fuel pressure

347 · #1 (**permalink**) 04-30-2003, 04:47 AM

how much fuel pressure do you guys run with your idf's??
i am running 5psi.......my engine builder says thats ok
the weber bible

i am reading says 3.5 to 4 max????

help?

Mike I · #2 (**permalink**) 04-30-2003, 05:43 AM

Way too much pressure, it shouldn't be any higher than 3.5 psi max!!

Most guys keep it below 3psi.

Mike

427Aggie · #3 (**permalink**) 04-30-2003, 08:36 AM

Your seals wont hold long with that pressure...NEVER above 3

Matt

walt347 · #4 (**permalink**) 04-30-2003, 08:47 AM

I bought a fuel pressure regulator and installed a fuel return line. Will keep it below 3psi.

Walter

347 · #5 (**permalink**) 05-02-2003, 12:53 AM

would excessive fuel pressure create any problems with low down drivability?

Mike I · #6 (**permalink**) 05-02-2003, 03:43 AM

Yes, it would. If you overpower the needle valve's ability to seat and shut off the flow of fuel into the bowl, the carbs will flood and cause all kinds of drivability problems.

Mike

347 · #7 (**permalink**) 05-02-2003, 06:34 AM

i adjusted the fuel presure down to 3.5 psi,unfortunatley didnt make it any better
this may sound strange but the stumble feels worse

i am putting 55idle jets in tommorow to see if it helps with my down low stumble
as for the main jets what should i use(i dont have any problems over 3000rpm,this is just for optimum performance)
i have 145 mains with 200 air correctors
should i have my old 150 mains with 200 air correctors?
or should i put my 175 correctors back in with my 145 mains?

Mike I · #8 (**permalink**) 05-02-2003, 07:02 AM

The low rpm/low speed drivability problems you described on the other thread indicates the idle circuit is at fault, nothing to do with the main circuit at all. However, I would recommend you go back to the baseline setup (150 main jets and 175 air correctors)and start again. Lowering the fuel pressure was just step one. You should definitely heavy up the idle jets next, go ahead and start with the 55s and see what happens.

Mike

walt347 · #9 (**permalink**) 05-02-2003, 07:13 AM

I support Mike's approach. I run 60 idle jets (and 150 main jets, 210 air correctors). No problems so far - but haven't set up process yet finished. Over the coming weekend I'm going to adjust the fuel pressure down to max. 3psi.

Walter

347 · #10 (**permalink**) 05-02-2003, 06:26 PM

i am going to change my idles to 55's
what confuses me is friend of mine has a 396 clevo stroker with 48 idfs he is running 55 idles, he has 50 cubes on me,13.5comp 643hp why does he only need 55's?

Mike I · #11 (**permalink**) 05-03-2003, 04:47 AM

Can't really answer that question with any conviction since there isn't enough info to make a valid comparison, but I'd guess it has a lot to do with volumetric efficiency (VE). BTW, what choke size are you running?

Mike

347 · #12 (**permalink**) 05-03-2003, 05:34 AM

36mm chokes......?

Mike I · #13 (**permalink**) 05-03-2003, 06:11 AM

How much initial ignition advance are you running? How much total advance? Initial advance should be set at the ragged edge of being able to start the engine, total advance should be set @ 36 - 38 and should come in by 3000 rpm. You should not have vacuum advance hooked up.

Mike

walt347 · #14 (**permalink**) 05-03-2003, 12:02 PM

Just came home from a hard working day. Did some fine tuning on the weber carbs today and installed a new throttle pedal. The car is still in the garage but runs fine. Wondering how it will run on the street (in 2-3 weeks hopefully !!!). I also have installed the fuel pressure regulator and the fuel return line. Pressure is set to 3.0 psi. Runs perfectly. Thanks to all of you who gave advice !!!

347,

There is one thing I gave up during the installation/set up process of my webers. Since each weber installation is really unique you shouldn't compare your engine with the ones of your friends. There is an initial jetting as a starting point from which you move towards your unique setup. It's more or less a trial and error process. I'm not yet at the end .......

For my 347 I have choosen the following initial set up which is a good street setup (as confirmed by CalMetal):

37mm venturis
60 idle jets
1.10 idle jet holders
F5 emulsion tubes (F7 are more common)
150 main jets
210 air corrector
50 pump jets

F303 Ford Racing hydraulic cam with no overlap (288int/288exh)

Walter

Excaliber · #15 (**permalink**) 05-03-2003, 12:15 PM

Over carburation is a VERY common problem. Most of the time, LESS is better.

Ernie

Mike I · #16 (**permalink**) 05-03-2003, 02:26 PM

Walt, that's a very nice looking installation. The setup you're using sounds right for an IDA system, CM knows his stuff. IDFs are tuned/set up a bit differently. The real test will be under a load, that's where the road test tuning begins. The fact that it runs well under no load indicates that you're probably on the right track. You will enjoy your car all the more for having a well tuned Weberized motor, it'll run like EFI on steroids. I couldn't believe the difference when I switched to Webers.

Ernie, what does that have to do with this discussion? 347's problem is not one of overcarburetion, 36mm chokes are well within the correct range (maybe even a bit on the small side) for a hot street setup of that displacement. Don't confuse the issue.

Mike

347 · #17 (**permalink**) 05-03-2003, 05:16 PM

i cannot tell tell you the timmimg specs(as i didnt build this engine and i havent asked what the specs are) and there is no vacuum adv
before i had the heads and manifold ported there never was a stumble, as i have now.
would porting change how the carbs need to be set up?or is it a coincidence?
how can i post a pic?

Excaliber · #18 (**permalink**) 05-03-2003, 05:56 PM

My comment was in reference to 347's observation about a bigger or smaller motor BOTH using the same carb setup. Doesn't surprise me, over carbing is WAY more common than "under carbing". Wether it be in Jet Size, CFM or what ever. A little rich is generally considered a good thing, MUCH better than running "lean".

Ernie

347 · #19 (**permalink**) 05-05-2003, 04:50 AM

before i had the heads and manifold ported there never was a stumble, as i have now.
would porting change how the carbs need to be set up?or is it a coincidence?
how can i post a pic?

Mike I · #20 (**permalink**) 05-05-2003, 05:34 AM

Assuming the porting was done properly, it would increase the flow of air into the cylinders, hence requiring additional fuel. One more reason why stepping up the idle jets should be your first move. Webers are very sensitive to engine mods and environmental changes, it wouldn't surprise me at all that your stumble is a direct result of the porting. BTW, I checked my setup this weekend, I have #55 idle jets in mine and the bleed screws are 3/4 turn open on all barrels. I have not done any porting on my heads or manifold, but I would expect that when I do it will require additional carb tuning afterwards.

Mike