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MG MGB Technical - Running on

Firstly, I would like to thank all of you for helping me bring back to life a '75 that had stood idle in a barn for almost 18 years....today it passed it's MOT !!

However this has brought another problem....a prolonged 'spin' through the sylvian roads of Surrey has resulted in a significant run on, which was not there before.

I have previously balanced and tuned the carbs and dynamically set the timing, whilst renovating. My suspicion is that exesive carbon deposits are retaining heat and causing the run on, but before I buy the latest decoking product I would welcome any ones thoughts on this.

I am not sure whether the valves have been hardened, so I am using a lead substitute just in case.... would this cause a 'run on'?

Thanks in advance

Dave G
Dave G

Mine does that. The one before did it too. I use 4th gear to shut it down. Let the clutck out to the friction point and turn the switch off.

Steve
Steve

Same here. I sometimes think there is a delay cct in the ignition that keeps the motor running for a few seconds to cool it down. Just like turbo diesels!

Tony
Tony Oliver

Gentlemen - See Paul Hunt's web site: Pages of Bee and Vee at: http://www.mgb-stuff.org.uk/, click on Spanners, Fuel, Running-on for an explanation of what run-on really is and how to combat it. Cheers - Dave
David DuBois

Thanks for that.

I read the link and understand it thus:

1, The octane rating is too high (due to lead replacement additive)causing ignition through compression and therefore run on after a long drive.

2, I either live with it, stop using lead replacement or modify the carbs as described.

Or have I missed something ? Is there no credibility in my carbon deposits on the pistons theory ?

Kind regards

Dave G
Dave G

Remember MGBs liked 5 star. If I use my local supermarkets 4 star LRP 97 octane it runs on. But if I use Shell V power 99 octane it doesan't.
c cummins

Dave,

This is not unusual in B series engines running today's fuels.
If the engine is "dieseling" by running on and spluttering, try opening the throttle a moment after you turn off the ignition. The gulp of air will prevent run-on.

A high idle will make the problem worse.

David
David Overington

I think I will try using different fuels for a while and see what happens.

A friend mentioned that it could be caused by a 'not so good' head skim, that leaves tiny bits of metal protruding into the cylinders, these get red hot and cause this run on..... sounds feasible to me.. in which case I'll just have to live with it.

Thanks again

Dave G
Dave G

B series engines are rather prone to running on. Mine stopped when I put the Peter Burgess fast road head on it.
Stan Best

The octane is too *low*, it gets worse the lower the octane you run. Nothing to do with unleaded valves/seats or unleaded additive. The early 70s era of MGBs seem to be particularly prone, and to pinking as well. On an Arden run up the Shelsley Walsh hill-climb course it was noticeable just how many of that era were pinking away like Billy-oh, whereas 60s and late 70s tended not to. A high idle does indeed make runon worse, but a low idle leads to engine shake and a tendency to stall in traffic. After the UK went unleaded mine became unbearable, and it was bad enough before. Flooring the throttle did nothing (as did the after-market valve fitted as per instructions) and I hated stalling it on the clutch. Which was why I persevered with the anti-runon valve, using it in a similar way to the North American system, which is 100% effective (the 'normal running' type of runon in 73 and later American emissions cars is because the anti-runon system, which includes all the emissions plumbing and charcoal canister, is defective in some way). Now I can have a high enough idle to avoid engine shake and stalling, and only very rarely get a single cough on switch-off no matter how hot it is.

These days very little carbon gets deposited, when did you last hear of anyone having to do regular decokes? It is down to cylinder head design resulting in hot-spots. Lyndsay Porter in 'Guide to Purchase and DIY Restoration of the MGB' in the section on performance tuning shows how to grind away some of the point that exists in the head between the two valves to counteract this, even though it has the effect of reducing compression ratio a little.
Paul Hunt 2

My car had terrible run on. People (even the rolling road operator) said it was the head design, but mine's a Burgess big valve job. I played around with plug types. I fitted an anti-run on valve, and this helped but was not altogether succesful - and the weld between the bracket on the Moss/MGOC valve fatigued so the valve fell off the bracket. Opening the throttle helped a bit (although makes it a bit worse to start with) but of course draws fuel into the cylinders which does not get burnt and hence washes oil from the bores ready for a dry start-up!

