Setting up single SU

Well, i am the opposite, i can undestand basic mechanicals, but when electronics are involved, i loose the plot.
Trying to tune up worn out and dirty carbs is a common waste of time. Single SUs can tolerate a lot, but twin set ups are a lot more picky.
The twin HIF6s i rebuilt and fitted one and a half years (and many - many miles) ago until now are remarkably trouble - free (touch wood :D ).
Excellent economy, low emissions, smooth idle, instant start hot or cold, no hessitations and they have not lost the original tune.
In fact, if it wasn't for the need to pull the choke for a cold start, and top up the dashpots every now and then, they have the behaviour of a reliable EFI system.
 
When stripping down and cleaning an SU, an item to check is the strength of the spring holding the carb jet nut in tension and which is supposed to prevent it from turning. A weak spring may allow the nut that locates the jet to rotate very slightly every time the choke mechanism is operated - and released. I experienced an SU that required a repeated need to reset the mixture on an almost monthly basis until cottoning on to the weak spring problem.

Agreed: once set up the standard [manual choke] SU requires only a periodic external clean and topping up the dashpot. [The SU on my P4 90 was set up professionally in 1980 and hasn't been fiddled with since - runs perfectly to this day.] The only carbs I know of that match the manual choke SU for bullet-proofness in service are the fixed jet Solex, Rochester and Weber designs.

Twin SU's are not all that difficult to set up in my experience - unless some PO has "had a go" - in which case it's necessary assume nothing and to check everything carefully [e.g. I once had rough running and traced it to Mr PO having swapped around the carb pistons]. [For detailed set-up adjustment of the linkages on the 2000TC, access isn't exactly brilliant and it may be necessary to pull it off and do some of this work on the bench: for example, important feeler gauge adjustments relate to free movement of the various operating linkages that are very hard to get right with the carbs in situ.]

As far as balancing twin SU's goes, I've not had much success with the Gunson Carbalancer system on the TC since the threaded pegs for the air filter box block the use of this device's vacuum pick up wheel, also the air flow through the carbs at idle was insufficient to create sufficient vacuum to get the tool to work reliably [even when one carb was blocked off completely]. The Gunson Colourtune system also gave me mixed results on the TC with frustratingly different readings coming from each cylinder leaving me to conclude that mixtures between each pot being significantly affected by inlet port design and inherent variations in the carbs, plus there's the time and huge inconvenience of having to fiddle around repeatedly switching the device between [extremely hot!] spark plugs after every adjustment all encumbered by limited access and a gimcrack two-piece plastic tube that persists in coming apart when most needed. That said, Colourtune is a brilliant tool for single carb SU set ups with easy access - it worked a treat on the Austin Mini.

Finally the old tube-in-ear system used to balance the carb airflow may only give very approximate readings that are good mainly to identify a gross imbalance between carbs. In practice, one carb may be found to be fairly loud and the other somewhat muted. In such instances sometimes adjusting mixtures and idles may be found to improve the sound balance, but at the expense of excessively high [or low] idle speed, richness, stumbling pick-up and rough running. Since a regular idle, good pick-up and smooth running etc. are the goals, adjustments should be made towards meeting these objectives rather than an unachievable 'equal sound' target and draw comfort from the fact that the manual says final adjustments must be made "consistent with even running".
 
Hi Keith

I wouldn't disagree with your list of difficulties setting up twin SU's. But I think you've over dramatised them somewhat. When I was a whippersnapper I would have accepted the burnt fingers from swapping hot plugs around. Today, I simply use a second colourtune! I get on pretty well with a piece of pipe to the ear. I use a pice of heater air duct tube around 2" diameter. I've not tried this on a 2000TC, but I wouldn't be surprised to be caught out by the low air velocities at idle. This is not so much a challenge to set up technique but a condemnation of Rover's carb selection. It's now firmly established that the 2000TC benefits hugely from binning the existing set up and substituting the twin HIF6's (or is it HIF44's -can never rememmber!) from the 2200TC.

I note your comments about setting up the linkages. I find that you are better off to do your own thing based on an understanding of what is trying to be achieved rather than the workshop manual's specific method of doing so. HArvey and I have a disagreement running on this. HArvey likes to set the carbs up with the interlink disconnected and then resonnect the llink to suit. I prefer to set the link to give an acceptable compromise between both carbs reaching full throttle together and both carbs shutting off fully together. I skew my settings to the full throttle being correct. This takes care of balance at high rpm, I then set low rpm mixture and balance from the idle adjustment.

But of course all that assumes the carbs are working correctly. And there are two key settings that must be checked first. These are the petrol level in the float chambers being exactly the same in both carbs and the fall time of the pistons from the dashpots being identical. For the latter it is necessary to clean out all varnish and dirt from the inner surfaces of both, then test the fall times minus the springs. I see no issues in swapping pistons and dashpots around until you have a matched pair - after all, that's only what the factory supposedly did. Then match the free length of the springs and you have a matched pair of carbs. The matching is far more important than getting the fall time precisely spot on. You can correct for that later by needle selection if really fussed.

