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MG Midget and Sprite Technical - Misfiring

Hello All.

Was out on Naviscat 10 days ago & this involves spirited driving with lots of stops & starts. 20 mile trip to start all OK. Into event about 15 miles & am getting misfire on brisk acceleration - at constant speeds it was OK.No sign of goo looking at oil filler cap so hopefully not start HGF. Mt thoughts turned to the coil of which I had retained a non blasted version prior fitting Magetronic ignition - which requires a ballasted coil. I've heard if they got hot they can cause misfiring. Anyway, I decided to obtain a new dizzie from Simon BBC. This is now fitted with my original coil.

This AM I got round to dynamic timing - 13 degrees at 1000rpm which is correct for midget 111 1275cc - using an Accuspark SP8000. I have run a single HIF44 for past 3 years and always found it reliable having been completely rebuilt.

It is still not happy on brisk acceleration. Oddly it's not bad in 1st gear but when changing to 2nd it then misfires.

I haven't touched the valves & expect some of you (Nigel) to give me a ticking off.

I'm guessing this is a timing/carb adjustment thing. Do I need to tune 'by the book' or have you any suggestions from info above. Any comments much appreciated.

Gavin Rowlesx

as Guy and others would ask - does the rev counter twitch when the misfire happens?
Nigel Atkins

Nigel - no twitching.
Gavin Rowlesx

1st. Dynamic timing is used to only get the engine started after a rebuild, not to be used for day to day driving.

You will need a timing gun to set the dizzy timing correctly... (static timing)

What nigel will say and I agree is to do a full on tune up...start with the valve timing and adjust all.the valves to spec, next adjust your timing, and dissy adjustments, check spark plugs... sometimes they dont fire very well at higher revs

Then move to adjusting carbs via a manual

If the new ignition system rquires a ballast, id put one in, or you could be replacing the new dissy shortly

But a twitchy tach will tell you if its electric or carb related...

But I think you need to do a full on recalabration with a timing gun and freash parts for those that are worn out

As to a timing gun, alot of people try to tune by ear,,,, this is a bad idea and not very possiable, you can pick up cheap timming guns at deep discount tool stores, ebay, and pawn shops, also you can barrow them from a friend thats a car enthusiest or the local car club

Good luck
Prop and the Blackhole Midget

Fuel filter clogged
fuel pump on way out
Leak in one of the hoses at the carb
Leak in vac advance
Vac advance u/s or disconnected
Spark plug(s)
Rotor arm - clean with 1500 emery
Dizzi contacts - as above

Don't fiddle with mix/timing until everything else has been eliminated.

Mark O

when shifting from 1st to 2nd at exactly what point does the misfire occure??

Easing off the gas? Engaging clutch ? Shifting? Releasing clutch? Applying gas peddle?

Prop and the Blackhole Midget

When did the misfire begin ... recently??after a modification or repair....always been there?

Misfires are tough....there just so many possiablities and it requires a well thought out plan of elimination to find the culprit

Needless to say its become a common occurance here, so hang in there

Prop and the Blackhole Midget

Plugs and leads?
Graeme Williams

no twitching, you or the rev counter

no twitching suggests the problems isn't with the low tension side of the ignition

don't assume because something is new that it can't be faulty

any other new parts in addition to Simon BBC dissy?

I'd check and clean as required:
. coil lead - check both ends are securely fitted and not held on or held back but tightness of rubber boot
. rotor arm
. dissy cap
. other HT leads - and as with coil lead
. spark plugs

to surprise Prop and you I probably wouldn't bother checking the the tappets gaps if they have been checked within a reasonable time and mileage

if you've adjusted the timing then you'll have to check that again but hopefully as you have a single carb you might not need to adjust that
Nigel Atkins


Misfiring started without any pre fettling.

The Simon BBC dizzie is the standard electronic version & you are advised to run without a ballast. I used a multimeter to check resistance was over 1.5 ohms in 200 mode and over 7,000 ohms in 20k mode. My old coil was in excess so I deemed it OK.

Prop - I think you have your Statics & Dynamics askew. Accuspak SP 8000 is a timing gun.

I checked for leaks around the carb & manifold using carb cleaner. All OK. The vac advance tube/ends are new but have had a good suck , as one does, & am confident this OK too.

Leeds have done under 500 miles but plugs is something I could consider. They look OK and done less than 5,000 miles. New Red rotor arm as supplied with new dizzie. Retained old cap as was under 500 miles old but will change over to new one as supplied with dizzie. Will check fuel filter & pump but fault here does not explain why it rev's happy in 1st but not 2nd gear!

I've messed about with the mixture already so am going to start again with 2 turns richer from the bridge of the carb as per SU manual.

Prop - in Ist up to 3.5K OK change gear get to about 3.0K it don't like it.

Apart from that I think I'll have to start with standard tuning route as prescribed in the good book.

Thanks for comments. Will update when I can.

