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Carb shield & new fuel lines for heat issues?


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I could do with some wisdom and opinion...

 

My TR4A has its original SU carbs. The engine was rebuilt over 20 years ago with the head updated for unleaded, but I'm not sure if the carbs were rebuilt. It has the original hard metal fuel pipe in place which runs from the pump, around the top of the thermostat then back to the carbs.

 

The present problem is that the engine will not re-start once hot, which unfortunately means I'm using the car less and less. Stop for fuel and you need to wait 20mins for the engine to cool to get a re-start. I suspect the problem is fuel getting too hot in the engine/carb area?

 

I live in the Lakes, miles from any specialised garage, hence I'd prefer to try some solutions before I take the step of taking it to a specialist - although I'm well aware that the engine probably needs a rebuild sooner rather than later.

 

Questions -

 

Do you think installing a heat shield behind the carbs might help? Ditto for changing the fuel pipe for a flexible metal-weave one?

 

Would a fuel additive help; if so, which one?

 

Could a carb rebuild help? Any suggestions on specialists?

 

Any other thoughts?

 

Many thanks!

 

 

 

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My thoughts are that your setup sounds to be completely normal, & the majority of TR's don't suffer from this problem.

I would therefore not concentrate on different fuel lines, but look elsewhere for the fault - maybe the float levels are wrong, or the valves sticking,

how about the fuel pump, is that pumping out fuel ok?

Even suspect ignition coil - they do not like to get too hot.

 

Good luck

Bob.

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I had this problem on my carb TR6 which got SS exhaust manifold. I installed a heat shield under the carbs, I used 1mm alu covered with heat shield material. I kept the original metal fuel pipe but got some silicone heat shield covers for them and the problem is now gone. Maybe modern fuels are worse in regard of this problem? On my car the SS manifold is probably the main cause though.

 

Also suspect the coil as my car would always start and run although not very well.

 

Magnus

Edited by TRseks
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My TR4 runs on Strombergs but I had fuel evaporation issues. I used Agriemach Cool tube 1/2" x 3 ft

split tube Stock code TD010414. It's good up to 500 degress F or 260 degree Centigrade.

As it's supplied as split tubing , it doesn't take too long to slip over the fuel pipe run from pump to carbs.Modern fuel seems to run at a higher temp.Never had a problem since.

https://www.agriemach.com/p428-cool-tube-1-2-x-3ft-slit

Bob

Edited by bob-menhennett
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My TR4A has not this problem and I don't know why yours should have it.

Try this: drive the car until it is "hot", make a stop, stop the engine, instantly open the bonnet to let the heat out.

Then make a restart when you expect the starting problem.

Does it start or not?

Edited by Z320
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Hi Clive,

when cold do you need much choke to get it going.

As mentioned what colour are the plugs.

 

Usually if the carb is worn it gives a weaker mixture (air getting in from all odd places) which may make it start easier when hot.

Having said that if the needle is worn it could be too rich at small throttle openings.

 

Have you contacted your local group (Cumbria) - they have some able chaps that may help.

 

Roger

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I think we've all heard stories of cars that are difficult to start when hot.

 

Why? What could the reasons be?

 

Carbs getting to hot and fuel in the float chambers vapourising?

What else?

 

AlanR

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What else? as mentioned above it could be a coil issue.

 

 

Magnus

Edited by TRseks
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Hi cliveallen ~

 

This is my S/S exhaust manifold. No1 branchpipe is double wrapped to protect the alternator and both pipes under the starter motor

are double wrapped. I sprayed the inside of the manifold with special heat resisting paint from Frost's.

The heat from 'modern' fuels is considerable.

 

Hope this helps.

 

Tom.

 

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The heat from 'modern' fuels is considerable.

 

Hope this helps.

 

Tom.

attachicon.gifIMG_1089 - Copy.JPG

Tom,

Modern fuels dont run hot.

But a TR with vac advance capsule that long ago gave up working will make cruise ignition too retarded and the engine will then run hot. The va capsule cannot easily be tested, and the take off on the carb is mm-critical too.

The answer is to replace the capsule with the correct one, and measure the vaccum while driving at cruise to see if it matches the capsule's spec. VA capsule code is here in note 2:

http://www.teglerizer.com/mgstuff/advance_curves.htm

Peter

Edited by Peter Cobbold
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Some SUs have nylon inserts on the underside of the piston to ensure a minimum air gap, maybe 0.5mm, over the bridge. If that gap has closed up I could see the mixture becoming too rich to allow hot starting.

Peter

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If the engine is hot when you stop the car, just release the bonnet catch - this will let the heat escape more rapidly and may prevent vaporisation of the fuel.

Just remember to close the bonnet before you re-start the engine!

