Air in Mechanical Fuel injection System

Thanks for the description -- is it set by that "T" handle on the mixture control unit?

Don't doubt your observation that some other designs could intentionally tie the fuel temperature to the water temperature. In addition to the possible advantage you noted, I think another could be that it keeps the fuel at a more consistent temperature (so the fuel density doesn't vary so much if the ambient temperature varies a lot) -- could be more of a concern on a fixed volume-based delivery system like yours. I still can't get my head around how fast that internal shuttle must be oscillating at redline on a 12 cyl -- seems not possible

 

. TR's also do this as the AC line runs through the fuel tank which also heats the fuel.

Umm, you have that backwards. The A/C hose that runs through the fuel tank is the low side hose, which gets cold when the A/C is on. They did this to cool the fuel on hot days (when you would be using the A/C) to reduce evaportative emissions.

Most other Euro cars from the same time period (BMW, Porsche, Jaugar) had fuel coolers of some kind. Usually the fuel returning from the engine to the tank would go through a cooler that was incorporated into the low side A/C hose.

 

The purpose of the by-pass valve inside the high pressure pump is to prevent a build up of excessive pressure within the pump if the delivery line becomes clogged or obstructed. Without it, an excessive build up of pressure would result in either hydraulic lock in the pump or rupture of a component resulting in a fuel leak of catastrophic consequences.

 

Sounds good. Keep us posted.

 

my guess would be that due to the P 3/4 being a race car and thus going flat out for hrs on end the air chill factor at speed in colder climates would cause fuel density problems and poor atomization, to reduce this it would need to be kept warm enough not to be problematic or even freeze in very cold temps.

I know that when I do the runs up the state here CA 450 miles of non-stop running at sustained speed will cause the car to run colder than normal even in summer. one winter the mondial ran 130F tops for 5hrs of freezing rain and fog, being that cold it chugged fuel!

my 2cents

 

This is what the thermostat is for.

Compared to driving in REAL winter, wind-chill is a negligible effect.

Pre-heating the fuel is more likely to cause vapor-lock in hot weather.

 

right, however in really cold climates you'll have to block off air flow to the radiator to keep the engine warm. even with the t'stat closed and even then it's not a complete seal and there is also the heater box which will cool the water as well.

but back on topic, in the case of Jims race car, I'm only guessing as to why.

 

Just a guess, but perhaps warming the fuel will actually reduce the chance of vapor lock by allowing a gradual increase in fuel temp - and thus a more gradual rise in fuel vapor pressure - rather than a 'sudden jump', which might be more prone to causing bubbles/disrupting flow by 'shocking' fuel to temperature.

 

I've had one of these that bypassed too much fuel from the distribution unit due to wear, but that showed on the pressure gage. We ran -8 to the pump and -6 to the distribution unit, the return is generally -4. The bypass off the unit should a fwe drops a minute on a tight unit. we often rran a little oil in the fuel especially in Sud Afrika where the altitude and availability of good fuel saw us using Avgas. Modern fuels are also a little'dry' for this type of injection imo so I would run 2% castor oil, (for the smeel factor as well!).

 

Is the return also into the collector tank? Should be, they move a lot of fuel in the Lucas system. That's why only a couple of Bosch pumps will do the job, I threw away the Lucas electric and mechanical pumps and run the Shadows on a Bosch full time.

But if you are seeing air in the injector lines, there is probably cavitation. What pumps do you run?

 

If you refer to the "aspirin" valve, it is there when turning off the electric pump to run on the mechanical. Very troublesome back in the day, many teams installed a variety of one way check valves externally. With the correct Bosch pump it will run at speed on just an electric. a lucas will not supply sufficient flow rate for higher rpm on a DFV- 7500 up.

 

again, back in the day, several teams made mistakes by using a check valve on the vent, the theory being that air could draw in as required but in event of rollover nothing would leak out. We had to make sure that there was no spring on the flapper so it wouldn't be closed as the return from the system would pressurise the tanks and cause problems.

the vent should allow air either way but shut off in event of rollover. we used a -6 size line.

 

I use a kart racing two stroke oil, about a 2% mix is enough to provide a little lubrication to the distribution unit. the tolerances are very tight and they can sieze.

 

So, can you run through the scenario again for me?

Do you have the system bled through initially? i.e. no air in the lines.

car will start and have what pressure?

How long before seeing air in injector lines?

Does it then quit?

Was pressure constant on the gauge?

Is there flow through the bleed off line?

 

certainly sounds like a fuel feed problem before the pump, I would look at the supply tank and lines first. Does the return line feed back into the collector tank? How big is the collector tank? Should be at least one gallon imo.

the pressure should be constant with running or stopped. Will it run for several minutes maintaining pressure (engine off)? In order to fall it has to run out of fuel supply. I run 110psi/120 max. You may turn the pressure down a little. I would be interested to see a diagram of the system with line sizes etc.

 

Is there a timeline leading to this problem? Forgive me, I have not paid much attention to your car until now. Interested what if anything has changed to manifest this problem.