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MG MGB Technical - Wierd or what?

I had driven approx 35 miles on the motorway today at around 75mph ish in o/d top. My car was running fine, when suddenly it felt down on power and made sounds like it was spitting back through the carbs, the speed dropped to just under 70, so I took it out of o/d and it was fine? then after about 30 seconds I put it back in o/d. It ran fine for a couple of miles, all power back etc, then did exactly the same, so took it out of o/d and fine again.

It did this 3 times all in, then ran perfectly for the rest of the way.

By the way, it had over half a tank of fuel, I've changed the fuel pump for solid state pump, and it has petrol filter just before the carbs. Its a 1971 BGT as well.

Any ideas? as it seems that any journey of a decent distance feels like playing mechanical roulette!

Cheers
John
J M Collins

Hi John.

I wonder if there could be an intermittant short in the overdrive switch area which is robbing the coil of power with the switch engaged ?... is your switch on the gear knob ?.

Don
Don

Hi Don

That would be worth checking. The switch is on the dashboard, but it doesnt always kick in first time, sometimes you have to wiggle the switch to engage o/d, it always switches out ok tho.

I had thought of changing the switch to cure this, but in light of todays problem, then perhaps that becomes more of an incentive to change it.

Thanks
John
J M Collins

John - The next time this happens, try removing the gas cap. If the problem stays away with the gas cap removed, then the problem is that the tank vent is clogged. You don't say what year your car is, but the early MGBs (up to at least 67) had a vented cap, while the newer ones are vented through the charcoal canister. Good luck - Dave
David DuBois

Sounds stupid, but something similar happened to me, and it turned out that one of my gross jets worked loose, flooding two cylinders and causing the engine to bog down. If I let the car "rest" for a few minutes it would start right up again and seem to run fine, but a few miles later I'd have to stop again.
Adam Birnbaum

Was the weather wet and cold?
Chris at Octarine Services

this sounds like fuel starvation , I lost the fuse on the fuel pump once , the car ran brilliantly until it emptied the float chambers , then suddenly went very dead. Might be worth cleaning out or substituting the fuel filter. Chris above may be thinking that this sounds just like carb icing and he is quite right that this happens in cool wet not neccesarily below freezing conditions . When you stop , heat soak melts the ice and the engine re starts so it's difficult to find .
S Best

Yup, happens to me every time after steady speed running on the motorway when it is damp & cold.

Switching the OD out and the increase in revs may clear it.

I find that varying speed helps but eventually I end up with an idle speed of 4000+ rpm as the throttle is iced up in the open position.
Chris at Octarine Services

Its a new fuel filter and Ive checked to make sure that its the right way round.

It was damp/raining & cold though, but I didnt think it was cold enough for icing of carbs? It was driving into wind though if that makes any difference. Any way to prevent this?

J M Collins

The icing of the carbs is caused by the latent heat of vapourisation of the fuel being drawn from the ambient , a domestic fridge exploits the same phenomenon using pump to condense the coolant again through pressure . Once you get to below 0c in the choke in a damp atmosphere ice starts to build up , this can happen at ambient temps well above freezing . Many a British Lancaster crew failed to return due to British classicaly educated "Generalists" at the Air ministry not understanding this bit of 5th form physics .I haver seen it on the B, my 99 Norton Domie with twin Amals never missed a chance though .This is also why cold boxes can be so effective , but clearly an everday driver has to have intake air at a temperature that will always work . Sorry to go on a bit
S Best

Would a good fuel de-icer (Isopropyl) help prevent this?
Robert Dougherty

I think the alcohol only helps the fuel to vaporise by virtue of having a lower boiling point it would have to be a substantial part by weight of the fuel , and have a dramatically lower latent heat of vapourisation to reduce the heat flow required . As anyone who has had some alchohol evaporate from the skin on their hands knows , this is not the case .There are textbooks where people have done the sums . It's the sort of thing I would do if a client was paying!
S Best

I've been driving SU-equipped (and other) cars including MGBs in all weathers for over 30 years and I've never had icing - or if they were it was never enough to affect running. But then I've never had vapour-lock either which is another 'cause' of problems which keeps being raised - under different climatic conditions of course.
Paul Hunt

I think it depends a lot on the set up under your bonnet - if you are running standard air cleaners with the curved spouts then you are picking up warmed air from behind the rad and the exhaust manifold - hence icing should not be a problem.

At the other extreme a pair of K&N filters in an engine compartment which does not have the rubber seal round the rear seam will be most prone to icing.

The other factor involved has also to be the mixture and timing settings of the engine, lean mixtures and advanced timing make the engine run hotter, rich mixtures and ideal timing will produce less waste heat and more cooling effect in the venturi.

