1998yz80 help

Clean your main jet. For the oil, I don't know your bike specifically, but Yamaha often used a little glass window on the right side. With the bike stood level, the oil should be in the middle of the window. The overflow screw is a Honda thing.

On my YZ125 and my son's YZ250 the oil level indicating screw is towards the rear, somewhat above the kickstarter, facing to the right. If you look at it closely it looks just a little different than the others, probably has a washer looking gasket under it. The proper screw will probably be shorter than the similar screws around it, and if you take it out and tip the bike that direction oil should run out.

As for your bogging problem it sounds like a jetting / carburetor issue. I wouldn't have expected a new plug, hotter or otherwise to help the situation.

If you have just made major changes to the exhaust, reeds and other intake/exhaust items then it is likely that the jetting needs to be adjusted.

The easist adjustment to experiment with is the throttle "needle". If you unscrew the top of the carb the throttle valve will pull out with the throttle cable attached. In the middle of that is a needle that is held in place with a small clip. The needle will have several slots that the clip could be placed which will change the relative height of the needle with respect to the valve.

First, make careful note of where it is so you can put it back.
Then remove the clip and reinstall one slot either up or down.
Try it.
If it got worse, move it one slot the other direction.

If it gets better, try two slots the same direction. Try a few more to see where it works best.

Note that by raising the clip to the next higher slot makes the needle go lower, which makes the needle stick into the jet more, which makes it run leaner.


If changing the needle position helps the high end but kills the bottom end then you might need to change the main jet.

Rod