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MG MGB Technical - Poor running during fan cooling cycle. - Solved

I have been having a persistent problem with my 73B, which when stationary in traffic, the tick-over becomes erratic, usually reducing, as the electric fan runs through a cooling-heating-cooling cycle. When needing to pull away from stationary after several minutes the engine will splutter, hesitate on acceleration and misfire on a minimal throttle when crawling in traffic. This problem generally clears once moving and the engine revs are raised. The onset of this problem happens sooner as the ambient temperature rises. This has been a characteristic for many years and I have struggled to resolve it - until now.

I have read widely on these boards, which I consider THE place for MG knowledge and are grateful to all contributors, looking for a solution. In resolving this problem I have read of many possible causes and solutions relating to LT, HT, cooling system, fans - electric and mechanical, timing, radiators, sludge in the block, MGs work fine in the US and Australia there is no problem, fuel vaporisation doesnt happen in MGs. and Im sure you can add to this list.

In the UK we have unusually hot weather at the moment so there is a great opportunity to experiment with this raised ambient temperature. I wanted to prove if or if not this behaviour was due to fuel evaporation in the float chambers. How to was the question. I am confident of the integrity of the cooling, HT, LT, and timing of the engine are all in good condition. I will post a future thread on why I know the HT is good, - mainly down to well documented new, poor quality HT components.

Working on the principle that vaporisation occurs more readily with raised temperatures of the float chamber, (HS4), reducing any rise in temperature of the float chamber should prevent this occurring. What could drive the raised temperatures of the float chamber in the engine bay? The exhaust manifold, the fan/radiator and no or little air flow through the engine bay as the car is stationary or crawling at <5mph. I am assuming that the heat shield is functioning as its condition is good. (newish, cadmium plated with riveted asbestos).

To protect the float chambers from these sources of heat I used some glass-fibre heat shield matting (see image) which I cut to shape to wrap around each float chamber. (see image of rough template). This matting has one side covered in a silver foil - aluminium I expect. Working on the inverse principle of a hot water tank, the heat shield wrapped around the float chamber should keep the fuel in it cool. I tie-warped this shaped matting around each float chamber as a way to provide insulation to prove the argument ahead of coming up with a better engineered solution. (See images attached). As youll see it is in-elegant but if this insulation did not solve the problem, I wouldve not have permanently changed the car. It would have just been a matter of cutting the tie wraps and going back to the drawing board.

Results:

Yesterday I drove 60 miles to Reading and back on the M4 and around West London when the temperature, according to the met office, was 35C at Heathrow airport using airport transfers tonbridge. I sat crawling on the M4 and around the suburbs and no such misfiring, hesitancy, spluttering was observed. The electric fan ran through its cooling cycle many times and the tick over was much more stable. Acceleration from standstill with the temperature gauge at its highest was as if the engine was operating at normal temperatures. - This gives much more confidence when queueing to enter a roundabout and not having a spluttering, powerless engine on your hands.

Initial conclusions:

Fuel evaporation must be occurring in the float chamber and starving the engine of fuel, or making the mixture very week, but why?

Conclusion:

It is my belief that air being blown through the radiator, by the electric fan, is heating up the float chambers as the air through the radiator can only initially pass either side of the engine block; past the alternator, oil filter, and distributor on the right, and the carburettors to the left. Assuming that the heat shield is doing its job, then this air flow is partially negating the heat shields effectiveness; effectively warming the carbs, which are on the cool side of the heat shield, causing the evaporation through this heating effect.

Thinking about this cycle, the fan sends a pulse of heat to the carbs, and the rest of the engine bay, and rapidly heats the float chambers and the fuel therein. There is only a small amount of new fuel entering the float chamber, as its not being emptied by being burnt in the engine, so no cooling fuel enters the float chamber. As the fan cycles the temperature of the float chamber gradually rises and the fuel evaporation increases and starvation increases. This manifests itself as increased spluttering/misfire which is a function of the number of cycles the fan goes through. When forward motion eventually restarts, everything cools until the next traffic queue two miles down the motorway and it all happens again.


Solutions:

- Design lagging for the float chambers
- Work out a hot air deflector around the front carb.
- Revert to mechanical fan as the cooling cycle is longer.
- It draws air through the radiator the instant the car stops moving forward, whereas there is a delay between that instant and cooling starting, as no cooling starts until the max temp is reached then the fan stat kicks in.

Im now researching how to make a lagging jacket for the float chambers in a different material because this fibre/aluminium mat is not ideal as it frays easily.

Ill post what this solution ultimately is and I hope this helps others. If anyone has a suggestion on a good material for this purpose please let me know.

Richard.



Richard Thompson

image2


Richard Thompson

image3


Richard Thompson

As it works, well done! But yes, I'm of the "They run in the US desert states" belief.

What happens if you have the fan(s) running constantly? ie no pulse of heat.

