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Pressure for Webers or other Carbs
I have a question. I understand the needle and seat can only handle a certain amount of pressure. However, if this is not an issue with the needle and seat why is pressure so important. Carbs are not like FI that it is important to have a high pressure for spray pattern. In a carb you are just filling a bowl which will then grab the fuel from engine vaccum.
Wouldnt volume be more important? So why is pressure in a carb so important. Thanks |
Too much pressure pushes the needle off the seat and floods the carb. The needle is only held against the seat by float pressure. Too little pressure and the carb bowls may not fill fast enough at higher rpm causing a lean condition.
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It only takes a small amount of pressure to force the needle open---this didn't used to be such a problem until higher hp cars needed more fuel and the enlarged the seat size which in turn increased the area of the needle for the pressure to push against which some floats can't overcome
for instantance---going from a .050 seat to a .100 seat is four times as much area so float has to overcome 4 times as much hydraulic pressure |
I understand there is a pressure break open for the needle, however a fuel pump like the Aeromotive (140 GPH ) provides enough volume. Why would excess pressure over the amount of breaking needle pressure. Would you want to set the pressure at the optimal pressure of the pump?
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No, that gives you an idea of the maximum capability of the pump when selecting one that can keep up with your radical engine combination! That's why they make pressure regulators and you shouldn't run an electric pump without one, preferably a bypass variety with a return. Deadheaded regulators have a tendancy to cause fuel pressure swings.
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Mreid,
Maybe I am stating this wrong. I am simply stating that pressure at say with a Weber IDA 48 is only important because of the needle and seat. So, if recommended is 3 psi and you run 2 or 4 psi you would probably not see a difference unless the needle has a minimum 3 psi cracking pressure. Also, if you exceed the 3 psi to 4 psi this may hold the float down from excessive pressure. If the previous condition do not happen then, there would be no difference in performance. Correct? |
Priobe---I have been following all your posts about your car and you have been battling these carb problems for over a year plus you also seemed to have inadquete experience'knowledge about various other things from pushrods/rocker arms. etc
So I have a couple suggestions so that you can start to drive your car and enjoy it----Don't take this the wrong way---First , get rid of the webers and put an 850-950 Holley/Demon type double pumper on it and after a little tweeking for your cam charistiques and driving style you'll be able to enjoy your car---Those cars have caused you over a year of heart break and its not getting any better--either you have a defective carb or you don't understand fuel systems and the engines fuel needs-- Go to something simpler even if it is a 2 barrel off a 360 truck engine |
If I understand you correctly, then yes any pressure is acceptable as long as it doesn't cause the needle to be pushed off the seat when the fuel bowl is full AND is sufficient to keep the bowl at the proper level at the highest engine output. Manufacturer stated fuel pressure requirements are specific to keep both of these in mind.
To Jerry's point above, I haven't followed your earlier posts, but Webers are outstanding carbs when the engine components are properly selected and the carbs are tuned properly. They are probably the most demanding of carbs for proper cam selection (duration and overlap), vacuum needs, and fuel pressure. They are the wrong carb for someone with no experience, unless they are tuned/maintained by an experienced Weber carb tuner. Fortunately, once proper engine component selection is complete and they are tuned properly, they run without the need for further tuning unless something breaks. They can be awesome! |
Jerry Clayton,
You mentioned that you have been following my post. Well if this is true then you would have seen that I am not asking this question pertaining to my car. This is a general question. Also, my car has been running fine for the past year and a half. Yes I have been experiencing problems such as rocker arms and push rods due to the rocker arms but again you have been following my post you would know that it was due to the parts used that many have claimed to be good for my application. Now I switched to Erson and new rockers and problem solved. In regards to this thread that I posted, this is a general thread. I am sure I am not the only person that post questions on CC just to keep the threads moving and in turn gathering information along the way. Please before making recommendation on what someone should do with their car please take some time to read the post to find out what the person is posting / asking. No offense taken. I hope you dont take this post the wrong way either ;) By the way My Carbs, Motor, are work Awsome :) |
I understand about 2psi-2.5psi is all that is needed, mechanical fuel pumps need to be limited (regulated) due to high pressure electric are unecessary for use on the Weber cars.
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My 48 IDA's are running 2.5 lbs. I'm using a Holley electric fuel pump with a regulator.
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