The only thing that worked was four star leaded (unleaded plus octane booster worked, although not as well, and was not without it's own problems). Then Shell introduced Optimax, I started filling up at the local Shell garage and the problem is a distant memory.

It could well be hot-spots caused by carbon build-up, from the fuel or from oil running down the valve guides. In which case a change of fuel is not going to totally solve the issue.

HTH,

Neil
Neil22

I had the same problem with my '64 and have found that using the highest octane fuel (premium here in the states) and making sure the idle isn't too fast has cured the problem.
don scott

I've always used Optimax (now V-Power) as it has always been the highest octane unleaded (well, short of the humungously expensive BP 102 at £2.50 per litre or $20 per US gallon!), but mine was still too bad for me to live with. YMMV.
Paul Hunt 2

Paul,

I have a question about the pinking. I have what I think is a valve noise (in my B)when under load but I call it a chirping. Some times if I give it more throttle the noise will go away. Do you think this is from using unleaded without lead substitute and is it harmful?

Thanks,
Jerry
Jerry Wood

Jerry - that is what pinking does on my car - more throttle will *stop* it, which seems a paradox, but maybe it is a factor of the bigger charge cooling the cylinder better. It could be less vacuum advance at the higher throttle opening as well, but I have my vacuum advance controlled from a switch using a TCSA vacuum solenoid, and it still happens (although is less likely) with no vacuum at all.

Pinking is due to a mis-match between octane and timing. I don't think it is anything to do with lead content per se for fuel of a given octane, but of course lead was used to raise the octane of lower grade fuel, which is cheaper than the extra refining required to produce fuel of the required octane without using lead. However since the UK at least doesn't have unleaded and leaded of the same octane to compare we may never know. Increase the octane, or retard the timing, and pinking can be stopped. At the end of the life of leaded petrol the lead content was minimal anyway, but I noticed a big change when going to unleaded, even the highest octane, which is 2 or 3 points lower than leaded was. I get less pinking with super unleaded than standard unleaded. In the UK super unleaded ratings of 97, 98 and 99 (although I've only had one fill of 99 I think) don't seem to vary much between themselves in my roadster. Standard unleaded is 95. 4* leaded was reputed to be just short of 100, as are the supplies currently available in the UK (99.7).

Pinking *is* harmful if the engine is left to do it for long periods and frequently. It can pit the top of the piston, and in extreme cases melt a hole in the piston. But occasionally and briefly shouldn't be a problem, many engine management systems rely on knock-sensing to get the greatest efficiency out fuel by running at the highest advance possible for the conditions, and if you rapidly change those conditions i.e. tromp heavily on the pedal whilst at a light-throttle cruise you can hear the pinking start then reduce as the engine management backs off the timing. It's an ambition of mine to apply that system to the MGB ... sometime.

Valve noise shouldn't change with throttle opening, only revs.

Again lead substitute isn't a factor in pinking, that only affects whether the valves and seats are likely to wear (recession) or not, but recession is a bit of a myth, unless you continually use your engine at wide throttle opening and high revs for long periods. If the engine has run several tens of thousands of miles on leaded without having replacement valves or the seats recut the chances of it are minimal. Nevertheless I choose to use an additive, and I haven't had to adjust the valves since we lost leaded in 2000.
Paul Hunt 2

Paul,

Thanks, You seem to know a good bit about this stuff. I will try your sugjestions of retarding the timing and or getting a tank full of the highest octane I can find.

It have used the medium grade of fuel and it seemed to make the pinking worse but I have not tried the premium grade.

Thanks Again,
Jerry
JR Wood

Check the vaccuum advance diaphragm in the distributor. They perish after a while and let air into the manifold enough to give weak mixture which in turn creates hot spots and consequential run-on. If you not worried about economy, blank off the vacuum advance - you will get better performance.
Richard Tinkler

Richard

An interesting comment... I think there is a problem with the vacuum advance... I'm getting intermittent, significant variations in tick over.
Do you know an easy way to check if it's perished (after 18 years in a barn it could be)
Kind regards

Dave G
Dave G

The vacuum advance is almost certainly useless after 18 years. To check it you suck on the pipe and see if the connecting rod moves. You kind of get a feel for it.

