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MG Midget and Sprite Technical - Engine running on 2 & 3 only

Hi all,

Mini engine, 1100, nicely tuned, all new. First start up, sounds like it's running sweetly but a bit of a putt putt from the exhaust, we thought it might be missing. So we try pulling the plug leads off, turns out it's not firing on 1 or 4! Took the plugs out, 1 and 4 are spotless as well as the piston crowns, 2 and 3 are sooty. Checked for sparks, 1 and 4 have a good spark, it's got a new leccy ignition setup on it.

I thought it's just running too lean as it needed full choke. We did find a problem with the dashpot sticking up, we think the needle is rubbing on the jet (not sprung), but even with that sorted it's still missing.

I'll ask Andy to put a pic up of the needle. We couldn't get it out of the dashpot to check the code, but it looks lean to my untrained eyes.

Any suggestions? Am I right in thinking that its just too lean? My thinking was that 2 and 3 are nicking all the fuel. Sounds logical to me anyway.



Andy Barrass

I don't really want to ask this question, but...are the plug leads on the right way?

You might be getting sparking plugs, but not necessarily at the right time.
Dave O'Neill2

Wow... id say thats a fat lean needle to me also

Are you sure yout getting spark at all 4 plugs

Are you valves set correctly

That is really strange...I hope that dosnt happen to me today

Any exhaust blockage by chance...(spaghetti agianst the wall)

If you dont mind the risk of trashing the needle...danial stapleton has an excellent chapter on how to re-profile the needles, I plan on that trying it ....but I think its easy enough to give it ago

Heck he might even give up some non published tips just for the asking

That needle would be the one to try... I think his book is now a download thur amazon

Prop and the Blackhole Midget


I don't know about with SUs but with side draught carbs it is quite common to see no difference in running when 1 and 4 plug leads are removed due to the uneven mixture distribution. Removing plug leads when an electronic ignition module is in use can also damage the electronics as IIRC the energy stored in the coil has no spark gap to jump and so the coil primary sees higher voltages which can damage the electronics.
David Billington

Thanks guys, I can't think how I would have 2 and 3 in the right positions and it still run, although thinking about it, 2 and 3 could be right, 1 and 4 wrong and we've got the rotation of the dizzy wrong as well. Andy is away this weekend so will check the positions properly mon/tues.


I do have a colorful T shirt that says across the front in neon yellow that 1432 will not run, very embarrassing...LoL

Your welcome to barrow it till you have earned your own


Prop and the Blackhole Midget

Yes, 1342 - anticlockwise.
Dave O'Neill2

yes I know 1342, come on!

That was just for the benefit of the archive ;o)
Dave O'Neill2

In many decades of working on A-series engines I've never known a carb problem to result in 1 and 4 not firing or, indeed, any particular cylinder not firng. If you have spark and in the corect order then your next stop should be valves and/or valve timing.
Nick Nakorn

"all the right sparks - but not necessarily in the right order"
David Smith

Dave O'Neill2

"In many decades of working on A-series engines I've never known a carb problem to result in 1 and 4 not firing or, indeed, any particular cylinder not firng."

David was referring to what happens when you have a lumpy cam and siamesed ports - you get a 2 cylinder idle. Happens on B series engines too. (Though I thought it was the other way round, but often manage to mix it up!)
Occurs because of the high vacuum under idle combined with valve overlap in the siamese port, results in one cylinder of the pair robbing the charge out of the other just before the valve closes.
Paul Walbran

Are you sure that 1 and 4 are not firing?
Try using one of those infra-red spot temperature guns to check that the temperatures are consistent across all 4 cylinders. Sometimes just pulling a lead doesn't really seem to make much difference anyway and it can be hard to be certain.
Guy W

Your reading my mind agian guy.

Prop and the Blackhole Midget

Paul, I think we kind of agree - it's semantics. Your 'lumpy cam' would be my 'valves or valve timing'; I wouldn't call that a carb fault if the cam is too 'lumpy' to idle. But, I have to say, it's not something I've ever experienced. Siamesed ports shouldn't cause a problem as the carb is only supplying one cylinder at a time anyway but one mod that helps smooth things out a bit is to un-sharpen the divide between the ports. We would normally do that before thinking about the cam anyway.
Nick Nakorn

As well as no change when the leads were pulled off, 1 and 4 plugs were spotless and shiny as well as the piston tops, whereas 2 & 3 were sooty. Maybe the sootiness is as a result of needing to have the choke fully out all the time.

We need to check the leads again properly, was late the other night when we discovered it.

I have only seen the effects of charge robbing to the extent of not firing on 1 and 4 on engines running Weber sidedrafts. Quite often we get calls asking about cause of engine only running on two cyls problem, caller surprised when we ask if 1 and 4 not firing and Weber sidedraft. It is the characteristic 'gurgle/chuckle' of weber sidedraft on a or b series.

