SU Carbs-Idle Speed

ALAN COCHRANE · Post by **ALAN COCHRANE** » Sat Jun 08, 2013 2:58 pm

Hi Everyone

I've returned to one of my original topics-high idle speed. I have set the idle speed to approximately 900rpm which is higher than the workshop manual recommends. The low volume screw on the rearmost carb is screwed onto it's rest to achieve this so I can't set it any lower.
I've managed to set the jets and balance the carbs so everything else seems OK.
When you press the accelerator and then lift off the revs refuse to drop below 1200rpm unless you hook your foot under the accelerator and lift it up. The throttle discs don't appear to move when you do this but the idle speed drops back to 900rpm.
I've checked and lubricated the throttle linkages and they seem OK. When you pull up the accelerator the U shaped linkage moves onto it's rest on the servo bracket, otherwise it is about 5mm shy of it. I've tried various adjustments but nothing seems to sort it. Can anyone give me some advice on how to cure this?
I have also noticed that anything more than gentle acceleration under load produces a brief rattle from under the bonnet. It sounds like the carb pistons rattling on their bridges before lifting. the noise dies away after a couple of seconds once the revs build up. I've never experience pinking before but having trawled the interweb, the symptoms described don't sound the same.
Any advice gratefully received.

Cheers

Alan

Moeregaard · Post by **Moeregaard** » Sat Jun 08, 2013 4:30 pm

Alan, I'll assume that your timing is correct, so here are the items I've found that can cause this problem:

1. Weak or broken carburettor return springs.

2. Incorrectly set throttle linkages. There should be some play in all three
linkages, per the workshop manual. If any one of them is set incorrectly,
that carb will not fully return to the idle position.

3. Fast-idle linkages binding where they pass through the carb body. I've
experienced this problem, since I rarely used the choke and the brass
pushrods became stuck in the alloy carb body.

From what you've described, your rear carb is not fully returning to the idle position. If the spring is not the culprit, I would verify that the linkage to that carb is properly set to allow it to close completely.

PeterCrespin · Post by **PeterCrespin** » Sat Jun 08, 2013 4:45 pm

Mark is right. And just because the throttle return springs are new doesn't mean they are the same tension as original SU springs, which are made of a heavier gauge wire than at least one eBay vendor's stock. But if hooking the pedal upwards settles things it also may be that the main linkage is binding or incorrectly adjusted. You don't mention RHD/LHD but they have similar issues.

The engine is basically an air pump. Cut off the air and it will stop. Give it air and it weill speed up. Find out where it is getting air from if not the idle circuit. Did you rebuild the carbs? Are the throttle plates correctly centred on the shafts? Getting a good idle does not involve much brain power but it can involve a lot of detailed working through from pedal to port, to see what's not up to spec. Idle is where maladjustment shows up, once off idle the small imbalances or leaks are swamped by the mass of air being pumped through the engine.

Don't know what research you did but your symptom appears identical to pinking and should be assumed to be pinking until you find another cause. Check your timing with a strobe and back it off a fraction at a time until you no longer get pinking, then forward again until you do, just to check. Then back again to the non-pinking position and re-strobe for future reference on that fuel grade.

Pete

christopher storey · Post by **christopher storey** » Sat Jun 08, 2013 9:27 pm

The first thing to realise with an E is that the pedal should NOT govern idle at all . The idle circuit should be modulated solely through the three large idle screws i.e. the throttles should be completely shut at idle, and air should be admitted solely through the idle circuit . It is common for the rubber lozenge shaped gaskets at the base of the 3 large screws to be perished, and in those circumstances it is impossible to get a proper idle . If the pedal is affecting idle, as your post suggests it is, then something is sticking or out of adjustment. Study of the carbs section on the knowledge base will deliver big dividnends . These carbs, although relatively simple and extremely reliable in terms of physics , are actually quite complex mechanically, and ALL the mechanical parts have to work without sticking for the full benefits to be realised

