SU HIF38 question

Don't worry folks, I already have the answer. It's an overrun depression valve and not really necessary - fitted to clean up exhaust emissions but probably not very effective. I'll get a plain disc.

It also restricts the minimum idle speed which can be a PITA with the Rover V-8. You can simply solder them up if you can't find a 'blank' butterfly.

It only operates on the over rub to reduce emmision BUT don't solder it up as on a fwd it reduces shock loads on the en gine mounts when you lift off the throttle.

Wonder how the countless FWD cars made before it arrived survived intact?

I replaced then with plain butterflies on my P6 3500S Rover. It made no difference when snapping the throttle closed, but allowed a slower smoother idle.

Over-rich idle mixture in those fuel-crazy days, and not closing the butterfly completely.

It really makes a difference to setting up the idle. I was never sure if this was because it allowed a smaller opening, or because it was such a pig of a job to balance the carbs when those valves could get sucked open while you were still twiddling.

-- Die Gotterspammerung - Junkmail of the Gods

With something like an SU, the idle mixture was about the only bit likely to be correct. ;-) And of course the butterfly (or idle bypass) has to be open slightly - for idle.

IIRC, snapping the throttle closed caused a vast output of hydrocarbons. So many makers fitted a throttle damper to slow this down and help reduce this. But it wasn't for any mechanical reason.

They would open slightly under the high vacuum conditions of idle. Ideally, they would only open under the high vacuum conditions of a closed throttle at running speeds.

Engine steady bar rubbers and exhausts on Minis lasted longer after they changed the carb

On twincarb MGs the set up procedure is different when the valves are fitted at least on MGs the traddional free play in the linkage between the front and rear carb was done away with after they changed the carbs. The story was the gap was there to stop halfshafts breaking on lifting off the throttle which was quite common on pre MGB MGs Your smoother idle was more likely down to overhauiling the carbs.

Hmm. The torque reaction which had to be absorbed would be greatest on a fast start or snatched gearchange rather than lifting off - which in any case would usually be in a high rather than low gear - few would be hard on and off the throttle in a low gear in normal driving. And IIRC, the design of the rubbers was changed several times in the life of the Mini. One of the first things I did to my early ones was to fit the cone kit which stiffened them up - and keep a careful eye on their condition. And I can't remember ever breaking an exhaust.

The snag with the transverse layout is the exhaust can't accommodate much movement - on an inline type it can rock with the engine.

That's a new one on me. I understood the delayed opening was to provide easier throttle control at low revs. And it was done away with purely on emission grounds.

All I changed was the butterflies for plain ones after reading an article in the club mag. The poppet valves used to flutter slightly giving the same sort of result as unbalanced carbs. Also, V-8s tend to have a slight rocking motion at certain idle speeds which can be made less obvious by either increasing or slowing the idle. I ended up at a perfectly reliable

500 rpm. Of course some gearboxes would be very noisy at this speed, so it was the luck of the draw. It also allowed you to go down to near walking pace in top gear without snatching - quite a party trick.