too rich or too lean ????

[dsrdave]

I'm helping a friend tune his carbs for racing.  At WFO the bike goes a bit flat.  If we back off the throttle a bit it picks up.  I know this is telling us something.  But I'm not 100% sure how to interpret it.  Is the main jet to small (lean) or the other way around ?  Thx for the help.

[yrunvs]

ROLL-OFF:
The Roll-Off technique is the quickest and is almost as accurate as the Roll-On method. First,
one gets the engine warm on the way to a safe roadway. If there is room, use fourth gear as this
allows more time to assess the result.
Now, get the engine rpm high enough that it is on the cam and in its power band. This may need
to be as high as 4000 rpm with some cam choices. Apply full throttle. Let the engine accelerate
for a couple of seconds until it has settled in and is pulling hard. Quickly roll the throttle off to
about the 7/8ths position. When you do this, the mixture richens slightly for a second or so.
If the engine gains power as you roll the throttle off, then the main jet is too small and you need
to fit a larger one.
If the engine staggers slightly or has a hard hesitation, then the main jet is too large and you
need to fit a smaller one.

[Triple Jim]

It's also possible to use the carb enrichers ("choke") if the bike has them.  In the trouble spot, open the enrichers briefly and see if the problem gets better or worse.

[rodekyll]

ROLL-OFF:
The Roll-Off technique is the quickest and is almost as accurate as the Roll-On method. First,
one gets the engine warm on the way to a safe roadway. If there is room, use fourth gear as this
allows more time to assess the result.
Now, get the engine rpm high enough that it is on the cam and in its power band. This may need
to be as high as 4000 rpm with some cam choices. Apply full throttle. Let the engine accelerate
for a couple of seconds until it has settled in and is pulling hard. Quickly roll the throttle off to
about the 7/8ths position. When you do this, the mixture richens slightly for a second or so.
If the engine gains power as you roll the throttle off, then the main jet is too small and you need
to fit a larger one.
If the engine staggers slightly or has a hard hesitation, then the main jet is too large and you
need to fit a smaller one.

Interesting empirical test.  I think I did this with my Convert a few days back.  What happened was I was looking for the rev limiter in low range and I think I found it -- engine acted like either spark or fuel was intermittent.  So I backed it off a few decibels and ran a few hundered rpm slower.  Then I backed the throttle off slightly, just to the point of overrun.  When I did that (remember WOT and near redline) the bike slowed for a moment and then gave a very powerful surge, as though I'd rolled the handle up tighter again.  That rattled my cage, so I backed it down to reasonable speeds and didn't think too much more about it, since I don't ride that way.  But now you're suggesting this might mean I'm underfueled at WOT?  Could it mean anything else?

[Perazzimx14]

If all else fails do a full throttle plug chop test . The plug readings should give an indication of what is happening .

 Dusty

If you have access to fuel w/o ethanol in it.

[oldbike54]

If you have access to fuel w/o ethanol in it.

 One can still get a reading .

 Dusty

[Chuck in Indiana]

If the engine gains power as you roll the throttle off, then the main jet is too small and you need
to fit a larger one.
If the engine staggers slightly or has a hard hesitation, then the main jet is too large and you
need to fit a smaller one.

That sounds right to me. As mentioned above, though.. a manual choke can tell you a lot.. as can an EGT, but most Guzzis don't come equipped with one.