May 1, 2000
I'm putting a 77mm big bore kit in my '86 XR. I run a Suppertrapp with 6 discs. everything else about the engine is stock. I ride mostly at sea level, although sometimes I'll ride at 3,000 -5,000 feet. Can someone suggest a starting point for jetting. Do I need to change the pilot jet?


Jun 12, 2000
I can't give you specific jetting, but here's some guidelines and resources: has correction factor tables, which account for altitude , temp. and airbox/exhaust changes.

To my surprise, my local dealer (who installs a lot of XR 400 big bore kits) told me that the bigger bore doesn't need much larger jets, if any. They recommend going up one size on the pilot (slow) and main jets.

According to Supertrapp, 4-6 discs creates about the same backpressure as stock pipes on later model XRs (not sure about your year). I notice a lot of guys run 12, which frees up exhaust, improves top-end and affects jetting (go a little richer on the main). Although, fewer is quieter, which is important too.

You should probably jet for your leanest condition, which means jet for sea level. You'll be rich at 5000'.

Given the above, you might need to increase the pilot and main one size. If you opened your airbox and exhaust, it would be more likely.

When I change altitudes, I switch between a K&N (higher altitudes) and the stock air filter (lower altitudes). This seems to help a little and avoids jet changes.


Aug 22, 2000
Go up three sizes on the mainjet, move the clip down one position on the needle. This is a good break-in setup, but will be rich.

After break-in, drop down one jet size at a time till the bike runs clean from 3/4 to full throttle. You may end up moving the clip back to the middle position on the needle also.

My educated guess says you'll end up with that clip back in the middle position, and one or two sizes bigger than stock on the main.

In most cases, you won't need to change the pilot. If it gets to be a real bear to start, then try bumping up one size. Otherwise, just leave it alone.


Mar 11, 2000
TobyS, Start with a #130 main, #45 pilot. This is pretty norm for your climate, elevation, and 77mm piston. Needle usually ends up in the middle (3), or 4 clip positon. These specs also are using decent pump fuel.
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