Fuel vaporization

[daverclasper]

Hi. If this is happening for whatever reasons (no info needed on that at the moment).

Is it the fuel expanding in volume/pressure, therefore throwing too much out of the jets making it too rich?.

Or. Causing air in the fuel, making it too weak?.

Have heard both ideas?.

Any info please?.

Cheers, Dave  

[johny]

no not air but the fuel vaporising too soon as it shouldnt do this until its sucked up in the jets. If it arrives at the carb already vaporised then I reckon it all goes wrong as the float wont operate correctly with a gas which will then come out of the overflow. The chamber level will be all over the place so this plus the liquid/gas mix in there will really upset the combustion mixture😳

[daverclasper]

Thanks Jonny. On the weak idea, I think it was lots of fine air bubbles in the fuel.

Anyway, so mixture all over the show, as a summery.

 

[johny]

well there will be bubbles but of vaporised fuel not air and they will certainly not have the energy density of liquid fuel...

[Nick Jones]

Part of the problem (and likely the greater part) will be actual fuel delivery reliability and resulting fuel level in the float chambers as one of the worst spots for vaporisation is in the fuel pump and the vapour messes with the action of the valves and displaces liquid fuel, realling spoiling its efficiency.

Nick

[Adrian]

As a lay person I'd say simple physics would sugest a liquid being delivered at the carbs (as designed) will produce the anticipated burn and consequently 'bang'. A vapour entering the carbs has a lower density and therfore weaker 'bang' as it is already a vapour.

Interesting point from Nick. Is it at the pump that vapourisation occurs (the last main contact with the block prior to the carbs) or just heating of the pipes within the engine bay? and what does vapourisation sound like?

 

Adrian 

[johny]

source of vaporisation seems to vary but the result is poor starting and running obviously worse the hotter the engine. However this symptom can also be caused by many other things so its easy to misdiagnose the problem. Saying that Triumph in their wisdom did fit a thermal spacer to the fuel pump on some of the last cars which seems to indicate they took the issue seriously....

[Peter Truman]

just check the float chamber stays vertical ie the bolt holding it hasn't come loose or the nylon/plastic locating bush broken down.

If either of these have occurred the float chamber can rotate to be near the exhaust manifold and cause vaporisation, yep it  can happen!

[Ian Foster]

I agree with Nick that the main cause is probably the pump suffering heat soak from the block. The fact that Triumph added an insulating spacer late in production confirms this was an issue.

When I experienced it on my GT6, it occurred on a reasonably hot day in traffic and actually stopped the engine running. Removal of the fuel pump cover produced a hiss of released pressure and the fuel could be seen boiling in the base of the pump chamber. Also, for a GT6 at least, the routing of the fuel line from pump to carbs around the front of the head, may also be a contributory cause which may be more significant when a hot car is stationary for a period.

I now use a Huco electric suction pump and have routed the fuel delivery pipe around the rear of the engine. I also have an insulating sleeve on the section of fuel line which is proximity to the exhaust manifold and carb heat shields.

Has been fine ever since.

Ian

[daverclasper]

Thanks.  If pump is absorbing heat from the block?, then would it be possible/useful to fit an alluminium (good for heat sink) spacer/shim between them and if so, what max thickness to still alloy the pump to function properly?.

[daverclasper]

Ian just got there before me, I didn't know Triumph did the spacer.

[Nigel Clark]

Just now, daverclasper said:

Thanks.  If pump is absorbing heat from the block?, then would it be possible/useful to fit an alluminium (good for heat sink) spacer/shim between them and if so, what max thickness to still alloy the pump to function properly?.

Unfortunately an aluminium spacer won't help, as aluminium is a very good conductor of heat. Also spacing the pump from the block will affect the stroke of the pump's operating arm, possibly reducing fuel delivery. Some pump and engine combinations use a fibre spacer.

General the Triumph 4- and 6-cylinder OHV engines don't seem to be badly affected by fuel vaporisation. When it does happen, most often it's due to the routing of the copper fuel pipe from the pump to the carbs, or possibly a weak fuel pump. Make sure the pipe from pump to carbs doesn't rest against the engine, and particularly that it's not sitting against the water pump housing. There should be a metal clip on one of the water pump housing bolts. Make sure the clip is in place with its rubber grommet, and that it holds the fuel pipe away from the pump housing.

You could also try wrapping the fuel pipe in insulating tube, like this:

https://www.ebay.co.uk/itm/114717830816?hash=item1ab5b72ea0:g:yT0AAOSwVLNfDKe4

I've used similar insulation successfully on the fuel feed to the Ford V6 in my Scimitar GTE, where vaporisation in the fuel pipe that runs over one cylinder head to the carb in the middle of the Vee is a big problem. I've never needed to do the same on my GT6 or Spitfire though.

Nigel

[Nick Jones]

11 hours ago, Adrian said:

Is it at the pump that vapourisation occurs (the last main contact with the block prior to the carbs) or just heating of the pipes within the engine bay? and what does vapourisation sound like?

They all contribute but the thing that actually stops the car is almost always the pump. 
Vapourisation is the fuel, or elements of it, boiling. Reaching boiling point is partly due to temperature, partly pressure and partly velocity.  All three worst case conditions tend to occur at the inlet valve of the mechanical pump where the lowest pressure in the fuel system occurs, alongside the turbulence of the valve and conducted heat from the engine. 
 

The vapour displaces liquid and although the pump will move vapour to an extent, its efficiency is drastically reduced.

Modern fuel has a higher vapour pressure (ie evaporates more easily) than the 60s and 70s brews which is no help.  This is not a problem for high pressure injection systems (less anyway) but does often cause problems with older carb systems.

