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Mon Feb 27, 2012 9:43 am

Does anyone know what would cause one carb to be unbalanced, one vernturi sucking 5, while the other is sucking 3.75. Worked fine for a long time, then all of a sudden it went unbalanced . I tried the carb on two different motors so it is not the motor, for this one it is on is newly rebuilt and has the same symptoms. The other carb is fine, just #1 sucks well and #2 does not. I have adjusted the butterflys, and have cleaned, and blown out many times over, with no better results. Is their an adjustment I am missing?
Any advice would be greatly appreciated, Fredrico

Mon Feb 27, 2012 9:50 am

A missing butterfly screw, a twisted throttle shaft, a bypass screw that is unscrewed, an open CO tube, vacuum leak, engine problems, to name the most common.

Look at the throttle plates as they stage the progression holes.. Are they exposing the same holes at exactly the same time?

If they are you can use your air bypass screws to adjust the balance. Just doing it all of a sudden makes me think something has changed that needs attention rather than needing a calibration.

Mon Feb 27, 2012 10:11 am

Thanks for the suggestions (I said all of a sudden but didn't mean it. Not sure when it happenned, the motor just started running worse, so I checked the carbs and found the issue. It has been in this condition since last August, thought it was the motor, replaced with new rebuilt and had the same issue.
We can rule out:
missing butterfly screw ( confirmed all their)
twisted throttle shaft (both closed at same point at same time covering the same progression holes)
vacuum leak (come back to idle normally)
engine problems (2nd engine same probelm)
bypass screw that is unscrewed (tight)

Need more info, what is this?
open CO tube

Tue Feb 28, 2012 10:19 pm

Some DRLA carbs have brass tubes right next to the mixture screws, that have a wrench fitting at the base. These are for tuning, and if one is loose or gone, then it will suck air there.

Wed Feb 29, 2012 5:58 pm

I did some research on line and think possibly that the air bypass screws although all tight, may have to be adjusted to tune each barrel. You adjust the strongest drawing barrel to match it weak partner, then adjust idle to match both carbs. I am told the the idle circuit is that, only for idle for once you apply the throttle the circuit is no longer used. If this is so, I see no harm balancing each carb this way. What are your thoughts?

Thu Mar 01, 2012 8:15 am

There is no harm as long as you don't have to do it too much.. any more than a point if flow difference is too much to me. You open the LOWER flowing screw, not the larger one.

It's just not typical, especially with Dellortos, for the flow difference to be that much when the screws are all in. I suspect a problem somewhere causing it. If you find that the screw doesn't affect it much, it may still be a vacuum leak or something.

Thu Mar 01, 2012 8:43 am

Tried it last night and you were correct the low flowing carb had to be adjusted. Adjusted out 1/4 turn put it right in range with the other barrel. Now I kick myself thinking on the old motor it was a bad cylinder. Oh well live and learn. Idles great now, so problem solved
Thanks for the help, Dennis