My V8 draws cold air straight from the rear of the engine bay which enters from the high pressure area in front of the screen and it set up for max power, rich mixture & moderate advance.

Solutions to the problem are to heat the carbs, the fuel or the air.

Since heating the air will reduce power and heating the fuel could be dangerous, my intended route to solve my problem is to devise an electrical heater element to wrap round the carbs which can be turned on in damp cold conditions.
Chris at Octarine Services

The electrical power is not free and you would need lots of it , exhaust heat/water heat is and in huge amounts you will get more power onto the road by setting up an arrangement that draws part of the air from over the exhaust manifold with a controlled flap or a heat exchanger controlled by a water valve . Clearly actually fitting this in may be easier said than done .
S Best

Hi all.

Heating the carb body using the cooling system (yes, it sounds silly but think about it) might be better.

Don
Don

Not difficult to fit at all - the exhaust heated air controlled by thermostatic flaps is exactly what the factory V8 had!

I was looking for a neater solution - the idea of heating using a copper coil fed from the hot water system is attractive but I am not sure whether I could get the coil to the critical spot which is the body round the venturi and throttle plate, whereas heating resistance wire could be wrapped round the carb body with relative ease.

Chris at Octarine Services

I think the carb body is aluminium which is an excellent conductor "all" you need to do is get a controlled flow of hot water in pipe which is in solid contact with the outside of the choke , thermal conduction will do the rest. You could use flexy for 90% with a centimetre or so of Al or Cu pipe which must be solidly attached to the carb body , a thin film of air will stop the heat transfer. Solex twin chokes were designed like this , and it drove the auto choke as well , atleast it did when it felt like it .
S Best

Hi all.

Perhaps the neatest way of electrically heating the carb body would be to use an aluminium cased power resistor.
These have one flat surface which is intended to be clamped to a heat sink, preferrably with some heat conductive paste (a special white grease) between the resistor and sink.
Some modern cars use such a resistor as coil ballast, they are usually gold or silver in colour.
A ballast one would be ABOUT 3 ohms, I guess, which would consume about 4 amps at 12V, giving a power dissipation of 48 watts, which might be too high.
Wiring two of these resistors in series (one per carb) would give a resistance of 6 ohms, resulting in a current of 2 amps, and a power dissipation of only 6 watts each, which might not be enough.

Resistors of this type are availible in various power ratings and resistance values from electronics suppliers.

Morris Minors have an air filter housing that can be manually rotated to point the air intake tube at the exhaust manifold if required.. I guess that this would be too much trouble for the modern driver !.

Don
Don

Assuming a rail of 14 v a 6.8 ohm resistor would give 29 watts . You would prolly have to attach it with gold loaded epoxy , and control it from a switch in the cockpit .A reminder light , or a 555 based timer circuit would be sensible additions you might be able to use a timer relay close to the load to give you a set time and avoid running heavy wires a long way . Last time I saw a new car with a summer/winter intake was when my father bought a 1980 Opel Kadett 1.3 .
S Best

Taking warm air is the simplest solution to this. As some of you car buffs will know, some downdraughts with auto chokes have water running through the carb body which must offer some heating. The Pierburg carb on VW's circa '91 has a heating element in the carb body which would be adaptable for an SU, but I can't see it being worth the effort.
I agree with Paul that SU's are not prone to icing. I've only had it when running the engine in the garage on a cold damp winters day, when the carbs get literally freezing cold at fast idle. The twin choke Weber on a Gilbern I had iced up all the time, but there was no hot air feed and a very open engine bay.
In the MG context I don't think K&N's add much except noise.
Dave
Dave Wellings

Yes , the Solex carbs had a water circuit , and an econostat which made them almost as good as an SU for fuel mileage
S Best

US spec Austin Americas -ADO16 1300 to you -had resistance heaters between the HS4 and the manifold, with additional heaters around the vacuum chambers. Thermostat on the vac chamber controlled both. These would fit right on a B. The thermostats would get a bit dodgy, so I wired switch and warning light on the dash. Oddly, I never had a problem with the single HS4, but a pair of HS2s on the AA would freeze up in very cold (0-15 F)weather. Before bad running, it would freeze the throttle plates in near wide open position. I ran long distances on deserted roads, and coming into a small town at wide open was most exciting!
Generally icing happens at 32 -40 F with high humidity. I figure the AA was heating the very cold air to this point, while also melting enough snow to supply the moisture. I made an aluminum baffle to fit on extended rocker cover studs, back over the carbs and my homemade air filter for winter only use. That also helped with the problem of serious condensation in the rocker cover in cold weather.
On "vapor lock" see my post on the TD BBS. (AT this time of the year it's not vapor lock)
Happy Motoring!
FR Millmore

