Tuning FAQ's

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Here are some messages covering tuning and other engine performance adjustments. I have included the e-mail addresses if you wish to reply directly to the original poster.

I will add more as we receive them. I'm also looking for volunteers to help with this effort.
This is a very popular subject. Especially adjusting carburetors and ignition conversions.
Subject:Re: Pinging & Timing
Date:Wed, 1 Oct 1997 23:15:42 -0400 (EDT)
From:Michael Frank mfrank@acm.org


At 12:12 PM 10/2/97 +1000, you wrote:
>So tell me, when you move the distributor so that the strobe 
>light shows the timing mark moving toward TDC is that what 
>you call advancing or retarding? 

Depends on which direction you are moving in. You are advancing the spark if the spark comes earlier in the ignition cycle. And of course, you making the spark come later is retarding.

>I also notice that moving away from TDC make the idle faster
>and smoother but the pinging on acceleration is definitely worse.

A tune up is an iterative process. The engine will speed up or slow down as you move the timing around. So you need to adjust the carbs to get the idle speed correct. But changing the idle speed can change the timing. Should converge to the perfect setting pretty quickly.

> Do the vacuum and centrifugal advance mechanisms act to 
>move the timing more toward TDC or the other way around? 
>When does the vacuum advance have most effect?

Centrifugal advance is easier: the faster the engine turns, the more advance you get. Vacuum advance will be determined by the amount of vacuum present at the carburetor port. Should be zero at idle, increase to max probably around 1800, then fall back to zero.

> >I have a small rough idle problem - it seems rougher than I would >expect from a Jag. It could be carburetor balancing or advance related.

Carb setup is a black art. Below is a posting from the original jag-lovers list on the subject.

Mike Frank
1969 E-Type 2+2

-Date: Tue, 8 Feb 94 12:43:02 PST
-From: sfisher@megatest.com (Scott Fisher)
-To: swedishbricks@me.rochester.edu
-Subject: Tuning Your SU Carburetors

I've been meaning to write this up for some time, ever since I did the SU Performance Tuning 101 a few months ago. This one is more like Basic SU Adjustment for Happy Driving.

The trick to tuning SU carbs is to understand that there are two things you need to get right: the air flow, and the fuel mixture. While they are interconnected, they are also independent, and need to be measured and adjusted independently.

Special Tools

You will probably need to arrange to buy or borrow a Unisyn flow meter. The Unisyn is the usual gauge for getting the air flow balanced between the two carbs. This costs about $20 and is simple to use. It consists of an adjustable opening (same size circumference, but with a disc on a threaded rod that you can screw tighter or looser) that you use to set the level of a little float that rises or falls in a glass tube at the side of the gauge.

For the fuel mixture, I have become sold on a device called the Gunson ColorTune (maybe ColourTune, as it's a British co.). This is a spark plug with a crystal pressure- and heat-resistant window in it that lets you see into the combustion chamber while the motor is running. The color of the flame indicates the mixture richness. It costs about $40, and while it's not absolutely essential, it makes life so much easier that it's worth the cost.

If you don't have a Gunson, I've included the standard directions here for determining correct mixture (step 4 of the Adjusting Mixture procedure).

To tune SU carbs, first locate the following components:

  • Throttle linkage nuts. These are the things that connect the throttle linkage (the bar connected to your foot through whatever means your car uses, cables or rods) to the carburetors' throttle levers.

  • Throttle stop screws. These set the idle speed for each carb, and are located typically behind the dashpot, on the same side of the carb to which the throttle linkage connects.

  • Mixture adjusting nut. This is the lower of the two nuts at the very bottom of the carburetor. Later SU carburetors of the HIF type have integral float chambers, on which the mixture is adjusted by turning a screw. You'll need to experiment (and I explain how) to see which way makes this richer and which way makes it leaner.

  • Lifting pins. These are little wobbly metal pins under the dashpot. When you push up on the pin, it raises the piston in the dashpot. Find these; they're crucial if you don't have a Colortune. If you don't have or can't find them, you can raise the piston with a flat-bladed screwdriver pushed down the throat of the carb and twisted to lift it.

