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Rough Idle issue - video included


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1983 Yamaha YT175

Fired up for the first time after total rebuild. Now I have no idea whats going on. Compression is 120, new plug, new fuel and oil lines, new choke and fresh fuel. Carb cleaned and reeds are good (no gaps).

Will not start without choke and stalls out when choke is off. Played with choke adjustment and doesn't change.

It fires up first pull and only idles if I play with idle screw. As soon as I touch throttle, it goes bezerk on me and I have to shut it down as you can see in video .......

You can here the recoil lever tinging in the video. I lost the spring. D"oh!!

So far I have swapped ignition coil, spark plug, CDI with no change. I have removed flywheel to check keyway and it is where it should be.

Checked resistance on stator ... I get 10ohms +/- 10% on pickup coil within specs as per manual and I get 273ohms on charge coil. Manual states 300ohms +/- 10%, so that is also within spec.

I have also swapped out jet needle and played around with different settings. Eventually I put it back to the 3rd line.

The engine already has new seals and bearing so I am completely baffled as to what I am missing here.

Any takers?

Any and all suggestions will be appreciated.


I have also swapped carbs from a running YT175. Same results.

Edited by oxidized_black
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Dream Car Giveaway

I went out today to look things over. Someone mentioned to check the flywheel (magneto) to see if it different than the one from the running trike. I actually have three.

So, I put all three side by side to compare and they are identical. Next I decide to see if maybe one of the wires from the stator was rubbing and was shorting out. Nope nothing there. After going back and forth between the 2 trikes looking at the stators, I noticed that the pulse coil and source coil was installed differently on the freshly rebuilt trike.

When I put everything back together after cleaning off the crud, I had the wires facing out on both of them, and they apparently need to be facing in. That is one thing the manual does not show. I had actually installed them backwards.

I put everything back together, primed it with a shot of carb cleaner, because I don't have the air box on yet, and it fired up. I threw on the plastics and took off out of the garage.

Now I just need to take care of the exhaust leak where the pipe meets the engine.

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