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MG TD TF 1500 - VAPOUR LOCK

Hi all,
Reading about problems some members have with vapour lock. ("Vapor" in the U.S.) Also remembering VL in my road TC on hot days.

Here is an experiment for someone to try. It might have already been done. I have been meaning to try it for decades but always too busy and then cooler weather takes my mind off the issue. Always something more pressing to do!

MY THEORY: Fuel is pre-heated in all sorts of places, like the petrol pump, pipes, hoses etc. However, in all places but one, the bubbles of vapour can usually rise to the top and move onwards without too much effect. Bubbles of vaporised fuel in the float bowls can rise to the top and exit via the vent. The one place where this can not happen is in the gallery in the float bowl which leads across to the underside of the carburettor? This is a solid alloy piece which is heated by the nearby exhaust manifold. Once inside that gallery, fuel and bubbles are destined for the jet only. Bubbles which form there, have to go through the jet where they each take a short time to clear causing the problem. I know that in hot weather, my VL symptoms immediately stop if I pull out the choke. I think that is giving the bubbles a bigger space to get out and faster fuel flow means less time to heat up.

THE CURE?: Drill out the gallery at the bottom of the float bowl. Then sleeve it back to size with a fuel/heat resistant plastic tubing. The plastic should insulate the fuel from surrounding alloy so the fuel does not get heated by the surrounding alloy. The tubing should protrude into the float bowl a bit and be cut at an angle, so bubbles rise past it. The Float bowl metal is plenty thick enough for this treatment. (Check yours first.) Tubing needs to be tight enough to stay in position. A slight complication is that this procedure would require drilling out and re-plugging the blanking plug at the bottom banjo-bolt end of each float bowl. When I fitted my racing TC supercharger in mid 1970s I drilled the gallery out a lot because I anticipated running the car on methanol (but never did). That non-angled float bowl did not have the blanking plug complication because non angled float bowls have an angled gallery. (Sorry, Unnecessary detail.) VL also not an issue in that car due to other factors.

I am only suggesting that someone try this if they understand my theory and know what they are doing. I am not guaranteeing that it will work. I think it will at least help, but I might be wrong. If someone tries it, I suggest they also paint the lower banjo bolts white. (White absorbs less heat than any other colour.) They should also cut the carby end of the tubing to a suitable shape so that if it shrinks and moves, it does not cut off fuel supply.

A test of the theory might be done by binding the whole area with heat resistant lagging. If that improves the situation at all, then try my cure?

Any thoughts? Has it been tried?

Cheers,
Bob Schapel
Bob Schapel

Bob, with the technology out there today and these new printers, how about printing an entire float bowl out of high-temperature plastic? It's being used a lot in modern vehicles. Just a thought. PJ
PJ Jennings

Ther was a serious amount of work done on a XPAG engine at Manchester University:

https://www.mgccyregister.co.uk/manchester-xpag-project-update/

In essence it suggests that fuel volatility is a variable factor and that the temperature of the venturi throat (where, of course the fuel enters from the carburettor base) is the major factor in vapour lock. As suggested above

It also proposes that the heat from the cylinder head is conducted to the venturi throat during rest, creating the problem.

On that basis, effective heat insulation between the cylinder head and the venturi is a key to management.

My own take on this is that the cooling caused by the vaporisation of the fuel in the venturi is also an important factor and once the car starts, this cooling prevents reocurrence of poor fuel mix and restart problems.
Ian Bowers

Bob Schapel's solution seems complicated and maybe not easily reversible if unsuccessful. I would first try this. Fit a split open section of rubber fuel hose over the exposed metal pipe leading up to the fuel pump preventing engine bay heat from getting directly to the pipe. Then wrap the two float bowls with foam home pipe insulation held in place with a nylon tie. These only address part of the fuel system but might be sufficient to make the difference. All completely reversible.
John Quilter (TD8986)

Hi bob, I completely agree with your theory. I even happen to observe it. One hot day, I stopped the engine in my garage with the intention to open the suction chamber to inquire about my VL issue. Short while to get the screw driver and other things. Few minutes and off the chamber. I could observe bubbles in the jet !

I tried to fit an insulator around the gallery. But no effect. My rational is that the gallery is not only heated by exhaust radiation but also by conduction from other parts of carb (also ramaining exhaust gaz when stopped, see Manchester). And so, the gallery needs also cooling by fresh air.