ALAN COCHRANE · Post by **ALAN COCHRANE** » Sun Jun 09, 2013 5:11 pm

Thanks for all the suggestions as to my carb/pinking problem.
Mark go to the top of the class. I disconnected the fast idle linkage to the rear carb and pushed it fully open and then released the brass lug, only for it to stay wide open. The throttle spindle was jamming in the carb body. I'm not sure if the return springs are original or not but I replaced the rear one with a spare higher tensioned one from my Triumph GT6. Bingo! it moved back to fully closed very smartly.
I re-balanced the carbs and used my Gunson's Colourtune to set the mixture and the "hot" idle settled to a very nice and even 950rpm. Flooring the accelerator and lifting off showed the revs dropping back to the 950rpm mark without hesitation.
I tried to check the timing but there doesn't appear to be any timing marks on the balance wheel, although the indicator is there-has anyone else come across this before?
I'm hoping all these adjustments will also solve the pinking problem without resorting to changing the timing. That will have to wait for tomorrow though as we're now enjoying the lovely weather and having a barbeque.

Thanks Again

Alan

Moeregaard · Post by **Moeregaard** » Mon Jun 10, 2013 12:10 am

Glad I could help, Alan. Your idle speed still seems a little high, which leads me to suspect that excess air is still getting inside when the throttle is closed. If it's a 4.2 (heavy flywheel), 600 RPM is not unrealistic if the throttle shafts aren't worn. 3.8s I believe are set closer to 750 RPM. Were you able to adjust the air-bleed screws to the factory setting? It sounds like you've addressed the major culprit behind the problem anyway.

Barbecue happening at this side of the planet as well :D

ALAN COCHRANE · Post by **ALAN COCHRANE** » Mon Jun 10, 2013 9:39 am

Mark

My E is a 3.8 so you're right the idle speed is a little high at 950rpm although this is with the engine fully warmed up. It idles at about 750-800rpm at start-up. It's not happy at anything below this. I tested this by fully depressing the brake and it was not a happy chappy when the servo dumped it's load of air and the revs dropped to about 550-600rpm.
I think the carbs will need an overhaul, but that'll have to wait till Wintertime. The rear carb slow running volume screw is almost on it's stop so that needs further investigation. In addition the float chamber seems to leak fuel so that's next on my list to fix. I'm not sure if the return spring or the the throttle shaft was at fault. The spring looked in good condition but as I said before it is of unknown vintage. There was some resistance when I pushed the throttle
closed using the external brass lug so on balance the shaft seems to be the prime suspect.
The pistons were quite dirty with black deposits on the main body.
I've only owned the car since October 2011 and although the engine was rebuilt in 2006 I'm not sure if the carbs have ever been touched.
You mentioned air bleed screws? Do you mean the fast idle screw? I have to confess I haven't but I'll do that today. I conveniently missed that section out in the manual. I was busy doing my victory jig after sorting the idle problem.
My GT6 has been running on twin HS6 SUs for 25years and I've never experienced any of these problems during this time. Both carbs were sourced from a scrapyard and I had to cut down the dashpot necks and dampers to allow the carbs to fit under the bonnet. Maybe I've just been lucky!

Thanks Again

Alan

Post by **Heuer** » Mon Jun 10, 2013 9:58 am

Alan

When you need your SU's rebuilt I can highly recommend David Lonsdale at Classic Carburettors - he does a superb job.

Moeregaard · Post by **Moeregaard** » Mon Jun 10, 2013 12:33 pm

Alan, from what you've just described, I suspect worn throttle shafts. With the slow-running screw all the way down on its seat, it sounds like air is still getting in somewhere else and raising the idle speed.

Regarding leaky float chambers, the first thing I do when a set of SU carburettors enters my life is to get rid of the Viton needles and seats, and fit Grose jets. They're one of the cheapest forms of insurance--and much cheaper than a fire--you can buy. You may have already done this, but I thought it was worth mentioning.

PeterCrespin · Post by **PeterCrespin** » Mon Jun 10, 2013 12:50 pm

Funny. I did the opposite. I recently rebuilt a set of SUs and a Grosse valve was leaking sio I put the correct parts in and all is good.

No system is foolproof and since swapping to an alternative almost invariably coincides with a carb clean or rebuild, who's to say it wasn't the refurbishment that did the trick, rather than the change of spec in either direction

Pete

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