Spitting Back Through Carburettor

[tonymce]

My 4.5 Litre runs exceptionally well at all speeds (up to about 85 mph) without fuel problems with the exception of going up a long, steep incline when, after some period of time (say one mile), the car loses power and suffers spitting back through the carbs. On levelling out it once again behaves normally. There is plenty of fuel in the float chambers. The problem exists both with Autovac and with electric pump and is not affected by enriching the mixture. Carburettors are HVG5 with CV needles. Has anyone experienced a similar problem?

[Chris Card]

Just a shot in the dark, but have you checked that both mags are doing what they should, by switching them off alternately at fast tickover and checking that the mag drop is similar for each mag.
Could it be that the winding on one mag is starting to break down under prolonged load and causing the sparks to happen at odd times. I had a similar, but not identical, problem with spitting back when my mags were starting to fail and the sparks were all over the place.

Chris

[tonymce]

Thanks Chris, Both mags seem in excellent condition and this "problem" never occurs at any time other than on a long incline, not even at a steady 2500rpm.

[Chris Card]

OK, if you've discounted the mags, the autovac and the mixture, what else is there? It does seem to be directly related to load, which equals heat so, is there a build up of carbon around the inlet valves which, under the load conditions that you describe, could start to glow and ignite the mixture in the inlet tract. I certainly had those symptoms with my old cylinder block and, yes, after storming up the hogs back it also spat back. It could also be an inlet valve that starts to bind when it reaches a critical temperature. The water temperature would probably not necessarily indicate overheating.
Beyond that, I'm stumped.

Chris

[John Murch]

It sounds like fuel to me, have you cleaned the filters, both the main and the 'thimbles' into the carbs?
John

[Robert Craven]

What about trying it on a rolling road where you can watch what's happening inside the bonnet and cut the engine as soon as the fault develops (assuming it does) and check for instance if the float chambers are still full? And they should also be able to measure other things such as timing.
With some expert assistance and a rolling road I was able to establish that my fuel starvation at high speed (and only high speed) was due to the rear float needle needing slight adjustment.
Ps, I've just noticed you said there is plenty of fuel in the chambers. But is that immediately upon the fault occurring or sometime after? When testing my car on the road I was never confident I'd stopped quickly enough for this test.

[tonymce]

Thank you all for your input.
I do not believe it is the fuel supply as I have put in a higher capacity fuel pump to see if that would resolve the problem obviously without success. I checked the float chamber levels by, as soon as the problem was evident, immediately switching off the engine and fuel pump and found levels to be at the correct level.
A rolling road exercise would certainly be a good idea. Not sure if the problem could be emulated, but it is certainly worth a try. However, here in South Africa it is difficult to find that facility with anyone who knows exactly what they are doing.
I would be surprised if there was a carbon build-up on the valves as I, every so often, feed Redex directly into the carbs to give the top end a bit of a decoke. However, your suggestion of a sticking inlet valve may have some merit. How to prove that is not that easy I imagine without taking off the block.
My only other thought, as the problem only presents itself on a steep gradient, is that it has something to do with the oil in the cam chest flowing to the back. Why that would cause the problem though I do not know.

[Chris Card]

The cam chest is divided into reservoirs, so the oil would not slop to the back.
Spitting back has to be related to ignition with the inlet valve still open. Have you checked the valve clearances lately. If one or more are a bit tight, this could cause a valve to remain partially open.
These passing thoughts are a bit like fencing in the dark!
Whereabouts in SA are you? My car is in the UK and I am in Namibia until December - not that I could call by to have a look!

Chris

[tonymce]

I have checked valve clearances at least a couple of times since this problem materialised and always found them to be very close to specification, but I shall certainly try that again.
I live in Knysna, which is not exactly next door to Namibia, but the BDC is having its Xmas lunch in Franschhoek on the 20th November should you wish to visit the winelands.

[Chris Card]

[Stephen Blakey]

Having wasted several weeks trying to come up with a diagnosis on a Bentley engine that displayed quite dramatic symptoms of combustion gases being in the wrong place at the wrong time, I feel your pain.

Having said that, I do think you need to categorically eliminate fuel starvation (or even excessive richness) from the likely causes and the best way to do that is to fit an AFR gauge and then drive the car up a hill to check if the mixture starts to go off as you go into misfire. (Once you're in misfire the AFR gauge won't work as it functions by measuring residual oxygen).

