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MG Midget and Sprite Technical - Running on

Entering final stages of tuning up the engine (1275) I believe it to have an uprated Kent cam, higher compression, 123 ignition thanks to POs, since which I have added perfornamce leads, BP8ES plugs, Lucas sports coil and balanced/tuned up the carbs - which incidentally are the larger H4s - AUC 6020 cast into the body. I ahve yet to fine tune the mixture using a gunson...later

It seems to now run much better, but for the very annoying and sometimes severe run on after ignition is switched off - It did this before all my mods, so not entirly my fault.

Question then is how to prevent it? Ignition static timing seems OK - but I guess it may not hurt to tap the distributer body clockwise/anti-clockwise a little to see if it helps - that said, I do not wish to sacrifice proper timing. I thought the colder BP8ES plugs may have helped - apparently not. Guess another solution is an anti-run on valve. Advice on best place to buy from (in UK) much appreciated - and more importantly, would it help if I fitted one?

Many thanks as always

Mark
Mark O

How did you determine what the static timing was? With such a tuned engine it would be wise to take out the strobe, even with a 123. Just put it where the led lits up might not be right the right spot.
Alex G Matla

Mark,
sorry you've got me again but my reply might encourage others

not sure about your choice of plugs, I assume you've seen or got this - http://www.ngksparkplugs.com/techinfo/spark_plugs/partnumberkey.pdf

how much higher compression is your engine and is it enlarged, head worked on, ect.?

what RON fuel are you using?

you've set tappets to their requirement then set timing and lastly mixture

is your idle speed high and/or throttles shutting right down?

any wiring issues, ignition switch, or additional electronic items fitted, relays?

if you have full details of your engine work that would help and photos of your engine bay

what temperate do you show on the gauge when fully warmed?

ETA: (Alex posted whilst I was typing, as usual) good point by Alex, standard tappet and timing figures wont apply to a non-stadard engine
Nigel Atkins

another thought/question - what setting/step is the 123 on ?
Nigel Atkins

Chaps

Cannot verify exactly what mods have been done to the engine. All I can say is that it has a fast road kent cam and 123 fitted - don't know what setting is as I ahve not removed it, but the few notes I have from PO suggest it is about right. I could knock distributor back and forth a bit to see if that improves run on, but I think I would be unduly affecting smooth running...

All other ignition stiff appears to be OK, and otherwise the engine is much better. Granted I have yet to fine tune using the plug colortune/Gunson thing, that will be next.

Think I will buy a Metro dump vv and retro fit to the screwed (currently) blanked off threaded boss on the manifold and see what happens. Until then, I will blip the throttle after switching off - to get air in and starve it.

Mark O

with a car new to me I'd be checking stuff even if it looks ok

personally I'm not keen on those colourtunes but others get along with them and it may help sort your problem

I'm sure I've probably already put that engine tuning isn't that important in relation to the car, brakes first, steering, tyres, suspension, lights, windows and carrying out a 36k-mile service/check will help to find, resolve and prevent problems

and photos of the engine bay may help spot things for you

all with you, you're obviously doing research but don't forget as well as t'net there are also books and manuals

main thing is to drive and enjoy your car, cheers
Nigel Atkins

Start by a little less pre ignition, Turn distributor CCW a tad.
Curve setting has no influence (or very little) on run-on I think...

Best is to get a strobe and set max advance at 28-30 degrees. (Wait till the rain stops :^P
Alex G Matla

what rain? :)

I'd suggest to set by engine sound and feel rather than strict figures especially while things are still being sorted
Nigel Atkins

Alex

Yes - I turned the distributor a little CCW and run on diminished a little. Agreed with Nigel - I tend to things by ear as there are so many other variables involved so as to mitigate exact settings.

Engine is first simply as it was the dirtiest part of the car!
Mark O

Mark, My 1275 has a fast road cam & a big valve cylinder head & "vizarded" 1.25" su's, probably making it a similar spec to yours....Mine ran on when I first bought it.

I used a strobe to adjust the ignition timing from about 7 degrees btdc @ 1000 rpm to about 9 degrees & approx 30 degrees @ 3000 rpm, this improved things.

The mixture was also rich...The plugs were black...I adjusted the carbs for correct idleing speed & then the mixture, and then fine tuned the mixture useing the plug colour as a guide, the car now runs o/k.