Gavin Rowlesx

Sounds daft, but is there oil in your damper? If it's low it can cause a problem on acceleration as the damper isn't held down and doesn't 'force' petrol suction on increase of vacuum.

Has the float suddenly packed in causing the fuel level to be wrong in the bowl? Take the damper off and the fuel should be visable in the jet without overflowing with the pump running.

Other than that, what everyone else said.

13 degrees is a bit much in my opinion...I think 7-10 is about normal

Prop and the Blackhole Midget

I set my newly reconditioned 25D4 dizzy with Simon BBC's Powerspark electronic ignition unit to 13 degrees at 1000rpm using a timing light. Whilst it runs very well, pulls well, no misfiring at all, initially when cold starting, it seems slightly reluctant to turn over TDC, almost as if the timing is set too far advanced. (new battery just fitted, so no problem there) Retarding the timing a few degrees makes it easier to turn over on starting. Any ideas? I can't hear any pinking to suggest overly advanced, but with noise levels being high, I doubt if I would anyway!
Dominic Excell

It's too advanced.I use about 7 degrees but I work backwards from setting it at 28 - 30 degrees at 4000RPM or whenever the distributor stops advancing. With unleaded the factory settings need retarding a bit.

Fuel pump fine. Not sure about float level. May look at but am going full tune first

Valves reset. Am setting static timing at 5 degrees to get started then use strobe & try 7-10 degrees.

Will swop out HT leads and cap & maybe use 'old' red rotor.

Gavin Rowlesx

HT leads - I was thinking of their fitting rather than their age or condition

if you are going swap out parts then do that one at a time and test or test drive otherwise if you swap out two or more parts at once you won't know which one was faulty out of the set

if you've now reset the valves then yes you'll need to check/adjust as required (points), plugs, timing, mixture

you could set things up by ear and test driving rather than sticking to figures as your car isn't standard or new

finish electric side and setting up before swapping to fuel side

good luck
Nigel Atkins


As Nigel suggested checked all leads & 1 of the connections was 'iffy' so chopped it a tad and re-attached to new cap - I know - naughty changing 2 things.

Got engine started (didn't mention earlier that could not get spark at static timing). 1000 rpm / advance blocked / 10 degrees. Same problem - mis fire under load 3/4K rpm. Revving freely in neutral.

At this point decided to check Simon BBC FAQ concerning lack of spark. Went through the checklist & could find no fault with what I had done either with coil/battery or on using their instructions.

So, rang Simon BBC. The advice was :-
1. Forget about obtaining a spark using static the system has to spin over fairly rapidly first.
2. Ignore the instructions to use a strobe. Allow engine to reach an even rpm then advance the timing till it reaches it's max then back off 100rpm.

Well this advice seems to have worked. I need to take the car for a longer trip but it now seems to be running as it should through the gears.

I guess comments made earlier about engine condition/ standard of fuel has a lot to do with this situation. I wonder if anyone else has had this issue with Simon BBC Electronic Dizzie?

I intend checking with my strobe what the advance actually is - although for future timing it seem a little academic!

Gavin Rowlesx

I'm a bit lost here Gavin, did SimonBBC say to ignore their instructions to use a strobe or are there instructions to not use a strobe and are they basically saying set it up by ear

I'd guess the backing off 100rpm would be just go back a very little unless you have certified calibrated equipment, which leads me to how do you know how accurate your strobe light is anyway, I'm not saying it's be out by much but like with the whole car you have to allow a bit of latitude, use figures to get you in the right area but not as absolutes

after you've run the car a bit to know how well it goes try putting in some new NGK plugs and gap them wider (30 thou) and see if you get improved performance/mpg - course you might also need to check/adjust timing and mixture :)

if it doesn't work you can always close the gaps
Nigel Atkins

I'm a bit lost here Gavin, did SimonBBC say to ignore their instructions to use a strobe or are there instructions to not use a strobe and are they basically saying set it up by ear

Im with nigel

That is truely odd, im not sure id be a fan of that advice...great way to burn a hole in the top of your piston

On the plug gap...I took peter burguss advice and gapped to .035, that counters the cheap and junk in the fuel ect.

Prop and the Blackhole Midget

I think the 35 thou might be for more performance engines like yours - but I could well be wrong and often am
Nigel Atkins


Id bet good money your correct...stock would be better at .030

Of coarse its easy to play and experiment with


Prop and the Blackhole Midget

The advise I got from Simon BBC is as I say above. Sounds crazy I know. Dominic has the same dizzie & it appears his responds to the instructions OK.

My battery is over 5 years old but still spins the starter well. Volts drop to 1.5/11.0 when cranking (meter connected to power feed on coil) which I think is OK.

I'll get some new plugs & gap them 30.

