427/435 Fuel Pressure

[Howard P.]

While attending this years Nationals at Novi and driving to the Milford Proving Grounds we encountered an issue with our 67 Coupe. When cold the car runs perfectly but after 10 miles or so and up to full operating temperature the engine no longer wants to hold an idle and will stall out. This is evident soon as you begin to decelerate as the engine stumbles and if you don't blip the throttle it dies.

As we were driving the approach to the Milford grounds I found the car becoming increasingly balky and dropping down gears and keeping the revs up kept it somewhat happy. When we entered the grounds the caravan slowed to a near crawl this was the kiss of death as the car just wouldn't run and as we pulled onto the "black lake" she died out. After about 20 minutes it fired up again and we were able to get a spot in last row of the line but again when we were about to caravan out it wasn't co-operating and refused to hold an idle. We flat bedded it back to the trailer.

My initial thoughts were that we were encountering fuel perculation however when trying to start the engine it would give a lean pop from the centre carb. One of the GM guys indicated that he felt my fuel pump may not be producing the correct pressure which he indicated 435's want not less than 9-1/2 lbs . This seems like a lot and my numbers correct stock pump delivers 6 PSI per my gauge when tested today. This was with engine warmed to 200 degrees on the dash gauge and 189 degrees per my Raytech on the upper rad hose.

I'm running a 50/50 mix of 101 Av-Gas and 91 Octane Shell unleaded fuel.

I've taken the carbs off, checked and replaced all three fuel filters, checked float levels etc etc. All seems fine.

Can any of you please verify the pump pressure requirement and can the pump I have be modified to deliver the necessary pressure ? If so who does this work.

Any other suggestions would be appreciated. I want to drive and enjoy the car !

[Joe C.]

Re: 427/435 Fuel Pressure

Fuel pump pressure should be between 5 and 7 psig.
9.5 psig is too much, and will cause the needle and seat to fail to hold back the pressure, causing flooding.
Try 100% avgas as a test........the alcohol in the 50% mogas will boil off because the partial vapor pressure of that component is high.
If it still percolates then you'll have to find a way to cool the intake manifold. Of course, you have already looked at all of the usual suspects, such as timing, vacuum advance, heat riser operation, float level.

[Domenic T.]

Re: 427/435 Fuel Pressure

Howard,
Sounds like you have the fuel right but you don't need much in the way for fuel preasure to run slow or idle.

I could only think that one of the jets is clogged in the center carb. Your filters would have cought any crap in the line.

If you check your filters make sure you blow thru them even if they are new and look clean. I found some SUPER clean ones on new trucks that were plugged with clear plastic, the gas etched the plastic container they used to put a few gallons of gas in to run them down the line at a motorhome outfit.

DOM

[Philip C.]

Re: 427/435 Fuel Pressure

Hi Howard, I had the same problem sometime back, we found a bad TI amp. replaced it no problem. Phil 8063

[Dick W.]

Re: 427/435 Fuel Pressure

Chances are Phil hit the nail on the head

[Timothy B.]

Re: 427/435 Fuel Pressure

Howard,

Can you replicate the problem with the car now that you are home? I don't think you have a fuel pressure problem if your gauge shows six lbs. If there is a testing procedure for the TI amp I would check that but IMO look at the carburetor.

If you close or cover the choke area with a rag does the engine want to idle after warm up, this would indicate a vacuum leak somewhere as you state the engine runs fine when it's cold. The problem could be a carburetor gasket not sealing causing either excess gasoline at idle (flooding) or vacuum leak internal in the carburetor. Did the engine backfire recently as this could rupture the power valve causing a rich condition.

[Joe R.]

Re: 427/435 Fuel Pressure

Howard,

Fixing your problem may be as simple as checking to make sure your gas cap is still VENTED!
May sound stupid but I've seen it happen many times.

JR

[Joe C.]

Re: 427/435 Fuel Pressure

Howard,

Fixing your problem may be as simple as checking to make sure your gas cap is still VENTED!
May sound stupid but I've seen it happen many times.

JR

Would he have 6 pounds fuel pressure if the tank were not vented?
Just asking.

[Howard P.]

Re: 427/435 Fuel Pressure

Thanks much for the advice. I will be putting the carbs back on tonite and will refire the engine. If it still doesn't want to run I will next look at the TI Amp though it does have a aftermarket unit inside the case which was put there a few years ago. My understanding was that either these devices work or don't and if the latter the engine simply won't fire or run .... am I mistaken on this point ?

I also have in my arsenal of parts phenolic spacers that can be inserted under the carbs to help prevent peculation. I was holding off on installing these as they are an obvious addition between intake and carb.

I will keep you all posted on how we make out with this, and many thanks to those who have provided their thoughts.

[Ken A.]

Re: 427/435 Fuel Pressure

The OE stock TI transistors give up @ 200 degrees. Make SURE the box ground is good.

[Michael W.]

Re: 427/435 Fuel Pressure

My understanding was that either these devices work or don't and if the latter the engine simply won't fire or run .... am I mistaken on this point ?

Not at all true, erratic operation can be an early warning sign of impending failure.

[Howard P.]

Re: 427/435 Fuel Pressure

Thanks Michael I will get another module and try that, after I confirm the ground is good.

I'm beginning to really suspect the TI as we drove the car approx 80 miles after completing the build and everything worked perfectly.

[Dick W.]

Re: 427/435 Fuel Pressure

Coil could also be the culprit. Sure glad I bought the old Sun 1150 Engine analyzer as you can bench test a coil on it.

[Joe R.]

Re: 427/435 Fuel Pressure

Would he have 6 pounds fuel pressure if the tank were not vented?
Just asking.

I'm sure he was checking the pressure when the car was running and not stopped on the street.

The line from the tank is gravity and the pressure side is between the pump and the carb.

And yes, the pump will pull fuel from the tank but a small vacuum after it has been running will stop that flow.

JR

[Richard E.]

Re: 427/435 Fuel Pressure

I had the same problem when i put a locking fuel cap on.My 67/435 also runs best with Sunoco 112.