Running rich on one carby

Roger's E Type · Post by **Roger's E Type** » Sun Feb 23, 2014 8:09 am

Hi all,
1966 Series 1 4.2 2 + 2: My issue at the moment is that the front carby runs rich - much richer than the other two as indicated by the black carbon deposits on the front two spark plugs against the medium brown deposits on the other cylinders.
I am adjusting the carbys in an identical manner and I have checked that the mixture control lever is not interfering. That is, I do not believe it is creating the richness issue.
What other things should I be looking at to fix this problem?
Incidentally, idle is quite reasonable - a little lumpy but sits at 700 - 750 rpm.
Cheers,
Roger

abowie · Post by **abowie** » Sun Feb 23, 2014 8:22 am

check that that carby's choke isn't sticking.

christopher storey · Post by **christopher storey** » Sun Feb 23, 2014 8:25 am

And also check the float level and that the needle is correctly seated in the base of the piston - its shoulder should be flush with the base of the piston . Another problem can be if you are using an aftermarket fuel pump and have excessive fuel pressure - the pressure should be in the region of 0.2 bar or 2.5 to 2.8 psi

Roger's E Type · Post by **Roger's E Type** » Tue Sep 09, 2014 12:17 am

Thank you for the replies - the answer was all that and more. Badly set up throttle linkages was a significant part of the problem. These had been set up by a specialist here in Melbourne who claimed he spent hours trying to get them right and charged accordingly. A friend with an E Type and I managed to sort it in less than an hour - it really was that obvious! Now just need a little more "tuning" and another issue will be done!
Thanks again for your input - love this Forum. Hopefully, one day I will be able to reciprocate!
Cheers, Roger

abowie · Post by **abowie** » Tue Sep 09, 2014 7:57 am

Glad you have got it sorted out.

Tuning SU's has a reputation as being a black art, and I have twice had one of my cars tuned by a local "expert" at significant cost.

Having been dissatisfied with his results I went to the trouble of working out how to do it myself.

My experience has been that "twiddling" a bit doesn't work. You need to start from scratch and eliminate all the potential errors in the existing setup, and follow the manual procedure with regard to initial settings. They work.

This includes correctly setting points and timing before touching the carbs. Well worth checking the leads and plug caps too if they're original ones.

Making sure that all 3 butterflies are completely closed and open exactly together by correctly adjusting the linkages is an absolute must.

You need to be certain that the jets are centred properly, you need to have all three needles set at exactly the same depth into the piston and you need to start with all 3 jets in exactly the same position. An excellent tip from I think Peter Crespin was to use a vernier caliper to measure jet depth from the top of the bridge ratgher than relying on number oif screw turens, as it is more accurate.

Once at this point I have found that the only reliable way I can balance the idle bypass is with a flowmeter and the only reliable way I can set mixtures is with a CO meter. I can't interpret exhaust noise, and I have never found it possible to lift the piston in a controlled manner to gauge rev increase or decrease. For me spending $200 total on those 2 tools was mandatory.

Roger's E Type · Post by **Roger's E Type** » Tue Sep 09, 2014 8:29 am

Hi Andrew, not sure they are completely sorted yet but well on the way. Yes, I agree with starting at the beginning.
The butterflies not all closing completely at the same time was, I think, at the heart of the issue and probably the easiest to sort by an "expert"! Effectively, the front carby butterfly was already partly open!
I have bought myself a flow meter but the CO meter sounds interesting - are you able to let me know the brand and the price - perhaps where you got it from?
Cheers, Roger

ChrisC · Post by **ChrisC** » Tue Sep 09, 2014 9:11 am

Can I also add a suggestion - which I found to be fun and ultimately resulted in a nice smooth engine and even mixture across all the cylinders.