If it is perished then that will not necessarily affect idle as only changes in timing (as opposed to wrong timing) will cause changes to idle speed.

I wouldn't think the pipe would pass enough air to affect mixture. And even if it did at idle, it certainly wouldn't with the throttle even slightly open (ie on the open roads).

Neil
Neil22

"blank off the vacuum advance - you will get better performance"

Can't agree with that, although it may vary from car to car. With my electrically controlled vacuum advance there is very definitely a difference in part throttle acceleration with vacuum enabled compared to not. I also noticed a difference when changing to a more aggressive capsule from a 'weaker' one, and so did the navigator. You can get the same acceleration without by pushing the throttle that bit harder, but it is just nicer to drive with the extra oomph on a lighter throttle.

Whilst I have had two V8 vacuum capsules fail in 10 years that is because the carb port is on the bottom of the HIF throat on that and fuel can run down to the capsule and rot the diaphragm. After the second very expensive replacement I fabricated a small separation chamber which is mounted higher than the carb port, which allows fuel to drain back into the carb instead of down to the distributor. Several further years on no fuether failures yet.

I haven't had any failures on the roadster, and I put that down to the vacuum port being on top of the throat on the HSs on that.

As Neil says suck on the carb end of the pipe, you shouldn't be able to draw any air through, and the points plate should move. However you have to suck pretty hard to get any movement, and if you can draw air through there is a leak in either the pipe or the vacuum capsule. If you can move the plate by suction, block the end of the pipe with your tongue i.e. don't suck any more, and the points plate should stay where it is. If it drifts back either you have a diaphargm with a small leak, or a very rough tongue. The points plate should return smartly when you release the vacuum.

A leak in the vacuum advance system very definitely affects mixture. In cars with the vacuum port on the carb there is no vacuum at idle, so no leak, and you get the correct mixture balance between the carbs. But as soon as you start to move the throttle you get high vacuum which *will* suck air through and affect the mixture, and more importantly the mixture balance between the carbs. As the throttle is opened further the vacuum reduces, which because of the greater airflow through the carbs anyway a vacuum capsule leak will have a progressively lower effect on mixture, but it is still weakening it. On cars with the vacuum port on the inlet manifold the effect of a leak is much worse because the vacuum capsule is under high vacuum at idle. This means the rear carb will have to be richened relative to the front carb to get the correct mixture at idle, but as soon as you go off-idle the effect of the leak starts getting less, which means the rear carb will become progressively *richer* than it should be, and richer than the front carb.
Paul Hunt 2

Amazingly, I noticed Paul's comments.But on early MK1, there was a separator device onto the vaccum circuit designed to prevent fuel incomes inside capsule, maybe on 18G and 18GA engines these installations were bolted onto dizzy instead of plugged. And on another hand, I didn't find that hard to suck for moving plate testing, i only unset the return spring. Good point about the difference between HS and HIF vacuum connecting way but is it possible to convert HIFs to an upside connection?
Regards.
Renou

Paul do u have a run-on valve for sale i need one for a modified MGB engine in my MGA the fuel in SA is never constant and the fuel Co's lie about the octane rating
Treor Burnett

Renou - I've noticed that capsule in the copper vacuum pipes, but don't really see how it works. Being lower than the carbs any fuel that runs down there can't get back up, and it will eventually be filled. Only if there is *another* diaphragm in there that *doesn't* rot could it work. I've not checked but I suspect that the HS and HIF carbs would differ in port position if the butterfly opens the other way. On the HS the *top* part of the butterfly moves away from the manifold, uncovering the vacuum port which then gets full manifold vacuum with a very small movement of the throttle. If it were on the other side of the butterfly it would only experience very small vacuum as it would then be on the 'constant depression' side of the butterfly. On my V8 at least it is the *bottom* of the butterfly which moves away from the manifold, hence the vacuum port also has to be at the bottom, and it isn't possible to move it. One more cross against the HIF as compared to the HS.