Ian Cornish

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By strange coincidence (or not, if you believe in Kammerers law of seriality) I suffered the same problem for the first time and I am also based in the Lakes. It happened last Wednesday night traveling home in the TR4 from Penrith. We had travelled 25miles on a cool evening and had stopped for a couple of minutes at the bottom of Whinlatter pass to answer the phone, with the engine running.When I came to set off we got a few hundred yards up hill and the engine started to stutter and fail.By knocking it down a gear and dipping the clutch to keep the revs up I managed to struggle our way to the top of the pass and freewheeled a long way down the other side. The next morning it started and ran fine. My local friendly expert diagnosed that it was most likely a fuel vaporisation problem based on the route of the copper fuel line. I have a ss extractor manifold which probably does not help. I will try the fuel line wrap to see it that cures the problem. My worry is that I have not experienced this before and obviously have stopped at traffic lights etc on long runs without any problem.

It does suffer from a tendency to overrun when hot.

I will feed back my results after testing.

Would carrying a bottle of cold water to pour on the copper pipe be a temporary solution?

John S

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Question for Peter Cobbold: does the increasing percentage of ethanol in modern fuels increase the likelihood of vaporisation in the hot areas under the bonnet?

I've never had my TR4's engine die as a result of the tremendous heat in the engine compartment, but I have had it fail to restart if switched off when very hot, and this must (I feel sure) be from vaporisation. Opening the bonnet (as described in my posr #15, above) soon cured the problem on these occasions.

Ian Cornish

Edited by ianc
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Question for Peter Cobbold: does the increasing percentage of ethanol in modern fuels increase the likelihood of vaporisation in the hot areas under the bonnet?

I've never had my TR4's engine die as a result of the tremendous heat in the engine compartment, but I have had it fail to restart if switched off when very hot, and this must (I feel sure) be from vaporisation. Opening the bonnet (as described in my posr #15, above) soon cured the problem on these occasions.

Ian Cornish

Ian,

Its not a simple question ! Note that the fuel itself would have to reach the temperatures mentioned below, and that means the pipe/hoses too. And it will take time for the heat flux to raise the fuel temperature. My view is the vaporistion idea has not been proven as the fuel temperature itself has never been measured. Instead I point the finger at the carbs delivering an over-rich E5 mixture during hot starting - too rich to burn.

Even SUs in perfect tune.

 

Pure Ethanol boils at atmospheric pressure at 78C. But a mixture of small percentages of ethanol say 5 or 10% seems to cause more gasoline to evaporate at a given temperature as seen in these distillation curves.

https://www.researchgate.net/figure/271964592_fig5_Figure-5-Predicted-distillation-curves-for-blends-of-ethanol-and-gasoline-with-non-ideal

At 60C the difference between the gasoline ( dotted line) and E10 ( diamonds) is marked- 0% evaporated compared with about 25%. data such as these are used to support the idea of vapour locking. But evaporation does not necessarily mean the vapour rises enough ot cause vapour lock. The vapour pressure of petrol is tightly regulated by a British Standard, Reid Vapour Pressure. The stipulated value measured with fuel at 38C varies with season. However RVP is mainly to ensure winter cold starting, not to avoid vapour locking at higher temperatures.

http://www.chevronwithtechron.ca/products/documents/69083_MotorGas_Tech_Review.pdf

This pdf page 11 descibes the factors that best relates to hot start issues.

 

So its difficult to pin the blame on ethanol vapour lock. I'm not sure fuel is boiling in the pipe/hoses. The f.c-to-jet pipe on a HS carb might be at most risk due to proximity to exh manifold. But we dont know if the fuel in it gets to say 60C or so. A thermocouple measurement in the fuel could hlep. However we dont know the critical temperature of the petrol to compare it with. Even so a bubble of vapour in that tube would surely shift in response to suction during cranking and the weight of fuel in the float chamber. Vapour lock in the main feed pipe I dont buy because the float chambers would have to evaporate dry - there's enough fuel in them to drive 100yards or so as well as start. Overall I'm not sold on vapour locking- I look to the carbs:

 

During cranking the mixture will depend upon the piston bridge distance and 'ease of air flow' through that gap. Cold starting needs a very rich mixture indeed, AFR arund 4, because little fuel will evaporate before the spark. But a hot manifold/cylinders/pistons will evaporate all the fuel during cranking and may make the mixture too rich to burn, around AFR10, despite not using the choke. I doubt fuel level in the f/c will make any difference as long as it is not overflowing! But the piston-bridge gap might. Lifting it maybe a half millimeter would reduce the depression over the jet and weaken the mixture during cranking enough to make it combustible.The slower air velocity over the jet will make for a smaller number of bigger droplets that evaporate much more slowly and will wet the maniolfd wall better -again this weakens the mixture that passes the inlet valve for a few cycles, giving a firable mixture at the plugs.

