Fuel Pressure Too Low?

[VaCat33]

Hi All

You are all familiar with the problem I am having with my engine stumbling/hesitating after I have driven for 30-40 minutes. In previous threads there was a general consensus that the fuel pump might be the source of the problem and CADREAMIN has been giving mucho guidance as I try to solve the problem.

I installed a fuel pressure gauge just before the carburetor. I got a reading of 2.0-2.3 with the ignition on and just about the same with the engine on and at 1500 rpms. I then took a drive. The stumbling started and I had to stop to let everything rest. At that point the pressure gauge was reading zero. After resting an hour I barely made it home and the pressure still showed zero. Now convinced it was a fuel pump problem I changed out the fuel pump with the help of another local Tiger guy. I installed a NEW stock fuel pump from Sunbeam Specialties in the factory location. I also correctly vented the vent tube into the trunk compartment. Previously, I shielded the muffler with heat wrap. I also inspected the old pump and found no evidence of foreign debris.

Once installed, I checked the pressure. With the ignition ON the pump sounded strong and the pressure again read 2.3. I got the same reading at idle and 1500 rpms…all which seem low. I did the following none of which changed the readings.

• Opened the filler cap to make sure it was not a venting problem
• Jumped the fuel pump power tab directly to the battery
• Grounded the fuel pump directly to the negative terminal on the battery
• Switched out fuel gauges just to be sure it was not a faulty gauge

Yesterday I took it for a test drive. Fuel pressure was 2.3 upon starting the car. Car ran great for 30-40 minutes…then it started to stumble badly at speed and nearly stalling when starting from a stop. I headed home and checked the fuel pressure as soon as I arrived. It was reading 2.0-2.3.

I am getting ready to move onto the carb (it’s an older Holley 1848 that has been rebuilt once) as the source of the problem since the pressure is being maintained, but I still have a nagging feeling that the fuel pressure is too low.

CADREAMIN still does not trust those little mechanical gauges so he has recommended another static check using a combo vacuum/fuel pressure gauge tapped in where the current gauge is located. I’ll pick up the necessary hardware tomorrow. In the meantime…My question is…

Is the fuel pressure too low? If so, why would a brand new pump have such low pressure? Interested in what others with stock fuel pumps might have for pressure and any other thoughts.

Thanks, Jim

 

[Moondoggie]

Jim,

Fuel gauges are nortorious for not working when hot especially if they
are sitting right over the manifold. Oil filled gauges are advertised to reduce
vibration on the mechanism but the oil expands when hot and shuts down the gauge. Your best bet is to use an air filled gauge near the carb and only trust it COLD. If you locate your pump in the trunk wheel recess like a MK II put
a gauge back there away from the heat. It's not uncommon for gauge to read
zero when hot !!!! I must have gone thru a bunch of pumps before I figured that one out. One last thing....you can remove the oil out of the gauge by removing the rubber plug on the back of the gauge.


Moondoggie

 

[Moondoggie]

Jim,

Your fuel pressure at the carb needs to be between 5.5 & 6 lbs for your carb to
work well. Most inline pumps have a hard time making that kind of pressure
and just put out a couple of psi at the carb. I like the Edelbrock billet pump
that has an adjustable regulator built into the base. You would have to mount that one like a MKII along with a relay and modify your fuel lines but it's worth the trouble

Moondoggie

 

[lillypad]

My fuel line was full of rust, changing it along with new fuel tank cured a similar problem. I had a little air in my fuel line indicating a leak as well.
John

 

[0neoffive]

Try this

My fuel line was full of rust, changing it along with new fuel tank cured a similar problem. I had a little air in my fuel line indicating a leak as well.
John

Drain tanks by any inconvenient method (pump them out). Disconnect the rear fuel line at pump and tank ends. Take a good rubber tipped air hose and blast thru the line and catch what comes out in a cloth or strainer. That should either eliminate or I.D. a problem for ya. I have found all kinds of rust/grime/kids toys/other strange goop in the lower balance pipe that restricts pick-up flow. As you drive, slosh slowly plugs the pick-up tube.