Engine adjustment advice pls.

Richard Decosta /

Finally got my new Yamahopper street legal and got to really test it out today. She goes great, scoots right up to 29mph in about 4 seconds. However, if I try to ride at about 10-15mph, the engine runs rough, sputtering, running up then down, etc. It idles ok, though a bit erratic, and at 29mph (full throttle) is smooth as silk.

Any advice?



Re: Engine adjustment advice pls.

Sounds like mid-range richness..

If your carburetor's throttle is a slide-type with a tapered needle that moves up and down in a "needle jet" hole, the needle will have positioning notches on it. Perhaps 4 or 5. A clip inside the slide cavity selects which notch is used.

So, the needle position can be raised (enrichening mid-range air:fuel mixture ratio) or lowered (leaning mid-throttle mix).

The particular taper and clip position and length of the needle partly determines the air:fuel mixture ratio delivered around mid-throttle.

The factory probably set the clip on the middle groove. Try moving the clip position one groove up or down and road test the bike.. You should be able to feel and hear a difference.

But first make sure the engine, bike and carb etc. are otherwise in good tune. I'd be specially attentive to the carb since you have minor idle problems.. Both problems could be due to something like a high float level, choke trouble or other rich-running condition. So, try to get that straightened out (check float level, air-filter cleanliness, carb cleanliness, cable adjustments etc.) before adjusting the needle.

Re: Engine adjustment advice pls.

Richard Decosta /

Thanks, I'll check it out. I do have a choke issue (choke cable is missing!). Fixing that today, so hopefully that'll help some too.


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