As for material I made a heatshield (since discarded!) from this stuff. It can be cut with tin snips and "shaped". I used it more recently to protect the cable from a temporary lambda sensor that ran up past the exhaust downpipes.
Michael Beswick

Richard, it's a topic of much discussion AND argument. Ethanol laced petrol is also a touch more volatile. My 1800's always had your issue and that was in the days B E!!
I habitually insulate all the engine bay fuel lines. When I did engine work I installed a SS tubular manifold and "wrapped" it. Some swear by ceramic coating too.
On my V8 (a facial sauna when you lift the bonnet), I have drilled large holes in the N/S rad support to at least let ambient air in when the car is moving. I have also removed the centre section of the rear bonnet seal which allows some of the hot air to escape when you are stationary, or nearly so!
I've often toyed with the idea of installing a small 12v fan to throw ambient air in the direction of the carbs, but haven't done that yet.
Allan Reeling

I'm glad you seem to have found a cure, but I'm a bit confused with your theory. Are you saying that you think the float chambers are becoming partially empty due to fuel evaporation? Only that can't happen because the fuel pump is continually topping them up. What do you think is happening? I'm intrigued.
Mike Howlett

Sorry missed the link!
https://www.merlinmotorsport.co.uk/s/heat-insulation/zircoflex-heat-insulation/zircoflex-non-adhesive-heat-insulation
Michael Beswick


Thanks for the comments and questions.

Michael,

I haven’t run a continuous fan test but this would be interesting to do. I’d expect that the carbs would gradual gain a raised temperature and the engine would run, run badly or ultimately stall. Thanks for the link. I’d found ZircoFlex in my search for a material and they make a self adhesive heat shield that I’ll investigate.

Mike,

Good point on the pump. In a nutshell I have only proved that shielding the float chambers from heat solves the problem. What actual is happening I haven’t pinpointed.

Thinking about your point. If the pump fills the float chamber and the valve closes and then the fan comes on, this will warm the float chamber. As we know warming things expands them. Maybe the fuel in the float chamber expands and the only way it can go is out through the jet onto the bridge in the carb. May be it’s sufficient fuel to cause mild flooding, rather than vaporisation as I originally thought, which could extinguish a spark sufficiently to give bad running. Once on the move again, the airflow dries up the excess fuel and off we go.

Richard.
Richard Thompson

My theory has always been that the heating of the fuel chambers does NOT cause a weak mixture, it causes the fuel to expand and gives a RICH mixture - hence the stumbling that clears as soon as you splutter away and the fuel level drops.

I have also found that if the overflow pipes are blocked by grease/oil/dirt the engine runs very badly especially when hot.
Chris at Octarine Services

Chris' theory seems to make sense.
Mike Howlett

Chris, Mike,

Thanks for your thoughts and I think you're right, having thought further.

This heating effect is causing enrichment similar to pulling out the choke on an engine that's at normal operational temperature. The engine will splutter if this is done.

Thinking about preventing this heating, I’ll start on a design for a heat shield rather than sticking lagging to the float chambers as is might look better and could be easily reversible.

Richard.
Richard Thompson

In looking for solutions, has any one had experience of ‘Cool-it Thermotech Hi-Heat Coating’? If this works then painting the floats with this would be a discrete solution.

https://www.merlinmotorsport.co.uk/p/thermo-tec-hi-heat-exhaust-wrap-coating-black-ex-wrap-aero

Richard.
Richard Thompson

float chamber, not floats!

R
Richard Thompson

Reading the blurb on Merlin's web page I would think this spray-on product is not an insulator, simply a paint that can withstand very high temperatures.

I have used the bandage type material to wrap the exhaust manifolds on my V8 and this does help to keep under-bonnet temperatures a wee bit lower. When you have experienced the heat generated by a Rover V8, you wonder why an 1800 MGB has any problems!
Mike Howlett

Richard, when I refurbished the heat shield on my roadster and got rid of what was left of the old (what looked like asbestos based) heat shield backing, I used fibreglass cotton matting. This has worked really well and I've had no problems even in the recent very hot weather. Like you I have an electric fan fitted (Revotec) and the only difference to your car is that I have HIF's. I'm not sure if HS4's are more prone to this sort of problem?

Here's a link to an example on ebay

https://www.ebay.co.uk/itm/6Pcs-Heat-Shield-Mat-Car-Exhaust-Muffler-Insulation-for-hood-Fiberglass-Cotton/282051257136?epid=1789398807&hash=item41ab90a330:g:clUAAOSwbsBXmtix


Andy
Andy Robinson

Mike,

Yes, I'd agree - I initially thought that ie might be an insulator.

Richard.
Richard Thompson

Hi Andy,

Interesting point. I have a Revotech fan setup too. I also understand that HIFs don’t suffer to the same extent as the float chamber is integral rather than remote. Being integral to the HIFs, the float chamber will have a greater heatsink in the body of the carb compared to the HS4s. This will have a slower heating coefficient compared to the HS4.

I’m looking into how the float chamber could be ‘ceramic’ coated as a heat shield, so that the under bonnet appearance is largely unchanged.

I’ve contacted Zircotec to ask if they could coat the float chambers rather than sticking insulation material to the float chamber or constructing a heat shield. This would maintain the original appearance. I’m awaiting their reply.

https://www.zircotec.com/

Any thoughts on this plan would be welcome.

