maybe a different carb than yours. Is it a relatively new carb, not likely blocked up with stale fuel. Had the pilot jet bunged in my DT1 recently so would only keep running with choke on--well, could choke it up through it then ok on full song but had to ride home with the choke on.
03 Dec 2021 18:55 #16

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

Yeah just got it last week, I have the air screw 2 and a half out. This is the carb i have, should i continue to run the stock filter with this carb? or should i run the filter provided?
Last edit: 03 Dec 2021 20:08 by DerveyZ.
03 Dec 2021 19:10 #17

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Congratulations!

(2.5 seems pretty far open on the air screw)
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
03 Dec 2021 19:33 #18

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Yes, screw it in to say half a turn for a test as it's 'in for rich'--less air-- & out is lean. 4 stroke carbs generally have a fuel screw so out for rich on them. Also a fuel screw is forward of the slide [nearer the motor] & vertical. The air screw is behind the slide & on an angle [just before the open mouth].
03 Dec 2021 20:36 #19

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

Another update guys, I can get it to fire over with one kick now, but it will not idle under 3k rpms.. ive messed with the idle screw and air screw, no such luck, do you think its because its a chinese carb? im thinking about putting my mikuni back on now that i have it somewhat running and see if thatll work. The throttle cables arent stuck either. I also have it running on premixed gas, but gonna switch back to the oil pump now that i have a brand new throttle cable.
05 Dec 2021 15:03 #20

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