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Discussion Starter #1
Hello Sirs,
I own (since some months) a 1968 Le Mans with a 400 cui engine and a Tri-Power carb.
The engine is running well except at low idling, and there leaks fuel out of all three carbs.
So, my first intention was, to reduce the fuel pressure by adding a fuel regulator.
Today, I had a look at the fuel pump, and I notice, that the return line is blocked.
(I do not know, if there was originally a return line on this model ?).
Could the pressure be to high when missing the return line ?
Till now, I did not measure the fuel pressure, and I think, the carbs looks like overhauled.
Maybe you can give me a good hint ?
Regards: Peter
 

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Hello Sirs,
I own (since some months) a 1968 Le Mans with a 400 cui engine and a Tri-Power carb.
The engine is running well except at low idling, and there leaks fuel out of all three carbs.
So, my first intention was, to reduce the fuel pressure by adding a fuel regulator.
Today, I had a look at the fuel pump, and I notice, that the return line is blocked.
(I do not know, if there was originally a return line on this model ?).
Could the pressure be to high when missing the return line ?
Till now, I did not measure the fuel pressure, and I think, the carbs looks like overhauled.
Maybe you can give me a good hint ?
Regards: Peter
The factory fuel pump rating seems to be one of those things that are of opinion, going from 2 psi up to 5-6 psi. I have a Sun Electric data sheet which states the fuel pump for non-AC cars is 5-6.5 psi @ 1000 RPM's and AC cars is 3-5.5 psi @ 1000 RPM's. Fuel flow test is 1 Pint in 30 seconds at idle speed. The AC cars had the 1/4" return line so pressure would be different.

The pump you have has the 1/4" return line (Vapor Line), but not all pumps had these. The 1/4" return line was used on the 1964-66 cars with AC and used a filter located near the front of the intake manifold that had an inlet for the gas line at the base and an outlet on the top for the gas line going into the carb and the smaller 1/4" nipple used for the return line back to the gas tank. The gas tank also has to have the 1/4" line fitted at the gas tank sender unit. I am not sure when the fuel filter 1/4" return line was deleted and the 1/4" fitting moved to the fuel pump, it could have been 1967-68?

That said, you can use a pump having the 1/4" return line as you do and cap/block it off. The 1/4" line helps fight gasoline vaporization due to engine heat by keeping gas flow moving. In order to use the 1/4" return line you will need a gas tank sending unit that has the addition 1/4" return line incorporated - you can't just add the 1/4" return line.

https://www.hotrod.com/articles/hppp-1105-pontiac-fuel-pumps/

It sounds like your carbs may have the float setting too high. If they flood over, especially at idle, the engine will run too rich and run poorly - not to mention the possibility of a fire and loss of your car.

Extreme fuel pump pressures could overpower the needle and seat, but not likely if you have a stock pump. If you have a fuel pump that does not deliver enough gas at higher RPM's, then your engine will "nose over" and you will feel the engine power drop off or go flat before you hit maximum RPM's of the engine. What is needed at idle is different than for wide-open-throttle at higher RPM's so an accurate fuel pressure reading would be at idle and then at an RPM higher into your power band, but most fuel pumps will simply have the same rating at all RPM's.

I would first check float settings as you said the carbs were rebuilt. Next, there are different styles of needle/seat assemblies. You want to make sure you have the correct rubber tipped needle that is ethanol friendly otherwise it will deteriorate and allow gas to flow past the needle and give you the same flooding-over experience like a wrong float setting. Some needles are solid metal with no rubber tip - I don't care for these and feel they don't seat so well. So you may want to purchase a set of needle/seats for all 3 carbs just for piece of mind in knowing you have the correct rubber compound on the ends. Then check your float setting at the same time with the tops removed.
 

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Discussion Starter #3
Hello Jim,
thanks a lot for your answer.
I know, that without the suitable sending unit I can't install the return line.
Mike (from pontiactripower.com) said, that the tripower cars only persist a pressure about 4 psi.
So, when it's getting a little bit warmer outside (here in germany), I'll check this (all).
Thanks, Peter
 

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Hello Jim,
thanks a lot for your answer.
I know, that without the suitable sending unit I can't install the return line.
Mike (from pontiactripower.com) said, that the tripower cars only persist a pressure about 4 psi.
So, when it's getting a little bit warmer outside (here in germany), I'll check this (all).
Thanks, Peter
OK, then you are aware of the return line going to the tank.

Have heard this about the fuel pressure before as another member was having issues as well and believe he had his rebuilt by the same service. I am OK with 4 psi AS LONG AS you have the needed gallons-per-hour (GPH) flow.

Simple example to keep in mind would be 4 psi pumping 40 GPH versus 4 psi pumping only 20 GPH. It is not necessarily the psi alone you need to be concerned with, but rather how much volume the fuel pump can move at that psi. The higher psi can overcome the needle and seat if too excessive, but not enough volume of gas and your carbs will go dry under wide-open-throttle.

This is condensed from a website on fuel pumps, flow, & HP;

"Often the custom shop will advise a fuel pump upgrade, suggesting a fuel pump rated to support as much as 4-6 times engine horsepower. The reason given is the effect acceleration has on filling the float bowl, driving the standing fuel back against the pump, raising head pressure, killing volume, and reducing the gas flow. The fuel system’s first priority is to keep the floats from running low enough to uncover the main jet, running the engine out of fuel. It may not seem significant but the weight of fuel above the main jet does impact fuel flow through it, and therefore the air/fuel ratio of the engine under load. This is critical if engine tune is to be held across the rpm band, achieving peak performance from cruise to hard accelerations. Under hard acceleration the typical fuel delivery system struggles because it must start flow up the line against the G-force developed during this time."

GPH fuel flow can be reduced as much as 50% from restrictions in the fuel system such as a clogged/dirty "sock" on the end of the fuel pick-up tube, fuel filters, bends, rubber line, elbows, heat, 1/4" return lines, etc..
 

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As a tripower owner-operator for decades, I concur with the 4-5 psi maximum pressure. A LOT of new fuel pumps are out of spec and can have as much as 8-10 psi, which will flood your carbs. You need to check fuel pressure and go from there. As for the vapor return, it came off the filter in '67 and not the pump...that began in '68. My '67 has the return line off the filter on the intake. My '65 has no provision for vapor return, and for some reason, has never needed it. The '67 absolutely needs it to run properly without vapor lock issues.
 

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Discussion Starter #6
Hello guys,
before some days, I installed the fuel pressure Regulator, and since then, the carbs didn't flood over...engine runs well, even at TWO (but couldn't test this for a Long time, streets in Germany are mostly too short ;-)

But I have another Problem (this existed from the beginning, maybe the Environment in Iowa was different as in Germany...):
Normal Idle is about 1000 RPM, when switching to Drive, the idle Speed slows down, sometimes the Engins stops, even when turning the (power-)steering.

My Intention is:
At idle Speed: increasing the idle Speed up to 1200-1500 RPM, adjust the mixture screws to max RPM, the lower the idle Speed to 1000 RPM.
Is this the right way ?
Or do I have some Problems with the Transmission ?
Hope you can give me some hints....
Regards: Peter
 
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