So I think your solution may work. But I won't try it.
Carbs are rare and very expensive. I have no more problem since I fitted a scoop above the cross member.

Laurent..

LC Laurent31

Good comments guys,
PJ's idea of plastic float bowl is good if my theory is the cause. It would deliver cooler fuel to the jet head. If that worked, then my idea might work at least in part. But I am old fashioned and don't trust plastic.
Ian, I agree the carbs need to be insulated from the head. Your comments have alerted me to the fact there might be two different types of VL. One is the re-start issue but the one my car suffered was while the car was running in hot weather, despite having insulators between carbs and manifold. My "cure" would not help the heat sink from head issue. Insulators would do that better. I should add that my problems were mainly before modern fuels because I don't take my road TC out much in very hot weather nowadays.
John, yes I agree that your solution would be good to try first, with particular attention to wrapping the bridge/gallery between bowl and carby thoroughly. I would think something which can withstand the radiant heat from the exhaust would be best.
Laurent, thanks I think it would work too but as pointed out by those above, it would only work in my situation where the car gets VL while running. My car would "cycle" between power and no-power. Cars where the VL issue is only re-starting might not benefit from it at all.
Thanks again guys for your comments. It seems that first thing might be to decide which sort of VL is giving the trouble.
Bob
Bob Schapel

Bob, I completely agree with your post. I'm not convinced that the plastic tube would work, but I'd love to be proven wrong. I don't think direct heat from the head and manifold are the only issue. Modern fuels, especially those laced with ethanol, have a ridiculously low boiling point. I've been told, but have not verified, that it can be as low as 60C / 140F with certain blends, and that most are closer to 82C / 180F. So the heat from the engine bay itself might be enough to push the fuel to the bleeding edge of boiling. Then the manifold perhaps pushes it over the edge.

The only thing helping might be that the fuel is flowing, albeit slowly. For this reason I've always felt that a return circuit would be a huge benefit, so that cooler fuel is always in the lines rather than sitting there crawling though hot pipes before entering the float bowls.

I've seen a lot of people install insulating spacers and heat shields and still have an issue. Yet my TC has none of these items and has no vapor lock problems except for a re-start on an extremely hot day, and that's cleared up in just a few seconds with a little choke. I know a lot of others in our local groups who also have no issues so perhaps it's a regional thing? We do have different gas here in California than the rest of the country (we refine and blend our own special higher quality mix, which is why it's so darn expensive here).

According to a lot of the old guard I regularly tour with, these vapor lock issues were not happening so easily back when the cars were new. So it seems to me that the fuel itself is our biggest battle.

Plastic float bowls... not a bad idea. But I"m old school also and I don't trust plastic either! A failure would cause a horrendous fire. I would constantly worry about it.
Steve Simmons

I seem to have opened up quite an issue on this item that shows itself for perhaps half a minute of embarrassment in a supermarket parking lot.

The technical issues that are coming up from a number of very knowledgeable individuals are fantastic. The fuel aspect hit me head on when I moved down here from MA to GA seven years ago. At the moment we have at least three threads running here plus one started in the NEMGTR TSO. It sure would great if an elemental solution can be found. Please keep going, folks. Bud
Bud Krueger

One option would be to raise the boiling point of the gasoline. I've heard, bt haven't tried it myself, that you can do that by adding other products such as kerosene or diesel. But it lowers the octane at the same time, so there's another battle to fight.
Steve Simmons

One of the T owners up here throws a quart or two of aviation gas in his tank, but I think it is to “revitalize” the fuel after winter maybe.

I do purposely avoid ethanol, I use an App called PureGas to locate stations that have it available. I have one within 2 miles that has 4 pumps dispensing ethanol-free. Often see classic cars there, even spotted a TD.

Even in Hawaii in traffic I never suffered from VL with my TD. I a relocating my TF south to SC, be interesting to see how it goes. I do have a heat shield, albeit I have not installed it.

Peter
P G Gilvarry

I drive all over the Midwest, at times in 100F+ temps in stop and go urban traffic and have NEVER had a vapor lock issue while the engine was running. Quick stops on those days while running errands are also not an issue with vapor lock. The ONLY time I have experienced vapor lock is on those hot days where I stop for more than ten minutes and less than an hour. Then all I do is pull the choke, it fires right up, I leave the choke on until I am out of the parking lot and push it off and I have no vapor lock issues the rest of the drive until the next :10+ minute stop.

