1990 Yamaha RT100 Compression test

mx724
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mx724
Came up with about 85-90 psi. Tested a few times. Bit will start but only for a few seconds. Cleaned carb and all the jets thoroughly. Cant find a spec on compression for this model? Getting spark and fuel. Will start off starter fluid but again won’t stay lit. I suspect new top end but would like to know what a RT 100 compression teat should yield? Any suggestions appreciated.
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Falcon
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Falcon 6/21/2022 1:37pm
6/21/2022 1:37pm
I'd say 100 psi at minimum, or maybe as much as 125. A playbike probably doesn't need a ton of compression, but you do need to be able to light the fire. Sounds like you need rings, at least.
mx724
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mx724 6/21/2022 2:16pm
6/21/2022 2:16pm
I also noticed its running one step colder plug. Br8es vs br7es. Gonna replace the pilot and plug to rule those out. I suspect rings as well though maybe. What are the odds you can replace the rings only tho. Im guessing it needs a new piston and bore. Might not be worth the cost tho.
Falcon
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Falcon 6/22/2022 9:39am
6/22/2022 9:39am
On an old steel (iron?) cylinder, I tend to agree; you may need a full piston kit and at least a hone.

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