SU HS2 idle speed gremlin

[PeteStupps]

Hi all. I've got a 1300fwd which kept finding new and exciting ways of breaking down in 2019, reaching its zenith on boxing day (see below for ref). 

Well now it's got a new (old) engine running twin SU HS2's because I broke the Stromberg... It's been running nicely and I made the mistake of feeling pleased with myself.

On Saturday morning however, on a 10-mile trip through town, the idle speed started creeping up as temperature increased. Expected something simple like a snagged throttle cable or sticky linkage - but no, everything was moving freely. Choke fully off, throttles fully closed, idle screws adjusted fully 'out'. But idling at about 2000rpm... 

Is it most likely an air leak? Any other possibilities? I drive it every weekend and it's abruptly started doing this. Two weeks ago plugs were all nice biscuit brown, and yesterday they looked the same. 

I put brand new spindles in but it made no difference. With engine running I removed the idle adjustment screws and pressed the throttle linkages to make sure they were fully closing, but still it raced on. The throttle discs have got those daft pop-valves, which make me suspicious so I've ordered plain ones. 

Any suggestions gratefully received. 

 

 

[PeteStupps]

I should add, it's got the later type carbs with engine-breather intake ports behind the venturi. These are blanked off and seem to be airtight. Removing one blanking plug made the engine stall. No breather on the manifold. Fixed jets and fixed needles. 

Ooh another thing: I sprayed brake cleaner on the exposed end of the front carb's throttle spindle (if that makes sense): this brought engine speed down temporarily, which I wasn't expecting. So there is a leak at the end of the throttle spindle, but if I stick my podgy finger over it to block it off, that has no effect at all. 

Edited March 1, 2021 by PeteStupps
More detail

[Nick Jones]

Poppet valves in the butterflies must be prime suspect, though there could be a small manifold air leak.

Is there indication that two of the cylinders are doing most of the extra work? You’ll probably be able to hear that one carb is pulling more if you take the air filters off (or use a carb balancer if you have one). This would prove the poppet valve theory. Otherwise suspect a leak nearer the engine.

[PeteStupps]

Reckon it was the front carb; I took the dashpot lids off and forced the pistons down in turn, which seemed to reveal that the front was doing most of the work. I'll have play with it tomorrow evening and see if there are any other clues.

What is the actual purpose of the poppet valves anyway? Presume it's emissions-related but can't see how. Especially not if they provoke 2000rpm idle...

[JumpingFrog]

Found this:

Quote

Another change was the fitment of "poppet valves" on the butterfly spindles.  This was designed to reduce the amount of fuel settling on the walls of the inlet manifold when the throttles were snapped closed as you took your foot off the pedal.  As the throttles opened again, the unburnt fuel would result in poor emissions.  To counter this the poppet valves would briefly open as the throttles closed allowing a small amount of air through to prevent fuel condensing.

and also this:

Quote

When the throttle is closed suddenly with the engine at higher rpm a massive and sudden vacuum is created within the induction system down stream of the butterfly. This instantly draws any fuel stuck to the port and manifold walls into the cylinders causing a short burst of extreme mixture richness. The consequence is high exhaust emissions. The valve rectifies the problem by opening at these high vacuum periods by reducing the vacuum and bleeding extra air into the induction system creating a more oxygenated burn resulting in far lower emissions. 

 I believe the second explanation more, I don't see why an engine on overrun would otherwise result in high emissions, given the current trend towards lean burn engines to reduce emissions?

Edited March 2, 2021 by JumpingFrog

[Nick Jones]

It is emissions related and supposed to prevent very high manifold vacuum. Not sure what the actual benefits achieved are. Unless it’s the belated satisfaction of causing the Nth owner a headache when the bloody things eventually fail. They can also reduce engine braking.

I treat them like locking wheel nuts- remove and throw far away.

 

swiped from the MGFexperience

 

According to http://www.mginfo.co.uk/upgrades4mgs/Engines/tuning_the_su_carbs.html

the purpose is 
 

Quote:Another change was the fitment of "poppet valves" on the butterfly spindles. This was designed to reduce the amount of fuel settling on the walls of the inlet manifold when the throttles were snapped closed as you took your foot off the pedal. As the throttles opened again, the unburnt fuel would result in poor emissions. To counter this the poppet valves would briefly open as the throttles closed allowing a small amount of air through to prevent fuel condensing.

 

[Nick Jones]

7 minutes ago, JumpingFrog said:

I think I believe the second explanation more, I don't see why an engine on overrun would otherwise result in high emissions, given the current trend towards lean burn engines to reduce emissions?

Agreed. Meshes with discussion I’ve seen on the MS forum about work done by Ford to quantify and control the amount of fuel lingering on port walls, with the aim being to keep it as constant as possible through throttle transitions. In injection systems it’s controlled by the acceleration enrichment settings which are hard to get right!

Means that the link I posted above has it backwards

[PeteStupps]

Ah, that's a good explanation thanks @JumpingFrog and @Nick Jones. The 2nd version seems more consistent with my hazy memory of pressure vs boiling point, and is worth remembering if I ever get round to fitting an AFR gauge.

Would you believe, I had noticed the lack of engine braking! Very glad to have a possible solution for it. And the new plain butterflies are in the post, so will soon find out if they're the culprit

[PeteStupps]

An update: plain throttle butterflies from Burlen were fitted Friday night, although not before having to tidy up some rough edges on the screw holes which fouled on the spindle when trying to slide them into place. Foolishly hoped that two small brass discs costing £21 + postage might have been finished to a reasonable standard 

Did it work?

Emphatically yes! Gold star for Nick Jones' remote diagnostics  

In fact the poppet valves were causing more of a problem than I realised because it almost felt like a different car. Whereas before it would stall embarrassingly often in stop-start traffic, today it was rock solid with a regular even tickover somewhere around 600-800 rpm. And such a difference to have engine-braking again.

All the bad rumours you've heard about poppet valves are true

[Nick Jones]

11 hours ago, PeteStupps said:

All the bad rumours you've heard about poppet valves are true

Presumably they worked better when new.....

some of the better running may because you’ve been able to set the carbs up properly for the first time with no pesky internal (and undetectable) air leak.

Good news anyway. Let’s hope you get some well-earned trouble-free service from the car now!

[PeteStupps]

On 3/7/2021 at 9:50 AM, Nick Jones said:

Good news anyway. Let’s hope you get some well-earned trouble-free service from the car now!

Thanks Nick, sadly it wasn't trouble-free for very long at all. This car...

Clutch plate seems to have broken, which is odd because it's the 2nd time it's happened

[Nick Jones]

Ah bollocks......  My apologies for the jinx 

So long as it’s not that funny little quill shaft snapped.....

We await the pathology report....