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"Warm" start issue has me stumped

goathearder

goathearder

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Hey guys, I have been chasing a "warm" start issue for months and it has me totally stumped. Here is some background on the issue and what I have done:

Issue:
1973 K5 with SBC 350. Starts perfectly (two gas pumps and key turn) when its been sitting for a couple of hours. Starts perfectly (no gas pumps and key turn) when its been shut off for less than 5 minutes. Anywhere between 5 minutes and a couple of hours (warm), its a major pain to start. Basically have to floor the pedal and crank for 10-15 seconds to get it to fire. Once it fires, all is good but I can't understand what is causing this.

Troubleshooting:
First thing I did was change the plugs and wires. After that, I thought it might be the dizzy so I threw an ignitor II in the original points dizzy. After that, I thought the electric choke might be sucking and I wanted a new carb anyway so I got a new carb with manual choke. I have tried starting with every possible combination of choke and throttle position and still nothing. I thought it might not be getting enough air so I threw a K&N filter on. Then I came back to thinking it was the ignition so I swapped to HEI with new plugs (gapped at .045) and new wires. Still nothing! I also thought it might be the compression so I checked that and its around 150psi per cylinder (give or take 10%) so I have no issues there. I also have a high torque mini starter.

Results:
With the new carb and HEI the truck runs way better when its running so I am happy about that but I am still having the "warm" starting issue. I can't fathom why it starts easy when cold and when hot but has issues when "warm".

Any ideas???
 
It seems as though this might be vapor lock from some reading I am doing which I never considered but I don't know about that because the issue just showed up one day a couple months ago and I didn't have it before.

Can a failing mechanical fuel pump cause vapor lock?
 
The pump could also be losing its prime, I would definitely put a pump on it, considering what you've done already. Also check all the lines, on the suction side of the pump especially. I've seen loose clamps cause a loss of prime, causing similar symptoms, even though theres no gas leaking.
 
My return was never hooked up, and when I put my motor in and headers, the fuel started boiling in the lines causing all kinds of crappy running.

Return working properly and everything's nice
 
bent wrench said:
The pump could also be losing its prime, I would definitely put a pump on it, considering what you've done already. Also check all the lines, on the suction side of the pump especially. I've seen loose clamps cause a loss of prime, causing similar symptoms, even though theres no gas leaking.
Click to expand...

Losing its prime huh? That sounds like it could be the issue. A new pump is only 20 bucks so I might as well. Any idea what size rubber hose I need, I might as well replace all that. From what I can tell there are two rubber hoses to two hard lines by the tank and then 1 rubber hose that goes from the hard line to the pump in the engine bay.

Deuling said:
My return was never hooked up, and when I put my motor in and headers, the fuel started boiling in the lines causing all kinds of crappy running.

Return working properly and everything's nice
Click to expand...

What do you mean by return? I don't think I have a return. I thought mechanical fuel pumps were return-less and had internal bypasses?
 
Have you checked that the carb has fuel before trying your warm start? Vapor lock won't be an issue on start IF the float bowl still has fuel in it.
 
goathearder said:
What do you mean by return? I don't think I have a return. I thought mechanical fuel pumps were return-less and had internal bypasses?
Click to expand...

Idk. All I know is my 85 had a return line somebody disconnected and plugged on the frame, and put a pump in with no return barb. I put a new pump in, didn't change anything, then I was givin an old mopar bypass fuel filter by my tranny guy. Hooked the extra barb to my return line and good as new :waytogo:
 
dyeager535 said:
Have you checked that the carb has fuel before trying your warm start? Vapor lock won't be an issue on start IF the float bowl still has fuel in it.
Click to expand...

Yea, the carb has fuel in it and the fuel filter has fuel in it too. That is why I never originally thought about vapor lock. I think the pump losing its prime seems like a more likely culprit.
 
If the carb has fuel, but it won't start, it has nothing to do with fuel delivery. I don't know the exact time, but the engine will start and run for a period of time with JUST the fuel in the float bowl. It's normally more than enough for the pump to re-prime.

I'd be more inclined to believe you have a choke issue.
 
dyeager535 said:
If the carb has fuel, but it won't start, it has nothing to do with fuel delivery. I don't know the exact time, but the engine will start and run for a period of time with JUST the fuel in the float bowl. It's normally more than enough for the pump to re-prime.

I'd be more inclined to believe you have a choke issue.
Click to expand...

Yea, I thought it was a choke issue too, thats why I switched to the manual choke carb but I can't seem to find a setting on the choke that makes it want to start normally.

