What Causes Petrol to Leak from the Exhaust

Talk about the E-Type Series 3

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George
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#21 Re: What Causes Petrol to Leak from the Exhaust

Post by George » Mon Aug 17, 2020 12:40 pm

Bob,
I checked the main jets and there is no fuel squirting from them with just the pump running.

With reference to the fuel non return valve when I look at the parts diagram it shows the larger connection going to the tank but on my car the larger connection is nearest the metal fuel pipe that feeds both sets of carbs therefore I suspect this may be the wrong way around. Can anyone let me know which way round it is on their car please.
George
1972 S3 V12 ROASTER

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MarekH
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#22 Re: What Causes Petrol to Leak from the Exhaust

Post by MarekH » Mon Aug 17, 2020 2:23 pm

The valve is simply a ball bearing pushing against a spring. The fuel pressure pushes the ball bearing back and so the excess fuel passing through returns to the tank. It may even have an arrow on it iirc. If it is the wrong way around, then no fuel will return back to the tank and all four float bowls will have to close against a higher fuel pressure. You have just reported that no fuel coming out of the carburettors with just the pump running, so I'm not sure how this is a problem. In any case, you can disconnect the outlet fuel pipe and run the excess fuel into a jar just to satisfy yourself that this valve is properly connected and doing its job.

You really ought to check your spark plugs first. The only realistic way to get raw fuel inside the exhaust is to not ignite it when it was drawn into the cylinders. So are any of the plugs wet? Are any black and sooty and wet? Do they all spark when left simply sitting on the manifold?

kind regards
Marek

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George
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#23 Re: What Causes Petrol to Leak from the Exhaust

Post by George » Mon Aug 17, 2020 2:50 pm

Hi Everyone,

I've solved the problem and I am posting it here for anyone to reference in the future.

My hunch was right the non return valve had been installed the wrong way round by a previous owner so I reversed it and she runs beautifully (wow the sound of a V12) with no petrol from the exhaust. Obviously excess pressure was being applied to the fuel feeds into the carbs and the surplus fuels only outlet was the exhaust because there was no pressure release via the non return valve.

Could I say thank you to all the people that have contributed to this thread and also to say that I am totally impressed with this website, to get an international response from people all over the world is just amazing.

Many thanks.
George
1972 S3 V12 ROASTER

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rswaffie
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#24 Re: What Causes Petrol to Leak from the Exhaust

Post by rswaffie » Mon Aug 17, 2020 3:26 pm

:drinkingcheers: :dance:

Now get out on the road and enjoy it!
Richard

S1 3.8 FHC Opalescent Golden Sand with Tan Trim 889504
Now running-in my diy nut ‘n’ bolt restoration. :wrench: :hammer: :fingerscrossed:

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jagwit
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#25 Re: What Causes Petrol to Leak from the Exhaust

Post by jagwit » Mon Aug 17, 2020 4:40 pm

George wrote:
Mon Aug 17, 2020 2:50 pm
excess pressure was being applied to the fuel feeds into the carbs and the surplus fuels only outlet was the exhaust
I'm glad the problem is sorted George, but I still can not see how the fuel gets into the exhaust without pouring out the main jet and being sucked into the engine via the inlet valve, survive the combustion process, gets blown out the exhaust valve, either in liquid or unburnt gas form.

What carbs does your car have? ZS Stromberg or SU's?
Best Regards
Philip
Jag: 72 E-type V12, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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George
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#26 Re: What Causes Petrol to Leak from the Exhaust

Post by George » Mon Aug 17, 2020 4:56 pm

Hi Philip
I have Stromberg carbs.
George
1972 S3 V12 ROASTER

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jagwit
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#27 Re: What Causes Petrol to Leak from the Exhaust

Post by jagwit » Mon Aug 17, 2020 5:17 pm

I am stumped!! :lol:
Best Regards
Philip
Jag: 72 E-type V12, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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71 V12
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#28 Re: What Causes Petrol to Leak from the Exhaust

Post by 71 V12 » Mon Aug 17, 2020 8:04 pm

Bob. wrote:
Sun Aug 16, 2020 4:38 pm
Hi George
Vacuum issues will only lead to weak mixtures, not rich. Have you checked that none of the carb float needles are sticking open. Remove air cleaners, turn ignition on to run fuel pump and look into each carb. (Don't start engine). Is fuel visibly squirting up from a main jet ?
Hi Bob,

In my experience a vacuum leak or any other source of air leak from the induction system leads to fuel enrichment. I have mentioned this a number of times on the forum, but it Is essential that all sources of air leak are fixed with new hoses, clips and blanking caps.