I think a compression test first, then tappet clearance check. Go through ignition system with fine toothed comb. Check for air leaks on manifolds etc etc. We did have one dizzy which would not work on all six cylinders, could rotate the ' magnetic trigger' and the misfiring cylinder would change!!!! Cured by points till the customer bought a new electronic conversion kit. Never got to the bottom of the problem, all six magnets were in place so cannot see why hall effect would not work, but it must have had a problem with one magnet.

Useful way of checking for manifold leaks is to spray penetrant around the carbs/ manifolds with the engine running.

Peter Burgess Tuning

Peter, thinking about it all the cars I've seen with cams with enough duration to cause this were running Webers. (Almost. In 1985 we had to run SU's to meet the regs in Rally of NZ, along with a Ford A6 cam profile as all the usual BMC profiles had too much lift for the regs. Only it was so long ago that I can't remember much about the idle. Not that it idled for very long or very often anyway!)
Paul Walbran


Peter Burgess Tuning


You may have stepped on that one concerning ignition drift

I had that samething happen with the pretronix chip, it was advancing by its self without ever of the theories I had to why I blew the HG

Theres a term for it, but I cant recall it...

"Phazing" ... it took a few minutes...

But I went back to P&C, and not going back to electronic ignition

Prop and the Blackhole Midget

Problem solved, dizzy 180 deg out! Can't believe it would still fire up.

its always the stupid things that get thur the cracks


Prop and the Blackhole Midget

180 out, didn't you know, c'mon! :)

do you regret any tetchy reply now(?) :D

these engines are ancient and quite simple and it's often the most basic things that are overlooked but someone usually suggests dissy 180 out

ah, thinking about it, you did ! :)

well done on getting it sorted anyway
Nigel Atkins

Some versions of Haynes manuals for midgets, Minis and even Morris Minors, give incorrect information about fitting the dissy drive gear. Don't ask me how I know.

Now, whenever I rebuild an engine, I set it so that No 1 is at TDC on the firing stroke, roatate the dissy (not fitted) until the rotor arm points where No 1 should be, then fit the drive gear to match the drive dog. Simples.

Later Haynes seem to have corrected the error.
Dave O'Neill2

there are also some other errors and omissions in earlier Haynes that are not corrected in later versions
Nigel Atkins

Tarquin, you've put a mini a-series in the porker, for better fuel consumption? ;)

Or did you buy Jims mini?
A de Best

Hi Tarquin,

for your megajolt... how is it set up ? ive been reading about it and there's a lot of ways to do it, crank trigger, wasted spark, EDIS etc.....
just out of interest......

Using wasted spark/EDIS system there would be no distributor at all....
Andy Phillips (frankenfrog)

Hi Andy,
whereabouts in Thailand are you?
Nick Nakorn

I work at laem chabang near pattaya during the week but live in suphanburi north of bangkok
Andy Phillips (frankenfrog)

Wonderful; that's a fair old drive back and forth! I've only been to Supanburi once and loved it - nice looking little town. My dad took me around the Summer Palace a few years ago. I saw quite a few classic Datsuns, a few Minis and some old Austins in BK near my family's place in Tippawan but no MGs, is there a good MG/classic club in Thailand do you hapen to know?
Nick Nakorn

Hi Nick, i'm in the vintage car club of Thailand, we have about three official events during the year, the biggest one is a run out from bangkok to hua hin every year in december.

there is also the classic car club and that i think is more active, they meet in bangkok every sunday morning, but i live far away from that..

There are a few MG's about but you never see them on the road, my friend has an MGA, and i have seen two midgets and one other frogeye.

Best place for old cars in Thailand is the jesada museum in Nakorn Chaisri, lots of interesting things there...
Andy Phillips (frankenfrog)

Must be great to be able to bomb about in Thailand in an old banger. I had some time in Hong Kong who I was younger. Never had a car there tho which I regret. There were some amazing roads as so much of Hong Kong is national park. Maybe when we finally get back there for a holiday we will hire a car.

like I said, it was late when we thought it was missing and decided to look into it. Too late to start pulling the dizzy. And we didn't want to jump in without thinking about things, been there done that.

Arie it's Andys inno. Have put a video on Facebook but you can't see it can you? Unless you look on Riannes account.

oh yes Andy, not my car. My midget had the works, wasted spark etc. Fantastic idle, revved brilliantly. 105 hp in the end. Only cost a couple grand (ish) to get there though :-)

Its the devils newest toy so I stay clear from that, unles it has p*rn?
It must have, why else would it have so many followers. :)

" 105 hp in the end. Only cost a couple grand (ish) to get there though :-) "

And six years... ;)

Good to hear Andy has his Inno back, great mini that was, and fast!!

A de Best

This thread was discussed between 14/03/2014 and 24/03/2014

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