1982 YT125J bogs in top end

[MudBug]

Just picked up a pretty low hour 125 trimoto, cleaned the carb, checked needle position, changed the fuel and injection oil, adjusted injection pump and throttle cable. Runs great, starts first pull every time , lots of power through gears except it’s low on power at the very top end, kinda bogging but not horribly, just stops pulling. First two stroke I’ve owned in a long time not sure if there’s something I’m missing. Reeds looked fine, stock airbox/filter is clean. Only thing I haven’t changed yet is the spark plug, hopefully that’s the issue

[350for350]

I'm not real familiar with the Yamaha two strokes. Most service/owner's manuals give you a method of cleaning out the silencer/spark arrestor. Maybe you need to try this?

[El Camexican]

Post a photo up of the color of your spark plug and what you have it gapped to.

Is your air box connected to the frame to bring air in from near the steering head? If so make sure there’s no mice nests in there.

Did you check you float height?

You mentioned adjusting the cables. With the carb off the engine, did you open the throttle up to make sure both edges of the slide completely clears the bore of the carb?

[MudBug]

Air intake is clean, float height was set to 21mm as per manual, both sides of slide clear carb bore. Noticed a bit of water in the airbox from me washing it a few days ago, may have been pulling some in at high rpm. Spark plug looks original, was gapped very tight, set it to .030” and seemed to run a little better, I’ll throw a new plug in it tomorrow and see what changes. One thing I noticed with the oil pump adjustment, when you adjust it like the manual says, it maxes out quite a bit before full throttle, not sure if it’s getting too much oil before throttle being wide open? Going to double check it now

[El Camexican]

.030” sounds a little on the big side, is that what the manual calls for? I’d try .026” to .028” at the most. Water could be a cause.

Are you sure you’re checking the float height correctly? The height is measured as the spring loaded pin bottoms out on the needle. Can be a little tricky. Better 1 mm low than 1 mm high for a couple reasons.

So the cable adjustment, I assume there are two cables that join to one that leads to the throttle? If so, is it possible that when the oil cable reaches wide open that the carb cable can’t advance any further? Sorry if I sound clueless on how your setup works, but it’s been about 35 years since I was near one.

[MudBug]

Manual calls for .028” - .031” got it back together and seems to struggle more now. Almost runs like the choke is half on but the cable is free and adjusted right.

[El Camexican]

What condition is the air filter in? Is it original?

What color is the plug?

I’d try gaping that plug at .026” - .028” to see if it improves.

[MudBug]

Original air filter (cleaned and oiled) and box. Gapped new plug to .028”. New fuel line, cap vent is clear, still doing the same thing. Acts as if choke is half on in the very top end. Might go through the jets again

[El Camexican]

Original air filter (cleaned and oiled) and box. Gapped new plug to .028”. New fuel line, cap vent is clear, still doing the same thing. Acts as if choke is half on in the very top end. Might go through the jets again

Until you post a photo of the plug I'm stumped. It sounds rich. You say the filter is clean, but I've seen filters that were only 10 years old that were crumbling and falling apart. You'd know better than me, but a 35 year old filter seems questionable. I'd tell you to try running it with the filter off for a but, but only if you're certain you're not going to be taking in any dust.

Have you looked at the spark you're getting from the plug? If it nice and blue in the dark? If not you can try cutting a 1/4" or so off the end of the wire and re-threading the cap back in.

Another possibility that would not apply to a low hour unit as you say yours is, is a worn needle. Those trikes vibrate like jackhammers if I recall and a worn needle will cause you to run rich. Maybe pull it out and look for any wear. Anything less than perfect could be your issue as well.

Stupid question, but is the fuel less than 2 months old?

[ATC King]

I'm no two stroke guru, but I've owned a few. Usually when they're rich up top, they cut out, almost like a rev limiter. It's real noticeable and not just a decrease in power. It's a choppy type of running.

If the power is backing off up top, but otherwise running smooth (ish), it may be lean. It could be something simple as a restricted fuel filter or fuel cap vent, to something else that'll need a more in-depth look.

Running lean on a two stroke means it's also not getting enough oil. I have seized one up from lack of oil, and been nervous about it ever since.

[MudBug]

Took this thing to (Canada/Alberta) trikefest and it seized up after about an hour of riding. Disappointed but I have a DT175e engine to swap in over the winter, should be a fun project.

[Yamaha Tri-Moto]

I'd ditch the oil injection and run pre-mix if you get it going again.