fuel pressure

Hayden · 09-29-2007, 05:58 AM

I know I brought this up a long time ago but cant find the thread. I replaced my 4.3 in my 94 K/1500 with an older 350 thats fairly built up a touch. My question.......Is the electric fuel pump in the tank providing to much fuel to the 750 holley double pumper which should only have 7 psi at the most. I do have the fuel coming from the pump in the tank to the pump on the engine.( I figured that'd help reduce the pressure some lol). If it is to much pressure (which is most likely causing the engine to diesel when I shut it off. Not the timing) Can I use a pressure regulator or would it be better to just get a pickup in the fuel tank instead of the pump?

J Long · 09-29-2007, 08:21 AM

New one on me. I thought excessive heat caused dieseling but my experience is limited. The first thing I would do is get the fuel delivery system to what the carb is designed for and go from there. An electric fuel pump designed for fuel injection boosted by a mechanical pump seems like way overkill. I would worry about raw fuel draining into the crank case after everything is shut down no matter what. I would also consider downsizing the jets on the carb a couple sizes from box stock. A holley 750 seems rather large for a street driven 350.

75K30 · 09-29-2007, 09:40 AM

You r combo is interesting to say the least. You should have a regulator in there no matter what.
What did you do with the return system that was originally there for there fuel injection system?
You can easily modify this to return unused fuel before it reaches the other pump. (might just get rid of the machanical pump anyway) and reglate it down to the desired pressure. Any residual fuel will simply be returned to the tank. This will allow the in tank pump to run at a max speed, and never over pressurize the system.
See carbureted bypass regulator.
Keeping a steady supply of fuel will be a good thing for a mechanical pump, but really not necessary in this case.
I too believe that there is more to your dieseling problem, and it is not fuel pressure related. Once the ignition is off, it can no longer supply the fire required to make an engine run. If there is an alternate form of ignition, such as a plug with the wrong heat range, or some carbon that is holding a bunch of temperature causing the ignition, the ebgine will continue to run, until it uses all of the fuel, all of the air (not likely) or the ignition source cools, and becomes so weak that it will no longer cause the fuel to fire.
The use af non copper plugs can cause this, as the small tips of platinum style plugs are so small they get very hot, and can cause this type of ignition.

J Long:
You bring some valid points to the table, except for the carb theory. The recommendation of jet downsizing may not be a valid one. Nor is the recommendation for a smaller carb.
The sentance would suggest that carb sizing and fuel metering is symetrical. Jets are just for calibration purposes and have nothing to do with the size of the carb, nor is there a prefered carb for displecement. There are CFM recommendations that use a generic cubic inch basis, but this is a very general rule of thumb.
Case in point, the Predator carb flows in excess of 1000 CFM and is used on small blocks all of the time.
I have used large carbs on many small displacement engines with great success, and fantastic throttle response. I have a little 377 chevy that I run a 950 cfm gas carb on, and it runs great, idles fine, and has throttle response like a SBC should.
I have said before that there is no such thing as running too large of a carb on an engine. Fuel metering, booster signal, and air speed is critical. Just easier to do on a smaller carb at the sacrifice of power.

09-29-2007, 05:58 AM	#1
Hayden Senior Member Join Date: May 2006 Location: Bay City,Michigan Posts: 387	fuel pressure I know I brought this up a long time ago but cant find the thread. I replaced my 4.3 in my 94 K/1500 with an older 350 thats fairly built up a touch. My question.......Is the electric fuel pump in the tank providing to much fuel to the 750 holley double pumper which should only have 7 psi at the most. I do have the fuel coming from the pump in the tank to the pump on the engine.( I figured that'd help reduce the pressure some lol). If it is to much pressure (which is most likely causing the engine to diesel when I shut it off. Not the timing) Can I use a pressure regulator or would it be better to just get a pickup in the fuel tank instead of the pump? __________________ There Is No Replacement For Displacement

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09-29-2007, 08:21 AM	#2
J Long Full Member Join Date: Sep 2007 Posts: 18	New one on me. I thought excessive heat caused dieseling but my experience is limited. The first thing I would do is get the fuel delivery system to what the carb is designed for and go from there. An electric fuel pump designed for fuel injection boosted by a mechanical pump seems like way overkill. I would worry about raw fuel draining into the crank case after everything is shut down no matter what. I would also consider downsizing the jets on the carb a couple sizes from box stock. A holley 750 seems rather large for a street driven 350.

09-29-2007, 09:40 AM	#3
75K30 Senior Member Join Date: Jul 2006 Posts: 3,312	You r combo is interesting to say the least. You should have a regulator in there no matter what. What did you do with the return system that was originally there for there fuel injection system? You can easily modify this to return unused fuel before it reaches the other pump. (might just get rid of the machanical pump anyway) and reglate it down to the desired pressure. Any residual fuel will simply be returned to the tank. This will allow the in tank pump to run at a max speed, and never over pressurize the system. See carbureted bypass regulator. Keeping a steady supply of fuel will be a good thing for a mechanical pump, but really not necessary in this case. I too believe that there is more to your dieseling problem, and it is not fuel pressure related. Once the ignition is off, it can no longer supply the fire required to make an engine run. If there is an alternate form of ignition, such as a plug with the wrong heat range, or some carbon that is holding a bunch of temperature causing the ignition, the ebgine will continue to run, until it uses all of the fuel, all of the air (not likely) or the ignition source cools, and becomes so weak that it will no longer cause the fuel to fire. The use af non copper plugs can cause this, as the small tips of platinum style plugs are so small they get very hot, and can cause this type of ignition. J Long: You bring some valid points to the table, except for the carb theory. The recommendation of jet downsizing may not be a valid one. Nor is the recommendation for a smaller carb. The sentance would suggest that carb sizing and fuel metering is symetrical. Jets are just for calibration purposes and have nothing to do with the size of the carb, nor is there a prefered carb for displecement. There are CFM recommendations that use a generic cubic inch basis, but this is a very general rule of thumb. Case in point, the Predator carb flows in excess of 1000 CFM and is used on small blocks all of the time. I have used large carbs on many small displacement engines with great success, and fantastic throttle response. I have a little 377 chevy that I run a 950 cfm gas carb on, and it runs great, idles fine, and has throttle response like a SBC should. I have said before that there is no such thing as running too large of a carb on an engine. Fuel metering, booster signal, and air speed is critical. Just easier to do on a smaller carb at the sacrifice of power.