TOPIC: 85 Subaru carb W/ thin iron throttle body

Howdy Mr. Hatton!

In response to your suggestion about installing a regulator to reduce fuel pressure to the carb I must respectfully disagree.

The only reason someone should ever reduce fuel pressure to a carb is if the fuel pressure is overpowering the float bowl in the carb.

...and like you said...such a condition will indeed result in the float bowl overfilling and of course that will definitely allow fuel to "dribble" into the venturi through the main metering jets like you said and all BUT....

most carbs are designed to operate under 3-4 pounds of pressure and reducing that pressure will result in low fuel levels in the float bowl.

Once the fuel gets into the carb float bowl it is no longer under under pressure anyway! (venturi vacuum is what pulls fuel from the float bowl through the jets)

Fuel injection systems are a different story however and your fuel pressure reduction suggestion might be a good idea for fuel injected cars!

respectfully yours...Carby-tater

Sorry, Carby Tater.

You will find this an admirable way to reduce consumption. You see, following reducing the jet size by three points (61 to 58) I found no reduction in fuel delivery. That's when I started pulling in information from my fellow researchers. They all agreed that, when they were able to see tremendous increases in fuel efficiency in carbureted applications, they had to reduce the pressure.

Further, others have stated the same from the performance field-experts in carburetors here in Phoenix.

Tell you what, why don't you try it? You may be pleasantly surprised!

Ron

Hmmmm.

after thinking about it is perfectly logical to assume that a low level in the float bowl would reduce pressure to the jets...

Some large carbs probly hold up to 6 or 8 ounces of fuel in the float bowls and reducing the float bowl level would indeed reduce jet pressure a little bit...hmmm

float bowls are definitely larger on most carbs than they need to be but I'm still a little hesitant.

I will be doing a carb equipped Dodge van next and like you implied you never know till you try it I guess...experimentation is a good thing!! ...and I just might get me an adjustable regulator.

thanks for the reply Ron!

p.s. You seem like a great guy from all your videos and I really appreciate the open sourcing of your absolutely great idea. and although I don't have the money right now... I do want to get a licence from you one day cuz I think your great idea is going to continue to take off...In the meantime I will continue to experiment and share what I learn with all they folks here.

(hats off to you good sir!)

The float LEVEL is not what I speak of. I speak only of the pressure.

While it is true the float level needn't be so high with reduced need,the pressure will force the fuel through the jets regardless of the float level. The fuel pump will keep a certain level of pressure on the bowl, causing a condition we find undesirable: wasted fuel.

Thank you for your kind words. I only wish the folks on the OTHER forums knew what we were doing! Maybe then we would be more popular!

Have fun with the Dodge!

Ron
PS: Please sign the posts with your real name, amigo! We want to know who we're talking to...

Hey Ron,

I would like to respectfully point out that all modern fuel float bowls (by law) are passively vented to the charcoal canisters (just like your fuel tank in miniature)...and consequently... there is no pressure. (unless the charcoal canister is plugged up or something like that....

once fuel passes the float needle and enters the float bowl it becomes "unpressurised" (except the accelerator pump circuit of course)

respectfully yours, Dave Yohn ...Birmingham Alabama.

Hi Dave,

So, I had this big drawn out explanation typed up ready to go until I was reminded by my 1984 Chevy 454 with the Edelbrock 4BBl on it.

Basically, without the fuel pump (pump is new) to supply enough pressure to maintain fuel flow through the carburator the engine quits. That is with fuel in the float bowl.

I can pump the pedal and throw fuel down the carb but without the pump pressure behind it the engine will not draw any additional fuel from the carb.

As far as 'modern' carburated engines, I'm not enough of a 'motorhead' to be familiar with any that are in use these days.

I may be wrong, I have been before... but, food for thought

Karl Fortner
Tacoma, Wa.

Hey Karl!

I'm gonna hold my ground on this one.

I can't explain why your Edelbrock won't idle for a minute or two while the fuel line is disconnected but I do know that my 318 truck will idle with the float bowl vent hose disconnected from the charcoal canister.

I also just got back from taking the float bowl cover off of an old carter two barrel and took a close look at the float bowl vent and mine has a one way valve that lets air OUT of the float bowl chamber just like a pcv valve. I blew some air through it and it might have a tiny amount of resistance indicating that a small amount of pressure can be contained in the float bowl... but definitely not 3-5 pounds. (maybe 1/2 pound at most)

I'm gonna firmly maintain that float bowl pressure is supposed to be equal to atmospheric pressure.

p.s. (what I meant by "modern carburator")

back in the 60's carburator float bowls were commonly just vented to the atmosphere or the fuel tank.

furthermore if carb float bowls were not fully vented then carbs would be subject to pressure pulses that come from mechanical fuel pumps.

also take look at your Edelbrock float bowl vent valve...if so a good experiment would be to remove it (if possible) so that the float bowl is totally vented to the atmosphere and see what happens...???

