DDK - Die Deutschen Klassiker

Hi,

I have a RoW spec 70T, so originally with carbs and no fuel return circuit that ends in the engine bay. My car is a hot rod (RoW 79SC based 3.2SS heavily tuned motor running 46mm PMO's). There is clearly some rust in the petrol tank as I had an issue recently and ended up needing to get my carbs cleaned (despite 3 inline fuel filters). I have other issues too. It takes a long time to start (lack of fuel). Also, if I brake heavily, the car sometimes stalls. I suspect that my 48 year old fuel pump is not up to the job. I'm thinking of making a number of changes and would like some advice.....

1) My local Porsche specialist in Zurich recommend getting the tank plastic coated to stop rust in the fuel system. Has anyone done this? It makes sense to me, but I'd like to hear if its been successful.
2) I'm thinking of fitting a fuel return system, as per the MFI engines and terminating at the PMO fuel pressure guage (70T fuel return system terminates at the fuel pump by the tank). I think this will keep the fuel cooler, which is supposedly better, and also mean that there is less chance of a stalling issue under braking. I will replace the current fuel line as a matter of course to ensure there are no rust/contamination issues with the current lines. Any thoughts on this?
3) I could also use a higher pressure fuel pump as the pressure can be regulated by the PMO pressure guage and install a MFI style filter in the engine bay. Any recommendations if this is ok and what pump to use. Standard Porsche 79 SC pump or something different?

Many thanks

Paul

Paul,

If your tank has internally corroded I would seriously consider replacing it with a new Dansk tank depending on capacity. I am not sure you will plastic coat the inside successfully and many of the ‘paint’ systems that are being used can deteriorate due to Ethanol in modern fuel. We have seen this problem on a number of occasions and have ended up junking tanks which have been internally painted.

I don't understand why the car is hard to start due to lack of fuel unless the reservoirs are not full and this could also account for cutting out under braking. Have you checked float levels? It is a myth that fuel drains from the carbs back to the tank.

When you turn on the ignition and the pump clicks rapidly and then slows it is just because the fuel pressure in the fuel lines has fallen and it takes the pump a little time to build pressure and slow down. The float valve prevents the fuel reservoir from seeing this pressure which would otherwise effect the jet calibration.

If you use the 'right' pump there is no need to use a regulator as all they do is tend to restrict flow which is a bad idea.

'Spill' return systems were commonly used in 1960's race cars which had massively high engine bay temperatures and fuel vaporisation at low speeds was common issue. We had a 300BHP Mk2 Jag we used for Historic Rallying which suffered from this problem and we did return the fuel the tank but you should never need to do this on a 911.

The MFI system uses 2 x By-Pass regulators to control pressure and to ensure a constant supply but this type of system isn't needed with carbs.

All you really need is a simple 8mm bore fuel line, a low pressure pump and a decent filter.

I find the idea of using a 75psi CIS pump and regulating the pressure down to 3 psi is bonkers.

Blocking regulators can also allow high pressure spikes from time to time and if the pump is ok they just aren’t needed and I guess this is why the MFI uses By-Pass regulators which need a return.

MFI By-Pass regulators in the engine bay operate at around 2 bar which is just too high a pressure.

There are two pumps which will work well.

The first is a Hardi Pump https://www.ebay.co.uk/itm/High-Quality ... .l4275.c10 These pumps should be readily available in Germany.

These pumps were used by Porsche on the 1967/86 911S – they work well but they are noisy.

You could also try an Airtex WP502 sold by Webcon which has an integral filter and runs at a maximum of 4.5psi and costs around £50.00. This is a quieter pump and is also quite reliable and should be adequate for around 300BHP.

Don’t forget any basic fuel pump produces peak flow at around 0.1bar - which is the pressure needed to crack the basic seals open – as pressure increases flow drops to zero at the pump’s maximum operating pressure so at 4.5 psi delivery will be zero.

It is important to use fuel lines with minimal flow restrictions and all of the unnecessary devices bolted into fuel lines just limit flow

Hi Paul,
Agree with Chris points re coating the tank, I've coated 2x motorbike tanks previously and the coat cracked after a couple of years, I think the tank on a 911 would be even worse re cracks as it wouldn't need much of a hit from below. Dansk tanks used to be pretty cheap on eurocarparts.
Cheers,
M

Many thanks for the advice Chris and Magnus. I will follow your advice. I have checked the carb float levels, but that was some time ago and my problem has become worse recently. The problem is only when the car is cold.

It sounds like I might have an issue with my pump as it does not slow down after it starts. Whilst it is quite loud, the sound is variable as if its struggling. I'll take a look at a replacement Dansk tank and replace the current fuel line with a new 8mm line together with the pump. I estimate my engine is somewhere between 250-280bhp, although it has not been tested.

Much appreciated

Paul

i think the only way to properly coat an old tank is something we have done at work, but its not necessarily practical as a one-off. you basically "rotomould" a new tank inside the old one, which is a case (very much simplified) of heating the tank, putting nylon beads inside it, and then rotating the tank so as the beads melt they form a new plastic tank inside the old steel one. we've done this for a few machines as part of a remanufacturing program to put them into countries where fuel quality isn't great and has a lot of water content (diesel) however we rotomould parts anyway and have the equipment and expertise, and so this was a feasible option!

