Motorcycle Cruiser

How to Index Your Motorcycle's Throttle to Troubleshoot Carburetion Problems

You can't solve a carburetor jetting problem until you know which fuel circuit is involved. Using a throttle index is an easy way to determine which circuit to investigate. From the December 2005 issue of Motorcycle Cruiser magazine. By Mark Zimmerm

April 8, 2010

All right guys, listen up. If I've said it once, I've said it a thousand times: Trying to diagnose a jetting or problem based on engine rpm is a waste of time.

Here's why: When you tell me or any other tech that the bike farts and burps at 3000 rpm, we've no way of knowing what circuit the carburetor is using at the time. As you can appreciate, 3000 rpm in fifth gear is a very different throttle opening from 3000 rpm in first gear. Even if you fill in the blank spots, the best anyone can do is guess which fuel circuit you're using when the problem occurs. And, if the problem is with carburetion, knowing which fuel circuit you are primarily using when it occurs is essential to correct the problem.

To make an accurate assessment of any carburetion problem, you need to know where the throttle is positioned, indicating which fuel delivery circuit is in use when things go haywire.

Trying to figure out where your throttle is while dodging traffic can get a little hectic, so try using this simple tool next time you've got jetting issues. Wrap a piece of masking tape around the throttle housing. On the tape, mark the closed position, wide open throttle, and the points in between, where the throttle is 1/4 open, 1/2 and so on. With the throttle closed (that is, released and at rest), stick a colored pushpin in the grip, adjacent to the Closed mark. Now, using your indexed markings, note the point on the throttle where the problem occurs. This will let you (or at least the tech) determine which carburetor circuit is primary when the problem occurs.

Once you know which fuel circuit is affected, solving your problem should prove much easier.

For more articles on how to maintain and modify your motorcycle, see the Tech section of MotorcycleCruiser.com.

Making a throttle index is quite simple and requires just a minute or two. Using a piece of tape, mark the position of your motorcycle's throttle grip when the throttle is fully closed, wide open, half open, and then and half way between those three points for the 1/4- and 3/4-open points. By observing where the throttle is positioned when the engine runs poorly, you can determine which fuel metering circuit needs to have its jetting or settings adjusted to get the right mixture and cure the problem. If there is a mark or seam on the grip, you won't need the pushpin.