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Alvis tuning
Lightning - 26/12/15 at 04:01 PM

On my straight 6 cylinder Alvis the exhausts are split in two with no balance pipe
It has twin SU carbs So it's in effect 2 3cylinder engines
It runs well but I have noticed on idle that there is more exhaust gas out of back 3 cylinders
Am I right in supposing that the linkage between should be loosened and the throttle on the front three opened slightly?

mark chandler - 26/12/15 at 04:12 PM

Yes

Dingz - 26/12/15 at 04:23 PM

If you haven't got a carb balancer you can get them very close by placing a length of hose at the mouth of the carbs with your ear at the other end and listen to the hiss of each carb at tick over. Loosen the linkage and adjust till you get the same hiss on both then tighten the linkage.

redturner - 26/12/15 at 10:43 PM

Why would you need a balance pipe between the exhaust pipes, surely you mean the inlets.......

scimjim - 26/12/15 at 11:35 PM

many twin exhaust systems use balance pipes to equalize gas pulses (and many don't )

owelly - 27/12/15 at 01:36 AM

I messed about with the chimneys on my Alfa V6. Adding a balance pipe made no difference to the power but made it quieter. A cross-over balance pipe had the same results.

jollygreengiant - 27/12/15 at 08:38 AM

Personally, I would get each exhaust on a Gas analyser FIRST to see what the difference was, in case it was something else, before I started to mess something up that was running "ok". First rule of mechanics - IF it ain't broke, DON'T fix it.

ReMan - 27/12/15 at 08:44 AM

I read that the op was not asking if a balance pipe was needed, but just making the point that the two exhaust are exclusive to the banks.
It sounds like they do need balancing as mentioned, roughly by ear or better with flow meter and gas analyser

mark chandler - 27/12/15 at 09:19 AM

18436572 v8's improve with a balance pipe as the exhausts are not balanced !

You could always go old school and dangle a £5 note behind each tail pipe and see which flutters more, rolls Royce £10 note, BMC £1 note

To check the mixtures have a feel around the bottom of the SU's for the lifting pin, engine running push until it connects to the base of the plunger then raise 1/8" more, the idle should either remain the same or slightly rise. If you do not have lift pins then filters off and use a screwdriver to carefully lift the Pistons.

If the engine RPM raises a lot the mixture is rich so twiddle the adjustment nut up, if it falls away it is weak so lower it.

Clever things SU carbs.

redturner - 27/12/15 at 10:11 AM

quote:
Originally posted by mark chandler
18436572 v8's improve with a balance pipe as the exhausts are not balanced !

You could always go old school and dangle a £5 note behind each tail pipe and see which flutters more, rolls Royce £10 note, BMC £1 note

To check the mixtures have a feel around the bottom of the SU's for the lifting pin, engine running push until it connects to the base of the plunger then raise 1/8" more, the idle should either remain the same or slightly rise. If you do not have lift pins then filters off and use a screwdriver to carefully lift the Pistons.

If the engine RPM raises a lot the mixture is rich so twiddle the adjustment nut up, if it falls away it is weak so lower it.

Clever things SU carbs.

Absolutely correct. V8s need one of the headers crossed over, but only because it makes it sound better and not flat. The amount of air coming out of the exhaust tail pipes will be the same whether or not you have a fuel issue. Of all the historic / classic cars that i have owned none have had a balance pipe in the exhaust system, though most of the SU set ups did......

britishtrident - 27/12/15 at 08:16 PM

Back to carbs I take is on SU. rather than Solex or similar so I would suggest finding a book on SU carbs.
If balancing using the hose stethoscope method then the end of those has to be held in exactly equivalent position in both carbs.

Setting twin carbs up on a 6 cylinder engine involves a lot of compromise.

If you ever want to set the mixture up a Gunsons Colour Tune is probably the best bet to get the initial setting, using an exhaust gas anyliser won't be much help until you get both carbs in the right ballpark.

jollygreengiant - 28/12/15 at 09:23 AM

quote:
Originally posted by britishtrident using an exhaust gas anyliser won't be much help until you get both carbs in the right ballpark.


Well I kind of disagree with that. Use an exhaust analyser first, on the individual tail pipes, IF both read the same (+/_ 0.1allowing for fluctuations) then it's not broken, LEAVE it alone.
IF the CO's the same but the Hydrocarbons are high on one then it could be an air leak, it could be oil scavenging/oil burning.(could be checked by colour of plugs). Using a Gas analyser FIRST would be a much more diagnostic tool than just playing for the sake of playing. The 'apparent ' more gas emission from one exhaust pipe, might not be in reality anything at all, sometimes we see something because we want to and then spend a lot of time trying to fix a fault which isn't there.

JMHO

cliftyhanger - 28/12/15 at 10:28 AM

quote:
Originally posted by mark chandler
18436572 v8's improve with a balance pipe as the exhausts are not balanced !

You could always go old school and dangle a £5 note behind each tail pipe and see which flutters more, rolls Royce £10 note, BMC £1 note

To check the mixtures have a feel around the bottom of the SU's for the lifting pin, engine running push until it connects to the base of the plunger then raise 1/8" more, the idle should either remain the same or slightly rise. If you do not have lift pins then filters off and use a screwdriver to carefully lift the Pistons.

If the engine RPM raises a lot the mixture is rich so twiddle the adjustment nut up, if it falls away it is weak so lower it.

Clever things SU carbs.


Worth reinforcing the bit about how much to lift the pistons by. It really is a very small amount

http://www.mintylamb.co.uk/?page=sutune.htm

is a very good how-to