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carb backfiring

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Jack... can you really do that... run a dual carb car off a single carb by holding one butterfly completely closed? I'm almost afraid to try that. Could you walk me through your procedure? PM me if its too long and off topic for this thread. I'd appreciate it.

Dave, I agree that balancing the air flow at idle should keep them balanced through their range but it's never seemed to work well for me. From memory, don't the Haynes and SU manuals discuss disconnecting the linkage between the carbs and using the idle screws to move the butterflies until the air flow is balanced, then lock the linkage in place? My problem is that once I do that, the carbs are balanced at idle but off idle they don't track each other well. (No matter how careful I am tightening the linkage!) By balancing them at 2000 RPM they seem to stay together through the RPM range.

Carb adjustments are the thing I enjoy the least. I know with careful practice I'd get better at it... but I'd just assume not.
 
Doug,
As I said, the piston return springs need to be closely MATCHED. Else the pistons will not rise the same amount even if the butterflies are perfectly synched.
D
 
Get it to do it again and check yer plugs....

I like NGK BP6ES but every thing else looks right.

Also float settings...
Check the level of fuel in the jet w/ a flashlight.
(Pistons/needles out & hit fuel pump w/ ign wire off coil)
Should both be about 1/8" to 1/16" below top of jet.
 
I think Doug is forgetting that the carb piston will impede air flow even if the butterflies are in sync.
 
Trevor, I see Dave's point and yours. I'm just baffled that off-idle air flow never seems well balanced on either of my LBCs. Compounding this confusion (for me at least) is the fact that the carb springs in the Mini were replaced in January just in case the old ones had fatigued. I admit I didn't perform the spring rate measurement that Dave suggested though.

Sorry for the thread hijack. Let's let my issue die for the moment and return to the through-carb backfiring question that started this thread.
 
Of course you can do that. That is how it was done at the factory and is the way all us old timers do it, I hope. Now that assumes all else is good of course. Oh yea don't forget to block vacume advance, carb end.

Fast easy and positive.

Engine actually runs preaty well on one carb, crossover pipe don't ya know.
 
OK Jack, I just don't remember ever hearing of that procedure before and don't remember it in any of the manuals I've read. So that's the way the factory tuned our cars?

So... how do you make those idle adjustments? Up a couple of hundred RPM on one carb then back the other off until the RPM return to where it was. Do that back and forth until one carb is closed off?
 
Aye, just a few hundred at a time. That procedure is going to put you really close to what your engine needs, they all different you know. The proof of the pudding is the color of your tail pipe after a few miles, a nice tan is ideal. Too black, too rich. Too white/tan too lean.

Honestly I have no idea where I picked that up. Don't forget to check final mix adjustment by raising pistons 1/32 inch, should increase RPM and hold.
 
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