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MG MGB GT V8 Factory Originals Technical - Factory V8 problem

Hi all,

I've had my Factory V8 now for a little over two weeks and I'm having a very frustrating problem with her. Ideas and opinions would be great.

If the car is allowed to idle for any period of time once warm, or say driven through stop-start traffic for 1/2hr, the idle starts to diminish, she misfires, then dies. If I try to accelerate during this time she just misfires badly, if I get the revs high enough she will smooth out - sort of, but back below 2000rpm she is toast. Pulling out the choke while this is happening has no effect. Shutting her off for 15mins allows me to restart her but shortly afterwards the symptons return. If I leave her off for an hour I'll get another 1/2hr out of her.

Now, I am thinking it could either be: a) Ignition timing/Dwell b) Fuel Vaporisation c) Carb icing (it is about 3deg C here).

a) As for the ignition timing, I have only just worked out the point gap is varied externally of the distributor (!) and unfortunately don't have a dwell meter yet. I have a timing light, and can see what I think is a pointer attached to the water pump but can't see any marks on the timing pulley at all to match up with this. When I installed a new set of points today, bumped her to TDC, the points were about 2/3rds of the way around the flat, not on a lobe! Could this mean she is way advanced/retarded? If I had a mark I could check.. very frustrating.. where should the mark and the pointer be?

b) Now here I am at a loss of how to diagnose and solve. This to me is the most likely scenario. The carbs are set at 3.8% CO2 and 14.4:1 A/F ratio. When she idles ok, it's a beautiful smooth idle and a tune up man reckoned the carbs were perfect so I'd like to be able to eliminate them. If it is fuel vaporisation, and to be honest, it feels like it, what can I do?

c) Someone mentioned carb icing to me. I guess this is possible but it isn't that cold here and the engine bay warms up to N and above reasonably quickly. The car has SS tubular headers and doesn't have the preheater (?) things to the air filters anymore. Would this be an issue?

I have replaced points, condensor, coil, plug leads, dist. cap, rotor so far thinking it was an ignition problem. I can't wait to get my hands on a Workshop manual but they seem a bit dificult to come by. I only got tools yesterday as I've only just moved to here.

I tell you the car is so much fun to drive when she is on the move, the acceleration is brutal <VBG> but this problem is unfortunately making the car undriveable for the moment. I'd like to be able to drive her to work now and then. Any ideas etc greatly appreciated!! :)

Neil Cotty

Gee, that almost sounds like overheating. Certainly the carbs freezing is simply a non-possibility with a V8! I have a V8 conversion and your description sounds like what used to happen to me if I forgot to click on the electric auxiliary fans driving around town -- subsequently I added a thermostatic switch and this became a non-issue -- but what happened when it would get really hot is that the fuel in the bowls would vaporize and the motor would start coughing and then die. Could it just possibly be that your temperature sensor's toast and the thing's heating up too much?


I've had a problem with stalling after a run down the motorway in winter and this has been put down to carb icing (probably more appropriately jet icing) and the Holley sits on top of the V8 heat. Using the old heel and toe I can keep it going. The solution appears to be ducting to the carb in winter, but the problen has not raised its head this year.


Humm, how does the fuel filter look?? This sounds similar to the problem I had with my weber on my 4 cyln. It turns out that the filter was either letting rust through, or the filter material was disintergrating and cloging the carb. The instant I changed the filter, it worked perfectly. It seems that only the idle jets were being clogged. If I kept the revs up (on the secondary jet), the engine would be fine. The instant I went to idle, the engine would die and/or run poorly.

Michael Hartwig

Thanks for the help so far everyone. Yes I changed out the fuel filter a couple of days ago - no change. The fuel and filter look perfectly clean.

As for the temp, not sure, the fans come on when the car is about 1/3rd past the N. The motor doesn't feel overly hot but - see below.

Today I managed to get my hands on a Dwell meter and the dwell was off some 12deg or so, I set her at 27. This made the car run a lot better throughout the rev range but I still unfortunately have the stumble when the car is allowed to sit for say 10mins at idle. She recovers much better now, a minute or two of 40mph, but the run I gave her was short so I expect with extended idling the same fault will reoccur and she will die on me, somewhere most incovenient too ;) I can really only think it is vaporisation at this stage. I will check the timing tomorrow with my new advance timing light but I doubt this will resolve the issue. I still can't find the pointer/pulley mark!

Could it be that the carbs are set too lean? What should the Co2% and AFR be for a V8? Mine is 3.8% and 14.4:1 respectively. This car is totally stock V8 with the exception of stainless headers and the lack of the air filter-exhaust manifold preheaters. I know a lot of 4cyl older MG's run best with a lot higher Co2 content, how about the V8? The PO had deliberately set her on the lean side concerned with bore wash and economy.. hmm

As for the heat, I know the heat under the bonnet is high-ish as I have fried two new plug leads! I have the plastic seperators but these don't seem to help much. In fact no.3 on the LHS bank fried around the underside of the spark plug rubber cover!! Is anyone suppling better heat resistant leads? I saw one lead tonight arcing against the rocker cover!!! I don't *think* this is what is causing the problem but I guess it could be related?!, this is a new set of leads, the old ones were better I think, but I still had the stumble too so thats why I can't say this is the problem...

