High Idle Speed

[Joss_D]

I bought a copy of Autosport off eBay which has an article on a Taurus tuned 998 Mini, it describes the engine as not threatening to stall until it drops below 200rpm. This has (rather annoyingly) reminded me how shockingly high my idle speed is. What could I do to fix it?

Engine spec:

pre A-plus 998cc block, bored +20 thou
12G 295 cylinder head (ported/polished/skimmed)
Twin HS2 carbs (blue springs and M needles)
Standard airbox
Maniflow 3-1 exhaust manifold, single box RC40
Metro rockers
Kent Cams 266 camshaft
Lightened flywheel

Everything else in the engine is standard. The one problem with it is the idle speed. It sits between 1000 - 1200rpm, the fact that it's sitting on a standard 850 3-synchro box makes it even more difficult to live with. I'm surprised first gear still has teeth.

I took it to a fairly well known rolling road in the Lake District a couple of years ago, but was told there wasn't much that could be done about the idle speed. Wouldn't it just be a case of changing the needles/springs and getting it tuned again?

I've not worked on the carbs in a while, so my terminology might be a little out; but the screws that adjust the position of the throttle linkage are already fully screwed out, so there's no room for adjustment there.

[Smiffy]

I bought a copy of Autosport off eBay which has an article on a Taurus tuned 998 Mini, it describes the engine as not threatening to stall until it drops below 200rpm. This has (rather annoyingly) reminded me how shockingly high my idle speed is. What could I do to fix it?

Doesn't need fixing, all have a idle speed that low does. Is wear the cam out.........

[1071 S]

Is the needle properly seated in the piston??? In any case with the adjuster fully out the butterfly should be shut..... Too many air leaks??? Springs don't really do anything near idle (unless there's something REALLY wrong) - and all needles are pretty much the same at idle...

Btw - if it crashes going into first just drop it into second and then go for first. Pretty much standard practice for those of us who grew up before they invented 4 synchro boxes.....

Cheers, Ian

[johnsimister]

Can't help with the idle speed other than to reiterate the possibility of air leaks somewhere - PCV valve or associated plumbing if you have one? - but I'd be very interested to know what power your engine produced on the rolling road.

Reason for asking is that my supercharged '59 ceased to be supercharged this morning when its Shorrock blower self-destructed. The rest of the engine spec is broadly as yours, but it has bigger-than-standard valves in its 12G 295 head and the capacity is 910cc (it's an overbored 850). I'll be fitting twin HS2s this weekend, which I have been keeping for exactly this moment when the Shorrock time-bomb explodes.

The 'charger will get rebuilt provided the carnage isn't too severe, but meanwhile I'm looking forward to the sweet running of a twin-carb, naturally-aspirated, small-bore engine.

John

[Joss_D]

Doesn't need fixing, all have a idle speed that low does. Is wear the cam out.........

I don't want an idle speed of 200rpm. Just one less than 1200rpm would be nice.

Is the needle properly seated in the piston??? In any case with the adjuster fully out the butterfly should be shut..... Too many air leaks??? Springs don't really do anything near idle (unless there's something REALLY wrong) - and all needles are pretty much the same at idle...

Btw - if it crashes going into first just drop it into second and then go for first. Pretty much standard practice for those of us who grew up before they invented 4 synchro boxes.....

Cheers, Ian

As far as I know the needles are seated properly. The wider part that slots into the piston certainly isn't clear of the piston surface, anyway.

I have tested it for air leaks in the past, and not found any. I might buy a new gasket set just to be on the safe side. And yeah, I've taken to putting it into second before selecting first. It's just a little frustrating, especially in standing traffic when it tends to overheat.

Can't help with the idle speed other than to reiterate the possibility of air leaks somewhere - PCV valve or associated plumbing if you have one? - but I'd be very interested to know what power your engine produced on the rolling road

Is the PCV valve that round thing that sits in front of the air filter between the two carbs? If so, it doesn't have one; the two holes on each inner side of the carb have been blocked off.

It put out 44hp at the wheels - so about 65bhp.

Was that the one at Kirkbride?

No, it was in Penrith.

[Spider]

Have you had the carbs off and checked that the butterfly does actually close right off? If the butterfly(s) become worn (and they do wear) or have been replaced and not centralised or even if there is side thrust for the linkages, it will prevent the butterfly(s) from closing that last crucial bit.

Best check with the carb(s) off, wind out the idle screw, remove the bell & piston and hold them up to the light.

[abri]

I had a similar problem on my car, well, sort of. It would not idle evenly unless the revs were about 1200rpm. Try as I may to set the carbs up properly balanced and tuned, no luck. Then one day I took her for a run and all of a sudden find myself running on three cylinders. Fearing the worst, I searched all possible causes until I found a small piece of dirt in one of the jets under the needle. With that removed I re-tuned the carbs and got an even idle speed of 800-1000rpm depending on cold/hot.

Not sure if this is of any help to you, but you may check that the carbs are balanced properly (or connect the two blocked off balance pipes with eachother, perhaps)

[johnsimister]

Yes, PCV valve is the round thing between the carbs.

That's an encouraging power output. On that basis I'm hoping mine might make around 60bhp with its smaller capacity, once it's set up properly.

Thanks

John

[Smiffy]

No, it was in Penrith.

Minimaster not there anymore, they shut up shop a year or 2 ago.......