As Ian has, I have also gone with an electric pump on my GT6 and kept all the lines away from the engine as far as possible. So far it seems to have worked, though I don’t consider it fully tested. A full test is 45 minutes in Monaco or Andorran traffic at 38C ambient. My injected Vitesse passed those tests (crew very uncomfortable though!) which it would certainly have failed miserably on carbs.

Nick

[dougbgt6]

I've replaced the metal pipe between pump and carb with rubber all the way. This was to lower the chances of the dread slivers, but the rubber doesn't get nearly as hot as the metal.

Doug

[johny]

never had a problem with the Vitesse and its been in some horrendous temperatures😍

[daverclasper]

Mines been ok (ish) for years, only stumbling at around 30 degrees in very slow London traffic, say.

It's got worse this year. The only thing I've done, is get the overflow bottle working properly, which had allowed about 3/4 litre more coolant and car now runs a bit cooler around town, according to the gauge (which I thought, if anything would have improved this issue?).

I think I will mess about with the mixture/timing in small increments, separately (though like you say, could be various things?).

Its livable at the moment, though don't want it to get worse in the hotter (hopefully) summer.

Car feels like it's running well in general conditions as far as the tuning goes ,aside from slow moving/idling, restarting a bit worse when hot.

Thanks Guys

 

 

 

 

Edited May 21, 2021 by daverclasper

[johny]

Strange its got worse and dont know if changes in fuel have had an effect. Ive got an electric fan on mine and although its not necessary for normal cooling I do like that fact that I can feel it pushing hot air out of the engine bay when Im stationary...

[Pete Lewis]

Dave  its not vaporisation     that occurs under  certain conditions and is not variable , and does need to be  pretty hot arduous conditions

and not a irregular now and again   

if hot starts are part of whats going on i would weaken your mixtures especially if it idle gets a bit wooly  when hot 

she should idle for hours without any drop or hic ups in idles speed 

Pete

[Tim Bell]

I have read the above thread with interest.

I have a 1961 Herald convertible fitted with a 1147 engine. It’s been running fine of late, but on the last couple of outings, it’s been running a bit lumpy (i.e. when you rev it, even from cold, it runs like it’s about to backfire, but it doesn’t backfire.

What could be causing this?

I’ve known dirt in the fuel to get into the carbs and cause this but tank not running on empty.

Also had the condenser go on a Spitfire once. This caused a backfire but this happened quickly and wasn’t preceded by ‘lumpiness’.

I poor running also tends to happen when you’ve been on a run and you either stop the car for a few minutes and cut the engine or if stuck in stationery traffic / kept at lights which take ages to change. This, I know, is due to heat build-up under the bonnet with no air flow and an electric fan controlled by a thermostat can help. It then usually starts off lumpy but improves as the flowing air cools things down again. So, I know this is a problem, (modern fuel evaporates more easily not helping – is this due to the ethanol content?).

When on a run today. Outside temp 25 degrees but engine running, not comfortably.

To try and help with reducing effects of heat generally, I’ve ordered some wrap for fuel lines from Funk Motorsport off the internet, and thinking of a fuel pump re-furb (I gather it’s better to re-furb than get an after-market  replacement). Interesting re the thermal spacer that Triumph latterly fitted, comment.

Took car out for a run today and made it back home, but it clearly wasn’t ‘happy’ and, of course, everything was hot under the bonnet (twin SUs have metal heat shields, but don’t think they have any real effect as they conduct heat too. My GT6 has fibre ones but these are much larger for larger HS6 carbs).

Coolant levels are all OK and radiator was re-furbished not all that long ago.

I will also clean the air filter on the K&N.

[Pete Lewis]

if i read this you have twin SU with KN pancake filters and electric fan ???

so you have all the things required for high under bonnet temperatures   

first has the fan got enough air flow, second  have it running all the time to keep underbonnet  air cooler  when caught up in traffic jams

KN pick up hot air  you really need cold air for carbs to work as well as possible theres  no simple answer to that

other than use a std air box 

if fuel lines have been meddled then expect the dreaded rubber sliver ts to block the float needle valves

to  use a plastic  pump spacer is hard  as there is in general  there are  no pumps available with and arm to suit now 

you should be running on 97=+ which has less ethanol and often none added 

if you run on 95 **  then you need to be retarded and that raises temperatures

many have problems with spark plugs when the ceramic insulator inside gets (carb /fuelling)  contaminated and they fail early life

Pete

 

 

[Tim Bell]

Thanks Pete. No don't have electric fan on Herald, though have one on GT6. Always hoped there'd be enough air space on a Herald to get away with mechanical fan only. Twin SUs on early Heralds, I think have one air filter for both carbs. Will look to see if a more conventional air filter and box might work better (if, indeed, available). 

I'll have to research what you call 'rubber silver' as this is new to me. 

Will consider new spark plugs and check out where to get fuel with less ethanol in it (think I've read something about this before).

Again, thanks for your thoughts :-).

[Pete Lewis]

 

its rubber slivers  ( not silvers)    quite simple especially with some of the awful fuel hose on sale 

you disturb the fuel hoses when chasing a leak or whatever  when fitting the hose over the metal pipework the tube slices a small section of inner hose off which now floats about and  ends up jammed in the small inlet hole to the float valve  they are persistant little sods and sure they breed overnight  they catch a good few out for performance problems 

Pete

[dougbgt6]

1 hour ago, Pete Lewis said:

its rubber slivers

Tim,

This is a depressingly regular occurrence on here and frequently disbelieved, it's not just rubbish new new hose, but also deteriorating ancient hose and crud from the bottom of the tank. Do you have a fuel filter? Best place to fit is just before the pump. However, there's nothing for it, the carbs have to come apart and the valves cleaned.

Doug