Help with tuning carbs please

[malcolm]

Seem to spend al my time on here asking for help - hope you don't mind!
Anyway, been playing with the carbs. Got the tickover speed right, and balanced ok listening to intake hiss of each carb.
Then tried adjusting mixture.
Handbook says adjust mixture screws to get fastest tickover. But adjusting the screws didn't seem to make any discernible difference to engine speed, although they must have done something as tickover is a bit lumpy now and can't get it back!
Question is, Should the mixture screws make the difference I expected in engine speed?
Have done this before on twin carbs with the older SU's that had the screw on the bottom of the carb. Small adjustments directly affected engine speed, and made it easy to get the mixture right, but doesn't seem to happen on these.
If I can't sort it out, who would members recommend for carb tuning in the Essex area? Near Chelmsford preferably.
Advice would be appreciated!

[Heuer]

Try these guides from the Forum KB:

Balancing SU and Stromberg Carburettors: http://dl.dropbox.com/u/8496016/carb%20balance.pdf
SU Carburettor Notes: http://www.jcna.com/library/tech/tech0006.html
SU Tuning Guide: http://dl.dropbox.com/u/8496016/SU_Tuning_1.pdf
SU Carbs.com Tuning guide: http://www.sucarbs.com/tuning2.html

[christopher storey]

Malcolm : you do not say if this is a 6 cylinder car with 3 SUs , but I shall assume it to be so. It is very easy to get confused about which screws do what . The idle speed is controlled by the very large screw ( about 1/4 inch /6mm diameter ) in the body of each carburetter. It is important to realise that the throttles should be completely closed at idle , the mixture merely going through the channels controlled by these screws . The fast idle screws are the ones nearest to the throttles i.e. nearest to the engine. The mixture screws are the lowest ones on the left of each carburetter nearest to the air intake trumpets . If you have got yourself into difficulties, the easiest thing to do is to take off each bellchamber and piston, and using the mixture screw, set each jet down the same amount. I start at about 65 thou down from the bridge and then adjust from there. You can get usually nice digital calipers which also operate as a depth gauge from Aldi at about a tenner, or rather more laboriously you can make a "depth stick" to achieve the same effect. There is noone in Essex that I would recommend at all - PM me for details

P.S. The fast idle is of course controlled by the initial movement of the choke lever . Beware of the fact that sometimes the brass rods which run vertically through the body of the carburetters seize up and require much wd40 and gentle persuasion with a hammer to free them off . If this happens it can affect both the idle speed and indeed the mixture strength because the choke can stick partially on if the rod is stuck in a position where it is actuating the shoe at the bottom of the linkage . If all of this sounds a bit worrying - don't let it depress you! It's not in fact that difficult once you understand the method of operation . Take your time, examine everything carefully until you see how it works

[malcolm]

Thanks for the replies. It is indeed a 6 cylinder 4.2 with triple HS8 SUs Christopher.
I'll read the literature, (thanks for the links), do some general checking, take the bell housing off and lok at settings, then try again!

[malcolm]

Just ordered some digital electronic calipers/depth guage for a tenner (inc delivery - no, this isn't an ad!) so looking forward to having a go when they arrive.

[Moeregaard]

Malcolm, the factory driver's handbook provides very good instructions for tuning the E-Type's carbs, along with a very nice illustration detailing the function of each adjusting screw and where they should be set as a starting point.

You mention that your idle got really lumpy, so you may have fouled a spark plug while messing with your carbs. It's virtually impossible to correctly tune an E-Type's (or any car's) carbs without the ignition bits being 100%, so it's probably worth pulling the plugs for cleaning and gapping.

As Christopher mentions, ensure that the fast-idle pushrods haven't seized in the carb bodies; this is fairly common with cars that aren't used much.

[malcolm]

Thanks all, and I'm glad to report that all seems fine now.
I took off the dashpots, and lifted out the piston, then measured where the jets were depth wise using an electronic set of digital calipers with a depth guage - ?10 from amazon, including postage!
They were very different, with depth from the bridge varying from 75 thou to 112 thou across the 3 carbs.
So I went back to ground zero.
Wound them up until the jets were flush with the bridge, then wound them down 2 turns as per the handbok.
This gave a depth for all 3 of 80 thou.
Cleaned plugs, checked rods weren't jammed for fast idle, topped up pistons in carbs with clean oil, and started up.
Ran a bit rough, and seemed a bit lean. Wound down to a further 1.25 turns eventyually, bit at a time, while revs rose a little with each adjustment. That gave a total of 3.25 turns on each carb in total, and tested again. Very smooth, no spitting, no smoke, accelerated very well. So pleased now, and thanks again to all for advice. Very helpful.

