Adjusting the carbs.

[richard btype]

I tried last evening to adjust the carbs for fuelling.

Following the workshop manual I did a static set up of the timing 10o btdc. (The points were fitted 1000 miles ago).

I set the slow running adjuster by screwing right down before backing up with two full turns.

Set the fuel adjusting screw with throat - level with bridge before screwing down 2.5 turns.

Set the fast running adjuster to around 700 revs with equal air intake between them.

This seems to be the first stage, but the exhaust note was unhappy - rather uneven. By lifting the carb piston nearest the bulkhead it responded by increasing momentarily which indicates the correct mixture, the middle carb had a tendency to slow down indicating a weak mix but sounded more unhealthy when adjusting to a richer setting. The front carb failed to respond at all.

The carbs were thoroughly stripped, cleaned and reassembled with a repair kit from Burlens including new jets and floats any comments or tips please as it would seem to be the carbs and not the timing seeing as how one of the carbs is responding?

[mgcjag]

Hi Richard..just check that the needles are in place and flush...

[cactusman]

I agree with MGCJAG

Check the needles are correctly set in the pistons...they go in as far as the groove on the needle shoulder and no futher....i spent hours fiddling before realising a numpty...not me....had got them all wrong!!! Measure very very carefully to make sure all three are precisely the same length...be worth checking all three are the same needles too....mistakes do happen....UM from memory for a standard set up.

Also...if you have rebuilt the carbs with new float chamber valves and floats....have you put in new fingers that rest on the float and operate the float chamber needle valve?

As supplied these may not all be identically formed meaning the fuel level in your float chambers differ...thie too will affect the fuel delivery between carbs...

Good luck...

[Heuer]

This 1968 guide is worth reading before you start: SU Tuning Guide: http://dl.dropbox.com/u/8496016/SU_Tuning_1.pdf

As others have suggested you need to make sure you have the needles correctly set in the pistons and ensure the top of the jet is even with the bridge on each carb before you start.

[abowie]

Needles. Have a groove in them. Set needle position by putting a 2 thou feeler gauge in the groove. All needles will be the same height.

Jet height. Use the 2.5 turns as a rough guide. Then measure the position of the jet below the bridge using a vernier caliper. Then set all equal. About 80 thou seems a fair starting point.

Other things to check:
Jet centering.
Sticking pistons.
Choke mechanism correctly adjusted ie all fully off when running.
That all the butterflies are fully closed with the linkages.
That the front carb has the vacuum advance either attached or plugged off so there's no air leak.

Personally I find the only way I can correctly tune SUs is with a CO meter. Do yourself (and your mates) a favour and go and buy a Gunson G4125 Gastester.

[cactusman]

As Andrew says...check his list and read the tuning guide carefully so you are familiar with the procedure. It is straight forward but must be done as described.

Having being tuning my MG twin SU set up for decades the E just added an extra one.

I use a carb balance tester...just tests the suck at each inlet plus an ear and a screw driver.

Follow the procedure and it can be done...after all Jag did it many times a day! Mine runs sweet as a nut and pulls like a train....even my MOT man says she goes like a rocket! I get 20 MPG on a long run.

Not used a gas tester. While the Gunsons device may provide an indication it might be worth remembering that a professional analyser will be several thousand pounds...the G4125 is less than ?200....does not take a rocket scientist to deduce there must be a difference in build quality and probably accuracy. Amazon UK reviews of the device are best described as mixed I have to say. I can think of better ways to spend ?200.

[ALAN COCHRANE]

Richard

I've just tuned my carbs as well. I had a problem with two of the pistons sticking. If this is the case one of the symptons is a tendancy to stall after revving the engine or at the very least a hesitancy to return to normal idle.
Check the inside of the bells for scoring which would indicate a sticky piston.
These can be filed down carefully with emery paper, but ensure the bell is thorougly cleaned afterwards. Oil the side of the piston and rotate it in the bell as well as moving it up and down. This worked for mine.
Have to agree about the Gunson gas tester-it worked well on my GT6 but not so well on the E-Type. Although it does have a handy electronic rev counter function on it.
+1 for an air flow meter-mine is a synchrometer-one of the best buys I ever made on Ebay.
Can anyone tell me why SU thought it was a good idea to make the piston lifting pins on the middle & front carbs so inaccessable?

