Carbs balance - strange problem.....

[MOTOMYSZOR]

My friend asked me to help with carburettors in his XJR400.

He striped them down, cleaned, and now "only" left to balance them....

Engine starts and works not bad, idling quite well, just losing revs slower then it should......

I connected my gauge (carbtune pro) with I used on many bikes and it shows no read.... It looks like there is no vacuum.
I discovered that after I block for a moment air filter inlet,  revs will jump up and gauge will start showing proper reads for all cylinders (they are werry lightly inbalanced). then revs going down and vacuum "disapears".
Actually I have no idea what is the reason for that.... anyone knows? 

[mr blackstock]

Perhaps increasing the idle stop so she sits at 2000 rpm might provide enough vacuum? Some xj650 owners have the same issue, one guy partially blocked the airbox to get a good reading.
Sounds like a silly thing to do, but then again, you wanna check the balance not mixture.

Cheers, Gareth

[ct7088]

Are you sure the idle circuits are open and functioning? The throttle valves could be open far enough to lower the vacuum. How similar are the XJR carbs to the FJ carbs?

[MOTOMYSZOR]

I believe that Chris is right - engine operates on fuel from main jet not from pilot system.

And because of that throttle is opened enough to lower vacuum.....

Because carburettors were stripped down to the bones and rebuild by my friend who is not good in this I can expect more surprises.....

Today I will take closer look.

[Earl Svorks]

  MotoM,
I suggest you check your valve clearances. I have seen symptoms like this a few times . It can be difficult to diagnose. It seems like a carb issue, it seems like ignition. Check those clearances!
   Cheers
Simon

[MOTOMYSZOR]

Today I striped down carburettors by myself. 3 pilot jets were clogged.... After proper clean bike running surprisingly well for "only" 400 ccm...... 

Proper working pilot system allowed close throttle much more and create required vacuum....