Fuel pressure on Weber DCNF

I would say 5 +/- is good and would not be a problem unless the neddle valves are leaking. I would stick to the OEM jets and adjust the mixture screws. Get the car back to the OEM settings .

 

My Lancia Montecarlo has twin 42 DCNF's and I've always run them at 2.5 psi without problems. I could have sworn I read somewhere that was the region they should be run at.

I use a Filter King filter/pressure regulator:

http://rallynuts.com/motorsport/FILTER_KING_Regulators_1862/Filter_King_Competition_Fuel_Regulator_cw_Gauge_399.asp

 

I don't know of any fuel pump that is pressure regulated (not saying they don't exist). Typically you want a fuel pump that puts out "too much" pressure and then regulate to the correct pressure (lower level). Kind of like the water pressure in a house. "Too much" coming in and then you regulate it down to where it should be.

Same thing with fuel injected cars. Pump supplies over-pressure and the regulator sends the correct pressure to the fuel rail.

The voltage from the alternator is going to vary to some extent and this varying voltage is going to affect the "speed" of the fuel pump. This in turn will affect the output pressure (along with engine load).

 

How were you measuring the pressure? Just "dead-ending" the fuel pump output into the pressure meter or inserting a "T" into the fuel line before the carbs.?

 

I'm not sure from you description if you had the pressure regulator and pressure meter installed at the same time but that would be the way to do it. Then adjust the regulator to see fuel pressure changes (rather than pinching a fuel line).

 

Just had a look at my official Weber manual (factory publication) and they state that for the MAJORITY of Weber carbs., normal fuel feed pressures are Max. 4.2 psi at idle or transition phase and Min. 2.8 psi on the road around the top rated vehicle speed.

Maybe I'm not analyzing that correctly but it seems to me they are indicating the fuel is NOT going through a pressure regulator under these circumstances.

 

No test I know of/read with the tops off.

King Filter is not inexpensive but is "well respected." Pretty sure it was standard on multi-carb. Alfas back in the day and probably some F-cars and other brands.

 

If you have a stock engine then my guess is that you are running too large an idle jet in the car. I have a 1976 dry sump with P6 cams and high compression pistons and I am currently running 155 idles. Step down to 145 or 150 and readjust your carb mixture screws out. Don't worry about being more than 3 turns out or not just get it to idle right with the smaller jets. Then road test. If the car bogs down accelerating from a stop then and only then install the next size larger accelerator pump jets. The reason that these carbureted cars run rich is that we are running too large an idle jet. For years I did the same but when I took the car to a dyno the guy said I was too rich in low rpms. Smaller idle jets solved the problem. When in doubt try the simplest most obvious thing---Occam's razor I believe. Be curious to hear how this works out. Tired of your plugs fouing all the time- it is running rich most likely because your idle jets are too big.


Don

 

The float valves should be able to handle 5psi just fine. You're right, the commonly accepted range is between 3psi and about 4.5psi for most all down draft webers.

If the float valves were being overrun by excessive pressure (leaking) then I don't think you'd see just a marginally richer mixture in the cc and tail pipe....I think you'd see the carbs actually flooding, and creating a fire hazard.

But nevertheless, good to get your pressure in the 3-4.5 range. Nothing wrong with a permanently installed pressure gauge downstream of the pump and regulator. Summit Racing (www.summitracing) sells a number of regulators and gauges that will work well.

Might be best to run her up on the dyno and check the AFR throughout the rev range. There seems to be a lot of dyno shops that will do an hour or two of time for a few hundred bucks, and you'll likely pick up a few hp and improve your fuel economy as well in the process. Good luck!

 

Just fitted a new Filter King to my Maserati Merak SS which uses 44DCNF's and the pressure regulator came as standard.I think it wasn't functioning correctly anymore since I was getting fouled plugs and lots of trouble starting the car. I suspect the needle valves are worn and the excess pressure was feeding too much fuel past them at tickover. I bought a new Filter King and started the car over the weekend. It started easily, did not foul its plugs and fuel pressure was down to 3psi. I have also bought new Grose jets from MIE Maserati in the US which are designed to cope with higher pressures and also don't wear since they use a ball which rotates a little thus preventing a wear ring. This should eliminate any remaining rich running at idle since the fuel feed will be properly regulated


Julian

 

I'm not sure if it's the same Grose jets, but in the Triumph / SU carbs community those things are famous for sticking and causing massive flooding. Most people have gone back to using regular needle valves.

I had the same problem when the fuel pump on my Abarth stopped working. I got another pump of the same type that supposedly put out the correct low pressure, but it was too high and I had to put in one of the Filter King types-- actually made in Italy, works great.

Those generic pressure regulators you get at the local auto parts store are junk-- I bought one, which worked for about 15 seconds before failing (no fuel flow). I took it back and got another, same thing happened. Took it back and got a Filter King. The FK type has to be adjusted with a gauge in line with the engine running-- there is no knob with numbers to turn to set it.

 

I had a dual DCOE Weber set-up on the Alfa GTV I had. When I received the car it ran like crap, smelled bad, idled high, etc. I went through the jetting number routine too. I then talked to a Weber expert, Gron Perry. He immediately said my pressures were too high and that it should run around 2.5-3.0. I had the car worked on by my mechanic who installed a new fuel pump and a pressure regulator set to 2.5 and it ran like a champ thereafter. All the issues were gone. He too said that Webers like less pressure than conventional wisdom suggests. Just my two cents worth and I claim no expertise, rather just my experience with Webers and pressure regulator.

 

Hi Julian,
Grose jets work fairly well, when there is no varnishing of fuel or contaminants, if they stick (like any float valve) you have a mess.

BUT... Grose jets do not flow as much as either the SU/Stromberg or Weber needle valves. If you ever have a problem with WOT in higher revs, try subbing the original needle valves for them, before going on a wild goose chase.

 

Facet makes two pumps, the low pressure is the one you want, (2 -4 PSI ?) the high pressure is like 5 to 7 PSI and will not do it properly.

There are inexpensive inline Regulators I have used to avoid changing out the 'wrong' pump, then throw it all away together next time you need a new one.

 

On one of my vehicles with an aftermarket electric pump back at the fuel tank. I put a Holley fuel regulator after the pump and ran a second line teed from the reg. back into the tank. This kept a more consistent pressure, and the pump ran cooler because the pump wasn't dead headed.




Ago

 

A pump that I have used without issue:
http://www.walbro.com/frb20-series-fuel-pump.aspx
Image Unavailable, Please Login