Rover P5b HIF Carburettor Modifications

[Deleted]

My P5b has the HIF6 carbs with integral float chambers. The throttle flap on each has a valve in it to presumably reduce the manifold depression on overun.
I'm sure that some while ago I read that there is an improvement in the engine running to be had by removing these valves and soldering up the holes. I suppose if nothing else removing them must improve the flow through the venturi. Has anyone tried this or does anyone remember reading something similar?. I keep looking for an excuse to take the manifold off as it could really do with a lick of paint.

[enigmas]

Hi Kev. My MG Magnette V8 coupe has a Rover SD1 engine fitted to it. It runs the autochoke Zenith Stromberg carbs which were also fitted with the overrun disc valve on the throttle butterflys. The aim was to lessen an overly rich mixture during closed throttle engine braking situations. I soldered them shut over 20 years ago...as was the practice after the cars warranty period expired.

Engine braking with these valves in operation is quite disconcerting.

The valves were implemented during the early stages of emission controls along with exhaust air pumps and all the other add on paraphernalia that ruined/compromised engine performance during the late 70s - 80s before EFI was properly sorted.

The sensation when driving a vehicle with this system in place, is that the throttle doesn't fully close and that engine braking never feels right. The springs also lose tension over time compromising other aspects of throttle control.

[harvey]

Feb 22, 2016 7:52:12 GMT @baconsdozen said:

My P5b has the HIF6 carbs with integral float chambers. The throttle flap on each has a valve in it to presumably reduce the manifold depression on overun.
I'm sure that some while ago I read that there is an improvement in the engine running to be had by removing these valves and soldering up the holes. I suppose if nothing else removing them must improve the flow through the venturi. Has anyone tried this or does anyone remember reading something similar?. I keep looking for an excuse to take the manifold off as it could really do with a lick of paint.

You can either replace the butterflies with plan ones, or heat the solder on the poppet valve with a soldering iron, screw it up until the spring is coilbound and the poppet won't open, and then solder it again to keep it all in place.

The poppet valves were for emission control, but a lot of the time the spring wasn't strong enough, so the poppet would stay open when returning to idle, artificially raising the idle speed beyond acceptable limits. I soldered up loads of them under warranty......

[Deleted]

Thanks for the replid chaps. I intend replacing the flaps with plain ones. A couple of times I found the engine braking felt suspect, maybe one or both valves are reluctant to seal.

[Deleted]

All full of enthusiasm I took the carbs off and dismantled them. The overrun valve projects into the venturi quite a bit and must upset the air flow if nothing else.



I dismantled the valve,removed the spring and rivetted the valve back together to block the hole then soldered it. The valves both had black stains around their edges on the downstream side,pssibly showing that both had been leaking.

[gingerbeer62]

Feb 27, 2016 9:18:14 GMT @baconsdozen said:

All full of enthusiasm I took the carbs off and dismantled them. The overrun valve projects into the venturi quite a bit and must upset the air flow if nothing else.



I dismantled the valve,removed the spring and rivetted the valve back together to block the hole then soldered it. The valves both had black stains around their edges on the downstream side,pssibly showing that both had been leaking.

My goodness, what a rubbish design that was! I can see what it is supposed to do but if anything drops off - where would it go!! The piston it gets involved with would love it !
Very Wise to solder it closed.
Cheers
Colin

[Deleted]

When you think about it an automatic can't really overrun like a manual car so the valves seem pretty irrelevant.
With the carbs and the intake trunking off,I used a Dremel like tool to grind out the casting flaws and smooth off some rough edges.Whilst doing so I was reminded of WO Bentley who talking about his mechanics polishing the heads and manifolds of his racing cars said "It doesn't really do anything but they seemed to like doing it". That pretty much describes my activities today.

[enigmas]

If it makes you happy Kev...just keep on doing it.

[Deleted]

I was pleasantly surprised with the engine all back together and running. The tick over was nice and even with the adjustments just on their initial settings. Probably the extra care in setting the butterflies etc helped. I've not taken it out on the road but the engine seems to pick up well from idle. Maybe I'm deluding myself but it kept me out of mischief for a few hours.

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[Deleted]

I just came back in from a test run,the performance on kick down is noticeably improved and as before its easier to get a nice steady tickover (presumably because there is no air leaking round the valves.
I had thought of fitting plain valves as the soldered shut valves must intrude a bit into the venturi but I doubt if there would be much difference.
For the sake of a bit of fiddling,a worthwhile modification.