  • The bridge. This is the part inside the carburetor, where the gas jet opens into the airstream. You'll see a needle inside the jet, and the jet itself should be a few fractions of an inch down from the bridge itself. The jet is the brass tube that sits in the center of the bridge, with a tapered needle poking down into it.

  • The choke linkage nuts. Comparable to the throttle linkage nuts (and usually the same size), but on the linkage that goes between the choke cable and the mixture adjustment mechanism. They make sure that both carbs are enriched when you pull on the choke.

Balancing The Air Flow

  1. Start with the engine warmed up to operating temperature and perform your standard ignition tune-up (points gap, timing, spark plug gap, new condenser, etc.) first. If you've got a timing light and a dwell meter, you can verify all that stuff independent of the way the car is running. When it's warm, shut the motor off and remove the air filters.

  2. Begin by balancing the air flow. To do this, first loosen the throttle linkage nuts. Leave them connected, just loosen them half a turn or so.

  3. Back out the throttle stop screws till you can see that they are just touching the throttle stop. Then open each carburetor (that is, lower the throttle stop screw) 1-1/2 turns of the throttle stop screw and start the engine. It will probably idle at about 2000 RPM; don't worry.

  4. Put the Unisyn over either carb and adjust the orifice in the Unisyn till the little float at the side rests at the middle of its graduated tube. (Pre-diagnostics: if the idle drops and the car wants to die when you slap on the Unisyn, the carb is too rich; if the idle soars upwards, it's too lean.) Hold the Unisyn over the carb for only long enough to see the level of the float, then remove it.

  5. Place the Unisyn on each carburetor in turn to check its flow, adjusting the throttle stop screws until both carburetors register the same position on the graduated tube of the Unisyn. (The float will probably move either up or down in the tube, which is why you want to center it in Step 4.) When both carburetors flow the same amount of air, tighten the throttle linkage nuts, adjusting for the amount of free-play between the linkage and the throttle stops that your manual calls for (probably about 0.006"). Your goal should be to achieve the lowest possible idle with both carbs balanced and the engine running smoothly. (Note that the idle speed will very probably rise as you get the mixture correct.)

  6. If you've taken more than five minutes to do this, rev the engine to over 2500 RPM (assuming the idle isn't already that high) for thirty seconds or so to clear the spark plugs. Then adjust the mixture.

Adjusting The Mixture:

Note: in the following procedure, one "flat" is the basic increment of adjustment, and refers to 1/6 of a turn of the mixture adjusting nut. This corresponds to the flat faces on the nut.

I'm going to give instructions for SUs with the separate float chambers. If you have the HIF integral-float carbs, you'll have to look in a manual to see whether you turn the mixture screw to the right or the left to make it richer or leaner; I've done that once but I can't remember. Alternatively, you can -- with the motor shut off -- peer down the throat of the carb and turn the mixture screw while watching the top of the jet. Remember that moving the top of the jet up will lean out that carb, while moving the top of the jet down will richen it.

  1. Shut the car off and loosen the choke linkage nuts.

  2. Adjust the mixture nuts (screws) fully lean.

    For separate float-chamber cars, this means raising the mixture nut all the way up against the bottom of the carb (or rather, against the spring). For HIF carbs, you can try turning the screw while looking down the throat to see which way the jet is moving. In either case, the idea is to zero out the jet: raise it all the way up in the bridge.

  3. Now drop the jet an equal amount -- two full turns for HS-type carbs, two full turns (I believe) for HIFs. Then start the car.

    Note: In the following step, you might want to consider adjusting the carburetors one-half a flat too lean, as the mixture will be enriched when you put the air filters (which restrict air flow) on at the end of the tuning process.

  4. Raise the lifting pin (or use a screwdriver if you don't have the pins) so that the piston rises no more than 1/16". Listen to the engine's exhaust note and compare it to the following conditions:

    - If the exhaust note rises and stays high till you drop the piston, this carburetor is adjusted too rich. Turn the mixture nut one flat (one-sixth of a turn) up, moving the jet toward the bridge, then repeat Step 4.

