Carb Flooding When Engine is Hot. Heat Riser Issue?
Thread Starter
7th Gear
Joined: Nov 2020
Posts: 7
Likes: 0
From: NC
Carb Flooding When Engine is Hot. Heat Riser Issue?
I'm rather new to carbureted vehicles, and I've been learning as I go with a '78 that I bought back in September of last year, so go easy on me. Recently, I've been having an issue where my carb will flood when the engine is hot. The car will start right up with no issues while cold and drive like a dream, but when I shut it off to go in a store or leave it for about 30mins to an hour, the engine has difficulty starting. It will turn over, fire up, and after a few seconds, it'll sputter out. Looking at the carb, I can tell that it's flooding, as small amounts of fuel are trickling down onto the intake manifold. I'll eventually get it started after depressing the gas pedal before turning the key (but it does take quite a few tries), and it drives like normal right after. I've been looking into this issue quite a bit, and I've heard that a stuck heat riser might be the culprit. How can I test if this is the case, and more importantly (and shamefully) where is the heat riser?
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
You don't have a heat riser issue - completely irrelevant. You have a fuel level control issue caused by dirt in the carb needle/seat or a float control problem.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,217
Likes: 9,354
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Sounds like your boiling your fuel. If you dont have this issue when the engine is cold then you should try a phenolic spacer between the intake and carb. You will need a gasket on both sides of the spacer. You need to verify you can add the height and not have the air cleaner stud hit the hood when you close it. This happens with Q jets and Edelbrocks a lot. If you have a dual plane intake you should try an open spacer. It tends to help. I started with a half inch spacer and it helped. I went to 1 inch for the performance gains but thats with a performance motor and alot of hood clearance. When I got my car it was in the fall and I never had an issue until spring and the ambient air temps got closer to 70*F.
You could also have higher than needed fuel pressure that could be causing this issue as well but you would see this on cold days and before the engine gets hot. As well as the issues that Lars mentioned.
Some things to try is not touching the throttle pedal when you try to hot start it. IF that doesnt work just hold the pedal to the floor to match the air to the fuel already in the intake. Once it starts feather the throttle to keep it idling.
You could also have higher than needed fuel pressure that could be causing this issue as well but you would see this on cold days and before the engine gets hot. As well as the issues that Lars mentioned.
Some things to try is not touching the throttle pedal when you try to hot start it. IF that doesnt work just hold the pedal to the floor to match the air to the fuel already in the intake. Once it starts feather the throttle to keep it idling.
Last edited by Rescue Rogers; Mar 26, 2021 at 08:15 AM.
Le Mans Master
Joined: Jun 2019
Posts: 5,485
Likes: 2,193
From: Loveland
I had exactly the same issue with my Holley 4175. The 3 things that helped. I restored the fuel return line that someone had disconnected, installed the original fuel filter, and timed the ignition according to Lar's spec. All the heat shields ETC didn't help. It was kind of fun tearing the carb apart seeing how they work though.
Last edited by kodpkd; Mar 26, 2021 at 10:35 AM.
