Ehtanol Fuel Boiling in Bowls

[Thamil]

I have major issues with fuel boiling in my bowls and flooding engine. After car is hot, idling is rough. When car is turned off after a drive you cant start it back up for 30-45 minutes as the fuel has boiled in the bowls and flooded engine. My heat shield is in place. Does anyone have any suggestions?

 

[Bob_Spidell]

Maybe raise the bonnet when you stop? I'm getting ready to do an overhaul on my BJ8 and plan to have the exhaust manifolds ceramic coated, hoping that helps.

 

[christophe]

Assuming you have checked your timing and other possible causes first... DEI on the back of the heat shield will help and insulate the fuel line with DEI sheath where it runs past the engine. I have had this issue with other cars as well, especially those with steel fuel lines that are near the engine. You are from Texas and I am from Alabama, so I am sure we share similar humidity. Sometimes it's just not even worth driving the cars when it is really hot outside.
https://www.summitracing.com/parts/dei-010400/overview/
https://www.summitracing.com/parts/dei-010457/overview/

 

[Michael Oritt]

Thamil--

My Elva Courier (race car) has an MGA engine which is configured like a Healey with the intake and exhaust manifolds on the same side of the head. In really hot weather, or when countdown to starts have been interrupted, on several occasions I had to turn the engine off to prevent overheating and often had a very hard time getting restarted because the fuel would boil, etc. etc.

The solution I came up with--it works every time--was to drill and tap the intake manifold, insert a 90 degree elbow and run AN10 hose from it through the firewall to a ball valve that terminates just under the dash alongside the steering column. Whenever I have difficulty restarting the engine due to high temps under the hood I simply open the valve while cranking. Engine vacuum draws a slug of fresh air into the manifold, allowing the engine to start instantly, and I quickly close the valve as otherwise the engine wants to go to redline, etc.

I have attached a photo of my present intake manifold and the air hose--it's the one with the red end. This manifold is an aftermarket one made by Maniflow which required brazing on the elbow but when I first came up with this concept I was using the stock cast MGA manifold which was much bigger and allowed for drilling and tapping--my point being that you can easily do this to a stock Healey manifold. Of course, you might have esthetic considerations and perhaps it is easier to wait 10-15 minutes for the engine to cool down but this is not an option on the starting grid!

 

[EV2239]

The simplest solution is to fit a banjo to the rear carburettor with two outlets/inlets, restrict one of them to about 1/16" and run a pipe back to the tank. This means there is a permanent slow return and it will cure vaporisation issues.

I apologise for being metric in temperature, but in response to heaters not working in winter, BMC changed the thermostat from 72 degrees to 82 degree in around 1960 with the proviso that you might need to use the lower opening temperature one in summer in hotter countries.

This problem and thermostat changes are common, Rolls-Royce Clouds have a similar issue as do other makes of Brit cars. Their manufacturers lived in and around our Birmingham and are unfamiliar with foreign weather.

 

[Michael Oritt]

EV--

I don't think a fuel return line alone will help. The problem is not with fuel supply to the float bowl but with the proper air/fuel mixture getting into the combustion chamber. Merely allowing fuel to circle through the bowls and back to the tank via what is normally an overflow line will not solve the problem.

My momentarily opening ball valve to the air line plumbed to the intake manifold downstream of the carbs brings in relatively cool air which more readily mixes with the fuel vapor producing--voila--combustion!

 

[British_Recovery]

With a return tube, wouldn't the fuel pump always be pumping?

 

[red57]

I think EV 2239 is right. An example; many years ago I noticed that Datsun 240zs ran a very small bore return line back to the tank which I always assumed was to keep a cool supply of fuel to combat percolation. I've considered doing it myself but never got around to it.

 

[EV2239]

Fuel would always be pumping, it always is anyway and has spare capacity, but the mod works well and is widely used to prevent vaporisation with modern fuel in old cars.

 

[Keith_M]

I confess that I don't see how either of the suggested solutions would work, but maybe I'm not understanding the problem. Having a return line to the fuel tank would work as long as the engine and fuel pump are running. But as soon as you turn off the engine, the fuel quits pumping, and the fuel in the carb bowls heats up.