I think I will install the old carb that I mentioned above onto my 318 (when I get to work on my van) and maybe just drill a hole in the top of the float chamber and see what happens...just for the heck of it. (just to see for sure)

Hi Dave,

Not a problem, there are a lot of things about the engines and fuel systems yet to be discovered with this whole process and everyones experiences being shared is part of that.

One of the things that has a potential on Ron's part is that liquids do not compress so any pressure that may exist has to go somewhere and the jets are an easy out.

Interesting link that may shed some light...
(Sometimes, I still like the internet. )



This one explains a lot more... (I did not know this!!)




Karl Fortner
Tacoma, Wa.

Thanks Karl, I hope I wasn't sounding like an ass, and I really hope I didn't diss you or Ron.

I most definitely agree with both of ya'll that too much fuel pressure will cause poor fuel economy. (no doubts at all)

I also totally agree that there ain't a thing wrong with lowering fuel pressure down to one or two psi as long as the float bowl remains full!!

Experiment onward!

knowledge is power!

may our endeavors become a friction unto the machine!

(rock on!)

p.s.

I'll get back to everyone in about a week or so with my hole in the float bowl experiment. (I got a busy week coming up) ...unless someone else performs this easy experiment before I do.

I took another look at my float bowl vent and I think i can just "prop" open the reed valve with a toothpick instead of drilling a hole so I don't ruin the old thing.

KUDOS to you for that last link you dropped...I too have never seen a bowl vent solenoid either. Now I know what you guys were getting at!

perhaps there are a few carbs out there? that are designed to run with "pressure" in the float bowl chamber but I have yet to see one. you guys might be "partially right" after all!

OK guys, I think everyone here is right about this in their own way...As I understand it, fuel is pressurized up to needle/seat; the needle held in seat by float, float of course regulated by fuel level. Ron is right about reducing fuel pressure to cut consumption. Eagle Research.com and many other sources agree here. Karl, your Edelbrock(an AFB I assume) float bowls hold a good amount of fuel when full, if you have an Edelbrock Q-Jet it holds less I think. Now if you have a full float bowl(s) the needle is seated against fuel pressure. Remember that the fuel level in the bowl ALSO is mirrored in the emulsion wells leading to the venturi discharge tubes, these wells are fed thru the main jets. The emulsion wells are where air from air bleeds is bled/mixed/carbureted into the fuel on its way to the main venturis and idle discharge ports and slots. When the fuel level drops in the bowl from no fuel pressure/input and idle consumption of fuel, the relative air//fuel mixture in the emulsion wells goes steadily leaner from fuel level dropping in these wells also. So maybe that's why your Rat dies from fuel starvation w/ no fuel pressure but fuel havung been put in fuel bowl(S). Even at idle a carb.'s inlet needle drops in just a second or three to admit more fuel, it doesn't take long to draw down the fuel level. The trick here is to give the carb what it needs but not over-feed it. George Wiseman of Eagle research has stated that most carbs sent to him for modification have Leaky inlet systems even w/ new needle seats in them! So, I'm with Ron here; Dave, try an adjustable fuel regulator and see what it does, if your press is 4 or 5PSI try 3 then 2?? My 2 cents, TracyG

Ok so, I went to my junk pile out back and found another two barrel carb from a Dodge van and ripped it open. (it's a 1986 holley) as opposed the the 1975 Carter.

the bowl vent valve was also a reed valve. but on this newer carb the valve was actually
opposite of the 1975 carb and at first glance appeared to actually hold pressure in the float bowl but...

the valve was much more complicated than the first one so I opened up and after further disassembly I looked inside and it has linkage connected to the accelerator pump.

Basically any "foot action" on the accelerator pedal causes the valve to open and freely vent to the charcoal canister.

The vent valve on the 75 carb I opened has one of the tiniest little springs possible and can't possibly hold more than an ounce or two of pressure inside the bowl before pressure overcomes it and sends vapor to the canister.

I can positively say that Dodge V-8 carburators (Carter or Holley two barrels) are not designed to run with any internal pressure more than an ounce or two...

Hi this is orrist up in portage la prairie manitoba canada have any of you heard about the carborater inhancer it puts a slit vacume on the flot boll of the carborater check it out at eagle-research.com.

orrist Michalenka