    - If the exhaust note falls and the car sounds as though it is going to stall, this carburetor is adjusted too lean. Turn the mixture nut one flat (one-sixth of a turn) down, moving the jet away from the bridge, then repeat Step 4.

    - If the exhaust note rises briefly and then settles back down to something like the original RPM level, this carburetor is set correctly. When you have achieved this setting for both carburetors, continue with Step 5.

  5. Tighten the choke linkage nuts so that the choke cable will pull an equal amount on both mixture nuts when you pull the knob.

  6. At this time, I find I usually have to adjust the idle again because getting the fuel mixture right usually changes the idle speed. Since you know you have the throttles synchronized, I normally just adjust the idle without loosening the throttle linkage. The easiest way is to screw one of the screws out till it doesnt' even touch the throttle stop, then use the other to get the idle speed right. When that's done, you can screw the other stop screw down till it just touches the stop on that carb and you're set.

  7. Replace the air filters and go for a test drive!


SU carburetors are most fuel-efficient when slightly lean, and provide the most power when they are slightly rich. You can use this knowledge to provide a certain amount of tuning for the kind of driving you do. If you learn to read spark plugs, you can get a basic idea of what your engine's condition is and make fine adjustments to the mixture nuts accordingly.

If you have a ColorTune, you simply install it in place of one of the plugs, then adjust the carburetor that feeds that cylinder (the front carburetor for 1 & 2, the rear for 3 & 4). The ColorTune will let you see the color of the flame. White flashes mean too lean; yellow flame means too rich. Blue (like a Bunsen burner) is correct, and blue with a faint orangish tinge is the best for power.

You can also modify your car's throttle response characteristics slightly by adjusting the viscosity of the oil in the dashpot damper. SUs are set up so that a thicker oil will resist the piston's attempt to rise in the dashpot for just long enough that the engine's increased load (when the throttle is opened) will pull more fuel across the bridge; this enriches the mixture and temporarily bumps power up to help the engine achieve higher speed more readily.

If you modify your engine, you will probably need to modify your needles, as it is the needle profile that determines the mixture curve for different air-fuel loads.

If you experience uneven idle, hunting, or an idle that changes (rises or falls) as the engine's temperature climbs or drops, you probably have vacuum leaks. The most serious fault on most old SUs is wear in the throttle shaft area. To test for this, spray some carburetor cleaner on the outside of the throttle shaft; carburetor cleaner is non-combustible, and if the engine speed drops, it means some of this is getting into the air stream from outside the carburetor. You may also have leaks from the manifolds, from tubing such as the vacuum advance line to the distributor (if fitted), or from other places; the carb cleaner trick works well for locating those leaks as well.

Other problems that SU carbs experience involve dirt in the dashpot and occasionally in the float chamber. The dashpot is a precision piece of machining that involves very close tolerances so that the piston doesn't stick or bind when it rises and falls. A little grit between the piston and the dashpot can make the car jerk and sputter. Take the dashpot off, wipe the insides down with carb cleaner and a lint-free, clean rag, then reinstall it, getting the screws down tight. Also, don't swap the pistons between dashpots; they're matched to one another so that the clearance between the piston and the wall of the dashpot makes a tight seal but permits easy rising and falling.

Dirt in the float bowl basically shuts off that carburetor (or can make it flood open, depending on whether the dirt is wedging the valve open or closed). You can try rapping on the float bowl with the handle of a screwdriver, but your best bet is to take the cover off, clean out the valve fittings, and reinstall everything, with a new fuel filter for good measure.

Some older SU models also have adjustable floats, in which you need to set the float height (which basically equals the fuel level in the float chamber) by bending a brass rod. These carburetors were replaced in the mid-1960s with carburetors that had fixed, plastic floats which are basically trouble-free unless abused. The stop at the back of the floats can break if they are installed badly, and the brass pin that holds them in place can wear an oval hole in the float pivot. New floats are fairly inexpensive and aren't a bad idea if you're doing a rebuild.