Regards,

Kevin

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angelw
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#29 Re: What Causes Petrol to Leak from the Exhaust

Post by angelw » Mon Aug 17, 2020 10:15 pm

Phillip Wrote:
I'm glad the problem is sorted George, but I still can not see how the fuel gets into the exhaust without pouring out the main jet and being sucked into the engine via the inlet valve, survive the combustion process, gets blown out the exhaust valve, either in liquid or unburnt gas form.
Hello Phillip,
Not only that, but the least path of resistance would be the breather passage from the Float Bowl, the exit for which is in the inlet face of the Carburetor. When a Needle and Seat assembly doesn't shut off completely and the Float Bowl over fills, that's where the fuel escapes.

Regards,

Bill

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AussieEtype
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#30 Re: What Causes Petrol to Leak from the Exhaust

Post by AussieEtype » Tue Aug 18, 2020 2:08 am

I must admit I have never understood the need for this ball/spring valve on the fuel return line - I have had vehicles in the past with stombergs and they never had a valve - just a simple line back to te tank.

Currently I have a Land Rover FC 101 with a V8 that runs twin stombergs and it does not have a valve in the fuel return line. I thought it might have been in the e-type to stop fuel in the carbs syphoning back to the tank when the car was stopped but the height above the tank of the LR carbs is twice as high as the Jags.

I suspects that the Jag would run just as well without the valve fitted to the return line, but fit it back to front and problems occur.

Garry
1971 Series 3 E-type OTS

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lowact
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#31 Re: What Causes Petrol to Leak from the Exhaust

Post by lowact » Tue Aug 18, 2020 4:17 am

I imagine it is to provide a bit of back pressure, just enough to be confident that the float bowls will always be filled, even around the Nurburgring …
If valve is in backwards could be significant back pressure (is positive displacement pump) enough to overpower the float valve? Maybe more so if carbs and pumps are modded, not OEM?
If the carbs overflow out the front face, wouldn’t this mean that all fuel would still end up in the engine, and if too rich, cannot be burnt?
Regards,
ColinL
'72 OTS manual V12

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jagwit
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#32 Re: What Causes Petrol to Leak from the Exhaust

Post by jagwit » Tue Aug 18, 2020 7:04 pm

lowact wrote:
Tue Aug 18, 2020 4:17 am
If the carbs overflow out the front face, wouldn’t this mean that all fuel would still end up in the engine, and if too rich, cannot be burnt?
I agree Colin. This by far one of the most inexplicable reports I've come across. Although the solution seems clear the symptoms remain a mystery - to me anyway.

I'm now left wondering if somehow fuel WAS pushed out the front face, ran out of the air filter housing (certainly NOT passing through the engine), down many of the chassis members, plates, pipes and somehow ended up on the exhaust pipe, running down and along the exhaust making it appear as if the fuel was coming from within.
Best Regards
Philip
Jag: 72 E-type V12, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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#33 Re: What Causes Petrol to Leak from the Exhaust

Post by lowact » Wed Aug 19, 2020 12:38 am

Not sure what yr agreeing with? George has already explained that he leak was from inside the exhaust?
I imagine that the carburetor float chamber vents are where they are so that any emanating petrol should not escape, instead should be evaporated and drawn into the engine via the carburettor/throttles/manifolds/inlet valves. If the resulting mixture is too rich, less than the stoic portion will burn, the rest will pass through into the exhaust.
If the non-return was forcing full pump flow into the float chambers, I know from personal experience that petrol can flood from the carb vents in larger quantities than can be contained by the above. I imported my car. When they tried to drive it off the car carrier petrol sprayed out all over the cars on the mezzanines below. They had to push my car off the ship. When I went to pick it up there was no evident problem, drove home, 3 hrs, no fuel issues evident, I was dismissive. A couple o weeks later I experienced the problem, solved by refurbishing the carbs. So on top of everything else, the problem can be intermittent, at least initially ...
Regards,
ColinL
'72 OTS manual V12

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MarekH
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#34 Re: What Causes Petrol to Leak from the Exhaust

Post by MarekH » Wed Aug 19, 2020 7:22 am

A couple of points....

