Modern fuel

[Martin Evans]

My mother has a basically standard 1978 MGB GT (Which hasn't been used a lot of late). It's done about 63000 from new and was converted to unleaded/gasflowed in 2003. It gives 91bhp (Which I understand is 7 up on the standard for a late car; can anyone confirm what late UK cars were meant to be?). It drives well but of late, if you get into stop/start traffic, it starts to misfire and loose power. Normally, this is about ten miles from where they live but on Tuesday I drove it five miles, parked it and when I restarted the car, the problem occurred. The symptoms are that if you open the throttle quickly, it loses power; if it's a bad attack, it will stall. If you able to keep moving, it doesn't miss a beat, though once the problem has occurred, it seems to need to be allowed to cool down fully, before it will run properly again. It doesn't overheat in traffic.

I have heard that modern petrol has a short shelf life and I have been advised to use Millers anti ethanol treatment, even with the super unleaded that we always use. Since the problem seems connected with a build up of heat, I am wondering whether the fuel is boiling in the float chambers. So far, the other cars we have have not been affected but they all have heavily vented bonnets (I have removed the sound deadening felt from under the bonnet of the B, as I am sure this keeps the heat in) or in the case of my 1275cc Morris Minor, more room above the engine. The MGB was last filled with fuel in February. I have observed the inline fuel filter and note the odd bubble as the fuel flows through.

This is quite a new phenomenon for me, that was brought home to me last year, when I tried to start my Midget, after I had fitted a new cam to the it and de coked the top end. In the past, when running on 4 star, it would always start after a long lay up but last year it just futted and wouldn't pick up. I disconnected the fuel pipe and pumped the fuel out into a can, put in a fresh gallon and it went first time. Has anyone else suffered with late model MGBs, when running in stop start traffic? If so what was found to be at fault?

[Charles Farran]

Hi Martin
I have a 1980 roadster which i have had from new. Like us all i have experienced problems similar to yours which are not necessarily caused by the same fault.i have had a waxed up fuel filter that intermittently caused poor running; carburettor pistons which clearly caused a similar experience(a quick removal & clean sorted this) & the hardest to spot was the fuel hose linkig the two carbs that had clearly lost its internal membrane (i spotted this by opening the thottle under the bonnet with air filters off and could see the hose flexing up which was then starving the rear carb of fuel).The car definitely runs better when used regularly & fresh petrol . Personally i try to use Shell V power or Tesco pentium & use castrol valve master + ( i don't think i need to use the "+" valve masrer but have found the engine runs smoothly (standard & unmodified from new/ original head/75000 apart from 123 electronic ignition ).i have left the bonnet insulation in place albeit replaced. IS your carb heatshield still in place? Running with that fuel combination is giving me plugs of the right colour & my MOT 2 weeks ago gave a reading of 3% CO at idle which is as per the hand book.
I think from memory that the standard BHP was 90,but can't quickly find the reference so stand to be corrected on this point.Have you checked the plugs & plug gaps?
Cheers, Charles

[Charles Farran]

Hi Martin,
It is not easy to arrive at what bhp a late standard MGB should be & how it is measured (under load on a rolling road).It is worth looking at Peter Burgess's website to get some understanding about the subject.
Cheers,
Charles

[Martin Evans]

Thank for that. The heatshield is still in place but I haven't looked at the plugs yet. It isn't that many miles since they were cleaned but it will be something I will look at; the car has tended to be serviced each MOT time, even if that means only 1000 or so miles. I tend to clean the plugs after 5000 miles, rather than take them in and out too often.

I will certainly look at the other things you mentioned but whatever it is, it's made worse by loss of airflow. When we run the tank down a bit and try fresh fuel, if that solves it, we'll know the answer.

Maybe some of the classic fuel would be good to keep, in case a longer lay up is anticipated. IF electric cars ever become feasible, we may be forced to buy our fuel in drums and store it. I certainly thinks it's better nowadays to run the car nearer empty after a run, than fill it up ready for the next time.

I like the sound of your having had the MGB from new. I wasn't quite old enough to buy one new but I was one who hoped to see Abingdon saved back in 1980 and when I bought my five year old Midget, in 1984, I bought it with the idea of keeping it....and I have!! Like the MGB, mine has an overdrive.

[Vic Butler]

I have had no trouble starting my 1977 Stage 2 BGT after hibernation except when the choke wasn't operating correctly. The head is bespoke built for me by a former Downton Engineering employee who was their ace cylinder head man. The HS6 carburettors are genuine BL Special Tuning ones purchased in 1981 and rebuilt by BFS in about 2006. The distributor was rebuilt by the Distributor Doctor to suit the state of tuning. Pancake K & N's with stub stacks are fitted as is a heat shield. The exhaust and manifold is a Double S 2 inch bore stainless steel system and the manifold is lagged. There is an 82C thermostat fitted and there are no overheating problems. Only Shell V Power is used.
The heat in the engine bay is likely to be coming from the cast iron exhaust manifold but cast iron ones must not be lagged otherwise they will crack.
The late rubber bumper B's with the forward mounted radiator and the vacuum advance taken from the inlet manifold must use the correct distributor which is number 41610. Anything else will have totally unsuitable advance characteristics.
My MG is tractable in heavy traffic as it is happy on the open road. The electric fan cuts in when it should and at 70 mph only a whiff of throttle is needed to keep it going.
. Vic Butler

