Is it possible that flow bench settings aren't always correct?

[ohiorider]

I mean, we do know that the Guzzi factory doesn't always get everything right.  Take for example un-greased swing arm bearings.  So, then, is it possible that some of the throttle bodies aren't balanced properly on flow benches?  Most of us who ride big block Guzzis know (by now) that the balance screw is located on the left throttle body, and the TPS is located in the right throttle body.  And that re-balancing the TBs results in having to reset the TPS, since we're altered the position of the right side TB.  (why didn't the factory design the TBs with the TPS on the left TB?) But I digress.

There are those of us who experience periodic high idle on these bikes.  On a trip several thousand miles from home, my 1200 Sport went into a high idle condition.  By high, I mean 1600-1700rpm.  I found it was possible to reach below the saddle and pull the throttle lever on the right TB toward the closed position, and viola, the idle would drop to 1100 rpm.  Release it and the idle would climb back to 1600-1700rpm.  And at the risk of slightly unbalancing the TBs, I turned the 'not-sacred' balance screw counter-clockwise about a quarter turn, and the idle dropped back to 1100rpm.

Makes me wonder if the original flow bench setting might not have been quite  right, and that we're fighting a maladjusted flow bench adjustment?

Are there any Guzzi techs out there who have found that the 'sacred screw' adjustment is not set properly from the factory, and tweaked it, or is it simply gospel that said 'sacred screw' is never to be touched?  And we continue to work around this issue with things like clamping off the vacuum hose to the stepper motor?  Which is probably not the correct solution, anyway.

[Vasco DG]

While adjusting the bell crank screw may well have a useful effect as you have discovered it will really simply be masking another problem.

While small differences in porting and pipes do seem on some bikes to lead to a minor imbalance, (This will be far more noticeable when running an open loop map as the ECU won't trim.) and this can sometimes be addressed by moving the bell crank screw the fact that when you physically yank the butterfly closed the idle drops is actually indicating that the butterfly isn't closing fully on its own under the influence of the spring. If the TPS was re-set after a TB balance and the butterflies aren't fully closed then the extra air being passed by the butterflies may overwhelm the ability of the stepper to compensate and you'll end up with the high idle and an incorrect AFR for the given opening.

In every case where I've been confronted by this I've found the TB's to be filthy which prevents the butterflies from closing. The cause of this is, as you probably know, oil that is expelled through the breather system into the airbox befouling the TB's. Guzzi actually suggest removing them every 10,000Km for cleaning but in al honesty I think this is grotesque overkill.

The pushrod 1200's seem particularly prone to this and its important not to overfill the sump. As you know I recommend running the oil between half way down the stick and the 'Add' mark as this minimises the expulsion. Even so though there is still likely to be some expulsion.

Best solution is to drop the TB's off and give them a very thorough cleaning on the bench. DO NOT mess with the stop screws while doing this. After they are all squeaky clean again pop them back on and then go through the TB balance and TPS calibrating ritual again. it usually fixes it.

pete

[ohiorider]

Pete, thanks for the thoughts re my findings.  I've taken your 'oil at mid level' advice. I was thinking the rod connecting l and r TBs might have been adjusted so that expansion and contraction due to engine heat might have been pulling on the right TB linkage.  Probably overthought the problem.

Bob 

While adjusting the bell crank screw may well have a useful effect as you have discovered it will really simply be masking another problem.

While small differences in porting and pipes do seem on some bikes to lead to a minor imbalance, (This will be far more noticeable when running an open loop map as the ECU won't trim.) and this can sometimes be addressed by moving the bell crank screw the fact that when you physically yank the butterfly closed the idle drops is actually indicating that the butterfly isn't closing fully on its own under the influence of the spring. If the TPS was re-set after a TB balance and the butterflies aren't fully closed then the extra air being passed by the butterflies may overwhelm the ability of the stepper to compensate and you'll end up with the high idle and an incorrect AFR for the given opening.

In every case where I've been confronted by this I've found the TB's to be filthy which prevents the butterflies from closing. The cause of this is, as you probably know, oil that is expelled through the breather system into the airbox befouling the TB's. Guzzi actually suggest removing them every 10,000Km for cleaning but in al honesty I think this is grotesque overkill.

The pushrod 1200's seem particularly prone to this and its important not to overfill the sump. As you know I recommend running the oil between half way down the stick and the 'Add' mark as this minimises the expulsion. Even so though there is still likely to be some expulsion.

Best solution is to drop the TB's off and give them a very thorough cleaning on the bench. DO NOT mess with the stop screws while doing this. After they are all squeaky clean again pop them back on and then go through the TB balance and TPS calibrating ritual again. it usually fixes it.

pete