Hey All. I recently picked my dad’s old 1978 Yamaha IT-175 and an identical parts bike out of my uncle’s machine shed in hopes of revival. It had not been started in 6 years so I drained fluids, cleaned filters and got the thing running. I ended up pulling parts off of the other bike and swapping them out as well as pulling apart the carb and cleaning it. It was apparent from riding that the bike was running a bit rich so I ended up resetting the pilot air screw to what the manual recommended (it was a bit off the recommended 1 1/4 turns out). I initially mixed 20:1 (what the manual said), but figured out quickly that 32:1 was better. 

After doing all of that work, and then some, the bike sounded great (for a 45 year old barn find) and started up on one kick. I began riding it for more than a couple miles at a time and noticed that after about 2 miles or so of riding, the acceleration and top end power were slightly diminished. Additionally, the bike will stall when I try to launch it from first gear after a short amount of riding and it has difficulty starting back up. The 2 times I’ve riden it after the second round of work, I’ve had to wait for 10 minutes to let it cool then give it a lot of gas on launch to get it rolling back home. I noticed some power loss before carb work, but like I said I had been riding short distances before that.

The bike sounds beautiful and runs great on the first kick at the start of a ride. My uncle’s theory was possibly a clogged exhaust (I put a new muffler on and I highly doubt that) and my theory was that it could be running too lean and thus overheating. Another thing that I forgot to mention was the when the bike gets started (no choke btw) the throttle opens up immediately and goes back down after a few seconds which I know that runaway idles on two strokes can be tied to too lean air:fuel ratio or something like running out of fuel (it has plenty of fuel). FYI the spark plug is the old one, have yet to fit a new one (if it ain’t broke don’t fix it), but it is stained dark in color from running rich for so long. It should be noted that this “runaway idle” was not an issue before carb got pulled apart and pilot air screw reset. Could it also be possible that leanness could be from some sort of air leak that opens up when things get hot (such as rubber engine manifold)?

I’m 17 years old and this is my first “big boy” bike and I’ve had a lot of fun learning on this project. I’d really appreciate some ideas or theories from vets. Let me know if there is anymore info that I can offer.



 

Welcome from NZ.
You're very lucky to pickup one let alone two IT's.
I had one once that'd been in a shed fire & had it running good before long, albeit looking a bit melted.
Was a free bike, 6 speed & all, but that's another long story.
So--first thought is how is compression to feel on the kick start comparing cold start & running well--to the power loss when hot.
Does comp get very low--soft to feel on the kick.
Are rings sticking in the grooves--although i doubt they'd come right when cooled if that was the case.
I get sidetracked on exhausts but sounds like muffler is new so look down in where it plugs into the main chamber for a ring of carbon where the pipe finishes.
But then again that's not going to change from cold to hot unless by chance hot loose bits are bouncing around under power & blocking it from the inside against the tail muffler then dropping away when cooled.
All an unlikely wild guess & probably way off the mark.
I had a yamaha LS3 100cc road bike that i'd say sat at least for 20 years.
Piston had carbon 3/4 of the way down the skirt [never seen that before] & on getting it started for the first time it kept tightening [seizing but not realy] but tightened to a halt. Had absolutely zero clearance due to this perfect pollished carbon down the skirt so it expanded a gnats whisker & tightened to halted every 10 seconds the freed after about 10 seconds.
Was weird until i lifted the cylinder off & was happy to find that easy fix.
So--it'd be a very long shot if you had a minor version of that & tightening but still running.
Be great to have pics sometime if pic on here are working again. Cheers & good luck. Sorry my ideas are of limited use.
By the way i'm 60 years older than 17. What's it like, i've forgotten haha.

A couple of heat cycles could easily enlarge the left crankshaft seal and because the spark plug was not replaced you could easily have a unknown lean running condition. I personally would do a bubble test and replace the spark plug. A second carburetor cleaning should also be considered. Rubber seals rot from sitting long periods of time and heat then shrinks the rubber…

a

Spider web in gas cap vent.

cliff

If possible it'd be good to have a video, nice long one, of cold start then heading off on ya ride. Then again when the condition arises. Makes it easier for us to have better guess--i guess.

Greetings from Tennessee  
 We can get you going video or pictures would help 

Appreciate all the advice. Going to look into some of the suggestions and pull apart some of the engine components. Here’s a couple video representations. Cold started for the first time in two weeks on the second kick. Rode for 2 miles and it won’t launch. For the person that was wondering about the compression, I tried kicking it after I got back from riding and it had noticeably less compression after it got hot.

If someone can let me know the best way to get videos to an approved format from my camera roll that would be great. Thanks.

I load mine on youtube & make it unlisted then link it to here. Can make it public if ya want of course.
I'm sure there's better ideas someone will come up with soon.
Look forward to seeing the vids.
Feel the comp by hand on the kick, cold then hot after the problem shows.
Get a better feel by hand sometimes.
By memory the kick gearing is quite low on those so only get it over one compression per stroke so low comp should be very obvious.
Or compression gauge test of course.

Congrats on the IT's. The 175's are very fun. My limited experience with old 2 stokes is to be careful when trying to kick start or rotate old engines. The cylinder will be bone dry from sitting and bad things can happen in there when they are forced to move abruptly with no lube. Your bikes may have been put away with tired top ends too. Not a big deal to get them going again. Good luck and have fun with those ITs!!