Twin SUs
 

To cut a long story short, there is a problem with the cross shaft between the two carbs (the one which attaches to the butterfly spindles, & links the carbs together). Where the shaft is fixed to the aft carb spindle, there is a huge amount of play, and as a result that carb opens considerably later than the forward one. This is reflected in the airflow (measured using a Carbalancer), as the forward carb is doing most of the work.

However the spindle itself seems to be fitting correctly in the carb body, as I can't find any play there.

There are adjusters on the bar, for the levers that clip to the spindles, but these are locked solid so I can't eliminate the problem that way. That would only be treating the symptoms anyway, as the play would still be there.

Does anyone have any tips, hints etc regarding having replaced these in the past, and how the whole assembly goes together? The factory parts books & manuals aren't much help.

In particular, how does the shaft fit onto the spindle? Hopefully the spindle will be ok! Also, will I need to remove one of the carbs to replace the shaft?

Thanks!

Paul

If the linking bar is clamped firmly to each of the carb spindles there should be no play as they then effectively become a single spindle. The two U clamps do 2 jobs. When loose they allow the carbs to be balanced individually and when tight they help to minimise the effects of the 2 carb spindles being possibly out of line. I'm more familiar with the clamps used with the B16b set-up which uses a separate zig-zag clamp which fits between the spindles. They have 2 bolts per clamp. There's a gap of about 1cm between the spindles on these. Misalignment hastens spindle/carb body wear which makes tuning SUs properly, impossible, due to air leaks. I think most people just loosen 1 clamp when balancing the carbs but having both loose is better. I'm also sure that there are Amazon owners out there, who think they can get by without a manual, that just set the tickover using the carb screws and haven't considered undoing them first.
By loosening the clamps and moving the rod sideways you might be able to remove the link bar if needed. The carb spindles and/or link bar might have grooves worn in them if the clamps have been slipping a bit so smooth with a bit of fine wet'n dry. Check the clamp nut and bolt for stripped threads. If the clamp squeezes shut and still doesn't grip the carb spindle you might be able to file a little bit off the inner faces to allow it to close up tighter. Refit. Balance carbs. Before retightening, check that the control rod will work to give you full throttle without binding or going over centre. Adjust if needed. Carefully retighten the 2 clamps, making sure the carb spindles stay where you set them. If your carbs are worn be prepared to compromise. Points setting, timing and valve clearances need to be spot-on for good results.
Useful links - http://www.vclassics.com/archive/stoichio.htm
http://www.vclassics.com/archive/sucarbs.htm

I may be one of those pesky owners who try to fiddle with the SUs to solve a problem.

My problem was that, after I had given the car some wellie in 3rd (going up to about 65-70), the revs remained really high even at idle; they calmed down a bit but were still much too high when I pulled up at home and idled for a few minutes. I tried to help by turning the two idle screws about half a turn anti-clockwise, and the revs lowered.

However, the revs were fine before I did my 3rd sprint, so what has gone wrong at a deeper level to make them shoot up. I also had quite a bit of overrun after shutting off the engine, which I've never had before. Any guidance much appreciated.

Tom

I hate to ask the obvious, but have you checked that the butterflies are properly secured to the carb spindles.
Your symptoms sound to me like something has come loose when you wellied it, and if its not visible on the outside, perhaps its inside. Worth a check just to be sure.

Loose butterflies is a possibility, you'd really need to take the carbs off to confirm that. More likely air leaks around the spindle/carb "bearing". If you can feel/see any movement when you try to move the shaft by hand, it will be sucking in air, weakening the mixture and increasing the revs. Tom's engine and carbs being HOT after a bit of a thrash probably means the spindles settled back to a slightly different position to when the temp is "normal". I'd guess he'll have to increase the revs again after things have cooled down or at the next cold start. A drip of oil both sides of each carb with engine running will seal it long enough to check how bad it is. Burlen
http://www.burlen.co.uk/ do an overhaul kit which isn't too expensive to fix this. Has new spindles and bushes in it.

SU Carbs
 

The quickest answer is to call Burlen Services in Salisbury and get a re-build kit, take yours to bits and start out afresh.Anthing else will take more time, effort and bad language!:Banane59:

Rebuilding SUs
 

If you decide to go down the rebuilding route, I would suggest studying very carefully whether there is in fact appreciable wear on the carburettor body itself where the spindles pass through the body, as opposed to on the spindles themselves. The kits contain new spindles and these can easily be checked in the carb. The reason I say this is that the kits do emphasise, quite correctly, the need to have the carburettor body drilled and reamed in line for the bushes to fit perfectly in line. If this is not done with 100% accuracy, the spindles bind.

While much has been written of spindle wear and leaks, personal experience on many cars tends to suggest that leaks of far greater magnitude arise at the manifold gaskets, either at the carbs or the head joint. Many pre war cars with original SUs or Zeniths made of pot metal are still running well with original spindles and no replacement bushes yet have covered mileages of 200,000+. I'm not saying they don't wear, only that to make significant sudden differences to reliability or performance, I would look for another culprit first. Just a suggestion. Good luck (and Burlen are excellent, by the way !)

I totaly agree. I have never needed to replace new parts on an SU carb. All I have ever done is strip, clean, rebuild and set up properley with new gaskets. I have come across the odd needle thats worn due to incorrect alignment in the jet. I have had a few problems with bad running even when the ignition side is perfect and found most problems with either the manifold itself or leaks from the gasket or pipes off it. I also have three cars on the road with twin carbs and two sets of carbs on the bench at the moment. All have some play in the butterfly spindles of about 8 thou'. My cars run spot on and get over thirty to the gallon on a run. It's all about setting them up properly, which is not rocket science. Specialists will always tell you that you need a rebuild because either they don't know anything about them, or that's the easiest route for them.

Many thanks for all the advice etc chaps :)

I found this article is especially good for the novice like myself! :D

Eventually managed to free off one of the cross bar adjusters without damaging it (so don't need to replace the worn parts there quite yet...) & have now balanced up the carbs properly following that article (used a Gunson Carbalancer to check the airflow, & also referenced the Haynes but only for the choke / fast idle setting) & it's now running so much more smoothly, even at idle where it has always previously been a bit grumpy. Fingers crossed that means that the spindles are ok, and now that it's set up properly, I hope to see some improvements in both performance & fuel consumption too!!

In other news, the strap holding up the rear silencer (big bore simonz system) decided to let go the other evening, but the mid mount was enough to keep the backbox off the road (just...). Now wearing a bodge hanger consisting of two jubilee clips slung off the bumper iron as the bumper bracket bolt that also holds the strap is not the slightest bit interested in being undone :(