Grose-Jets are very popular with some people and a big pain for others. It appears -- and this is just conjecture -- that Grose-Jets work best in cars with adjustable floats, as they are longer than the stock SU float valves. The standard failure for Grose-Jets is to flood the carburetor. I have never had problems with the stock SU float valves or floats.

--Scott "I like SU carbs -- they're expert-friendly" Fisher

Subject:Wipes & Ignitor
Date:Wed, 1 Oct 1997 13:31:43 +1000
From:Arnold Vonk arnold_vonk@innovations.com.au

Hi all,

Wipers again -
Thanks for your help on wipers, from 3 of you I think - Martyn, Noel, Larry. It turned out that it was mechanical in nature again. The drive from the motor into the "drive arm" was not in place properly. I didn't find it myself as I was too depressed at the thought of being beaten once again by my wipers. The auto-electrician is a bit of an expert at removing the motor now so he does it so quickly that it doesn't cost me that much.

My Ignitor story -
Got mine yesterday. At 8.45 pm I decided to have a look at how it would go in. I boldly decided to give it a go at about 8.55 pm (rather foolish after working all day and being pretty tired but hey, why not add a bit more stress to my life!). I popped the unit in and while doing it thought "this is too easy, it'll be a miracle if it works". Well, the engine started on the 2nd try - I was amazed.

However, the timing was out (the instructions said you had to time it in the usual way) and I drove it around my local area. The car ran badly (timing too advanced) and died about 1 km from home. I luckily got it going again (I didn't bring any tools and it was pitch dark) and it stalled up-hill from my house so I coasted back. I adjusted the timing in the "usual way" i.e. by ear and put my car away for the night at just after 10 pm.

This morning I found that my ear had been a bit tired at 10 pm last night as my car died in peak traffic. Got it started again and back home, advanced the timing and drove away. It is still pinging a bit so I'll advance it just a little more I think. By the way, is it "pinging" or "pinking"? I'm sure I heard people say both. I also notice the "spastic" tach. needle that someone mentioned before. It seems more normal now that the car is running better. Why would that be?

I'm looking forward to never changing points again.

1970 S11 FHC

Subject:Pertronix Ignitor Ignition System
Date:Wed, 8 Oct 1997 09:03:32 +1000
From:"ANNETT,Noel" noel.annett@deetya.gov.au

Hi everyone

Just thought I'd do a post to let you know of my experiences in bringing my E's ignition system into the 20th century. Thanks to advice from George Cohn, Martyn Ward, Alan Mandell, Arnold Vonk and others on the list, I breathed in deeply and ordered a Pertronix Ignitor unit from British Auto USA ($99 US).

The unit duly arrived by post last week and I installed it in the car on the weekend. I took Arnold's advice and did it on Saturday. The manufacturers claim installation can be completed in 15 minutes, and in some cars this may be so. In my case, I discovered that someone had installed an XJ6 rotor button in the distributor at some stage and its rear overhang would not clear the Ignitor unit when installed on the distributor plate. So....hunt around for the correct rotor button (which incidentally, has a round shank with no rear counterbalance). Luckily I had access to one in another "parts" distributor so no problem. I have ordered another couple as spares. The rotor button is Bosch Part number GL254 and as well as 1964 to 1968 E types, is common to Austin trucks and a few other rather ordinary British marques BTW. This rotor button presents no clearance difficulties when the Ignitor unit is in place in the distributor.

The second problem encountered (which was the subject of a recent post by Arnold Vonk) concerned the plastic water shield which fits under the distributor cap. The main function of this plastic "sheath" is to prevent moisture from entering the distibutor body proper. Unless you modify it in the manner described by Arnold you cannot use it. I have discarded it for the time being.

The Ignitor comes complete with a formed rubber "grommet" which fits into the distributor body and through which its connector leads are passed. Unfortunately, the person (idiot) who manufactured/assembled my unit at Pertronix fed the wires through the grommet from the wrong side.