I have to keep the pedal on the floor to get it going (plus the 10 seconds of cranking) so I feel like its a fuel or air issue but I'm pretty sure the truck should run for at least 30 second on just the fuel in the bowls.
 
Having to keep the pedal all the way down but not pumping actually leans out the mixture. It sounds like its getting rich somehow. Maybe fuel is leaking out of the carb and into the intake manifold? Try seeing if its rich by pumping pedal vs just holding the throttle open when you crank it open.
 
Whats a good way to check that? The carb is brand new and it was doing the same thing with the old carb. It does need to be pumped sometimes too so i cant say for sure that holding the pedal down solves it.

Both the new and old carb are edelbrocks and have spring loaded needles for offroading. Could that possibly be causing issues? If the fuel pump wasnt working maybe the needles are having trouble opening?
 
http://www.ifsja.org/forums/vb/archive/index.php/t-27311.html
http://www.chevelles.com/forums/showthread.php?t=188680

Common problem it looks like. Maybe try keeping the hood open after a long run and see if it gets better. Or leave a bag of ice on the carb and verify if this is the problem.
Good luck!

EDIT: I should have given a little more info on how to solve this. I have to assume that the intake manifold is causing this extra heat to build up, and I think either a carb spacer, or maybe an air-gap manifold would help. Just throwing out ideas.
 
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I have exactly the same problem with edelbrock carbs. I have no issue with quadrajets, holleys, or the old style "sea erchin-looking" rochester mechanical carbs. I've been stumped on this issue too for years, finally just giving up and holding the pedal to the floor when cranking and saving up for fuel injection. I'd swap the edelbrock, but my truck runs too damn good to justify building another one for it. Good luck bud, hope you get it figured out!
 
samsterman99 said:
http://www.ifsja.org/forums/vb/archive/index.php/t-27311.html
http://www.chevelles.com/forums/showthread.php?t=188680

Common problem it looks like. Maybe try keeping the hood open after a long run and see if it gets better. Or leave a bag of ice on the carb and verify if this is the problem.
Good luck!

EDIT: I should have given a little more info on how to solve this. I have to assume that the intake manifold is causing this extra heat to build up, and I think either a carb spacer, or maybe an air-gap manifold would help. Just throwing out ideas.
Click to expand...

Well now that makes a ton of sense! I have the off-road Thunder AVS so the float drop is set to 7/8th which would certainly make the situation worse. Come to think of it, this happened right around the time I did a spring loaded needle and seat in my 1406 and adjusted the float drop for better off-road performance.

I use a dual fuel propane kit now so I am not too worried about the gas performance off-road so I'll get in there and adjust the float drop to a larger volume and throw one of those spacers in. Hopefully the answer is that easy.

In retrospect, I should have just gotten fuel injection instead of the propane but I wasn't super interested in running a bunch of electronics in a simple car :doah:
 
If you have to hold the pedal to the floor for 10 seconds that indicates a a sign of flooding. Holding the pedal down allows the engine to suck in air a lean fuel to point where the engine can fire up. Lean problem you would have to pump the throttle to start.

On a hot engine if the fuel pump does not relieve pressure after shutting off the engine fuel pressure in the lines increases due the heat expansion and can creep past the needle/seat and cause some flooding.

Your 73 probably doesn't have a return on the fuel pump, my 74 K5 doesn't. If you get an inline fuel pressure gauge and put in line between the pump and carb you will see the fuel pressure increase after shut down due to heat if the pump is not releiving pressure. Mine was holding pressure and would climb over 10 psi when hot after a few minutes.

I had this problem and went with an edelbrock pump and it drops the pressure off within a few minutes. This cured mine of having to hold the pedal to floor just likes happening to you.

Id start with a least a stock new pump. Don't buy a cheap one, get a gm or napa. you can also bypass the metal line thats up against the block by using 3/8 regular or fuel injection hose and route it away from the block. Fuel Inj hose is pricey tho.

Hope that helps a little from my experiences.
 
Thanks Blazer74, good tips. I am definitely gonna put a new fuel pump in, what the heck at this point.

In good news, we seem to be on to something. I put the edelbrock spacer gasket in tonight to help isolate the carb from getting hot and it significantly helped! I have a 10 to 15 minute restart window now instead of the 1 to 5 minute restart window it had before. Maybe the combo of that and the fuel pump will do the trick.

I also have an electric fan to put in and some hood vents so maybe that will cool the engine and the engine bay enough to put this to rest once and for all.
 
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