[Charles Farran]

I had a T registered Midget with a 1500cc (which would have benefited from an overdrive as i always felt the gearing wasn't long enough ) from new before the MGB .One further thought on the MGB is that if the temp rises under the bonnet when sitting in a queue,the petrol in the carb area evaporates, when you increase the throttle more fuel is sucked through which floods the carb & creates a too rich situation & the engine struggles. Once the engine has cooled down, the excess fuel has evaporated & the problem goes away. Do you get drips from the overflow pipes when the car has displayed the problem you have?
Cheers,
Charles

[Martin Evans]

I will check the overflows.

As to the exhaust manifold, it is the OE iron job. All the other cars we have are fitted with tubular ones, which are also lagged (Except the V8, which I have never attempted to lag but they are at least the other side of the heads to the carb. Added to that, both it and the Midget 1500 have vented bonnets.

Going off topic (As these things often do) my Midget was rolling road tuned by an ex Downton man called Peter Vaughan. It runs on Dellorto carbs and so it had to be set up for that. Going back further, when my father had a Lotus 7, with a 948 A Series, Daniel Richmond used to hillclimb a similar car and beat the ones that had Coventry Climax engines. Of course Daniel Richmond could drive and I'm sure that in amateur motorsport, the driver often matters more than the car (Within reason). I went to classic Le Mans in 2014 (In the V8) and the category for the late 1950s was won by (As you might expect) a D Type Jaguar BUT 2nd (Could have been 3rd) went to an Austin Healey 3000, which clearly had someone rather good at the wheel. Anyway, after the 7, my father had an early Austin (Mini) 7, which had some Downton goodies on it. That was when the national speed limit sign meant de restricted and one occasion, a local Police officer followed my father on the original road to bypass Merthyr Tydfil (An uphill drag known locally as the Slip Road), to see how fast he was going (Not with any intention of booking him, just out of interest). The Policeman was in a Zephyr 6 and all he could say was that the Ford was doing 75mph and the Mini was pulling away.

[Peter Cresswell]

As far as I can determine all UK spec MGBs sold in the UK had the high(er) compression engine and gave a quoted 95bhp. This is despite the different cylinder head castings used through the production life and despite some of the heads having larger inlet valves as standard. The 95 bhp was a bench tested engine (or flywheel power) without dynamo or alternator and fan being fitted, and an installed value of 90 bhp is often quoted as normal. Rolling road figure take into account the losses through the drive train, and are usually 2/3 rds of the flywheel power so around 60 bhp would be normal. This concurs largely with the information Peter Burgess quotes in his book of 64 for early and late cars, but 66 for the cars with the larger inlet valves. To maintain these figures over many miles is unlikely unless the head in particular is carefully maintained by reseating the valves every few thousand miles, which of course you are not going to day for an everyday car.

I think Charles and Vic are on the right lines as regards the sudden loss of power or difficulty starting when the engine is hot that Martin is experiencing. I would add that if there are fuel filters fitted, they should be cleaned or renewed, and if the flexible fuel lines have not been replaced with ethanol compatible pipes then the is a growing number of reports of cars with sudden rupture of the fuel lines which is caused by the ethanol content. Older fuel pumps had a rubber diaphragm in them which is not ethanol resistant either.
In my up speccing of my B, I have added a 4in cold air duct from behind the grille that passes through the radiator support panel to get more cold air to the carb side of the engine. I'm not sure if will make a difference given that the car didn't overheat in 35+ deg in France last summer, but it must help!

[Vic Butler]

I forgot to mention that the spark plugs in my BGT are the 4 electrode Bosch ones.
. Vic Butler

[Charles Farran]

Hi Martin,
Three further things to consider.
I am not sure when the fuel filter was changed/ removed to examine.Was it put on the right way round? (From personal experience, i changed mine last year & neither i nor my neighbour who is also a classic car nut could see any direction of flow arrow on the filter but convinced ourselves which way round it should go.The car ran alright at slow speeds but when opening the throttle it started to struugle.After checking everything i could think of i shone a torch out of desperation at the fuel filter & just caught sight of a poorly stamped arrow pointing the wrong way! I replaced the fuel filter, not risking that i hadn't damaged the internal membrane & the car then ran beautifully )!
Are the carb overflow pipes blocked as they are prone to this at the bottom on the side of the engine block. (Also check the rubber connectors where they are joined to the outlets to the carbs. (Blockage of these can create your problem if the car is idling ,rather than burning off the fuel as demanded at cruising speed.The fuel can't escape & floods the engine making restarting difficult).
If the car has original style saucepan air filter cans & f they have been removed recently to change the filters, have the metal spacers between the carbs & the back of the cans been put on the right way round with the 2 breather holes at the top to line up with those on the carbs & are both gaskets in place?
Cheers, Charles