Setting my SU carbs

[UKPhilTR7]

I am looking to set my SU carbs. I have heard that it is not as bad as it sounds once you start. I also hear that you to check that your needles are correct. Can anyone point me in the direction of any good youtube clips or instructions/guides for our cars? I have a standard 2lt engine with k&n air filters. I do not think I have any strange needles in or if I do, I do not know about them.

[Stag76]

If you take the bells off, and use the depth gauge of a vernier caliper to set the jets .125"
below the top of the holder, you will have the needles synchronized to start with.
It's much more accurate than screwing the jets up until you think they are level with the
top of the holder, then screwing them down 2.5 turns (or whatever).
When you're finished and it's running well, re-measure them for future reference.
Being vacuum operated, they are very susceptible to air leaks and riding inlet valves.

[Cobber]

If you don't have a vernier caliper Aldi had digital vernier calipers for $19 on Saturday, get down there quick and they might still have some left.

[Hasbeen]

If you haven't already done it, check & adjust your valve clearances. 7s are quite sensitive to valve clearances.

With K&N air filters you will need BAL needles.

SUs are sensitive to fuel pressure. Even in perfect condition they will flood with more than 2 PSI. The pumps sold in Oz as suitable for TR7s are a generic pump, suitable for a range of pommy cars. They should come with 2 spacers that can fit between the pump & the cylinder block. The thickness of the spacer controls the pressure the pump supplies.

Usually it is the thicker spacer that suits the 7, but not always. Sometimes a short arm pump can be sold. WARNING. With the usual pump if no spacer is fitted the pump arm will be broken.

Once you have sorted the valve clearances, & the pump pressure, you have done most of the carb tuning. It is then just a matter of equalising the suction through each carb, & adjusting the mixture.

Air leaks between the carb & the head are the thing most likely to then give you any problem.

Hasbeen

[UKPhilTR7]

Hi guys thank you for the comments and I will try to address them one by one.

Thankfully, I have a vernier calliper (funny enough from Aldi in the UK, but I will still look at the one this weekend), I just need to get some batteries for it now.

So I take of the bells are the pots. With the the jets set to .125" below the top of the holder and having the needles synchronized. Am I right in thinking this is the starting point? The car may run like a dog, but it is a good place to start and then you can start screwing the mixture screws down 2.5 turns (or whatever your engine needs).

I was thinking if I needed BAL needles for the k&n filters, thanks for confirming. I will have to order a pair. I have the fuel pump on with the black rubber spacer. From memory I have the one for the later models, the one with the bigger arm.

When we say valve clearance, is that being set when the jets are set to .125”?

I have been looking at the Gunson colortune from Accuspark. This looks like it is easy enough to use. Has anyone got one of these and is it any good for our cars? If I got one of these would I need to still set the jets or would the colourtune take away that need?

[Hasbeen]

Hi Phil.

We have got you confused. The instructions for starting tuning is to bring the main jet level with the top of the bridge in the carb, then wined them down 2 & a half turns of the jet nut. The .125 measurement is instead of this. You then raise or lower the jet to fine tune.

Valve clearance is the cylinder head valve clearance on the cam shaft. You can’t really tune an engine if the valve clearances are not close to right.

A funny example. I once started to try to tune an engine of a car I had just bought, enough to get it onto a trailer. The front carb was flooding, & we discovered the No1 inlet valve was riding, & being held a little open.

The ensuing backfire through the flooding carb blew burning petrol out of the carb, setting fire for a few moments, to my son’s arm.

The Gunson colortune is only an aid to help you see what to do. The tuning still has to be done.

One other thing, you should check the ignition timing, A lot of poor running that is due to ignition is blamed on the poor SU.

There is much mystique about carb tuning, but it really is quite simple. Just don’t be frightened by it, think slowly & clearly, & it is not a black art at all. It is just the gurus who want you to think so, SO THEY CAN CHARGE MORE.

Hasbeen

[UKPhilTR7]

Thanks Hasbeen for that. I went down the garage yesterday and had a mess about with the carbs and see what I could do.

My desire to sort out the carbs has been driven by a number of factors. The main one being that when she is cold and idling she will run at about 850 rpm and then when the electric fan comes on, the revs will go down with the extra load on it to about 650 rpm. Then when the fan goes off the revs would stay at around 650 rpm figure and she sounds like she is going to cut out when on idle. The other reason was due to the fact that the carb mixture screw at the front when turned would result in the revs going up or down. However, when the carb mixture screw close to the bulkhead was turned, nothing happened to the revs.

So in the garage I turned the bulkhead mixture screw (a few turns) until I could hear the revs move. So now when I turn both mixture screws, the revs move. So now the revs will hold at around 850 to 900 rpm, when on idle (picture below).

