I have been working on this little moto 4 80cc as a fun little project but cant seem to get it running just right. I have done a few things to it...

- carb has been cleaned/ rebuilt with 85 main jet and 15 pilot jet.
- high flow cone filter attached right to carb inlet.
- valve adjustment
- removed baffle from muffler

Ive been messing with the carb for too long and I dont know what Im doing wrong or if something is wrong with the carb. As soon as I think its running good, the next time I start it cold it doesnt run right. It wont start unless the choke is on and I give it a little gas while starting. Once its warm it seems to run fine but getting it there is too much of a challenge. Now Ive been trying to tune the carb and I turn the idle up a bit, then turn the mixture screw in or out until I hear the idle bump up and get smooth. Although I cant seem to find the right spot. Turning the mixture screw to the right is leaner and turning it to the left is richer correct? One symptom Ive noticed is when I press the throttle, it cuts out and almost stalls. I want to make sure Im tuning it right. So what is the correct way to tune this carb? What symptoms mean what?

Heres a picture of her!

 

Beautiful bike! Temperamental though, I understand

Yes loosen is richer, tighten is leaner on this carb

The choke on the YMF80, the little plastic wheel dealio has detent's in it that keep it from being closed too far, it is really easy to go past the detent and have too much choke.

I have a 1989 YMF80 I just did a full rebuild on like you (I have been watching your thread, very nice work!) And it is also cold blooded. It has a carb on it from a later model Raptor 90 which I have heard is a nice upgrade for the earlier moto4's (dealership installed)
So make sure you stop the choke lever on a detent, don't go past it.

I got into the same game with the carb. Adjust the carb when the bike is up and running and warm, now I have a perfect idle and it runs great! Next time I go to start it, my idle setting was too low and it required throttle to keep it running.

Now I have found the middle ground and a technique that works each time we start it.
Choke must be on the last detent (choke on) and the bike will fire up with the touch of the button. It runs for about 5-8 seconds with a slightly increased idle due to full choke. It runs great for these 5-8 seconds and then begins to slowly bog down. I reach down and turn the choke off and BOOM perfect idle, no throttle needed, no carb adjusting needed. If I miss the mark, turn the choke too quickly, it will die and I have to do it again. Once its warmed up no choke needed and it fires up with a perfect idle
Only idle issues I have are at first cold start of the day and it only takes 1-2 minutes of running before it acts "normal"

My screws are set at 2.5 turns out on idle speed and 1.5 turns out on mixture screw. Jets came with the bike (carb replaced by a dealership two years ago, I just had to clean the snot out of it) so Im not sure what jets are in there but they are correct for my altitude.

My other 1984 moto4 80 just fires up and runs, no choke needed, no weird choke tricks to get it started, its very odd. I did the top end, valve adjust, and carb requild on both of these bikes at the same time. I also have a cone filter on the carb and it works fine.
The only difference between my two moto480's is one has 1984 carb and one has a 90's something carb, otherwise identical

Have you broken in the engine yet?

After driving the sucker around alot getting it hot a few times you gotta check the valve lash again and verify the timing marks.
Also DON'T USE ethanol fuel....causes all sorts of a mess on these tiny older carbs.

 

its been a while since iv messed with the mixture screw. Cant remember the exact procedure. But its something like set the machine to fast idle, Turn the mixture screw to the top of its range until ending rps levels out, then turn it to the bottom of the range until it levels out then set in the middle. This is all done with the mixture screw turned all the way in until it lightly bottoms out. I would look up the correct way but it goes something like above

 

That brings back memories, I had a blue ymf80, dad bought new off the showroom floor in 1985. It was ridden harder than anyone can imagine. One suggestion, keep the diff serviced. My reply is pointless I know, just cool to see something I had when I was a kid, that's all.

 

Good looking moto4. Can I ask, who did your plastics?

 

I did the plastics actually. Thank you for the compliments!