hello I just put together a 400+ horsepower 383 small block this spring and have noticed something interesting with my fuel pressure. I had originally bought the carter muscle car series pump but was told by carter this was a mistake. I sent that one back and got the 6900 series pump from them. I hooked it up and got a 3 psi reading on my summit guage on the fuel line. I was disapointed so sent the 70ish dollar pump back. I was in a rush to get this thing running so I went to the local parts store and got a master pump for 13.00 hooked it up and got 6 psi on the summit guage. All seemed good. I have been using it for about 4500 miles. All has been well untill I did my high speed run down a local section of road for approx 2 miles at 155mph (approx 3/4 throttle to keep tranny in overdrive). I noticed my temp guage went up approx to 215-220 degrees which it always runs at 170-175 while cruising 75mph. This was strange. so I fabbed up a 0-15 liquid filled guage(quality guage from work) and some instrument fittings and rubber hose to come of the fuel line and taped the guage to the windshield to monitor the pressure. here are the results 6.5 psi while at idle. 5 psi while cruising. 2.5 psi while primaries wide open 1 psi to the carpet all 4 barrels wide open on the mechanical 750 holley. this cannot be good for horsepower. I was told by holley 4 psi wide open on my carb is a minimum.
I just ordered a summit 110 gph pump in a chrome finish i will test the results with this pump. what is weird is Ii have not been dissapointed with the cars performance and this really cannot be helping performance at all. What do you guys think?

 

Check all rubber hose on the inlet (feed) side of the pump. Must be very short and not kinked. Will collapse under suction of the pump. Check for over heated fuel lines. Near exhaust etc. will cause vapour lock.

i gave up on the mechanical pump Carter 14PSi 130GPH race as I also could not get a steady fuel pressure at WOT.
Went to twin Carter 4594 7psi 72GPH (140total) electrics and twin 3/8" fuel lines from tank pick up to carb. No regulator is required. And rerouted the fuel lines away from the hot headers. Seems there is just too much heat soak on these cars with their cramped engine bay. Haveing the fuel lines under pressure from the back to the front instead of under a vacuum helps a lot.

 

all rubber lines are new 3/8" and are not near headers. the headers are also coated to reduce under hood heat. the hood also has functional hood louvers to aid airflow. the carb sits on the air gap rpm intake, which also helps less heat soak in the carb...thanks for responding i may also end up with the electric set up too... i hope the new pump takes care of the low psi..

Ryan

 

Most likely you have the crusties in your gas tank; the fuel line is half stopped up ( or more) with the funk of 100,000 miles.

Very very very common. You can change fuel pumps from now til Doomsday, and it won't fix that, if that's what you've got.

3 psi is plenty. For that matter, ¼ psi would be plenty, as long as the carb bowls stayed full (not that it will; but if it did, it would be fine). Don't get all hung up on the absolute value of the presssure. i.e. whether it's 3, or 5, or 2.5, or what; as long as IT DOESN'T DROP DURING SUSTAINED HIGH RPM AND HIGH POWER RUNNING. The drop in pressure is the sign of a problem, not the 3 psi or whatever.

 

What Sofa said. A drop in pressure when you go wide open is a problem.

Even a stock pump shouldn't be that bad. This almost certainly points towards a restriction on the suction side of some kind. Heat-related issues go away when you hit WOT for a few seconds as the system is moving a considerable volume of (cold) fuel through the lines. Heat-related issues are usually when idling, hot-start, low speed cruising or stumbles upon initial WOT application on a hot motor. Not SUSTAINED WOT running.

I beleive original L-69 305 HO carbureted cars and any carbureted V8 car from 1985-up had a low pressure electric pump in the tank to "help" the block mounted mechanical. I can imagine if this pump isn't working for any reason it would be a restriction, rather than a help.

If your car was originally an FI car (TPI would be the only possibility in 1986) and you are trying to suck fuel through a non-functional in-tank EFI fuel pump this is almost certainly your problem.

 

My intank electric pump was removed and the inside of the tank looked brand new. The stock tpi style canister filter is inline on the frame rail was changed and was left in place is this a restriction for a mechanical pump?
I believe the l69 had fuel boiling problems and they installed that electric pump to try to stop that. The l69 problems in my opinion were all the related to emissions equipment and high engine temps that poor carb never had a chance. but that is just my opinion.
I think that i am just overpowering the stock pump and thats when the pressure drops due to this. I guess I should have given the carter 6900 series pump a chance. it was 3 psi at idle i guess if it stayed at 3 psi wide open i would have been fine. i just figured that it was bad right off and mailed it back. well i guess i will tell when i get the performance summit pump and if it does the same thing i will no that in fact a restriction does exist somewhere. thanks for the replies....ryan