PMO carb starting issue

[PMNorris]

Hi,
I have a starting issue with my car. It starts fine if I restart it fairly soon after its last run, even if its cold. I think my issue is that my 46mm carbs are draining slowly.

After leaving my car for a while the windows on the carbs show that the chambers are empty






After turning the car over many times, pumping the gas and finally getting the car to fire, the chambers are around 50% full, which is correct.




Is it expected that the chambers drain when the car is not used. I would expect them to stay half full. Any idea what could cause this?

Many thanks
Paul

[PMNorris]

I might have answered my own query as its at the top of the PMO carb technical bulletins...

2. Float Needle Valves

We started out with viton float needle valves which have a somewhat blunt viton tip. Historically, these have been the choice for off-road cars.
We changed to the Weber float needle valves when we found quality control problems with the viton valves. After that, we went to the double-ball type. Both didn’t work as well as the viton ones for holding fuel level. Consequently, we went back to viton, but we now individually inspect every valve.

As a footnote, not even the viton valves will hold the fuel level in some cases where solid motor and transmission mounts are used. For race cars, we recommend the Clubsport mounts which can be stiffened by modification. WEVO recently developed urethane motor mounts which should be the best of both worlds.

PMO has different thickness float needle valve gaskets available to set the float height. Ideally, the tang remains parallel to the rest of the metal stamping with the float set at the correct height. The float tang should be at 90 degrees to the float needle valve when adjustment is completed. The gaskets are available in .010, .020, .030, .040 and .060 thicknesses.

I will follow up with Richard at PMO.

[MikeB]

Hmm, the float needle valve allows fuel in at the top of the carb and to fill the float chamber with fuel, I wouldn’t think that they would be responsible for the fuel level to drop.

How long does it take the fuel level to drop? Webers and PMOs do have vents that will let fuel evaporate over time (a number of days). Generally if you leave them for a week or so you will have to let them refill the carb bowls and give them a few pumps of the accelerator to reprime the carbs.

[911hillclimber]

I have 50mm PMOs on my 3.2
From dry, the red top pump takes a good 20 seconds to 1/2 fill the float chambers.
The engine will start after 2 good stabs at the throttle or more and run.

The fuel level never drops much after for weeks, though the evaporation is high, the garage smell certainly changes when the LOla is in the garage.

Do your carbs have full air filters? With the carbs fully open, I could see the fuel evaporation being rapid.
The
Graham.

[fetuhoe]

The questions is what is happening to the fuel.

As has been suggested it won't drain back to the tank as the inlet is higher than the fuel level.



Fuel may evaporate through the vent tubes but this could take a significant amount of time and there will be a variation with ambient temperature.

There may also be some issues due to fuel boiling that has caused problems with IDA carbs for years.

When the engine is running carb temperatures are relatively low but when an engine is turned off heat from the engine will tend to warm up the carb body and this can cause fuel to evaporate via the vent tube into the air filter and can cause a fire hazard.

Fuel will also be discharged through the auxiliary venturi and jets, it will flow past the throttle plate and then sit in the cylinder of the engine resulting in flooding.

PMO's are fitted with an anti-percolation mod developed by Ferrari and don't normally suffer from this problem but engine running temperature can cause some issues.

Sticky needle valves can cause fuel height variation but they are just as likely to stick up and allow overfilling as they are to stick low and cause underfilling. I would suggest that the fuel level in the first photo is too high and in the second photo is a little low.

In any case the low fuel level may not cause a cold start issue. As long as there is sufficient fuel in the reservoir for the accelerator pump to operate then you can provide the cylinder with sufficient fuel for the engine to run - albeit even briefly.

There is also some fuel loss occurring as indicated by the yellow staining on the carb bodies and this may be due to leaks from the gasket between the carb throttle body and the top cover if the reservoir is overfull.

The old measure of a 'clicking' fuel pump can also be misleading as the pump will not only cycle to deliver fuel but it will cycle to develop the pumps cut off pressure in the fuel lines even when the float valve is closed.

I would check fuel level with the fuel pump running and then check again with the engine running.

I would also check fuel height just after a drive and then check every day to see how the level changes.

I would also be interested to know what fuel pump you use.

[fetuhoe]

I have just seen the comment on the Early 911S Registry which suggests that if the idle jet air correctors (24) are blocked fuel will siphon from the reservoir.

I am not sure I understand how this happens. The fuel head in the reservoir is normally set so that the fuel presented to the jet is just below the level required for it to flow into the cylinder and the vacuum pressure in the manifold then causes fuel to be drawn through the jet.

This mixes with the air being drawn through the air corrector and the volume of this mixture delivered to the engine is regulated by the mixture screw (20).

If the engine is running and the air corrector is blocked it is possible that neat fuel will be drawn through the idle jet and the engine will become very rich and is likely to idle badly as there may not be sufficient air.

If the engine isn't running I am not sure how blocking the air corrector will cause any problems and cause the fuel to siphon as the head in the reservoir is not affected.

[911hillclimber]

I ran my 3.2/PMO carb' engine on Friday last week.
The fuel levels were 1/2 way up the glass which I think is right for the engine at tick over for a minute or longer, thus the engine demand and pump supply are at equilibrium, everything settled.

I have just been out to the car and the fuel level is just to the bottom of the glass, level with the brass clamp ring. All 4 are the same, car on the level.
I would say this is down to general evaporation, strong petrol smell on Saturday!

Your carbs are very fuel stained, mine have never been that bad and they are 9 years old, about 2000 miles.
They came off a rally car before I got them.

I would be tempted to clean the carbs out, be very careful if using carb cleaner, the spindle bearings will suffer if washed in the stuff.

[fetuhoe]

Interesting numbers.

What is the fuel level with the pump running without the engine being started?