[MT2+2]

This is my first post, so to start at the beginning, thank you all for sharing your knowledge so freely! It has given me the confidence to have a go at fine tuning the 3x SU carbs on my Series 1 4.2 myself!
I have couple of items of feedback:-
1. I bought the Uni-syn type flowmeter to adjust the slow idle, but found that I could hardly get the red ball to move up the tube at all, and the engine nearly stalled within seconds of offering it up to the body. On the basis of the better reviews of the Dellorto (snail) type flowmeter in some of the related threads, I have ordered one of them!
2. I also bought a vernier gauge and found it easy to measure the jet position. However I was surprised to find that my jets are only about 40 thousands of an inch below the bridge, yet the car runs pretty well having been set up last by a trusted Classic Jag garage. Is this because the floats have been set high? Any thoughts appreciated.

[mgcjag]

Hi..Whats your name....what needles do you have...for standard road i think its UM...not sure what affect richer needles and higher jet setting would have.... Steve

[christopher storey]

MT 2+2 : yes, UM are the standard needles , and all the available needles are pretty much the same at idle , so a different needle should not account for the jet settings being high. The first thing I suggest you do is check that the needles are correctly installed in the pistons : they should have the shoulder of the needle exactly flush with the base of the piston . If those are correct, then I would indeed check float levels ; also, what fuel pump is being used ? If a facet or similar pump has been used, there can often be excess pressure from the pump. The correct pressure ( which can be set by using a filter king in the line ) is no more than about 0.2 bar / 2.8 lbs/sq.in

[MT2+2]

Steve, Christopher, thanks for your prompt replies.

The garage invoice shows the new needles to be UB, and the fuel pump was a standard SNGB supply. (I thought it was fitted with the modern electronic valves, but it still clicks like the original).

I will investigate further and let you know if I find anything, but the funny thing is that the car seems to run well with the jets where they are!

Best regards,
Martin

[christopher storey]

UB is only slightly richer than UM, so not a lot of difference there . The electronic SU should not cause over pressure either . It could be the float level, or a mis-setting of the needles . Either way, if it's running well, it's probably best to leave it for the moment . What is the air filter setup ?

[JagWaugh]

This is my first post, so to start at the beginning, thank you all for sharing your knowledge so freely! It has given me the confidence to have a go at fine tuning the 3x SU carbs on my Series 1 4.2 myself!
I have couple of items of feedback:-
1. I bought the Uni-syn type flowmeter to adjust the slow idle, but found that I could hardly get the red ball to move up the tube at all, and the engine nearly stalled within seconds of offering it up to the body. On the basis of the better reviews of the Dellorto (snail) type flowmeter in some of the related threads, I have ordered one of them!
2. I also bought a vernier gauge and found it easy to measure the jet position. However I was surprised to find that my jets are only about 40 thousands of an inch below the bridge, yet the car runs pretty well having been set up last by a trusted Classic Jag garage. Is this because the floats have been set high? Any thoughts appreciated.

MT,
You need to "calibrate" the unisyn. The disc in the middle of the intake can be moved in and out put it on one carb set to the "default" settings and adjust the disk until the red ball is in the middle of the range. Now compare the other two carbs.

Have you checked that your dwell and timing are bang on? No use messing with the carbs if that isn't right.

Take the tops off the float chambers and check that they are right.

Although it is possible to adjust the jet height to compensate for a high float level, the car will tend to go rich on an incline, which won't do your oil comsumption, or Cylinder bores any good.

The correct procedure for ANY carburated car is:

1) Verify TDC on the flywheel/Harmonic balancer.
2) Check that the makring is correct (measure the marking on the harmonic balancer with respect to TDC)
3) Set the points gap/dwell
4) Use a timing light, mighty vac and tach to verify that the mechanical advance and vacuum advance/retard are working correctly.
5) Check/Set the float level.
6) Set the carbs to the default settings, and check that the butterflys are properly closed.
7) Balance the carbs with the unisyn.
8 ) Adjust the mixture (colortune is more important than a CO meter, having both is better).
9) after you've set the jet heights on all 3 carbs, go back to the first one and check it again via colortune/CO.

Getting it perfect requires multiple iterations. If the intake manifolds have interconnects then the front carb mixture affects the other banks, if the carb/exhaust branches are not 1:1 (or 2:2, or 3:3) the same holds even if the carbs have no interconnects on the intake.

[MT2+2]

JagWaugh, Many thanks for your thoughts and the sequenced list. Most of the items were set when the car was in the garage last time, so on this occasion Im wanting to fine tune the carbs rather than start from scratch.

I think that for now I will take Christopher's advice and leave it as it is, after all it seems to be running well. I will monitor fuel consumption though and set the slow idle with the Dellorto type flow-meter when it arrives. This was my original problem and although much improved, Im hoping the meter will get it spot on.

best regards,
Martin