Further to my comments above, Moss shows the "induction heater" between carb & manifold as factory fitted to MGB w/ either HS or HIF for cold climates. They don't show the vac chamber heaters though. I mis-stated that the thermo control was on the vac heaters; it is part of the "induction heater". I have never seen a factory installation on a B, and I have never seen an original installation that didn't have both bits. I ran the vac chamber heaters only on the AA, since the IH wouldn't fit the HS2 manifold. That and my air baffle fixed my problem. Note that the vac heater will cause the chamber to expand more than the piston, causing the piston to drop and thereby richening the mixture, which you probably need under these conditions. The vac chamber heaters are a semi-flexible aluminum cased unit that just wraps around the chamber, with a terminal for a wire. They easily fit HS2-6, and supply enough heat to be useful on any car so equipped. Resistance is 3 ohm. Probably available from an industrial supply place if you can't find originals.
Regarding the alcohol additives mentioned above, the alcohol combines with any water in the fuel, preventing freezing. Whether it would also be significant in preventing airborne moisture from freezing is questionable, though possible.
The early stage of freezeup feels just like a bad throttle cable, sticky operation and slow shut-off.
FR Millmore

According to the books North American spec cars got the induction manifold heater but it was just an emissions reducing device promoting quicker warm-up. It replaced spacers for both the later twin carb and Zenith cars. It wasn't thermostatically controlled but powered all the time. UK spec cars never got it.
Paul Hunt

Who writes the books? Based on what rumors?
In the 1100/1300 factory shop manual - NOT in the "Emission Control" section - it says "Heaters are fitted between the carburetter and the induction manifold and to the suction chamber on models exported to countries where conditions of extreme cold exist." "the induction heater is fitted with the bulb of the thermostat pointing..."
In the wiring diagrams, virtually all 1100/1300 models from '62 on show it as optional or "optional/std on some"; this is well before emissions requirements.

In the (Moss MGB)picture of the IH the bit sticking out to the side is the thermo control. The feed wire is attached to this. I have been working on these cars commercially since 1964; I have serviced hundreds or thousands of BMC/BL cars, and I repeat, I have never seen an MGB with these fitted, so it is NOT an emissions required item. The wiring for it is not shown in any of the MGB diagrams I have, but it is only a single green lead, which would have been easily added if they were fitted as an option. Perhaps our friends in Canada or Scandinavia have seen Bs with them. It would seem a necessity in seaside northerly places like Scotland.
On cars with them fitted, they are all thermocontrolled, never always on. The IH is app. 2 ohms = 6 amp; the vac chamber unit is 3 ohm=4 A. So we have 10 Amp for 1 carb, 20 for 2. The dynamos were rated at 22A, most early alts less than 35A, and people had the silly idea of wanting lights, wipers and heater blowers under these operating conditions! I believe they were primarily an extreme cold start aid; I seem to recall that the thermo shut off at 40F measured right above or beneath the manifold, which is too soon for some icing conditions. While they do allow earlier choke shut-off by providing better vaporisation, and so reduce emissions, they existed before emission controls.

My initial point was to let folk know that there are factory parts which will cure icing, and I have tried same with success. There is a nice 9.5 ohm alum. cased resistor which is part of the wretched Lucas electronic ignition system, usually left on the car when the rest is thrown out. At about 20W at 13.5V, It could no doubt be used as a gentle heat source, preferably fastened to the bottom of the throttle body.
Cheers!
FRM
http://www.usachoice.net/gofanu
FR Millmore

Can't speak for the 1100/1300. For the MGB Clausager states in his Emissions section that from 1975 on (and not before) North American spec cars got the "Zenith carb with an automatic choke device and an induction heater for the inlet manifold" i.e. all cars after that time regardless of market climate (and after some point North American spec cars were supplied to all markets outside the UK), and many have spoken of it on their cars. The wiring for it *is* shown on MGB wiring diagrams as factory fitted from 1975 on, not even as optional before that. Other than in Moss I have not come across any source that shows it as an optional item before that, including Clausager, even though he mentions the use of a higher-rated stat for cold climates, and other sites list 'cold climate' stats.
Paul Hunt

OK, I am willing to stand corrected on the post 75 B. I do have a tendency to forget that non-SU cars were Bs too! Most of my large scale work was before that, and I don't have a post 75 manual. My latest wiring diagram labeled "Later North American models" (after 74)doesn't show it. I have a number of post 75 customers, all but one of which has some sort of carb conversion. I don't think that one has the IH, but haven't seen the car for 2 years. It's coming soon for a wire wheel conversion.
Cheers!
FRM
FR Millmore

Indeed so: Having had a 75 US-spec B, it actually was fitted with an induction heater, and used to run in California, definitely not what you would call a cold climate.
Jochen Beyer

This thread was discussed between 18/04/2004 and 29/04/2004

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