My car is a standard tune, normally aspirated TD.

Heat shields and insulation will not help to a heat soaked engine compartment. It MAY give some relief if the car vapor locks while moving.

I cannot speak to supercharged or higher stage tuned normally aspirated XPAG engines as I have no experience with those.

Regards, Tom
tm peterson

Dont forget to read the very good book that reports the manchester university study. Everything is in it : Ethanol, boiling point, heat sources, starter, kerosene, ignition, shields, insulator...
https://classicenginesmodernfuel.org.uk/Clubs/ModernFuel/Book.aspx?DYN_MENU_MainMenu=1000001&AspxAutoDetectCookieSupport=1

Laurent.
LC Laurent31

Naturally, it's Out of Stock. I have the complete set of TTT2. Wonder if I can create an index to search out the editions with the Fuel Book entries? Hmmm?

Tom, in almost 30 years of TD'ing I've experienced the same sort of thing that you're talking about. The only VL problem while running has been down here in GA toward the end of summer parades. Sends my guest passengers to wonder what's going on.
Bud
Bud Krueger

Peter G: Where in SC did you relocate your TF? I'm in the Upstate about 15 miles north of Greenville. We have a very active British Car Club that you might want to check out.

https://fbccsc.org/

We have a major one-day driving event scheduled for 24 Oct. Let me know if you'll be in the area and are interested.

Jud
TD 25009
J K Chapin

Heading to Latitude MargaritaVille Hilton Head, Hardeeville, about 2.5 miles off the I-95, few miles north of GA border.

Will be snow birds for a few years, before selling up here.

Peter
P G Gilvarry

Hi Bud,
Sorry, I probably should have put my suggested experiment re VL under your thread. I did not want to de-rail your fuel pressure issue with my suggested experiment, so I started a new thread. Looking back, I now see that discussion re VL had already developed on your thread. Pity I can't tag this thread under yours. My apologies Bud.
If reading this thread, also see Bud's thread re fuel pressure.
Bob Schapel
Bob Schapel

peter G, too bad. You can't be much farther away from me and still be in South Carolina. Nonetheless, even this mountain boy has to admit that HH is a nice place to be.

Jud
J K Chapin

I read this thread and I have to admit that I don't know what it is like to have vapor lock. While I don't drive my TD the amount I used to, It has taken a number of trips where I drove it very long distances without issue.

Any input on why some have issues and some don't?
Bruce Cunha

High Bruce.
Depends on climate, fuel (type, brand and country).
Probably depends also of tuning stage and cooling circuit health, way of driving, carb tuning.

Laurent.
LC Laurent31

Hi Bruce. As an old Marine Corps saying says, 'it depends upon the terrain and the circumstances'. The car will have been driven in warm/hot weather to the point where the temperature gauge is above the thermostat value. Two happenings -- one is where you stop long enough for your underbonnet temperatures to rise high enough to vaporize fuel in the carburetor system and the car balks at the idea of starting unless you give it lots of choke for a short while. Another is where you have to slow down and your temperature gauge climbs toward the oil pressure zone. The car wants to stall and you have to keep goosing the throttle, with the clutch pedal depressed, to keep it running.
In either situation the culprit is the vaporization of fuel that is supposed to be busy in the needle and jet.
Other victims may see it quite differently from my description, but it's the same culprit. Bud
Bud Krueger

Bob, IMHO your theory of the float bowl gallery is the most reasonable theory that I've seen put forward. Unfortunately, the cost of experimenting with your solution is going to hold up a lot of us from trying it. Here, in The States, we have a parade, called Veterans Day, coming up in about three weeks where the ugly VL villain can raise its head if it's a warm day. I warn the folks behind me to leave a good space toward the end if they hear me goosing the throttle.
I have a relatively inexpensive idea wrapped upon your theory that I'm hoping to try out.

I think this is going to my sixth response in a row to things on the site. Embarrassing, but I'm doing some exercises for my 88 year old memory. Bud
Bud Krueger

This thread was discussed between 19/10/2023 and 23/10/2023

MG TD TF 1500 index

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