I am thinking of wrapping the header pipes to reduce the heat, and covering as much of the fuel lines as possible. What can people out there recommend to use to do this and where in the UK could I get it from? I also hear that header wraps can cause the manifolds to crack... *sigh*

Wishing the V8 was as reliable as it is fun (when it's going <VBG>)

Thanks again,
Neil Cotty


You expressed the wish that the factory V8 was as reliable as it is fun.

I have had two, the second of which I have owned since 1973, and can assure you and any other potential owners that they can be totally reliable.

I have had very few problems in over 100k miles, and only one which left the car immobile - a failed Facit fuel pump - was not in respect of original equipment.

Regrettably I can't help with your problem, as I haven't experienced it.

Safety Fast


Wishing the V8 was as reliable as it is fun
Nigel Steward

As a general rule, it is sensible to change ignition leads every three or four years after heat degredation.

Check the vacuum advance too - the diaphragm corrodes quickly - I must have had three in 15 years.

I would have thought your carbs were a littlw rich - yes legend has it that they are best that way but on carbs mine was best at 3% and while you are about it - check the pots and springs.

Legend also has it that they use water - they do but you should be able to account for every drop - ie; they only lose it by leaks - most of them visible so if you are adding water consider testing the head gaskets for leaks.

In terms of probability - the problems are likely to be electrics (don't totally rule out replacement coil which couls also be duff) or carbs tho.

good luck



Demon Tweeks is a good place to start for uprated bits also merlin motorsport, suggest you get a free catalogue although wrapping exhausts should not be necessary. I'm running a 4.6 and have not fried anything under the bonnet. In case your guage is faulty you mignt try some Temperature strips on the Rad and Oil filter.


This may seem kind of obvious (then again, maybe not), but if your spark plug wires are arcing to the engine block, I think it's time to change them. I'm surprised your engine will even run with them like this! I had an old Civic wagon whose wires were only a year old, and the wires all of a sudden went bad on me one rainy day. One wire was arcing as it was turning over, and that's all it took to keep it from running. Granted, the V8 has 7 more cylinders to take up the slack, but if one wire is arcing, i'm sure the other ones are just as bad.

Joe Pitassi
Joe Pitassi

Hi Neil,

So if you've eliminated everything in the ignition by replacing it, then it's got to be fuel. How about the mixture strength? try using the "lift the piston" test to check your mixture. The HIF carbs have bi-metallic strips which compensate for fuel temperature, if your mixture is marginal these could be changing the mixture such that it goes too rich and loads up or too lean and dies when everything gets hot.

I'd be interested to know how you finally solve this problem because my factory V8 is intermittently doing the same thing at the moment. Only it doesnt cut out altogether, the idle just drops a little. Mind you I've just put a new cam in mine and it needs a new pair of air filters, I'm going to get K&Ns and new needles so I kind of just assumed the carburation is all over the place at the moment.

Does anybody know if the HIFs raise or lower the jets when the fuel gets hot I'd just be interested to know

Philip Shingler

You mentioned that the fuel and filter look clean, but there is a possibility you might have crud in the bottom of your tank that is intermittently clogging the fuel pickup tube. Whatever is in the tank (leaves, rust flakes, etc) could be too large to pass through the fuel line to the filter, but large enough to block off the intake when you agitate the tank while driving. Stopping for awhile would allow the crud to settle out, allowing a few more miles of smooth driving.

Just a thought,
Paul Kile
Paul Kile

Hi all,

Thanks again for the help. I have solved the problem and am feeling a little stupid because I didn't check this earlier but I only got the right tool a day ago and the parts...

SPARK BL**DY SPARK PLUGS!!! I pulled them last night. They are NGK BP5HS. Now from what I know with my other three MG's, 5 is a cold plug. I would have thought with the low compression V8, a warmer plug was required or at least a 6 like on a B? 7 being warmer yes? At any rate, some PO had installed a Sports Coil (which I replaced) and had gapped these plugs to over 040"!! So IMO this accounts for the misfiring. I always set mine at 028", I am not one of the believers that say you can go to 35, this didn't work on my A/B/C so I only ever go a few thou over the factory spec. I was recommended by an engineer friend of mine (Hans - supercharger site) to only go 2-3 more.

The electrodes of the plugs didn't look in very good nick either. I think these plugs could have been in there a very long time. I really can't believe the total lack of maintenance our cars receive sometimes.. Oh well, now I have had the car only two weeks, at least I have familiarised myself with the igniton and fuel system and got myself a set of tools! <VBG> The dwell was also a mile off. I replaced the plugs with a set of Uniparts (sorry don't know model) from Brooklands in Northern Ireland. I took her through 1 1/2 hrs of traffic chaos through the centre of Dublin today with absolutely no signs of dying!

Oh and to Nigel, sorry I didn't mean it to sound like all V8's were unreliable I was just meaning mine! :) Hey it isn't now add another reliable one - (all digits crossed <G>)... :))

Cheers & thanks again all,
Neil Cotty

This thread was discussed between 07/01/2001 and 10/01/2001

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