But first you have to do the obvious...
1. Make sure the ignition is in good condition and as you say the butterflies close etc!
2. adjust all the choke linkages so that they increase the revs BEFORE they richen the misture. Make sure they all operate at the same time.
3. Set the float height as per the manual and then look down the jet. with the fuel pump turned on - the fuel should be at the same height on all 3 carburettors. If it isnt and assuming none of the floats has a leak - this is where you ignore the manual - if the fuel in one jet is higher or lower than than the others adjust the float on teh offending carb until the fuel level is the near enough the same as the others.
4. Now set all 3 jets at the same height as the bridge. If the adjusting screw goes loose before the jet is parallel with the bridge - then set the adjusting screw so that it just touches and has no free movement. This is not ideal but it is okay...
5. wind down all 3 jets 2.5 full turns
6. wind down the idle screws 2 full turns
7. Start the engine and bring to running temperature
8. using your flow meter adjust the idle to where it is smooth even if smooth is at 800 rpm...

Now the fun bit - drive the car for 5 miles or so then take it back home and look at the spark plugs. If you have a spark plug that is darker than the others - the appropriate carburettor is running rich. weaken the mixture on that carb by turning the mixture screw anticlockwise 1/4 of a turn. Put the spark plugs back in, check the idle with your flow meter and adjust as necessary - take the car for a drive again :D

Repeat as necessary until all teh plugs are the same coulour, the car accelerates as you think it should and has a smooth tickover which may or may not be as per the manual 8)

It took me a little bit of playing but doing this as an experiment - I could make the plugs go from a very light tan (weak mixture) - to black (way too rich). I found my car performed best when the plugs were around a mid tan colour.

Now I know this is not the fastest way of doing it but that last bit of adjustment will get all the carbs balanced. I am old school - a Co2 meter will tell you a number, but call me old fashioned: 6 spark plugs all the same colour is the real indicator.

christopher storey · Post by **christopher storey** » Thu Sep 11, 2014 3:14 pm

I'm afraid I can't agree about plug colour being reliable any more. In the days of leaded fuels, it was a very good guide,( as was a nice dark silver exhaust pipe colour ) but since the advent of what is called unleaded ( it actually has minute quantities of TEL still ) colour is not a good guide except in the case of a hopelessly overrich setting . I start with the jets about 65 thou down from the bridge and then adjust from there, using a CO meter alternately in each pipe. I use an idle setting of 4.5 to 5% as my ideal, although sometimes this is a little on the weak side for running . I then recheck the depth settings and if there are differences, I add them up and divide by 3 and then set all 3 to that average, and see what happens !

2 things to remember I suggest :

1.the pitch on the adjuster screws is by no means uniform - thus a quarter turn down on one will not necessarily produce the same result as a quarter turn on another carb

2. Since the Jaguars have very generous balance passages, altering one carb has an effect - albeit lesser - on the other 2 carbs

ChrisC · Post by **ChrisC** » Thu Sep 11, 2014 6:49 pm

Nope sorry have to disagree with you on that Sir.

Exhaust PIPE colour is no longer a reliable indicator of whether a car is running rich or not. But having 6 spark plugs all the same colour (lean/rich or just right) IS a good indicator that all the carburettors are pretty much matched. So its now a question of whether they are optimally burning all the fuel or not

I grant you that a 1/4 turn of the adjuster screw is not 100% accurate but if adjusting it too much makes the plugs the wrong colour you do an 1/8th turn or a 1/16th turn or just s tweak - until they are all pretty much the same colour (which is subjective)

All I am saying is you dont need fancy kit to get all 6 spark plugs about the same colour. But I also grant you that once you have matched the carbs.... THEN using a CO meter to see if you are rich or lean might be appropriate but I dont think its entirely necessary unless you are racing.

abowie · Post by **abowie** » Fri Sep 12, 2014 9:24 am

ChrisC wrote:

Nope sorry have to disagree with you on that Sir.

And I Sir, disagree with you :D

I've come to the conclusion that reading plugs is next to useless with my cars. The only thing that has made any sense has been my Gunson CO meter, and it has been a game changer.

PeterCrespin · Post by **PeterCrespin** » Sat Sep 13, 2014 1:37 am

I don't recognise the bit about 'generous balance passages'.

