Very Hard Start Hot With Dual DCNFs

jimmyx

jimmyx

True Classic
Getting dual 40 DCNFs dialed in usually takes a bit of work, but while my cold start and driveability was close to perfection, my hot start was a horror. If the car was inadvertently stalled hot and immediately restarted, no problem. If it was hot and shut down to refuel and restarted a few minutes later it took an extended crank while holding the pedal to the floor. It is symptomatic of flooding, not starvation so we can skip the heat shield, carb spacer, and vapor lock discussion. The car is equipped with a Facet 40-148 cube pump and a standard Holley regulator set to 2.5 psi. If you engage the pump cold and shut it down a minute or two after starting, it will show 2.5 psi on the gauge and return to zero after 15-20 minutes. If you shut down hot, the gauge reads 2.5, but slowly rises to 6.0 within a minute or two. It will eventually drop to zero within 30 minutes. The root cause of the problem is the pump. This model pump is commonly sold by our favorite vendors, but it contains an anti drain back check valve which is not a problem when running carburetion that employs a fuel return line, but causes issues when the under hood temps spike and there is no return line. The correct Facet cube in my case is a 40-104. Similar in specs to the 40-148, but has no internal check valve allowing fuel to move in any direction when the pump is powered down. The 40-104 maintains 2.5 at all times and drops to zero immediately upon shutdown. Once the correct pump was installed, problem gone. Unfortunately, Facet cubes are one of the most counterfeited parts out there, so my usual go-to source for the genuine article is Pegasus Racing. I would stay away from ebay for this particular part for sure if you find yourself in need. Hope this might bee helpful to somebody in the future facing the same situation.
 
Very good and convincing story.
So far, I did in fact believe in the theory that the heat evaporates the fuel in the bowl, but always wondered why it does‘t just refill when the electric fuel pump is turned on before cranking.

I will check if I still have this issue with my new pump. If I do, I will temporarily hook up a return line and see if that relieves the problem and the excess pressure.
 
fiatfactory said:
Why not just use a return fuel line as Eduardo recommends?
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As stated in my original post, if you have the additional plumbing then you won't encounter the problem. If you choose to run without a return then select the right pump for the application. Either method will cure the problem, so it just comes down to builder preference.
 
NEG said:
I’m a little unclear, your tank doesn’t have the return pipe?
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Yes the tank is equipped with a return, as I believe they all were. The difference is in the plumbing for fuel delivery when deviating from a stock set-up; there are two different methods one can use. If you incorporate a return line, running a pump with a check valve will not introduce a hot start problem, if you don't run a return line a check valve will cause you problems.
 
Just so you don't get target fixation, I have never run return lines on my dual IDFs on the spiders and Xs or the DCOEs I had on a few Xs and never had a hot start problem.
 
And my buddy has dual DCNF with a severe hot start problem with what turns out to be the correct Facet without the valve (and no return line).
But it is still a piece of the puzzle and we will test it with a return line.
 
Ulix said:
And my buddy has dual DCNF with a severe hot start problem with what turns out to be the correct Facet without the valve (and no return line).
But it is still a piece of the puzzle and we will test it with a return line.
Click to expand...
Remember that this is something to investigate when you have verified with a fuel pressure gauge that your pressure spikes high after shutdown hot. It is not the only cause of a hard start hot condition.
 
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