Dan Suskin has had success with an AFR gauge on one of his cars. Maybe he will see this?

Good luck,

Stephen

[Christopher Carnley]

Stephen,

You were "dreaming" about the EGR in the first sentence,I found none such, the exhaust was blowing beautifully.
What you wrote really muddied the pool.

[Iain Warner]

Tony,
My 3 litre would exhibit exactly the same symptoms with spitting back after slogging up a long incline at speed and this I put down to fuel starvation as after cleaning all the filters and checking the SU pump there problem would go away. (until the next time the filters needed cleaning). If I slowed right down on the hill during the misfire the fuel level would catch up and the misfire would cease. But this does not seem to the cause of your problems.
Is this problem something you have always lived with or has it come on recently?
Iain

[John Murch]

I note that you have both an autovac and electric pump, I assume that these draw fuel from a common feed and that there are then a series of taps to isolate one or the other supply. If there were a crack in a pipe or fitting you could be drawing air into the system, the fault not resulting in a petrol leak as the system is always under suction. But not too sure how that would affect the autovac!
Could you have some flakes of paint in the main tank getting drawn up against the fuel pick up and restricting the flow?
John

[Chris Card]

Tony,
Picking up on John's thoughts, if you do your hillclimb blast and, as soon as it starts to spit back, cut the ignition and coast to a stop. Then, with care, remove the exhaust side plugs from 1 and 4, and check the colour of the electrodes. They should be a mid brown colour. If they are white, then you are running too lean. This may be a matter of adjustment or, as John suggests, a temporary blockage.

Chris

[tonymce]

Once again, thank you all for your input. Google has come to the rescue to explain AFR and EGR. If I can find an AFR gauge that could certainly help identify the problem.

When I get a chance, I shall try your suggestions and let you know if any of them bear fruit.

[William Highley]

Tony,
Many years ago I had a similar problem. The car was fine up to about 80mph or so but above that it started to spit back through the carbs. Slopers in my case. Having tried several cures it was by pure chance that I found the problem. One day I was turning the petrol off on the bottom of the AutoVac and the handle of the petrol tap broke off.

My father years before had fitted a modern petrol tap as the original brass one leaked. As a temporary repair I re-fitted the old tap while waiting for a new one. Next time I took the car out it went straight up to it's maximum speed without any problems.

What had happened was that the modern petrol tap had a nylon insert with a hole through it's centre which the petrol went through. Over the years the nylon had swelled reducing the size of the hole therefore restricting how much petrol could flow through it. The size of the hole was fine for normal speeds but obviously too small for high speeds.

If you do have a modern fuel tap with a nylon insert it may be worth looking to see if the hole is smaller than it should be.

Regards,

William Highley

[tonymce]

Gentlemen,

I am cautiously optimistic that I have found the problem on my car and there is probably a lesson to learn here.

For a number of years I have used Champion D16 spark plugs, probably bought when my previously used A6 and A7 NGK plugs were not available. The D16 plugs were (and still are) shown as equivalent to the A6 or AB6 plugs, albeit at the hotter end of the range.

These performed without any trouble for many years. I am assuming that there may been a change in formulation of the fuel here in SA but it appears that what was happening was that the plugs were getting too hot and, under load, were igniting the fuel as it entered the combustion chamber through the inlet valves.

I tried brand new Champion D16 plugs which I have with no improvement.

I then reinstalled some old A6 and A7 plugs that I took out, probably 10 years ago, but being a Scot kept in case I needed them someday. My "problem" immediately disappeared. I still need to have a long drive to make sure that it has indeed disappeared and I need to find some new AB6 or AB7 plugs here in SA which may not be that easy. However, it looks as if the problem has been identified and if it proves to be a good solution I shall communicate this to the wider audience through the Review.

I'll let you know the result after some new plugs are found and a decent run completed.

[Chris Card]

Tony - that's good news and, fingers crossed, you won't have any more problems. Keep us informed.

Chris

[John Murch]

It does seem to be the norm, that when you think the problem is fuel, it turns out to be ignition and the other way round!
Great to read that you seem to have found the solution.
John