 

Opening the bonnet to let cool air in will have two prompt effects. Denser air will lean the mixture....but only 3% for 60C to 20C. Fuel evaporation in the cool air flow will be slower which might allow more fuel to be deposited on the manifold walls, leaning the mixture that passes the iv for a few cycles. The more fuel that evaporates before the iv closes the more its vapour will displace air- richening the mixture. So cold air helps hot re-starting, in my analysis, by reducing the mixture richness at the iv enough to allow the hot engine to fire.

 

To me hot restarting might well be a carb issue not vapour locking. I think I can argue on the basis of the distillation curves that E5 will volatilise a lot faster than E0 and that makes the hot start mixture over-rich and impairs hot re-starting, even in correctly tuned SUs. The shim on the bridge is the test I would do.

 

Measures to keep the fuel cool such as shiny ali wrapping on the lines willl also help - the warmer the fuel the less heat has to be added to evaporate it after the jet - but not becaue of vapour locking. A heat shield to reduce inlet manifold getting hot will also help, but again not because of vapour locking, in my book, but by reducing fuel evaporation rate. And a cold air intake will help rather than those bling underbonnet air filters.

 

Peter

Edited by Peter Cobbold
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By strange coincidence (or not, if you believe in Kammerers law of seriality) I suffered the same problem for the first time and I am also based in the Lakes. It happened last Wednesday night traveling home in the TR4 from Penrith. We had travelled 25miles on a cool evening and had stopped for a couple of minutes at the bottom of Whinlatter pass to answer the phone, with the engine running.When I came to set off we got a few hundred yards up hill and the engine started to stutter and fail.By knocking it down a gear and dipping the clutch to keep the revs up I managed to struggle our way to the top of the pass and freewheeled a long way down the other side. The next morning it started and ran fine. My local friendly expert diagnosed that it was most likely a fuel vaporisation problem based on the route of the copper fuel line. I have a ss extractor manifold which probably does not help. I will try the fuel line wrap to see it that cures the problem. My worry is that I have not experienced this before and obviously have stopped at traffic lights etc on long runs without any problem.

It does suffer from a tendency to overrun when hot.

I will feed back my results after testing.

Would carrying a bottle of cold water to pour on the copper pipe be a temporary solution?

John S

A clue is the short distance it ran before stuttering - probably a float chamber's worth. Could be one f/c valve sticking shut.

 

Running on when hot is unrelated. If higher octane fuel prevents it then its autoignition - try a coupls of grades colder plugs and check the vac advance works ( easier said than done!)

 

Peter

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Without having the car this is a difficult one to diagnose. Carry an aerosol

of cold release agent (Loctite 8040), when you experience the problem spray

suspect items, ie. coil, float chambers etc. to try and identify where

the problem lies. Health & safety advise, these cooling products are highly

flammable.

 

Harvey S.Maitland

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Does the car start on the button when cold? ie no churning before it fires and runs.

 

No one has mentioned the non return valves in the fuel pump. Could the one on the supply side to the carbs be not operating as designed.

 

PI cars suffer with non return valve failures, usually the banjo bolt on Nos 2 & 5 making the cars misfire & run lumpy on start up.

 

or am I off the song sheet here?

 

Peter W

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Does the car start on the button when cold? ie no churning before it fires and runs.

 

No one has mentioned the non return valves in the fuel pump. Could the one on the supply side to the carbs be not operating as designed.

 

PI cars suffer with non return valve failures, usually the banjo bolt on Nos 2 & 5 making the cars misfire & run lumpy on start up.

 

or am I off the song sheet here?

 

Peter W

Pete,

But there would still be fuel in the float chambers, that wont drain back. There's enough to start and run for 100 yards or so.

If the fuel level in the f/c , or jet, is checked immediatelty after a failed hot start we'd be closer to finding the cause.

 

My expectation is the f/cs don't dry out** and that narrows the fault to either a vapour block between f/c and jet OR both carbs are delivering an over-rich non-ignitable mixture. Harvey's freezer spray and a check on the f/c fuel level would confirm a bubble in the fc-jet tube. If that doesn't work then it points to the hot start mixture.

Peter

 

** because the distillation curve shows only half could evaporate even if the fuel in f/c reached 90C !!

Edited by Peter Cobbold
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  • 2 weeks later...

I had a chance to test the fuel line vaporisation theory today by taking the car for a run along the A66 until hot then parking up and then sitting for 5 mins with the engine running.Temp went up to 185f and then e fan cut in, oil stayed at 55psi and engine ticked over fairly steady for the duration.Came to pull away and no problems. Had a further joyous hours run around the back roads, confidence restored once more. Looks like Peter Cs diagnosis was correct in that a sticking float most likely cause. I will leave the copper pipe unwrapped and following it's original route to the carbs.

Thanks for guidance,

JS

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