Would this really "balance" the carbs ??

[rickm]

OK, just thought I'd throw this out there for some opinions/comments.
Was just perusing fleabay and spotted these J carbs being offered by a well known dealer of used spares.
You can see that the vacuum balancing take-offs have been linked by tubes.
What would be the effect of this ?
Would it balance the air flow rate across the linked carbs ?
Would the carbs even work like that ?
I suppose I'm asking why has somebody done this ?

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[KWACKERZ1]

Your probably right i expect on standard they would all run into the air-box to give a standard pressure i suspect these have been run with pod filters and someone has tried to equalize the pressures.

Al will be along shortly he is the J guru.

[rickm]

Normally 3 of them are blanked off with a rubber bung, and one serves the vacuum tap on the fuel tank. So they all run “independently” as standard.

[Al]

Ha, i think one pair of linked hoses operate the fuel tap diaphragm and the other pair operate the air switch diaphragm. Neither are open to atmosphere so have no effect on balance or airflow. I believe some US models ran this system for smog controls and knowing that, 'that' company imports most of its stuff from the US points in that direction for me.

AL

[Al]

[rickm]

But you balance the 4 carbs with vacuum gauges to run separately but equally ?
If they were balanced initially and you join 2 of them then fine.
If you join 2 carbs which were not initially balanced for vacuum, then the vacuum between them must to some extent balance out at the mean point when joined ?

[kev edwards]

The UK 750 Zephyrs had the same set up with the open end going to the Vac operated tap, carbs are balanced in the normal way, pipes put back on makes no difference in the running, i just capped the 3 stubs in the end to tidy it up.

[moizeau]

My LC with TZ top end had a 'balance pipe' between the carbs,It was on the intake rubbers, not the carbs.To balance the carbs, static, then gauges, each carb feeds a cylinder.

[zed1015]

Having them linked won't affect carb balance as long as they aren't open to atmosphere.
Vac is only used as a measurement as an easy way to ensure each running cylinder is working/pulling air equally to the others as this is rarely achievable by simply static bench syncing all the slides to the same height.
Once set, having them plugged or linked won't change the balance.

[Julian_Boolean]

My LC with TZ top end had a 'balance pipe' between the carbs,It was on the intake rubbers, not the carbs.To balance the carbs, static, then gauges, each carb feeds a cylinder.

All LC twins are like this standard.

[rickm]

My LC with TZ top end had a 'balance pipe' between the carbs,It was on the intake rubbers, not the carbs.To balance the carbs, static, then gauges, each carb feeds a cylinder.

All LC twins are like this standard.

So is the pipe joining them actually there to balance the carbs in some way while the engine is in normal use?

[Julian_Boolean]

My LC with TZ top end had a 'balance pipe' between the carbs,It was on the intake rubbers, not the carbs.To balance the carbs, static, then gauges, each carb feeds a cylinder.

All LC twins are like this standard.

So is the pipe joining them actually there to balance the carbs in some way while the engine is in normal use?

I believe so, LCs don't run very well without the pipe and the holes blocked up if after a rebuild you can't find the pipe and think "it'll be alright"

Also the boost bottles you used to get for the twins did nothing, but the standard one on my 1980 YZ125E improved the negligible mid range by a small amount (you still had to change down a couple of gears in the air from a biggish jump though)