Issues balancing SU Carbs

[UKPhilTR7]

Some of you may of seen my previous post about tuning my SU carbs. Everyone was great and gave me some really good advice. I since brought a colour tune and carb balancer, which I have used quite a bit. However, there is a bit of an issue that I am hoping someone can help me with.

I can tune the carbs, so they are running about 6.5 on both sides and she runs nice about 900. After the electric fan has kicked in and then gone off, the reves return to 850 - 900 quite quickly. However, when I have been pressing the accelerator the revs will only come down to about 1000 or just above. She will not return to 850 - 900, which she was running at before I pressed the gas.

After the gas was been pressed the front carb (by fan) will be running at about 7.5 and the back carb (by the bulkhead) will be running at about 6. I pressed the bracket - front carb by fan (in the picture below) and the revs went down. No matter what I did, I could not get the revs to stay down. Have anyone come across this where the front carb will not come down but the back carb will?

[Spike]

had a similar problem on mine, it was down to spindle wear.

Replaced with new ones and everything sorted.

[FI Spyder]

That's what I was thinkin. We had a carb tech session at our last club meeting by our resident carb specialist and he says he often has to ream out the carb body and insert new bronze bushings that are available from the carb people.

[littlepippin]

You can adjust the lifters by slackening off the two nuts that are in the top of your piccie, and set them with a feeler guage between the lifter and the sprung catch, so that they are not lifting at idle.
Also, adjust the throttle cable linkage so that it is operating around the middle of its span/travel, clean it, lubricate and check it is not snagging.

Hope I am making sense....!

Sean.

[Beans]

Indeed first check should be for spindle wear, and it is very easy (check the spindle on the float chamber's side)
As they start to wear the throttle discs alignment will suffer, causing random differences in tune
Hence the variation in idle speed.

[Hasbeen]

Yes spindle wear allowing misalignment of the butterfly, preventing it from quite fully closing, drag in the throttle cable should effect both carbs equally.

If you are getting a bit lazy & cheap, [which you don't appear to be], you can overcome this with a couple of small tension springs.

I removed, [& binned] that ridiculous exhaust gas rebreathing valve, & made a 3/16 steel blanking off plate with 2 tangs to take fairly light tension springs, one from each carb. They are not heavy enough to notice using the accelerator, but return the carbs to fully closed every time.

If your throttle cable is sticking or dragging a bit, try washing it out with petrol, to remove any dust & dirt gathering oil residue, then lubricate it with graphite powder. You will be amazed at how smooth your throttle becomes when kept dry & lubricated this way.

Hasbeen

[UKPhilTR7]

Thanks for that guys, it seems to be unanimously that my issue most certainly is due to spindle wear. I will check out the cost of some new ones (spindle and bronze bushings). Too true Hasbeen, I have never been one to take the easy option with my car , been a bit of a pain in the past but learnt so much.

From what I can see it looks like I might need the bits highlighted yellow in the picture below. However, my main thought is how would I replace them and can it be done while the carbs are still on the car or do they need to be taken off? I am thinking that I need to take the float chambers out and then remove the butterfly disks then slide the spindle and the bushing out. Sounds a bit too easy for my car and I know that an hours job will take four for me lol. Any tips or advice on how to change them while the carbs are still on the car would be great.

[Jolyon39]

Have you seen this accessory Link for the carbs?

Perhaps this will add more consistency to the throttle balance? (Yes, it will be a worn spindle but would this add control to the situation?)

http://www.ebay.co.uk/itm/Throttle-Link ... XQROxRF4Wy

[Spike]

On mine I just replaced the spindles and throttle discs and that sorted out the issue. So you might be able to get away without rebushing.

The throttle linkage probably won't make any difference to your idle, it will give you an smother/easier throttle action.

regards

Graham

[Beans]

... Perhaps this will add more consistency to the throttle balance?

I have this conversion on my car for many years and from own experience I can tell it won't have any influence on worn spindles.
My SU powered DHC has the same issue mentioned above: the revs won't come down to idle completely, settling anywhere between 1000 and 1500 RPM.
(but that is currently being addressed, as a friend is overhauling one of my spare carburettor sets)

... So you might be able to get away without rebushing ...

Indeed it usually is the spindle that is worn. So just a matter of replacing the spindle and maybe the throttle disc.
But the carburettors need to come of to do that.
When the spindle hole is worn the carburettor housing will need machining to enable the bushes to be fitted.

[sonscar]

I replaced the spindles and butterflies on my car.The carb body needs machining to fit the bushes.There are some cringe worthy photos in my blog.Sreve.

[UKPhilTR7]

What I am getting from this, is that if I want my idle to return back to its original position after I have been pressing the gas, I need to replace the spindles, bushes and the butterflies to be on the safe side. No worries I can look at this.

From what I am reading some people have been able to replace these with the carbs on the car and some without. I personally would like to go for doing this work with the carbs on the car to hopefully reduce the risk of things going wrong an mucking up the carbs/engine. If anyone has any pictures or description of how they did this, it would be very useful.

There are talk of machining the spindle hole to get the bushes in. Is that only if they are very warn or does this need to be done every time?

[UKPhilTR7]

Just looking on the net and around and I have seen that there should be three throttle return springs on our SU carbs. All on the throttle; one on the center cam, and one on each of the individual throttles levers. If you look on the Rimmer site below they have three listed. Mine has only ever had the one return spring.

Do we need three?

https://www.rimmerbros.co.uk/Item--i-GRID006070

[littlepippin]

I'm not sure there is provision for attaching 3 springs where they meet the carb end of things, although there are three holes on the anchor plate at the opposite end ?
As I said before, I would exhaust all avenues of altering the adjustable clamping points for the lifters, and the clamp that joins the throttle cable linkage to the spindle, and the even checking the peddle spring and cable action.

Is the choke returning cleanly and fully - I even have had to disconnect and reset that as well in the past.

The springs for the return of the butterfly valves to me seem pretty strong and should overcome some wear of the spindle, but I do agree with the guys that if they are very worn and binding they gotta be attended to.......

[andyf]

Phil, I had the same issue and rebuilding the carbs solved it including new spindles but I did not have to rout the carb body. Before I rebuilt them I added an extra spring to the front carb spindle, effectively pulling it shut. It will make the throttle heavier but it did the trick for me as a temporary measure (about 3 years!)