Hope this helps, Alan.
Alan Cotterill

Alan - Indeed.. sounds very similar - Many thanks for info. I hope to get colour plug on it soon to refine current mixture setting.
Mark O

I thought it was only show cars with clean engine bays not go cars :)

anyway you clean the engine bay as you service and check it over :)

cleaning is alright as long as it's a tiny ratio to driving

I do think you should get this run on sorted but unless your engine is quite a bit uprated you shouldn't get run on

only way to tell, without taking it apart, is to have it running reasonably and comparing it with another similar Spridget or sticking it on roller or diagnostic machines

did you drive any other Spridget before getting yours?

personally unless the engine is very much uprated I'd do away with the oil cooler or fit full thermostat on it

now you've got it running better perhaps you could try adjusting again using the old plugs if they're standard and in good condition and not too old

ETA: so a vote for idle speed and colourtuner there and of course if you alter one thing you have to follow on with adjustments further down the chain
Nigel Atkins

....but I am getting it as it is uprated! As aforementioned, it was there when I bought it. There are various solutions, but I am keen to get as much right with the engine/carbs as possible before putting the sticking plaster of essentially a dump valve on the manifold.

Clean engine bays are the easiest method of allowing correct fault finding to take place - besides, I (usually) enjoy tinkering about...
Mark O

yes I do think it's agood idea to have a clean engine bay shortly after buying and just before selling, I use the spray cleaner, aggrivate, flush, spray polish, leave to dry method intitially about 20 minutes from getting stuff ready to put stuff away

sorry I'm opposite to dump vales and tinkering

tinkering to driving ratio needs to be a lot lowere than cleaning ratio for me

I like to spend a lot of time at the start to get the car running well and be very reliable

unfortunately 'the start' time on my present midget has dragged into years and loads of bits and money, I now put off doing even small jobs because I feel I've done them too often on the car already and not near enough driving of the car

nipped up the newish rear brakes yesterday and spent double the time sorting out the sissor jack handle (it's not the proper one for the jack)

once things are running reasonably a building up to a few good blow out runs can help the car no end (if only to highlight faults - no I'm joking)
Nigel Atkins

Alan said "I used a strobe to adjust the ignition timing from about 7 degrees btdc @ 1000 rpm to about 9 degrees & approx 30 degrees @ 3000 rpm, this improved things."

This doesn't make any sort of sense to me (maybe I'm confused in my thinking). For a start the timing figure of 7 degrees BTDC is the static setting, not the dynamic setting at 1000 rpm. For the 12CE engine the setting at 1000 rpm is 13 degrees BTDC, and for the 12V engine it is 16 degrees BTDC, both with the vacuum disconnected.

Alan then says that he made an improvement to running-on by changing the timing from 7 to 9 degrees, but this is going the wrong way. That way is advancing the spark which would make running-on worse. Of course if Alan's engine IS timed at 9 degrees BTDC at 1000 rpm, the ignition is significantly retarded, and it ought to run poorly, although it would be unlikely to run-on.

I only mention all this in case someone reads this and tries adjusting their engine to Alan's formula. They could get in a bit of pickle if they follow that advice.

If I've got this a*se about face, please let me know.
Mike Howlett

My 1320cc engine runs on too! I've got a Kent 236 cam and a bigger valve head from an MG Metro. The car's been set up on a rolling road and they couldn't stop it! I've got about 10 degrees dynamic advance. I'm now fairly sure that the 1275 engine is very prone to this, my last standard car also did it. It may be that the head gasket 'hangs' over the edge of a cylinder in some place and that's what heats up and causes it. It's certainly not plugs overheating..they have never showed any sign of it and I've run much colder plugs and they make little difference. The running on is directly related to water termperature, ie a hot engine does it, when it's colder not at all.

So, my conclusion (and it's only a theory!) is that the design of the head, probably the gasket, leaves something like a fire ring protruding into the head enought that it glows and causes the problems. Interestingly, my Caterham with a far more tuned, high compression, hot cammed, twin 40 DCOEs never does it!
Robin Cohen

My 1275 also has a Kent cam with venier chain wheel, and a worked head with Titan manifold and HIF44 carb, with Maniflow header on Magic Midget exhaust. Ignition is a standard 25D4 dissy with Simon BBC Powerspark unit replacing the points. Mine doesn't run on at all. So they don't all do it!! I have retarded the ignition to 14 before TDC at 1000 rpm to prevent pinking when pulling uphill.
Mike Howlett

1275 has nothing on the B series when it comes to run-on, and generally isn't a problem. Certainly in our three 1275's all modded similar to above with CR around 10 to 10.5:1.

No 1 cause of run-on is air leaking through the spindles, aggravated by higher idle speed (and higher idle is usually needed if air is leaking as it makes the idle rough).