Trevor - the MGOC sells the after-market valve, others may do also. Whilst this had no effect on my car when tee'd into the vacuum servo pipe it may do if it can be screwed directly into the manifold, or at least not have the servo non-return valve between it and the manifold. Failing that, you may like to see what I did at http://www.mgb-stuff.org.uk/wn_fuelframe.htm and click on 'Running-on'.
Paul Hunt 2

"I've noticed that capsule in the copper vacuum pipes, but don't really see how it works." In the shop manual that capsule is billed as a flame arrestor ?? Cheers - Dave
David DuBois

In my Leyland Workshop Manual it describes it thus: "A fuel trap is incorporated in the vacuum line from the induction manifold", so you pays your money and you takes your choice. The last two words are interesting, as by the time manifold vacuum was being used the engines had changed to black 18V with a plastic pipe and no capsule. However looking at the drawing of the pipe in the Parts Catalogue - if it is representaive of the one use on the MGB - the capsule *is* positioned above the carb port.
Paul Hunt 2

Paul,

That capsule should be mounted in a bracket attached to the rear head stud and thus higher than the carb, so fuel will run back towards the carb.

I guess it was eliminated as a cost cutting measure! I certainly see enough advance capsules that have perished diaphragms!!
Chris at Octarine Services

Hi

This 'running on' thread just keeps running.....great stuff..I wonder what the record number of posts are ?

Any way a quick update.

I'm now trying 4 star LRP..£1.03 a ltr !!!!. BP super at 97 rating didn't help.

It seems the vacuum advance ( V A ) has perished, so I have blocked it off at the manifold. This has resulted in a drop of circa 3000rpm to the tick over...which has helped eliminate the run on but is too low.

I guess this is because the carbs are now out of tune. Before I retune and live without the V A, is the V A purely for economy of fuel or does it have another crutial function?

Kind regards

Dave G
Dave G

Oops sorry

That should read 'drop of circa 300 rpm'

Dave G

Chris - that figures, however I haven't experienced any failure of the diaphragm on my HS equipped roadster (plastic pipe no capsule) unlike the V8.

The vacuum advances gives improved economy when cruising plus noticeably sharper part-throttle acceleration. Even (ooops) the Navigator noticed a difference when I swapped a 5 degree capsule to a 10, let alone the difference between a standard 10 and none at all. I would advise replacing the capsule, then setting-up the carbs from scratch, which should only be done after valves, plugs, points, timing and centrifugal and vacuum advance are good and there are no other defects. A vacuum leak i.e. from a faulty vacuum advance capsule *will* give a higher idle. At the very least screw both idle screws in *by the same amount* to get the desired idle speed while the capsule is blocked off, or replaced. If you turn the idle or mixture screws/jets by different amounts you will muck up the settings and have to start again from first principles anyway.

LRP isn't 4-star but simply 97 octane super unleaded (i.e. same as your BP) with an anti-wear additive. Sainsbury's have been passing-off LRP as 4-star but have had to withdraw it following complaints and treats of court action. Even then LRP only contains half the amount of additive said to be required. The additive won't make any difference to running-on or pinking, only valve seat recession which is virtually unknown in practice anyway. But I have found no difference in pinking with Shell 98 or 99 octane which I use regularly, or Tesco 99 albeit only a tankful. Proper 4-star *may* be better, if you can get it.
Paul Hunt 2

Thanks Paul

Sainsbury's haven't withdrawn it around here, as thats where I got it from !
I'll try the 99 octane next.
Best wishes.
Dave G.
Dave G

I have found my idle to be much smoother with the vacuum advance attached. Without it attached, the idle was lumpy. With it attached I could back off the idle screws quite a bit which helps improve fuel economy. Mine is a 73 with rebuilt HIF4s, and a rebuilt dizzy from Jeff Schlemmer and new wires/plugs on rebuilt engine. I get no running on like this.
Still having a hard time however with a slow reduction in idle speed while sitting awhile eg at a long light (eventually it stalls), if I increase the idle speed to counter this, its often too high at other times and leads to running on.

Erick
Erick Vesterback

This thread was discussed between 19/10/2007 and 10/11/2007

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