I'm intrigued by your comment about inequality between cylinders on the 2000TC. I suspect that could be an effect of the very low gas velocity amplifying any minor errors in the passages etc. I suspect it might sort itself out at higher revs. It would be interesting to see if the smaller carbs correct this - implying that it is the carbs at fault - or whether there is no difference implying that it is the head porting.

Chris
 
DaveHerns said:
If I'm wrong I've been putting ATF in my carbs for the wrong reason yet still getting the desired result
Click to expand...

My tuner always uses ATF, and I'm not going to argue with him.
 
chrisyork said:
Hi Keith

I wouldn't disagree with your list of difficulties setting up twin SU's. But I think you've over dramatised them somewhat. When I was a whippersnapper I would have accepted the burnt fingers from swapping hot plugs around. Today, I simply use a second colourtune! I get on pretty well with a piece of pipe to the ear. I use a pice of heater air duct tube around 2" diameter. I've not tried this on a 2000TC, but I wouldn't be surprised to be caught out by the low air velocities at idle. This is not so much a challenge to set up technique but a condemnation of Rover's carb selection. It's now firmly established that the 2000TC benefits hugely from binning the existing set up and substituting the twin HIF6's (or is it HIF44's -can never rememmber!) from the 2200TC.

I note your comments about setting up the linkages. I find that you are better off to do your own thing based on an understanding of what is trying to be achieved rather than the workshop manual's specific method of doing so. HArvey and I have a disagreement running on this. HArvey likes to set the carbs up with the interlink disconnected and then resonnect the llink to suit. I prefer to set the link to give an acceptable compromise between both carbs reaching full throttle together and both carbs shutting off fully together. I skew my settings to the full throttle being correct. This takes care of balance at high rpm, I then set low rpm mixture and balance from the idle adjustment.

But of course all that assumes the carbs are working correctly. And there are two key settings that must be checked first. These are the petrol level in the float chambers being exactly the same in both carbs and the fall time of the pistons from the dashpots being identical. For the latter it is necessary to clean out all varnish and dirt from the inner surfaces of both, then test the fall times minus the springs. I see no issues in swapping pistons and dashpots around until you have a matched pair - after all, that's only what the factory supposedly did. Then match the free length of the springs and you have a matched pair of carbs. The matching is far more important than getting the fall time precisely spot on. You can correct for that later by needle selection if really fussed.

I'm intrigued by your comment about inequality between cylinders on the 2000TC. I suspect that could be an effect of the very low gas velocity amplifying any minor errors in the passages etc. I suspect it might sort itself out at higher revs. It would be interesting to see if the smaller carbs correct this - implying that it is the carbs at fault - or whether there is no difference implying that it is the head porting.

Chris
Click to expand...

Thanks for the comments Chris. Yeah - I get pretty emotional about twin-SU's since they gave me complete grief for months before getting on top of their foibles and the problems caused by PO's seeking to "tune" them. My efforts were intended to provide some guidance regarding pitfalls and the techniques that work for me to maybe one or two folk reading this forum who have carb problems but are perhaps too nervous to have a go at the job themselves. In this regard a real coup was acquiring an SU workshop manual that explained in clear detail how to set up the linkages between the carbs - this resulted in an immediate improvement in running smoothness.

Otherwise I'm happy to report that my TC runs perfectly at idle and cruise with one fast and one slow sucking carb - and takes off like a rocket otherwise.
 
Just a quick addendum....

This line of discussion got me motivated to have another go at balancing the carbs. This time I plucked up the nerve to cut out a small notch on the Gunson Carbalancer to accept the air filter post so that it fitted snug into the carb inlet. What a difference! Perfectly balanced carbs in less time it takes to write this.

However a subsequent road test out on the open road at 60mph [the first since a recent cylinder head rebuild] showed ugly signs of very lean mixture on hard acceleration. Back in the shed, the idle mixture was puzzlingly just fine. The problem was traced by hearing a loud hiss from the brake booster when the engine was switched off: the rubber grommet that accepts the vacuum pipe non return valve had split on its inner circumference after 43 years of service allowing extra air to be drawn into the carb under high vacuum running conditions.

Apart from the hiss, the dead giveaway of a duff sealing grommet is that the non-return valve is so incredibly loose that a simple tug is enough to remove it from the booster - it should be a really tight, hard fit.

Big ups to Nelson Brake Services who supplied a new grommet and [what the heck] non-return valve off the shelf.

Barry Cade Hint: When installing a new grommet, use rubber grease or brake fluid to lubricate the non-return valve if not, the chances are that the grommet will suddenly pop out and fall inside the booster necessitating the booster's removal to fish the jolly thing out [yes - it happened on the day]. Hello brake bleed job!
 
Congratulations on a successful afternoon's work, Keith!
It's great when a plan comes together - especially when previous attempts have yielded very little.
I had exactly the same issue with the servo vacuum hose as few years ago. The jubilee clips hides all the cracks and damage quite well doesn't it!
 
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