I have a page on run-on causes and cures on my website at http://www.mgparts.co.nz/advice/technical-notes/run-on/

Mark - I note you have H4's. Did you check the condition of them before fitting?
Paul Walbran

last time I put I'd never had run on with my two Spridgets I got it soon after ! I was never quite sure if it was the cause but I rewired the electric cooling fan and it went away and has never returned

Paul,
Mark got the carbs with the car but they came without full linkage and cams and there was some question about the spindles so you might have won the cigar!
Nigel Atkins

Paul

Yes indeed - point taken. Not thoroughly checked spindles, but tried the old spray spindles with carb cleaner trick to see if engine died away a little - seemed OK.

Agreed that it is much more prone to running on when hot - that is for sure. For the momnet I will get the mix correct with a gunson plug, hopefully reduce idle, fit an old Ford anti-run on valve to threaded boss in manifold, and take it from there. For the moment, I blip the throtlle after switch off - this seems to help.
Mark O

Best way to check the spindles is to grab an end in each hand and see if there is any movement. There shouldn't be. Any noticeable mvement and enough air will leak in to cause problems ... and that's on the MGB engine. For the much smaller 1275 engine a given amount of will be even more significant for run-on though mitigated somewhat by the A series being not as troublesome as the B series for run-on.
Paul Walbran

I've got valve seal hats on the outside of the spindles. Keeps them clean and seals.
Alex G Matla

So I need to recheck the spindles! I put new spindles in when I rebuilt them but did not change the bushes. Alex, any chance of a photo of the valve seal hats, can't work out how they fit on as not much protrudes. Do you have two on each carb, on either end of the spindle?
Robin Cohen

I've always had running on with my car.
1330 with Kent 276 cam. Peter maye fast road head. About 11:1 compression. BP8ES plugs, Slightly mucked about with Hif44 on titan manifiold. 123 dizzy.(set up by rule of thumb by Peter Burgess based on my verbal descripion of the engine) maniflow manifold and RC40 box.
On Aldons rolling road last year, they said running pretty much perfect fueling through the rev range but slightly weak at tickover.
I always get running on.
frogeye Gary

11:1 will be pushing the limits of avaoiding run-on without a dump valve, but having said that my B has 11.5:1 and usually doesn't run-on.

Weak mixture is a definite cause.

But there is always the odd stubborn case which will run-on despite everything being right. My theory is that it's something to do with the cooling cores in the head affecting the way heat is conducted away from the head. No evidence other than if you swap the plugs for a fresh set then start the engine & turn off straight away it doesn't run-on. But if you let it run for a minute it will.

And if it persists in spite of everything being right then a dump valve will always solve it.
Paul Walbran

Number one cause of run-on is too fast idle speed, frequently to cover some other fault, Number two cause is retarded timing, which overheats the exhaust valves in particular. If you have retarded timing to prevent pinking, and your advance curve is wrong, you are likely quite retarded, and can expect problems. Carbon deposits from rich running, slow running, or just plain wear also cause it. Most of these engines should idle below 1000 rpm if anywhere close to correct, and rarely run on. Hot cams sometimes require higher idle speed, but the same factors reduce effective compression at low speed, so are self correcting. Also helps a lot to let the engine drop to its true idle speed before switching off; shutting down as soon as you stop commonly gives run on.

FRM
FR Millmore

Robin, the hats are only on the outside, so not between the carbs. Spindles protrude far enough, I just cut of a small bit of the valve hat to make it shorter.
Alex G Matla

Just reading this thread with interest, don't have the problem with the 948 (flowed head, LCB, K & N's rejeted Carbs & Needles,Modded Inlet) even though the spindles do leak a bit now.
To put my two penneth in does anyone have the running on issue with a 1275 running a 45 Weber ? If not then it would suggest that its SU related. Or is it just a oddity with the 1275 engine ?
Ed
Ed H

Has anyone thought of machining a groove into the brass spindle shaft and putting a small o ring on that so that the oring seals in the bore that it sits in? If the shaft is approx 6mm OD, then the shaft thickness will be reduced to around 4mm at the point of the O ring but I should think that will be plenty as it's under no strain except for the return springs pulling down. Any thoughts?

I'm at beaulieu in a couple of weeks so will see if I can pick up a new spindle from Burlen fuel systems and machine it up with O rings.
Robin Cohen

Could be doable, and with a slimmer O-ring you don't have to go very deep. Think I'll give it a go when one of my two SU set-ups needs a rebushing.
Alex G Matla

And mine also ran-on with the weber 45 on it IIRC
Alex G Matla

Remember that you'll need to use nitrile O Rings so that the fuel doesn't eat them.
Robin Cohen

Interesting that Alex's ran on with the 45 weber. So maybe its not just an SU problem and could be somthing else causing the running on. Could it be octane related, has anyone tried putting in an octane booster and see if the problem stops ?
Ed
Ed H

This thread was discussed between 21/08/2012 and 29/08/2012

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