1975 C10 high idle and sea foam, won't start

andrew-75c10

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Location
San Diego
First Name
Andrew
Truck Year
1975
Truck Model
C10
Engine Size
350
I've been working on a 1975 C10. I got it driving last month, but the idle has been high. I drove it down the street today (to get out of my neighborhood and get the engine warmed up). I adjusted the high idle screw until the engine was sounding pretty quiet. I don't have a tach, but when I shift into 1st gear and let the truck go without touching the gas, I was cruising at 5-8 mph which seemed reasonable.

Next I left the truck running, and fed about half a can of Sea Foam in through the vacuum line that normally runs to the charcoal canister. The engine would rev up when I was pouring the sea foam in, but I never saw smoke and the engine did not stall. I stopped the engine and put the remaining half can of sea foam into the gas tank I got into my truck to drive home, and the engine would turn over but it would not start.

I really wasn't sure what the problem was, but I followed the section of the manual for flooded engines - fully depress the pedal and start the car. After a few tries, the engine roared to life but I took my foot off the gas and instead of idling the engine died. I was not able to get it started again.

Would greatly appreciated any suggestions. I'm scratching my head on this one, and I'd like to get it back home without towing.
 

75gmck25

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Location
Northern Virginia
First Name
Bruce
Truck Year
1975
Truck Model
K25 Camper Special TH350 NP203
Engine Size
5.7
Is this an original Quadrajet?

First go back to the carb and make sure all rubber lines are properly connected so you have no vacuum leaks. I once had the PCV line pop off the carb when it backfired and it ran like I just lost a couple cylinders. I spent an hour of troubleshooting before I noticed that the hose was off.

The engine is cold now, so it should not be flooded.
Get a can of gas and a small cup (cap from a spray can works well). Pull off the top of the air cleaner, hold open the choke, and pour in some gas. Now reinstall the air cleaner cover and verify the engine is ready to drive away. Then try starting the truck. If it starts, just let it idle until it is warm, and drive it home.

You probably need to rebuild the carb. However, you also have a lot of old rubber fuel hoses , and maybe a flaky fuel pump or clogged filter. Gas directly into the carb can compensate for cracked rubber lines or a plugged filter that is restricting flow. The mechanical pump Pulls vacuum to get gas through old lines that run through a tank switch and sending unit. Even some small cracks in a rubber line reduce its efficiency.

Bruce
 
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andrew-75c10

Junior Member
Joined
Jun 20, 2020
Posts
17
Reaction score
6
Location
San Diego
First Name
Andrew
Truck Year
1975
Truck Model
C10
Engine Size
350
Thanks Bruce! Yes, it's the original Quadrajet.

I haven't driven it much since I got it running - a trip to the gas station, to Home Depot, and to the auto store. And a few trips around the neighborhood to test it out. I've been learning a lot about old trucks, all my other vehicles have been fuel injected.

The fuel system was shot when I started working on this thing. I replaced the tank, fuel sender, all rubber fuel hoses, added an inline fuel filter, and finally rebuilt the carb before I got it running. I also replaced all the hoses coming off the carb before I got it to start.

After reading your advice, I'm wondering if I have a vacuum leak. I'm going to make sure I tightened things down on the carb, and check all the hoses and connections. I'm be sure to bring some gas with me. Thanks for all the tips!

Update: I went back to my truck today and found that the choke was cracked open. I thought it was supposed to open as the truck warmed up, but close when the truck cooled off. I had to push it close, so I probably messed something up when I rebuilt the carb. I also checkout all the vacuum lines, but didn't see anything suspicious. Either way, it started after a few turns of the engine. Thanks for your help!
 
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