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MG Midget and Sprite Technical - Frogeye won't idle

Newly rebuilt engine: bored to 998ccs; 10:1 compression ratio; flat-topped slipper pistons on press-fit rods, 12G206 (12G295) head; Swiftune SW10 cam; LCB and twin 1&1/4 SUs (AUD327 with AN needles); electronic ignition.

Runs lovely from 2,000rpm up, plugtips just the right colour. But can't get a consistent idle at anything less, engine rpm falls away, becomes erratic and stalls leaving plugs heavily sooted.

AN needle dimensions very close to V3 needle which is standard for a 948 on 1&1/4 carbs.

I was expecting some lumpiness at idle, but not this.

Any suggestions?

dennis
D Stanfield

Dennis,
Generally modified engines will need a richer needle - the V3 is actually weaker all the way up than a standard AN so if nothing else a change of profile is probably required. However to eliminate there is an air leak I would initially see if the engine will idle at tickover at a regular 1000 RPM. All SU needle profiles start pretty much in the same way so you should be able to find a happy mixture around 15 flats down using the V3's. If it can idle without drama it will be a case of finding which of the needles is the correct profile for you. If you look up the Teglerizer SU database you'll see the problems in suggesting a needle, but as a guess I'd have thought an E3 on a blue spring would get the car up and running but real answer is to get the car booked in to a rolling road which has a mixture analyser.
f pollock

f pollock. Sorry to come off thread but I have an article featuring your car getting a new fibreglass front. I think it's from the 1970's. Are you aware of it?


G Lazarus

First off apologies to Dennis on misreading his question. So my answer on needles was about face though the rest should be OK. As for the article on 117EPO - no I have never seen it. I bought the Frog as an ex development rolling shell from Oselli Engineering in London back in 1971, complete with the fiberglass front (but fitted) and the bootrack you see in the picture. Today it still has the Armstrong adjustable rear dampers though most of the rest has been rebuilt 2 or 3 times. I'd be very interested in a copy of the article - what a great surprise! My thanks.
f pollock

I will put it on flickr tonight so that you can download it.
I saw your car at Spridget50 and recognised the reg number (how sad is that!) but no one was standing around looking like they owned it. If I can find the original article I will sort out posting that to you too.
G Lazarus

The article.
Now I've just got to remember where I put the original magazine (I have a few).
G Lazarus

Oops

http://farm4.static.flickr.com/3087/2668782546_5a9e6ca8a6_b.jpg
G Lazarus

I tend to agree with F.P. A car that will not idle is generally due to extremely weak mixture. That can be induced by a couple of things including manifold leakage.
Another common cause can be burnt out valves or poorly adjusted tappets. If all else fails try a compression test.

Re reading have you ever been able to set up the carbs correctly or is this your first try without success?
Bob (robert) Midget Turbo

Gary,
Thanks for the article. Now I'm intrigued to learn which magazine ran the car. FP
f pollock

I (tend) to agree...BUT...

He make an odd statment >>>>>>> leaving plugs heavily sooted<<<<< He is running too rich it would appear...if he had an air intake leak, that would actually be a good thing, to thin out the fuel...well okay an air leak is not really a good thing...but you get my drift.


prop
Prop

Bob, weak mixture was my first thought, but the sooted plugs put me off that idea and, as Prop suggests, I reckoned it was running too rich. But no amount of weakening made any difference. Valves are refaced inlets, new exhausts, all properly seated, tappets adjusted per manual and compressions good on all cylinders. Ignition timing checked about 6 times.

I rebuilt the carbs with new spindles, and these seem to be operating correctly.

If I screw the idle screws in 2 or 3 turns and start on an open throttle she will run and pick up well but idles at 2,000rpm. I start to back off the idle screws and as soon as idle drops to about 1,800rpm she becomes erratic, idle drops away and engine stalls, sometimes, one in ten times, with a blow back of exhaust gases through the carbs.