Alan

[cactusman]

Nope...maybe Mr SU had extra finger joints!!!

Air flow meter is a must and really only a few pounds.

Most automotive multimeter's have a tacho setting and are less than 20 quid.

Sure the Gunsons meter will work. Trouble is the E has two pipes....and the middle carb feeds both....so what ever comes out of a pipe is a mix of two carbs input and will be an average of the two up to a point. I would imagine it would be possible to get two wrongly set either side of OK and for the avg CO to look not bad. Useful as a final check but I would not use one to set them up.

[christopher storey]

I always use my Gunson, and have done for over 20 years. Provided you think logically, and check the depths of all 3 jets both before and after adjustment ( equalising them before you start, and then using the average after you have finished ) you will get a good result . Incidentally, if one uses the correct UM needles, 20 mpg suggests over richness - both mine have always done 23 mpg

[richard btype]

As always many thanks to you all for the tips and advice.

A most interesting exercise!

It took a couple of attempts to get my thinking into harness. At the end of the day it was to understand that every aspect has to be accurate and consistent as gains were made in adjusting the float levels but the biggest factor was around the position of the needles in the pistons.

The engine note is far more consistent/even but feel there are still further gains to be found further round the power band as it starts to feel a bit flat around the 3500 rpm mark (or I've become used to fuel injection).

On another subject (rear wheel bearings) there is a little play - not enough to fail the MOT, on reading up what needs to be done and observing the quantity of 'end float' shims available - helps one to appreciate how back in the day when you visited your garage one would have the work done by a 'Motor Engineer' where as these days it is more of a 'fitting exercise' as very little or no tolerance exists in the modern motor car.

Over the last 5/10 years I have taken on the repair and maintenance of my daily car and this has become a hobby - the E type is proving to be an altogether different challenge but extremely satisfying and rewarding to get right... :D :D

[cactusman]

Hi Chris..

UM needles...I would say avg about 20 as I do take her to the supermarket once in a while...10 ish mile round trip depending on how much frozen stuff and the route there and back!!! Always causes a stir... Did a long run to Bristol and back and got close to 25..plugs look fine and less than the 2.5 turns on the main jets set.

[christopher storey]

Richard : the rear wheel bearings are designed to have a small amount of play, and at the wheel rim this is multiplied significantly . It is possible to check the end float with a dial gauge, but the accuracy can be questionable because of the tendency for the car itself to move as one pulls or pushes the hub. In the end, I'm afraid I do it by feel, looking for just a "jog" of movement at the hub so that it is almost imperceptible and yet definitely there . Incidentally, I have found that it is rearely necessary to alter the shimming even if the bearings are replaced - it appears that the design variable is in the driveshaft rather than in the hub or bearings, and thus unless the driveshaft itself is changed, the shim distance remains the same


Julian : You sound to be well set up. I've never managed as much as 25 even though by Heuer's standards ( and most other people's these days ) I am a pedestrian driver

[Alty Ian]

I have brimmed the tank on the S1 twice now and over approx 450 miles the current average mpg is 25.6mpg which is very pleasing albeit with the car being driven quite conservatively whilst running in.

Compared with my S3 its brilliant

[cactusman]

Bristol was a long run. With the local JDC lot...lovely long weekend away..gentle driving...mostly...ahem...and got around the 24 mark which I thought was a..maz...ing. Just going for a local tootle more like 18 or 19 really. Still not bad for a 50+ year old 3.8 . she is well worn in!! Goes well though...quick spin out to St Albans and on to Harpenden at the weekend was startled to see the wee needle very close to or maybe slightly past a number with an eight in it! Thought St Albans arrived rather sooner than in my crappy van. Was not even trying.. I know I preach to the entirely besotted but what an amazing car.

[Simon P]

Long time since I've purchased a gallon. Where does one go to buy one these days...?

[cactusman]

Ah...can't buy those gallons any more. Would probably fall foul of some sort of rule from...ahem...somewhere.

You can of course buy precisely 4.54609188 metric litres which is equivalent to the imperial gallon.

For those in the US you only need to buy 3.785 litres to get a US gallon....but then you can probably still buy gallons anyhow :D