The reason I mention this is, we tried a bunch of other processes (tumbling and repainting, resealing, etc., etc) and none of it really worked very well, so I wouldn't recommend doing it on a tank which is already in poor shape, unless its a 2-piece tank (bmw 2002 for example) where you can split it and fully clean/repair before recoating and resealing

Sounds like a new Dansk tank might be a cheaper and simpler solution. They are £212.80 at Design911 ( or £1,213.62 for a geniune Porsche one)

There is one small issue with the current Dansk Tank and this concerns the angle of the threaded fuel ports that you will find on the rear face of the tank.

If you look at an original tank these ports are at a slight angle so the threaded plug that fits into the tank will easily clear the steering rack crossmember.

On the replacement Dansk tank the port is not angled and when fitted the tank connector is uncomfortably close to the steering crossmember and in some cases can make contact - a feature that doesn't make us happy.

We made a threaded insert which allows us to 'tweak' the angle of the port by a few degrees so we obtain an adequate clearance between the fuel connector and the crossmember.

I am not sure if current factory tanks are better in this respect and as the fix only takes 5 minutes it is unlikely we will ever find out.

I have played with a few old tanks in the past couple of years, sometimes successfully, and in the beginning, sometimes not. The key, as always is preparation and cleanliness. In the past it wasn't usually easy to see inside a tank unless you cut it to pieces, but the advent of cheap and simple USB borescope cameras has meant that peering inside is now fairly easy to do.

In my experience, tank coatings fail for two reasons. Old style tank sealants like the clear Petseal don't like ethanol so much, and can fail after a few years, but more modern ones like POR15 are better, however they can have sticking issues if the tank isn't scrupulously clean.

In the event of having to resurect a tank with a failed internal coating, there are products available to get it out.

I have successfully coated a rusty Ducati tank with the POR15 3 stage method, and it has been in regular use for 5 years without any sign of faliure. This tank is a good one to use as an example, because the filler aperture is large, due to the need to replace in tank filters from time to time, so easy to see inside. The POR15 process works well, but is quite expensive for what it is. I have never used it on a car tank, but I can see how it might fail to provide the necessary protection if debris is left inside, or the chemicals have not been left for long enough to do their work, which is very easy to do.

The most recent tank I worked on was my 356 SC tank, which was in very good condition, no rust, but being 50+ years old, there was quite a quantity of petrol varnish residue visible inside (which looks remarkably like rust). I decided to do the cheap way by making up a strong caustic soda solution, leaving that in for a few days, to take out the varnish, then following that up with spriits of salts to etch out any rust, then a phosphate coating to prevent any rust coming back. This cost about £20 and a couple of days.

I've posted this link before, but it really does show how to get the job done cheaply and effectively.

http://www.realclassic.co.uk/techfiles/ ... moval.html

On inspection with a borescope, the tank is beautifully clean and like new inside.

I think that there is probably no reason to cut holes in tanks to clean them, or to scrap tanks just because the rusty inside. The Dansk tank at just over £200 seems like a bargain though.

I took a look at the pics on the Design911 website for tanks and pumps.

The Porsche tank does appear to have angled outlets. It also comes painted and looks much nicer than the Dansk version.
https://www.design911.co.uk/fu/prod1375 ... -63-07-73/

Its not too clear from the Dansk version, but the outlets look quite different.
https://www.design911.co.uk/fu/prod1297 ... 120190402/

Not sure if I can justify spending an extra GBP 1000 though, but I checked the prices here in Switzerland where VAT is only 7.8%, but the Dansk tank is CHF 690 here (approx GBP 500).

I didn't get a chance to inspect my fuel pump over the weekend (I have manflu). I see the Hardi and Airtex fuel pumps have 1 input and 1 output. However, I see that the original Porsche part: 91160810703 has 2 outputs and 1 input, so circulates excess fuel back into the tank.
Porsche version: https://www.design911.co.uk/fu/prod1319 ... 160810703/. Design911 advertise the Hardy version for 65-69 cars only.

The Porsche version is GBP 572 though vs the Hardy on eBay for GBP 80!!

The original pump had an inlet an outlet and a by-pass regulator.

To describe this as 'fuel re-circulation' is not quite correct.

If the pump delivers a pressure that is greater than the By-Pass regulator setting the fuel will return to the tank.

If fuel pressure does not exceed the set pressure then fuel will not return to the tank and will be delivered to the carbs in a normal manner.

If you want to use a Bosch Pump then you will need to buy Part Number 911.608.107.00 (Bosch No. 0580960009) as this pump has the regulator set at to operate from 2.8 to 4.3 psi. This pump used to have a 'Blue Dot' paint code. The MFI version which looks almost identical has the pressure relief set at 28 psi and will almost certainly overwhelm your carbs.

Design 911 List the MFI version at £572 and the T version for carbs at £601.00

My original point was that the Hardi Pump develops sufficient pressure to overcome fuel line restrictions but dos not risk over-pressurising the float needle valves and is a good match.

The fact Design 911 specify this pump for 1965-1969 cars clearly demonstrates a lack of understanding.

1965 cars ran with Solex 40Pl cabs, they used a Bendix Silver Top fuel pump to take fuel from the tank to a reservoir built into the intake manifold. There was also a Pierberg Dual Mechanical pump driven from the LH cam which delivered and re-circulated fuel to the carbs.

These carbs were used up to 1966 when Weber carbs were introduced and the Hardi fuel pump was used up to 1968.

IMHO the hardi pump will work well on any carburetted engine and is a good replacement unless you are looking to maintain originality. All you need to do is blank the fuel return port in the tank and run a single fuel line to the connector in the engine bay.