Thanks for all the advice, we will be trying it out this weekend.
Air/fuel sounds like the route to go.

 

RAPTORAZ -

Although a #42 pilot jet may work fine, it's overkill on a stock machine. It can even cause the 400ex run too rich at idle.

Consider this, when using a microscope, say the power is set on 10x. In order to see a slide - you look through the viewfinder and see that the image is blurry. You wouldn't turn the power up to 20x (that would be like increasing the pilot jet to size #40 or #42). Instead, you would use the fine adjustment on the microscope to get the image in focus. Likewise, the air/fuel mixture screw is a fine adjustment for the pilot jet.

I know this isn't a perfect analogy, but it may help some understand the difference between the air/fuel mixture screw and the pilot jet.

ogre

 

So if I understand you correctly, every 2 digits on the jet numbers increases the size by double. Am I understanding you right??

 

Ogre is correct in that you could fix your problem by adjusting the air/fuel idle screw. However, that will only adjust your air/fuel ratio at idle where as a #42 pilot jet will also give you more throttle response from idle to 1/4 throttle. And going from the stock #38 to a #42 is a very MINOR change.

 

Deathbringer - sorry about the misunderstanding; no, each size increase in the pilot jet doesn't double the size of the "hole" in the jet. I was generalizing on how one needs to adjust the fine settings before you make a course adjustment.

Let me make myself clear, if not just for the fun of it. If I remember back to science classes, the microscopes we used had magnification settings of 10x, 20x, 50x, and maybe 100x. These represent the magnification level (e.g., with 10x, you could see 10 times "closer" than with the naked eye). Anyway, The microscopes had a fine adjustment, where you could focus so you could see clearly at every power level. That's where the analogy ends.

With the carb on the 400ex, the stock size pilot jet is #38. The next sizes up are #40, #42, #45, #48, #50... Back to the microscope analogy, one needs to use the fine adjustment (i.e., air/fuel mixture screw) with the pilot jet. If, at four turns out of the air/fuel mixture screw, the quad doesn't start/idle the way you want, then increase the size of the pilot (i.e., from #38 to #40), and start over with 1 turn out of the air/fuel mixture screw.


RAPTORAZ -

You're right, with a stock machine, increasing the pilot from #38 to a #42 is a minor change. And a textbook answer says if you're lean in the low-end circuit, increasing the pilot jet size will give better power from 1/8-1/4 throttle. What I've found however, is that when you've got the air/fuel mixture screw dialed-in (from 1 to 3 turns out) - that is usually the best pilot jet for the application.

From my experience, the pilot jet is a VERY minor factor in overall jetting. When adjusting my jet needle, I've found that it affects 3/16 to 1/4 throttle much more than the pilot jet.

But I've found that the pilot jet does affect starting a lot. I've run too big a pilot before (#45 and #48), and it was harder to start than ever before.

These are just my experiences, however, I can't claim to be an "expert" tuner.

ogre