Bob, re your question, previously engine was standard (but dying of old age) and no issues with carbs setup(1&1/8ths) but this is first time setup with all the new bits mentioned in my initial post.

dennis
D Stanfield

It might be worth checking out this site which discusses various cams, it would seem that some do not like to run much below 2000rpm. Yours is described as the modern equivelant of a 649 which would seem to be very lumpy
Carl Bintcliffe

And I suppose you might want the link!!




http://www.minifinity.com/distribution/viewtopic.php?f=5&t=10708&start=0

Sorry its late Carl
Carl Bintcliffe

Carl, thanks this is an interesting read. Maybe I don't have a problem, just need to accept that a high idle speed is necessary.

But I had a 649 in a 1293 mini for a while (too) many years ago and while it was lumpy I don't recall it being as bad as this. Maybe the lumpiness is accentuated by the smaller engine - per Vizard.

dennis
D Stanfield

>>>>Maybe the lumpiness is accentuated by the smaller engine - per Vizard.
=== definitely !
David Smith

Dennis what happens if you set the carbs up a idle? can you get it to idle well when you have them fully balanced and mixture set correctly?
Bob (robert) Midget Turbo

Have you tried advancing the ignition just a little?
Guy Weller

could be the carb is to big, or ....you need a longer smaller in diameter throat in the intake manifold....


prop
Prop

what about valves are they to small in diameter....did you put in bigger valves? if you stayed standard size...hard to say..., if thats the case maybe a 1.5 ration rocker assembly could help....granted more so at the top end...

prop
Prop

Dennis
The cam timings of the SW10 are not so extreme as to preclude a regular tickover. The numbers suggest it's quite a way short of a 649 or even the 544 - both of which can be set up to give a good 1000-1200 RPM idle on a 998cc. Check for an airleak with WD40 sprayed on the inlet joints when its stumbling and see if the revs pickup. Also do a full drop test - as installed - to check both pistons are falling and clonking down onto the bridge in unison and both air inlets are clear. Do both pistons start to chatter up and down as it struggles below 1800RPM. Does it run on or die cleanly? If everythig checks out one possibility is that the cam is advanced throwing all the timing out.
f pollock

Another thought while I'm still at it - there's been no mention of which distributor you've fitted. If it has the standard curve it will be wrong for the engine you have - your spec needs a yellow Aldon timed in around 30 - 32 degrees. FP
f pollock

That is why I suggested advancing the timing a little. If this enabled it to hold a proper tick-over then it might suggest that either it was incorrectly timed to start with, or that the advance curve was wrong for the specification of the engine.

Guy
Guy Weller

Thanks for all the thoughts and suggestions. It will be the weekend before I can get back to trying to sort this, but I've tried all sorts of carb settings and am beginning to think it might be the distributor which is an unknown unit that I sort of inherited but was told was suitable. May be time to think about an Aldon unit or similar.

One thought though, the carb slides move smoothly but don't actually come down with the customary click. I've 20/50 in the dashpots - should I try a lighter oil?

dennis
D Stanfield

Dennis,
Using a distributor with the wrong advance curve won't mess the idle up to the degree you've got - but will give too much advance from midrange - so its safer for the engine's longevity to use one with the correct profile. I once melted a piston running a 649 on a standard distributor so be prepared to spend a little money to fix the curve or you'll be rebuilding the engine prematurely. What you're describing on the carburettor will certainly give the crazy idle and with it the petrol wash, as the pistons and with them the needles are being held off the seats. What causes this is usually down to 2 or 3 things. Either the jets are not centered relative to the needle or the dashpot is not centered to the piston, or there is grit or dirt inside the dashpot preventing full movement. Using 20/50 is fine. I won't describe how to center a jet as set up details will all be in the workshop manuals and the archives, but what's imperative is that when you push up the pistons - and they are at the top of their stroke, they glide down smoothly and hit the bridge together with a definate clonk. And do it 5 times to ensure its consistant. If one is marginally faster than the other you can add thicker oil to equalise the drop speed. Then you can go about and balance and fine tune the idle. But equally important is to always fit a K&N to keep the intake air clean, an RC40 to help it breath and book a RR session once it's run in. FP
f pollock

dennis,

When I check for centered jets by listening for the click on the carb bridge, I pull the oil cap / rod out of the top of the dash pot so that the oil damping doesn't interfere. Under that condition, I do not raise the piston to the top of the stroke before dropping it.

Charley
C R Huff

This thread was discussed between 13/07/2008 and 17/07/2008

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