I don't get the blast of cool air in the intake manifold at all. I thought the problem with fuel boiling is that not enough gas gets into the carbs because of vaporization. Adding more air would seem to make the situation worse.

 

[EV2239]

Fuel evaporating in the fuel pipes under the bonnet/hood cause a pressure build that the pump can't overcome. A return feed avoids this so fuel is always available to the flat chambers. Doesn't matter than they empty by whatever means because now there's always fuel to refill them.

As I said, it's a common enough mod and recommended even by some fuel pump manufacturers.

 

[John Turney]

Fuel evaporating in the fuel pipes under the bonnet/hood cause a pressure build that the pump can't overcome. A return feed avoids this so fuel is always available to the flat chambers. Doesn't matter than they empty by whatever means because now there's always fuel to refill them.

As I said, it's a common enough mod and recommended even by some fuel pump manufacturers.

But the SU carb bowls are vented via the overflows, so there is no pressure buildup in the bowls.

 

[EV2239]

But the SU carb bowls are vented via the overflows, so there is no pressure buildup in the bowls.

Correct. The problem is in the pipe.

https://en.m.wikipedia.org/wiki/Vapor_lock

 

[Michael Oritt]

The problem is not in the "pipe" but in the intake manifold/combustion chamber. Running a return line to draw away heat from the fuel system might produce some relief--so would pouring ice water on the lines--but let's remember that the object of the exercise is to get the engine restarted.

When the carb bodies and/or the intake manifold get overheated due to high engine temperature and heat coming off the exhaust manifold the engine gets "heat soaked", the fuel turns to vapor and the mixture becomes too rich and will not ignite. The air inlet corrects the situation and once the engine has fired it can draw fuel past the needles/jets and into the combustion chamber in a burnable fuel/air ratio.

I don't suggest that the modification is for everyone--I'll repeat that I did this to one of my race cars where originality is not an issue. But it works and if anyone will be in the Pittsburgh area next weekend I'll be racing the Courier at Schenley Park and I will be happy to give you a demonstration.

 

[Keith_M]

Interesting... I always assumed that the problem was fuel boiling in the carb bowls so that liquid fuel couldn't move through the jet and into the throat of the carb.

 

[John Turney]

Correct. The problem is in the pipe.

https://en.m.wikipedia.org/wiki/Vapor_lock

The Wikipedia article cites a problem with fuel in the pump inlet pipe vaporizing because of the fuel pump being engine-driven. Not the case with a SU, or other pump, next to the fuel tank unless the pump inlet is overheated from the exhaust, something we have not discussed.

 

[John Turney]

There should be small rectangular gaskets or spacers between the exhaust manifold and intake manifold to moderate the heat transfer between the two. Are they in place?

 

[Michael Oritt]

Interesting... I always assumed that the problem was fuel boiling in the carb bowls so that liquid fuel couldn't move through the jet and into the throat of the carb.

Keith--

We are all guilty of using the terms "fuel percolation" and "vapor lock" indiscriminately but they describe different problems. The solution that EV proposes might help with vapor lock that occurs in the fuel lines, and maybe that is more of a problem in Scotland than fuel percolation which is what we Healey owners in The Colonies suffer from due to the intake manifold's close proximity to the exhaust manifold, not to mention high temps under the hood.

Lest we start feeling sorry for ourselves this problem is common to a lot of older cars with carbs--for an interesting PM article go to:

https://books.google.com/books?id=0...NAhWEOSYKHQD9BrUQ6AEIKzAC#v=onepage&q&f=false

 

[EV2239]

The problem first surfaced for me to hear about it was with an R Type BENTLEY owning chum living in Las Vegas. He actually measured the pressure rise in the fuel lines.

It it barely ever occurs in the UK, but it does happen from time to time and as here, produces pages and pages of discussion, but my friend in Vegas has had no repetition for about ten years now. I am of the opinion the problem is on the rise as fuel evolves. Some blame ethanol, but I don't know what causes it.

 

[John Turney]

.... Some blame ethanol, but I don't know what causes it.

In our discussion with Lake Speed at Yosemite, he is of the opinion that, because ethanol increases octane, the underlying base gasoline is of poorer quality. Ethanol also has a higher boiling point than gasoline. The lighter and lower octane fractions of gasoline have a lower boiling point than the heavier, higher octane fractions.