Twin carb issue

[MGB Boy]

So we've been trying to balance the carbs on my '79 MGB GT for a while now (on and off on the weekends, spare time etc).
But no matter what we do we can't get the carb closest to the front of the car to 'suck' as much as the one to the rear of the engine bay.
The rear carb seems to be going like the clappers, When you use the carb balancing tool the rear will read 4/5 and the front will read around 2.
Before we did anything with them they were removed and cleaned internally and externally (had to come off to get to the manifold gasket)
Since then we've tried winding the mixture screws right in and right out (a lot!) and 'balanced' the bar across the two which determines when they open (so they both open at the same time). I'm not 100% sure if we've done the bar right to be honest so if anyone knows how to do that properly please post away!

Can't see much coming up on Google so please help!

[Sam_68]

What sort of carbs are we talking about? Webers/Dellortos?

If so, you should be able to balance them on the centre link. The mixture screws have negligible influence on the amount of air they will be sucking throughm - it's down to how wide the throttle butterfiles are opening.

If you have one choke of a twin-choke carb that can't be synchronised with the others, then the most likely reason is a twisted tthottle shaft, resulting in one of the throttle butterflies not closing fully, but I can't think of any reason why you get a big imbalance between two full carburettors that can't be sorted on the linkage.

[falkster]

Wont they be SUs??

[MGB Boy]

They are the standard HIF 4s (I think)

[Uhura_Fighter]

MGB Boy said:

They are the standard HIF 4s (I think)

If they are for the rubber vermillion one in your profile you are right.

Did you fit new needles and do a carb service while you had them off?

[varsas]

Some problem with one of the cylinders the 'poor' carb is feeding? Some blockage in the inlet manifold?

I assume the air intakes are clean and the dashpots have the correct amount/type of oil? (try unscrewing the dashpot tops, both the...plunger type things...should move up/down by the same amount)

[tr7v8]

OK far too long for a post. But basically check ignition condition & valve clearances first.
Split the linkages so each carb is independent & then set mixtures by piston lift or colourtune. Then set the idle on both idle screws so each carb is set about the same. And not to fast otherwise it'll run on & over heat. Then nip up the linkage so it is connected to the throttle cable WITHOUT MOVING anything! THen pull the cable to WOT & check they track properly. Synch at WOT is not to critical but at idle is.

[poo at Paul's]

Have you done a compression test on the cylinders first? Also, if they are inch and 3/4 SUs as I think they are (or inch and a halfs??) you need to endure the piston damper oil is identical in each carb to get the to balance anyway near.
But first check the cylinders are sucking the same, and compressions are in tollerance, (valve clearance all in spec etn), then I'd empty the damper oil out and start again, fill the plunger and insert the damper rod, I used to use a light 3 in one oil. Dont go too thick, the chokes wont open enough, too think and they will bounce and flutter and the car will jackrabbit like Michael J Fox after a double Espresso.

[Sam_68]

As Varsas and Poo say... with SU's, first thing I'd check is the damper oil.

[ARH]

Check for wear in the spindles, these leak when there is play in them. not too familar with HIF's but with hs su's.

separate the linkages
adjust mixture screw till closed then open 2.5 turns
adjust idle on each one till they flow the same
check mixture and adjust
adjust idle on each one untill the tickover is about 900 rpm
recheck flow and adjust
recheck mixture
adjust idle again
repeat this loop untill correct this can take some time
reconnect linkages

take for a hoon

this may be slightly different for hif's

[MGB Boy]

Well we have them off now and have found that a few of the rubber seals on the spindles etc are missing / buggered (allowing air to get in when you don't want it to) and The valves on the butterflies have been welded up.

I did the oil in the pots last time we had them off but it would seem now that one is still full and the one that sucks more has drunk a fair bit.

[ARH]

oops that doesn't sound good. you could get them reconditioned http://www.burlen.co.uk/ but personaly I would try to get a decent second hand pair from somewhere. su are a pig to set up when they are worn.

[davepoth]

Welding the butterfly valves is fairly normal, it helps with mixture control (it's an emissions addition that makes the car run bad) so not really a problem. The only real issue is if the spindles are able to move, stopping a proper vacuum from being created around the butterfly.

[MGB Boy]

Should be able to get the gaskets, rubber seals etc for a reasonable price. For the butterflies the welding is (supposedly) fine as you say, Just as far as I'm aware there should be a small nick out from around the edge to make up for the valve being permanently closed.