1982 XT125 = down on power

Bike was purchased last summer. It is in excellent condition. 3840 miles. I'm about 99.99% sure the engine has never been touched.

It wouldnt run properly when I got the bike, because the previous owner had a Mikuni carb on it that was way too big.

So I watched eBay for about 8 months....and finally someone listed the proper TK carb from an '82 that was being parted out.

Now the bike runs nice, starts easy, etc.

However..... power is maybe 60-70% what it should be. How do I know? I had one of these bikes back in 1982, and I only wiegh 25 lbs. more now than back then.

Bike has a new plug and a new coil.

I have a Yamaha shop manual for this year and model.

What to check first? Adjust the valves maybe? Do a compression check?

I've never messed with these little 4-stroke engines (but 2-smokers are a different story).

Any help or tips here would be greatly appreciated.


I have the same bike, I too felt like the power was seriously lacking. So over the winter I pulled the motor apart had the cylinder honed, new piston and rings, and took the head and had the valve's lapped. It definitely made a difference but remember it is still just an 82 125.

Plug is pretty sooty and the spark isnt the greatest-looking.

I hope something electrical isnt faulty.

Bike runs nice, it just lacks power.

I would check your compression. If the compression is low then do a leak down test to see were you lack of compression.

I would check your compression. If the compression is low then do a leak down test to see were you lack of compression.

Agreed...It could also be fond memories from the past :smirk:


Fond memories, yes.:smirk:

But.... the bike has 6 speeds, and it will not pull 5th and 6th.

"Back in the day", 6th gear was pretty tall for the little motor, but it would get the bike up to 50 mph at least.

And right now.... 35 mph at best.

Tear your carb down completely and clean it good. Then clean it again.

Do a compression test, then a leakdown test if you have the equipment

Check your valve clearances. Check for a plugged muffler.

Replace the spark plug. There's a million little things that could be wrong, and without workin on your bike first hand, it's hard to say whats wrong

If the plug is sooty you're running too rich or you're not getting complete combustion. I'd assess the ignition circuit. On a bike that age, it could be corroded connections, a bad plug wire, or plug boot. Could be a low battery too, make sure it's good and charged and survives a load test. Once you get a good spark your problems might be solved. If you get good spark and it still doesn't run well take the carb apart, clean it, and make sure you reassemble it properly. It's easy to make a mistake while setting float height or installing jets.

I had an 81 XT125 in high school. I was 150 lbs and it went ok for me. I think they only make like 10 hp or something, but an additional 25lbs shouldn't slow it down that much. I used to 2-up on it with girls in the woods and it did ok.

EDIT: I forgot- check the air filter, make sure it's not filthy or over-oiled. That would make you run rich.

Compression checked out today at 145psi; which is on the upper end of the range listed in the factory service manual.

Air filter is clean, exhaust pipe is clear. battery is new. coil, plug wire, and plug cap are new.


Going to recheck float height this weekend. Bike acted like it ran out of gas this evening. I have a new float needle valve on order.

Hi, did you get it to run ok... Have the same bike and problem, acts like out of gas. When i go full throttle (all 11hp screaming) after a few seconds it cuts out and stops so i'm guessing fuel problem. I have cleaned the carb several times and know that gas is in the bottom of carb since it poors out when i open the drain screw. Also changed coil, plug, wire, battery.. It's charging 8v on 1/3 throttle

You don't happend to have the orginal carb settings and valve clearing ? I have order the servicebook but it's atleast a week away.

Any other tips appreciated.


Dakota Boy.....did you figure out anything else? I have very similar issue with my 82....need all the help i can get! thanks !

Martin, if it's starving for fuel at WOT, it could be float height, bad float, or restricted line, like a clogged filter or crud in the line.

Maks sure your gas cap is venting properly:thumbsup:

I'd guess these have a CDI box It may not be advancing properly, though without replacing with a known working part it's difficult to tell

same kind of problems on my 82 XT 125 no top end what so ever! any fixes guys. where can you find an aftermarket CDI unit for these that's reliable? that one guy on EBAY only makes them for XT 250's and bigger. I PMed him and he just said he couldn't help me with my model. I wonder if the CDI'S off the bigger bikes will work on the 125's really want to get this thing running right as it supposed to get 78MPG!

I havent solved it yet, but then I havent been giving it much attention lately.

I'm going to give it another shot this winter. Goal is to have it running decent and licensed for the street by early next summer.

Took the bike to a local shop known for working on vintage Japanese bikes.

CDI was bad. Replaced it.

Also, some cretin had drilled out the pilot and main jets in the TK carb.

And last but not least, the timing chain was off by one tooth.


I have an XT125 from 82 and the cdi doesnt advance a second cdi does sod all and a third cdi one spark and out,call me lucky .Inthe meantime I have ordered one from an XT 500 also 6 volt near as dammit same timing properties when I get it I will let you know .The co is called "electrex world" in England ,also check out "transmic.net " they have plans for you to make your own cdi , I made one for an XT600 parts cost 9euros and it works.

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