1800: Mixture setting on HS6 carbs - Why is the Volvo way unusual?

[fishyboy]

Hi all,
I've been trying to set the mixture on a set of triple H4's on another car I own without success. Then I remembered something mentioned in the Volvo "green" service book which I went back to check (Service Manual 6 Engine_B18_B_D, 1969 version).

A common method described in numerous SU books and publications related to the "lifting pin method", which why have all probably tried with mixed success.

The accepted method is to lift the pin 1/32"or approximately 1mm (effectively lifting the piston) and if the engine starts to stall the mixture is too lean, if the revs start to increase steadily the mixture is too rich and if the revs rise around 50 rpm then fall back to normal the mixture is spot on. Normally an adjustment is then made to the jet on the same carb as the pin was lifted on.

However, this is not the Volvo method. According to page 24 of the Service Manual 6 Engine_B18_B_D book (reproduced below) if the pin is lifted on the Front carb and rich conditions are present the adjustments are then made to the Rear carb. News to me and a little unusual.

However, Volvo are not alone. Charles Bishop on one of the Jaguar XK forums describe the same method and implies this is the Jag workshop manual method.

Has anyone followed the Volvo/Jag method and had any success?
Does the method make any sense and how would it work with three carbs

Phil

[Burdekin]

I’ve seen that and also lifting the piston with a screwdriver but I think the Volvo method I use is to wind down the jet increasing fuel mixture until revs stop increasing then just back up one flat is the best and most accurate. A SU should idle rich, about 13.2:1 so when you back off the throttle when driving the carb doesn’t go lean and backfire. You can also use a O2 wideband gauge to measure the fuel ratio or a digital multimeter with a tachometer attachment to easily read the revs as you adjust the jet.

I read the attachment after replying so I lean the mixture a little rather than make it richer. Checked on my O2 sensor it was 13.2:1.

[amazon69]

I've never seen that advice elsewhere and often wondered how it makes sense. I've never seen it mentioned in Des Hammill's book for example. Not that that helps you at all, be interesting to see if someone comes along and explains why this should work. All I can think is that it's due to the intake manifold somehow, but you would expect it then to be advice for other similar 4 cylinder set ups.

[Derek UK]

When you adjust one carb the other ones mixture changes slightly due to the cross over tube. It's normal to juggle them back and forth a bit. Before setting the mixture the carbs have to be balanced for "suck" using a suitable gauge/synchroniser/balancer/manometer.

[Bob Meadows]

This is a revised method that works on Riley SU carbs:~
I'm not certain that you can use it though- could be worth a try?
Bob.
----------------------------------------- " -----------------------------------
Correct setting of the float level is essential if the engine is to run well with good fuel economy.* If the float level is wrong it can make starting difficult and lead to very poor mpg figures.* All carburettor tuning should start with ensuring that the float level is correct.
*
The Workshop Manual talks of setting the level by measuring the gap between the lid of the float chamber and the bent down the portion of the fork against which the float pushes.* The problem with this method of checking is that it assumes that all floats have the same buoyancy.* In practice some floats float higher than others so a setting method which looks at the actual petrol level is needed.
*
Fortunately it is quite easy to see the actual level of the petrol in the carburettor.* Remove the dashpot and the piston holding the needle and put these in a safe place, somewhere where the needle cannot get bent. Now look down into the carburettor and you will see the top of the jet with the small hole (90 thou.) into which the needle fits.* You will also see the top of the guide in which the jet slides.* If you pull out the choke so that the choke lever pulls the jet head down, you will see the jet slide down the guide.* Make sure that the choke is working properly and that the jet slides down to its full extent.
*
With the choke fully in, pump up the petrol into the float chamber until it is full and look down into the carburettor and note the level of the petrol in the jet guide.* It should be below the top of the jet.* Now pull the choke right out and check the petrol level again.* This time it should be level with the top of jet or slightly above.
*
If the level is too high or too low,* bend the float lever a little at a time until the correct level is obtained making sure that the flat part of the lever which pushes the needle valve remains flat.
*
If two carburettors are fitted repeat this process on the other one.* You can now go on to synchronising the carburettors and adjusting the mixture.

[blueosprey90]

My guess is that it is an error in the factory manual.

I run dual SU H6s. I've pretty much given up on the lifting pin method and on the alternate lifting piston method because I can't hear well enough to determine if my revs are rising or falling. Instead, I remove the dashpots and measure the distance from the top of the jet to the bridge with the slide on a vernier caliper. When they are all adjusted the same, I know my carbs are balanced. I usually shoot for 0.07" to 0.09".

I've been reprimanded for adjusting for leanness/richness by this method. I'm told that once I set my carbs, I should leave them alone and adjust lean/rich by changing my needles. But I'm mostly running at close to wide open throttle.

See step #9 here: http://www.dbraun99.com/SU%20Carbure...0tV-ZK8frtdV8M