I would guess that at idle a 3.2 litre engine would use around 2.0 litres an hour which compares to a typical Facet Red Top pump which will deliver around 150 litres per hour into an open bucket and around 120 litres per hour at 3.5 psi fuel pressure and effectively 0 litres per hour at the limiting pressure of 7.5psi.

At full throttle and full load it is possible that the pump may just about run out of capacity and any fuel line restrictions that means the pump needs to provide more than 3.5psi fuel pressure could result in fuel starvation on long straights.

This should mean that the fuel level at idle is more about the float height setting than anything to do with fuel flow.

The amount of evaporation that occurs is also a function of the surface area of the vent and as this is quite small the evaporation rate should be quite slow.

The PMO makes it quite easy to make this measurement but as we have an external float height gauge I will fill a Weber with fuel and see how long it takes for the fuel level to fall.

[911hillclimber]

As you know Chris, my engine spends most of it's time at WOT and to the rev limiter.
There are 2 hills with long straights, Shelesly Walsh and Loton Park and the engine has always been on song, not a stutter in sight.
Same goes for a tight corner after a long straight climb, perfect pick-up out of the bend every time.

3psi delivery to the carbs via a local T piece very close to the pair of carbs, 6mm ID hoses. Fuel line length about 1.5 meters from pump to carbs.

To PM Morris:
My engine is mounted in simple '911' engine mounts with blue poly bushes, not much give in them!

[fetuhoe]

Graham,

I am sure you aren't suffering from fuel starvation and this should mean that the float height will always be maintained at about the same level as the pump has sufficient capacity to deal with peak demand.

We suffered starvation on a Rally prepared Escort with a BDA engine some years ago as it had been fitted with a fuel line that was too small a diameter and the restriction caused the pump delivery to fall. It was only noticeable on straights longer than around 1.5 miles long when it started to miss slightly. We always use a hose with a nominal 7.5mm ID which is the old standard used on 2.0 litre engines and with the 2.4T running on Zeniths.

I was trying to make the point that the pump will easily deliver enough fuel for idle and your float height must be set at the level shown in the window.

If the float height changes when the pump, but not the engine is running, then I would be surprised and I was just interested to know.

It is common for needle valves to be affected by vibration but this is often due to fuel frothing and side draft carbs always have some form of vibration isolation. this used to be provided by Thackery Washers and O Rings but these have generally been replaced by rubber bobbins and Misab plates.

The sooth running nature of a 911 compared to they typical inline 4 pot engine seems to mitigate against the need for any isolation but I guess solid mounts may not be the best idea.

Transmitted vibrations will tend to cause the float valves to leak very slightly and this is likely to raise the fuel level and cause a slight enrichment.

I must say that a 1-2mm variation in float height will be difficult to detect in terms of engine behaviour as the metering provided by a carb is not totally accurate or repeatable.

There is an old definition of a carb that I believe is still true.

Carburettor - an instrument that has been scientifically designed to provide the wrong mixture under all conditions.

[PMNorris]

I ran my 3.2/PMO carb' engine on Friday last week.
The fuel levels were 1/2 way up the glass which I think is right for the engine at tick over for a minute or longer, thus the engine demand and pump supply are at equilibrium, everything settled.

I have just been out to the car and the fuel level is just to the bottom of the glass, level with the brass clamp ring. All 4 are the same, car on the level.
I would say this is down to general evaporation, strong petrol smell on Saturday!

Your carbs are very fuel stained, mine have never been that bad and they are 9 years old, about 2000 miles.
They came off a rally car before I got them.

I would be tempted to clean the carbs out, be very careful if using carb cleaner, the spindle bearings will suffer if washed in the stuff.

The fuel staining is a bit odd. I am thinking that if the air corrector holes are blocked and I'm getting some sort of siphoning that could explain my issues. I have ordered some new parts from Richard as he has modified the gaskets to ensure they only fit one way and don't block the air holes. Also my air filter is not very air tight. Its a retro 3.0 RS one and poor quality. I plan to buy an original 70T air filter with snorkel but they are silly money on eBay.

[911hillclimber]

You did well to get hold of Richard. Do you really still have access to a fax?

As said earlier, maybe time for a jets-out good clean through.
I've had real problems with my PMO's using their carb-to-manifold and manifold-to-engine 'paper' gaskets.

All mine literally dissolved due to fuel erosion leaving several large gaps on most cylinders.

I would replace these gaskets (and both sides of the heat insulators) with some made from proper material.

I bought some material sheet from ebay and cut my own gaskets, and no probs since.

[PMNorris]

I have Richard's email address. I had to buy some fax software for my laptop when I ordered the carbs from him in the first instance though. He has been very responsive and helpful. He is sending me some new needle valves as mine were replaced by someone who tried to balance the carbs (he made the problem worse), but the needle valves manage the flow into the carbs, not out, so are not the cause of my fuel draining. Richard is convinced the cause is evaporation, probably not helped by my poorly fitting air filter. PMO have been using the modified gaskets for the last 5 years at least, so if they have been fitted correctly then they are not a problem. I will make sure I am not blocking any of the idle jet air connectors.

Making some new gaskets is a good idea though. I will inspect mine and take a look.

[911hillclimber]

Richard is excellent indeed.
I made my gaskets by sandwiching fresh proper material between two of the insulator blocks (bolted together) and a sharp modelling knife to cut around the block profile.

[PMNorris]

I bought a complete set of PMO replacement gaskets, insulators and needle valves from PartsKlassik. I'm in the US this week, so cheap delivery to one of my lucky colleagues. I'm going to get someone local to me in Zurich to replace the gaskets and tune it up as all my tools are in my house in the UK. Finding someone who knows about carbs is becoming increasingly difficult, but I have a couple of potentials to choose from.

Paul