Additional Fuel Filter

[Porsche_911s]

'74 2.7 with original CIS

Would it hurt to put an additonal fuel filter between the fuel pump and the gas tank?

-Stephen

[Grady Clay]

Stephen,

You don’t need an additional filter – there is already a filter screen in the tank. That tank screen is a service item and should be cleaned and inspected. If there is any signs of muck in the fuel tank, you should clean the tank.

The fuel pump doesn’t like a filter (like the CIS filter) in the inlet. It makes the pump more prone to cavitate and damage itself.

Best,
Grady

[Jim Sims]

I have installed an additional filter (an inexpensive NAPA in-line filter) on the suction side down by the rear mounted CIS fuel pump on a 1973.5 T without ill effect. Despite having the fuel lines cleaned out, the fuel tank cleaned and refurbished and a new tank screen filter installed there was still debris caught by this additional filter The debris was visible in the NAPA filter when it was changed out. I'm not sure of the flow configuration in the fuel tank but given the design, the OEM tank fuel screen doesn't appear to catch everything that could be in the tank (i.e. material in the tank chamber(s) downstream of the screen). I do agree with Grady that adding this filter does increase the load on the fuel pump by reducing the suction side head but felt it was worth it to keep debris out of the roller cell in the pump. It also has the benefit that the NAPA filters are a lot less expensive than the Bosch fuel filters which don't have to be changed quite as often given the fact that the NAPA filter is removing most of the debris. Note: the NAPA filters should only be installed between the pump and the tank as they are not rated for the pressures seen on the discharge side of the pump in CIS systems. Jim.

[Porsche_911s]

'74 2.7 CIS

Thanks Grady and Jim! I was considering putting a NAPA type filer and seeing if it would catch some debris. I thought about only putting the filer in once in a while to do spot checking to see if there was trash in the line. I actually had the tank cleaned (but not lined), I cleaned out the lines, fuel pump, fuel accumlator, and new fuel filter. I now have a fuel distributor that is acting up.

I guess it's not uncommon for a fuel distributor to start having problems after 30 years? Or normally does it take some dirt or grim?

[Porsche_911s]

Jim- Were you able to find a filter for a 1/2 line? My line appears to be 1/2 instead of 3/8's like I was expecting.

-Stephen

[Jim Sims]

The 1973.5T CIS system uses a 9mm ID x 15 mm OD braided cover hose from the tunnel to the suction intake of the pump. I used a NAPA Gold #3033 fuel filter which is basically set up to work with 3/8" ID hose which worked with the 9 mm ID OEM hose size. In 1974 this suction hose was changed to 11mm ID x 17.5 mm OD which is closer to 1/2" so you will likely need to find a fuel filter with 1/2" barbs. I found the filter for the 1973.5T by going into the local NAPA store and looking through the in-line fuel filters they had in stock with a helpful parts counter man. Cheers, Jim

[Formerly Steve Wilkinson]

A good additional fuel filter of the type intended for use on serious diesel trucks--mine is a Rayco, or something like that, as I remember--is absolutely required for reliable carburetor operation, but not nearly as useful with CIS. I have the stock tank filter, the truck filter and an in-line filter on each carb....and I've still twice in the last year had to blow an invisible mote out of an idle circuit (easy to do) when I've suddenly gotten the Five-Cylinder Stumble.

Stephan

[Porsche_911s]

Well IF I could not find a 1/2 fitting filter I was thinking about splitting the 1/2 (or 17.5) down to (2) 3/8's lines with two filers.

My reasoning was (2) 3/8's lines are 2/8's greater than one 1/2 line. I am talking about 1 foot of line (not the entire line).

Thanks for the suggestion Stephan, I am looking at the Racor online.

-Stephen

[Randy Webb]

"absolutely required"

Hmmm, well I talked to Richard parr about this re his PMO carbs. He told me I did NOT need anything other than the 2 small metal filters he includes in his kit. This was in the context of a 73.5 CIS fuel pump, etc. I specificially told him I had removed the big netal can filter with the rest of the CIS system. He said not to worry.

However, as long as you are not increasing the reistance too much, a big filter sure won't hurt - and it will gie you an added margin of safety in case some dirty gas is sold to you.

[Formerly Steve Wilkinson]

Well, try running PMOs--I've got 'em--on whatever you get out of today's gas stations and you'll be blowing out idle circuits monthly, I think. Go for it, though, if you and/or Richard are that confident. Doesn't work for me.

I do, however, pass along my late father-in-law's advice, which my wife and I kid about because it's like all his other compulsive theories...but make sense. Don't EVER refuel at a gas station where a tanker is parked, downloading fuel into the underground reservoirs. They'll be a maelstrom of whatever scheiss was parked at the bottom while the truck is pouring a hundred gallons a minute into the thing.

Stephan

[Randy Webb]

That last makes good sense.

Sounds like you've had a lot of trouble in NY. Never had any toruble with the Webers on my prev. car here in Oregon (about 8 years). don't recall what filters it had. My confidence is that somebody like R. Parr would be obsessive about filters.

BTW, your comment relates to the size of the filter media "holes" -- a small filter with only small passages would jam up and do so quickly when faced with dirty gas. It would not allow crud into the carb jets. So our problem must relate to the particle size passed by whatever media you are using.

Also, I "only" have $30,000 in my car. If I had put $70,000 into it, I might be more paranoid.