I have filled in the rough vacuum rail apertures where they disrupt gas flow at the upper carb port knife edge. I have flowed them to be a smooth mirror image of the bottom of the three manifold ports, with small holes to the air rail instead.

The purpose of the air rail is to supply some vacuum to the servo bellows and there is no net flow of air into the intakes as it is a closed system - it just maintains a low pressure.

It does this via a single modest hose to the check valve and thence to the bellows depending on 3.8 or 4.2 system. Three small holes, one per carb, draw easily enough to serve one larger (but still modest) hose. There isn't any to and fro flow of fuel/air mixture surely along the vacuum rail? I've never seen fuel or smelled it at the vacuum nipple on the back of an E-type triple SU manifold. Maybe I've missed that?

christopher storey · Post by **christopher storey** » Sat Sep 13, 2014 11:41 am

AFAIK the 3.8 passages are in the cast rail bolted on to the separate manifolds. The 4.2 manifold has the passages cast into it. They are quite large and are to equalise depression . Incidentally, I have never seen a mulitple SU setup without them.All of my 6 multiple SU cars have them, and all are of a bore not that much smaller than the venturis

PeterCrespin · Post by **PeterCrespin** » Sat Sep 13, 2014 12:11 pm

Never thought of it in those terms, more as a combined vacuum take-off, which is why it jarred when you mentioned it in the context of this mixture adjustment thread.

But equalising depression is fair enough. In the last few years I've worked working mostly on 4.2 (no separate rail as you say), a three-part Weber manifold car (no vac take-off) and monoblockand latterly D-type (monoblockand but with neither vac balance pipe nor even thermostat). So I had never had cause to look up the actual part reference.

If in doubt, RTFM and it is indeed referred to as the air balance pipe so you're quite right , thanks. I still will gas flow my 4.2s in figure if I have them apart as the triple SUs still worked fine, but maybe my idle will never be super smooth?

christopher storey · Post by **christopher storey** » Sat Sep 13, 2014 1:36 pm

Peter : I think also that one of the reasons for the balance pipes is to alter resonance effects in the inlet tracts. Although this probably does not apply to Jaguars, that function is particularly important in siamesed port engines such as the BMC A and B series. Anyway, in the context of this thread, the relevance was merely that altering one carburetter has a small effect also on the other cylinders because during the "idle" time of one inlet tract , the other active tracts can draw some mixture from it

Robert Hiland · Post by **Robert Hiland** » Sat Nov 08, 2014 12:14 pm

Others may wish to comment on this ...

The throttle return spring on the rear carburetor is longer than the other two. Most suppliers send you identical springs ... check it out.

Some people suggest that preloading the springs (half turn) helps with a positive return.

Check to ensure that all springs have the same tension.

MT2+2 · Post by **MT2+2** » Wed Jul 15, 2015 9:08 pm

Ref the valuable info in this thread about setting the correct fuel and jet levels, would anybody like to comment on a couple of questions:-
1. Is the level of fuel in the jet crucial or not? Chris Rooke says in his book & articles about his epic restoration that it is, wheras the Burlens website (who now manufacture the SU HS8 carbs) states that it is not important. Who is 'right'?
2. How does the level of the jet actually affect the mixture? I understand that as the piston and needle move up, the area of tiny annulus of fuel exposed to the airflow increases, which provides more fuel for acceleration. However the level of fuel is always a fixed distance below the carb bridge, and is independent of the level the jet is set at.
Thanks for any insights,
Martin

Geoff Green · Post by **Geoff Green** » Thu Jul 16, 2015 3:21 am

The level of the jet will change the location on the taper of the needle making the jet "larger" when moved down, therefore richer. The air passing over the jet in the depression causes the fuel to be pulled into the carb. The level of the fuel in the float chamber will change the mixture also. The amount of air pulled past the jet will change the mixture with more fuel with more air as compared to an adjacent carb with less air flow. Many settings change the mixture.

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