Starting Issues

[Olon]

Hello, I've got a 76 k10 with a 350 and an edelbrock 1405 (600cfm manual choke) and I've been experiencing a strange starting issue since the fall. I'll go out in the morning to start it: throttle down, choke on, throttle up so it's on the high idle cam. It'll fire up after a moment of cranking, then die off quickly, unless I "jazz" the throttle a bit. Seems like it's just running off the accelerator pump. After a few tries, (presumably after the fuel pump has filled up the bowl?) it'll keep running fine and idle on its own.

What's my issue here? Does it sound like the fuel is just evaporating from the bowl so there's not enough pressure difference to pull it through the idle circuit? After it gets going it runs great, it's just that initial startup that sucks.

Thanks in advance for the help.

Olon

P.S. I live in a place that gets cold, so I know there's winter mix at the pumps. Not sure if its volatility would cause this, if heat from the engine having just ran can cause fuel to evaporate from the bowl...

[JohnnyPopper]

I have the same problem with a QJet.

I have heard that the fuel pump has a check valve that is supposed to keep the gas from gravity feeding back to tank level, but cannot verify with certainty.

Hard to believe that heat can evaporate it back into the feed line, but it might 

Perhaps someone here can comment?

If it doesn't have one I might consider installing one to check my theory.

[Mike81K10]

You probably thought of it already. However, did you check your fuel filters? You may not be getting enough fuel if clogged or if there is floating debris in the line it could occasionally clog the flow. You may want to check your fuel pressure as well where your line goes to the carb.

[Olon]

I have the same problem with a QJet.

I have heard that the fuel pump has a check valve that is supposed to keep the gas from gravity feeding back to tank level, but cannot verify with certainty.

Hard to believe that heat can evaporate it back into the feed line, but it might 

Perhaps someone here can comment?

If it doesn't have one I might consider installing one to check my theory.

I've read that too, and that debris in the fuel tank can clog said check valve w/o the presence of a pre-pump filter, which I don't have. Certainly a possibility thank you for your reply.

[Olon]

You probably thought of it already. However, did you check your fuel filters? You may not be getting enough fuel if clogged or if there is floating debris in the line it could occasionally clog the flow. You may want to check your fuel pressure as well where your line goes to the carb.

I only have a pre-carb filter, which is pretty new so I think it's good. I'll go ahead and check the fuel pressure tomorrow but I think it's probably alright since it can keep up at WOT. Truck runs great other than this strange starting anomaly which is what baffles me.

[Mr Diesel]

Kinda sounds like not enough high idle. Maybe set the choke idle screw for a little more RPM. I have a 1406 on a 305 and I just tap the gas pedal halfway to floor once, let off, and crank it over. Always fires instantly and stays running. My cold idle screw is set to somewhere around 2,000rpm but that isnt needed for long. I let it idle high for 10-15 seconds, maybe more if very cold outside, then tap the throttle and it comes off the choke idle circuit.

[Olon]

One thing I did notice is the fuel filter is pretty well empty after sitting overnight, could that be an indicator of a stuck/leaking check valve?

Sent from my SM-G960U using Tapatalk

[bd]

Factory fuel pumps contain check valves.  If they didn't, the pumps couldn't move fuel in only one direction toward the carburetor....  If you think drain-back is an issue, pinch off the rubber fuel line at the pump when you shut the engine off.  Let it sit overnight.  Remove the clamp right before starting the following morning to see whether the symptoms disappear.

With respect to Mr Diesel's post, what is the cold running RPM on the high step of the choke cam?

[JohnnyPopper]

So you have a clear plastic or glass filter that is empty? Sounds like leak back.

I like bd's idea, but I will have to pinch mine pre pump, as the tube to the carb is steel.

[Olon]

Yeah it's clear plastic. It's a good idea, I will try it and report back

[Olon]

Kinda sounds like not enough high idle. Maybe set the choke idle screw for a little more RPM. I have a 1406 on a 305 and I just tap the gas pedal halfway to floor once, let off, and crank it over. Always fires instantly and stays running. My cold idle screw is set to somewhere around 2,000rpm but that isnt needed for long. I let it idle high for 10-15 seconds, maybe more if very cold outside, then tap the throttle and it comes off the choke idle circuit.

High idle is right around 1500, which I think should be plenty. I initially thought that was my problem, so I bumped it up but the issue remained.

[Olon]

You probably thought of it already. However, did you check your fuel filters? You may not be getting enough fuel if clogged or if there is floating debris in the line it could occasionally clog the flow. You may want to check your fuel pressure as well where your line goes to the carb.

Failed to address this earlier, but fuel pressure is right at 10psi, which I know is a little high but I don't seem to have any problems with it running rich or flooding. And that reading is taken right before the carb, after the inline filter.