Richard
Richard Thompson

Rather then covering the floatchambers could you extend the heat shield so that the hot air from the fan was ducted away down and sideways from the carb and air cleaners. Another quick test would be to pull the bonnet so the hot air escapes over the wings (you're stationary!).

This leads on to how to let the hot air out: removing the rear seal of the engine bay is often used.

As it's a Revotech can you set it to come on at a lower temperature? This would prevent it getting as hot so that on cutting in, the hot air blown past the carbs was not as hot.(it would start earlier of course)
Michael Beswick

I once fitted a vertical sheet of aluminium to the edge of the radiator opening so that the hot air from the rad was directed onto the exhaust manifold and under the heat shield for the carbs. With the large hole in the rad diaphragm letting cool air to the carbs I never had problems.

Removing the seal at the rear of the bonnet actually lets cool air IN at speed as it is a high pressure area in front of the screen. Hot air will only exit at a standstill.

Better is to cut holes in the inner wings since the wheelarch is a low pressure area at speed and will suck hot air from the engine bay.
Chris at Octarine Services

Never had a problem like this, roadster or V8, including with the V8 stuck in traffic on the M6 and M42 in mid-afternoon in the last month more than once. At switch-off I can hear the fuel boiling in the carbs, and they boil dry, with fumes pouring out of the vent hose and even dripping as it condenses lower down.

I can't see how the float chamber can empty - unless the pump stops pumping.

And whilst expansion and a rich mixture IS a possibility, that will stop any more fuel entering - assuming your float valves are working correctly! and the level will soon drop anyway. Expansion is more likely to cause a hot start problem.

If the vent pipes are blocked the floats can't rise, the pump continues to pump, and neat fuel goes down the intake - very obvious from the clicking.

The 'desert states' comment is the one I always make. If it were a general problem loads of us would be suffering from it, and would be well known over the last 40 years or so. As far as ethanol goes America has had it from the 70s and far more exotic brews, and don't report problems like this, including in their hot weather which is hotter than ours.

I just don't buy that it's a generic problem, just one with your car, but what is another issue.

Having said that HSs do gradually bog down in my experience of those on the roadster and HIFs on the V8, which I one of the reasons I keep my idle speeds at about 1k. The HIFs don't suffer, either because of the thermostatically controlled jet, or the bypass passage from just after the jet to just before the butterfly, but I'd rather keep a bit of visible oil pressure especially with the cooling fan running.

paulh4

Hi Paul,

Thanks for your comments.

I agree that this is unlikely to be a generic problem but my car is not the generic model, as it has an electric fan, K&N filters, different needles and an electronic ignition. This is probably sufficient to push it outside the normal operational envelope for a generic, factory definitive car. - I guess this is the penalty for modifying the car away from standard. If it was a generic problem then I guess Abbingdon would have engineered a fix. However it does keep the grey matter exercised and our interest in our cars up.

I've ordered some of the heat shield material Mike suggested and will start on a better lagging design to that which I bodged up to prove what was causing the poor running.

Richard.
Richard Thompson

Thanks for this useful posting Richard. Last time I had the car sprayed it returned with the later sealing strip around the bonnet. I left it there as you can get a wet boot with the MkI arrangement of rubber stops at both ends of the drain channels.
I do suffer from overheating issues and its my fault as the car is modified. I also have Revotech fitted.
I removed both the sealing strip and the top rubber bumpers and the bonnet works fine just locating on the front two and the slam panel. To minimise restriction in the water channel I put a cut to size piece of duct tape to seal the screw holes and stop water being dumped onto the voltage regulator. I adjusted the cut in temperature of the Revotech until the car behaved itself in traffic.
With this arrangement I no longer get water coming in through the pedal hole and the car doesn't hesitate in traffic. We did the Wallingford parade on hot day and the car was perfect, it always used to boil its petrol before that.
I do think its lean running as one year at Kop Hill the car ran beautifully in the first session, and then suffered fuel starvation after the 2nd to 3rd change on the second run. The temperature had climbed and we queued a lot longer in the second one. This is the good ascent, it had to be lack of fuel that caused the problems in the second one. The car run flawlessly on the return circuit and going home.
https://www.youtube.com/watch?v=nF_2sGUt6gM
Stan Best

HSs can be affected by heat on the jet pipes if the heat shield is damaged.

From deliberately running weak for the MOT recently I've discovered that HIFs can also be affected - the thermostatically controlled jet can weaken the mixture still further and cause problems, if there is insufficient air circulating through the engine compartment.

But apart from that i.e. if everything is as it should be then you shouldn't get problems, especially in the UK, even in the temps we have had recently. Richard's mods shouldn't be enough to cause a problem. I was stuck several times on the M6 in 30 degree heat stop-start and crawling, the V8 didn't have any problems, and that has K&Ns and different needles, electric fans as standard. Also fully sealed half-way up the sides and across the back of the engine compartment as it should be.

It's still very clear to me that if you get problems, then there is a defect somewhere.
paulh4

This thread was discussed between 27/07/2018 and 13/08/2018

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