It is unfortunate that the original poster didn't check his plugs. It would tell us whether all four carburettors were affected or whether the problem was more local than that. He also didn't give any steer as to whether the car was running well, running poorly or misfiring. We also don't know how the car performed when fully warmed up.

This last point is important because of the what it takes to fire up a carburettored v12. With a fuel injected car, the computer commands exactly the correct amount of fuel for the ambient temperature to be squirted right onto the inlet valve where almost all of it goes straight into the cylinder. The car starts instantly and the computer then moves a tapering enrichment fueling to a warmup algorithm which ends at ~82'c with normal running. With a carburettored car, this is all somewhat in the lap of the gods. The choke supplies more fuel and this has to be atomised by very little air flow. The fuel then hits silver water jacket which is full of stone cold water where some it will condense onto the cold surface. It then goes into the black plenum and the same happens. Still cranking (to get airflow), it then goes into the inlet runners up and over the camshafts and more of it condenses on these cold surfaces. Eventually, enough of it hits the cylinders and gets sparked. All of that remaining fuel has to be boiled off again so the airflow can give it a second chance to reach the cylinders and this won't happen until the engine is thoroughly warmed up.

If you look into the aluminium water jackets, you'll see a channel at the bottom where the condensed fuel sits, waiting for warm enough coolant to vapourise the heavier fuel fractions. This won't happen in the first few minutes of normal running.

There is plenty of unburnt fuel sloshing around at startup, but even with a direct link like EGR tubing, I don't see it being sucked directly into the exhaust.

What I think everyone is stuggling with is to explain how the car can appear to run well enough not to report any significant faults, yet still dump enough raw fuel into the exhaust without it vapourising. A look at the plugs would have been helpful. I suspect some of the cylinders were flooded out with excess petrol.

I'd also love to know whether the volume of "oil" in the sump has increased.

kind regards
Marek

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angelw
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#35 Re: What Causes Petrol to Leak from the Exhaust

Post by angelw » Wed Aug 19, 2020 9:00 am

Colin Wrote:
If the carbs overflow out the front face, wouldn’t this mean that all fuel would still end up in the engine, and if too rich, cannot be burnt?
Hello Colin,
If all the kit is still assembled in the air filter housings and the amount of fuel emanating from the breather hole profound, its likely that the fuel will end up in the carbon canister. If not, then some would make it into the throat of the carburetors.

Its my experience with fuel in the exhaust, that it will be ignited inside the exhaust by whatever bank of cylinders that has viable combustion. I have one car in work where all six spark plugs of the "B" Bank of the engine were fouled and therefore, all fuel being delivered to that side of the engine was being pumped into the Left side of the exhaust. The Left half of the Stainless Steel exhaust, from the front muffler to the Twin Fantail Trim, is heat discoloured navy blue from fuel burning inside.

Regards,

Bill

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#36 Re: What Causes Petrol to Leak from the Exhaust

Post by AussieEtype » Wed Aug 19, 2020 9:18 am

This is what happened to the exhaust of my landie when excess fuel from my Land Rover twin Strombergs got into the exhaust - bang - so I am surprised the OP did not have something similar if there was enough fuel in there for it to run out.


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1971 Series 3 E-type OTS

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#37 Re: What Causes Petrol to Leak from the Exhaust

Post by angelw » Wed Aug 19, 2020 9:47 am

Garry Wrote:
so I am surprised the OP did not have something similar if there was enough fuel in there for it to run out.
Hello Garry,
I agree. The car at work was backfiring profusely, with flames shooting out the Fantail.

Regards,

Bill

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