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Car Forum / UK Car Forums / Classic Cars (UK group) / October 2003SU HIF38 question
I am rebuilding my HIF38, and following Vizard's procedure for improving its flow. The throttle plate has a big pop-off valve in it, which must be a huge obstruction to the air stream. I have the Haynes SU manual but there's no mention of what this valve is for. It looks as if it provides an air bleed at idling, but I have not seen it on other SU types. Is it really necessary? > I am rebuilding my HIF38, and following Vizard's procedure for improving its > flow. The throttle plate has a big pop-off valve in it, which must be a huge > obstruction to the air stream. I have the Haynes SU manual but there's no > mention of what this valve is for. It looks as if it provides an air bleed > at idling, but I have not seen it on other SU types. Is it really necessary? 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. > 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.  Signature*If you must choose between two evils, pick the one you've never tried before Dave Plowman dave.sound@argonet.co.uk London SW 12 RIP Acorn > I am rebuilding my HIF38, and following Vizard's procedure for improving its > flow. The throttle plate has a big pop-off valve in it, which must be a huge > obstruction to the air stream. I have the Haynes SU manual but there's no > mention of what this valve is for. It looks as if it provides an air bleed > at idling, but I have not seen it on other SU types. Is it really necessary? 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. > 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. Signature*It's o.k. to laugh during sex?.?.just don't point! Dave Plowman dave.sound@argonet.co.uk London SW 12 RIP Acorn >Wonder how the countless FWD cars made before it arrived survived intact? Over-rich idle mixture in those fuel-crazy days, and not closing the butterfly completely. >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. 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 > >Wonder how the countless FWD cars made before it arrived survived intact? > Over-rich idle mixture in those fuel-crazy days, and not closing the > butterfly completely. 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. > >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. > 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. 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. Signature*I am a nobody, and nobody is perfect; therefore I am perfect* Dave Plowman dave.sound@argonet.co.uk London SW 12 RIP Acorn > In article <bnjso1$3d7$2@sparta.btinternet.com>, > Wonder how the countless FWD cars made before it arrived survived intact? Engine steady bar rubbers and exhausts on Minis lasted longer after they changed the carb > 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. 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. > > Wonder how the countless FWD cars made before it arrived survived > > intact? > Engine steady bar rubbers and exhausts on Minis lasted longer after > they changed the carb 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. > > 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. > 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 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. > Your smoother idle was more likely down to overhauiling the carbs. 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. Signature*Dancing is a perpendicular expression of a horizontal desire * Dave Plowman dave.sound@argonet.co.uk London SW 12 RIP Acorn |
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