Will give it a run tomorrow early evening & report back.
Gavin Rowles

Working back from maximum centrifugal advance is the way i would go, then what you get at idle, is what you get. Critical you don't get it over advanced which can cause high speed pinking when the bob weights spun out, not good!!.
Bit concerned about the coil issue as well. Is Simon telling you to use a 6volt coil, without ballast resistor (i.e., primary coil resistance 1.5 ohms)? Is it mounted at least horizontally, preferably upside down?
Don't get the 7000ohms as well, all that happens when you click up the range is that the decimal point moves.
When you dynamically set the timing at idle you must disconnect and plug the vacuum pipe. Static won't work on an electronic system unless there is some form of indication, LED etc., on the dizzy.
Allan Reeling


Coil is 12v - was working prior fitment of Magnetronic ignition circa 8 years ago.

Anyway, 10 mile run this evening & running better than it's ever done. Smooth pick up from low revs. no misfire at all & very consistent tick-over from the start & at end of run. No overheating either which was a concern with ref Allan's comments.

Took the opportunity to find out the advance (dizzie blocked/ 1000rpm)
and it was18 degrees! This seems too high so will be a tad cautious on the next few runs but I'm now wondering if the almost new gun I have is naff. Clearly if i've been setting advance at what I thought was 13 degrees it could have been 8 degrees or even less. When I tried 10 degrees as someone suggested the running was even rougher.

Other factor I need to address though is the fuelling side. Am going tom address this aspect hopefully at the weekend.

Gavin Rowlesx

10 miles, more of a walk than a run ;)

how accurate are these guns, I thought good ones where quite accurate, perhaps if borrow a know good gun and see if/what the variation is between them

how about getting new plugs and gap as per standard and test then wider and test
Nigel Atkins

Gavin, the straightforward, fire when the plug does type, have never had a problem with. But the "dial the timing" in guns I've had some very dubious results with. My last Sealey was over 5 deg out at idle. Have now gone back to simplicity.
Allan Reeling

Hello All.

For the record yesterday PM I had a 40 mile blast around Kent/Sussex border. Some 40/50mph cruising,some town driving, some foot down hard acceleration & a 65mph cruise of about a mile - and no hesitation when I asked for more.

I'm confident I have got both more bhp & torque than I had with the previous set-up. The plugs look perfect to me - greyish with a touch of tan & the electrode itself is shiny metallic & clean.

Granted I set the valves & roughly adjusted the carb too (single HIF 44) but I'm convinced my old dizzie (of unknown vintage) was the weaklink. I think the lesson is if you want electronic ignition for reliabilty then you may want to consider changing /rebuilding the dizzie for performance considerations too. For that matter applies to points dizzie too.

As far as m timing gun is concerned Alan Anstead has kindly offered to check it for me in the future.

Gavin Rowles

I agree and having been putting so from day one, fitting a new electronic igniter head is good but it still leaves the old and possibly very worn dissy mechanism

plus as has been put if they remain on CB points the rest of the dissy is still worn

it's why I went for a fully new (and fully electronic) dissy

I've read that dissys were well out within 3 years or so from the cars leaving the factory so I expect they'd be worse after decades

the Distributor Doctor rebuilds dissys but I don't know how long he can guarantee the rebuild for

now you know things are running well you could if you wanted experiment with a wider plug gap to see if it improves things even more (don't expect miracles) I run NGK at the manufacturer's 0.8mm as I have a metric gauge (32 thou I think)
Nigel Atkins

Gavin - what make timing gun are you using?
G Williams

Graham - you are very naughty. My very first thread said 'Accuspark SP8000'.

Are you going on the Bridges Run 27/4? Am thinking of rebelling and going to The Bell at Smarden or the Dering Arms at Pluckley Station. Both likely to have a good turn-out of classics.

Ralph W would have been up for this but he has a Granddaughter due to land on said day. Bit inconsiderate of the parents if you ask me!

Gavin Rowlesx

Alright Rowles, don't get a*sy! I have studied your thread from the outset, but even with one of this gravitas, reading from the start more than once is more than I can be bothered to do. And of course with failing memory, I didn't recall that bit of information! Yours isn't the unit I had a particular comment about so (yawn) I have nothing to add now!

I'm not doing the Bridges and we have visitors that day anyway, so it's a shame to miss out on Drive It day but unavoidable unfortunately.

Who is orgnising the two runs you mention?
G Williams

No organisation it's just a happening man. Anyway you can't make it so what's the beef brother.

Gosh I think I'm falling into a 70's time warp.
Gavin Rowlesx

You on the run on Thursday Gavin? Put the repairs to the test and a chance to get some expert advice from The Masc Men of Kent? Alan might bring his timing light, just don't mention "electronic ignition".
G Williams

Sadly I am working that day. Have fun & make sure you put plenty of sunscreen on.
Gavin Rowlesx

Gavin, sure you have your dates right?
The Dering meet was last Sunday and Jim did not mention one on the 27th and as far as I know the Bell at Smarden no longer welcomes classics, think the kit car brigade still go sometimes (happy to be wrong about this).
27th is the 5 bells at Braebourne IIRC.

d brenchley

Oh sorry Gavin, I thought you were retired. You look old enough to be retired!
G Williams

This thread was discussed between 28/03/2014 and 16/04/2014

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