This morning I went and started the car and she was running at around 850 to 900 rpm and then after the fan came on and went off, she dropped down a bit in the revs. Nothing like before and she didn’t seem like she was going to cut out, but the revs were lower than 850 to 900.

So I looked at the vacuum pipe from the carb to the dizzy. The one side had the correct Rubber Coupling - Right Angled (128262). However, the other side just has a small pipe. This after sucking in the pipe and connectors was letting in air. I used insulation tape and sealed the gap between the vacuum pipe and the end connector. I will get a new Rubber Coupling on order. This seemed to help on the second time or running the cars and her revs after the fan had gone off.

I still feel that I need to look at the timing and the carb settings.

I have been speaking to a guy in the UK and he said that there are three things that need to be looked at; the timing, the carb vacuum and the fine tune (possible using the colour tune).

I have since got a timing gun and hope to have a look at this the weekend. This I know may result in the dizzy being moved. I just hope that my timing markings are all and I can see them.

I have not got the carb flow meter or the Gunson, but will order them off ebay soon.

At the moment, I am just getting the process and the bits together. She is running at 850 to 900 rpm now and I will keep her like that. The last thing I want to do is take the rocker cover off as I have only just stopped a leak from it.

[sydney.wedgehead]

Hi Phil,

The thing that hasn't been mentioned so far is balancing the carbs so they are both pulling an equal amount if air. If your hearing is good you can do this by placing a flexible tube against the same section of the carb throat for each carb and listening to the induction.

Obviously, you should set the fuel pressure, jets and float valves first, using whatever measurement tools are available, ie, what Bruce and Phil are suggesting.

Also, look closely at your throttle cable and linkage setup. Both have caused me erratic induction problems in the past.

Owen

ps. addendum: I wouldn't bother with the Gunsen expense. I prefer to set a tune on new plugs then go out and thrash it a bit before returning to do a plug inspection. It's easy to tell if you are rich or lean.

[UKPhilTR7]

Oh yes Sydneywedghead that was the second thing the guy in the UK told me that needed to be considered. Was going to get the vacuume from eBay.

Last night I went down to the car with the gun a friend lent me. I have a marking on the flywheel and I have two yellow paint marks on the comb like thing on the engine. Lol not sure what it is called.

When I put the gun on the lead by the bulkhead the mark on the flywheel lined up perfect with the bottom mark on the engine. When I put the gun on the lean closest to the fan the marks didn't line up.

Am I right in thinking that lead 1 is the one closets to the fan?

The next that I need to do is work out what the two marks are in yellow are. Also what the original mark should be. I will add a picture of it soon.

[UKPhilTR7]

This is the mark on the flywheel:



These are the yellow marks on the engine:


The bottom mark is the one that my mark on the flywheel lines up with perfect when on the lead next to the bulkhead.

[Beans]

Setting the timing with a strobe light should be done from #1 spark plug lead.
Could be yours are fitted in the wrong sequence.
At idle the dynamic advance should be the same as static; 10°BTDC

[Stag76]

No 1 plug is the one closest to the fan.
The firing order is 1-3-4-2, so 1 and 4 should fire at the same place on the crank pulley,
360 degrees apart, while 3 and 2 will fire 180 degrees from the mark on the pulley.
No 1 should line up with the pulley the same as no 4 (rear plug).
No1 is used (as a convention) for setting timing.

The top yellow mark on the comb ( the bigger pointer with a line scribed against it) is Top Dead Centre,
and the bottom yellow mark is 2 1/2 teeth before the Top Dead Centre mark, or 12.5 degrees before
top dead centre. The points are 5 degrees apart.

What sort of ignition do you have...if it's points then this is the place to start.

[Stag76]

Where were the marks aligned when you attached the timing light to No 1 Lead?

[Stag76]

Phil,
Which mixture screws are you adjusting.

[UKPhilTR7]

Setting the timing with a strobe light should be done from #1 spark plug lead.
Could be yours are fitted in the wrong sequence.
At idle the dynamic advance should be the same as static; 10°BTDC

It may be possible that they have been put on in the wrong order, but I would of thought that it this was the case the car would of been running like a dog before now. She runs OK apart from running rich (I think and not holding idle after the fan too well).

What I will do is lign up the marks on the comb and the flywheel and then take off the dizzy and see if it aligns with plug 1 TDC.

@Stag76, that makes is clear for me and now it is starting to all fail into place

Thanks for pointing out that the top marker pointer was slightly larger than the rest, I had not noticed that. Now it is starting to make sense.

What sort of ignition do you have...if it's points then this is the place to start.

I replaced the points on my car to electronic ignition.

Where were the marks aligned when you attached the timing light to No 1 Lead?

Yes they were aligned to plug 1, the mark 2 1/2 teeth before the TDC mark was lining up with the mark on the pully.

Phil, Which mixture screws are you adjusting.

I am adjusting both of the screws.