As the connection "spades" are fastened after this process in manufacture, I had to cut the spades off, remove the grommet, turn it around, feed the wires back through it (the right way) and re-solder the spades. As my coil is located at the front of the head on the engine on my car, this did not matter particularly, as I had to extend the leads in any case. The magnetic pulse pickup sensor is located on the points plate by the old points mounting post and locked in place by the condenser mounting screw...so it is not going anywhere. The condenser gets the "heave ho" along with the points.

The rotating unit containing the six actuating magnets which slips over the cam on the distibutor drive shaft fitted snugly and sat right down on the lobes. This is important because the rotor button locating "nipple" needs to "seat" properly into its cut-out in the distributor shaft (something the XJ6 rotor button could not do) to get the correct clearances inside the distributor cap. Unlike Arnold, I experienced no problem with clearance of the central high tension pickup "brush" with the removal of the moulded plastic water shield from underneath the cap.

As the timing would need to be adjusted, I took the opportunity to remove the distributor (bastard of a job to get at the 1/4 inch UNF mounting bolt between the block and the distributor base plate) and dismantle it and clean and lubricate the advance mechanism etc.

All of this messing around, cleaning and rewiring etc. took about two hours. In the end, I decided to dispense with the plastic water shield and its connector. As Arnold did, I may modify the shield after summer and place it back in position. The distributor in an E Type is well shielded from the elements and the Ignitor supplied grommet is a snug fit with the cap in place, so I don't envisage any significant problems with moisture entry.

So, I placed the distributor back in position, placing its base-plate adjustment in mid-position. The engine fired straight away and ran smoothly. Some rough adjustment achieved good idle and a road test with a couple of quick excursions in excess of 5000rpm revealed smoother, more powerful acceleration from rest and strong pulling right through the rev range with no flat spots or hesitation. The engine fires instantly and idles quietly. Proper timing will be undertaken this weekend. It will be interesting to see how the car now performs in terms of fuel economy and plug wear.

Personally, I am very happy with the result for the relatively small outlay. No more points and no more timing and condenser hassles ever again. Sorry about the bandwidth, but I thought this detailed coverage of the installation may help and encourage others contemplating a similar performance and reliability improvement.

Noel Annett
2+2 (Electronique with 9 inch spark)
Canberra, Australia

Date:Wed, 8 Oct 1997 10:05:26 +1000
From:"ANNETT,Noel" noel.annett@deetya.gov.au

Hi Arnold

You will see my post earlier today about installing my Pertronix. It has not affected my tachometer at all. Do you have a balast resistor? If yes...did you wire the red Ignitor lead to the supply side of the ballast resistor? If wired to the coil side can affect supply voltage to the Ignitor unit. Thanks for your post about the pitfalls of your installation....it was most helpful.

Noel Annett
68 E (2+2) Manual (His)
67 Daimler (420) (Hers)
71 XJ6 Manual (His - being painted)
85 BMW 528i (Ours)
Canberra, Australia

>From:  Arnold Vonk[SMTP:arnold_vonk@innovations.com.au]
>Sent:  Tuesday, October 07, 1997 1:39 PM
>To:    e-type-digest@jag-lovers.org
>Subject:       Crazy Tach.
>Hi all,
>You may recall that in my Ignitor story I noted that since the 
>fitting my tach went wacko.
>The facts:
>- The ignitor unit plugs into the (+) and (-)ve connections on 
>the coil. The unit replaces the points and condenser completely
>and works by sensing the magnets in a sleeve around the shaft.
>- When I connect my hand-held tach, gauge I get a steady 
>reading but it is about half what I think the revs are on the 6 cyl. 
>- my tach. in the car is jumping all over the place but seems to 
>peak at the revs the car is actually doing.
>An auto electrician (who referred me to an instrument specialist) 
>said that the tach may now need some changes due to the 
>different response curves (or something like that).
>So what do you electronics gurus say is the problem & solution?
>1970 SII FHC