Vapor lock?
Thread Starter
Intermediate
Joined: Apr 2020
Posts: 34
Likes: 6
From: Tennessee
Vapor lock?
My 1978 with original 350 was hard to start after warm up and short run to the store. I seem to remember it starting after I replaced rubber fuel line with stainless steel. Thinking it was heating the fuel, I switched back to a rubber line. Still doing it. Found a station that sold ethanol free gas, won't boil as fast. Still doing it!!
What could be causing the rough start? Getting to the point I only take it out for a ride that I don't have to shut it off.
What could be causing the rough start? Getting to the point I only take it out for a ride that I don't have to shut it off.
Safety Car
Joined: Jul 2005
Posts: 3,700
Likes: 836
From: WI
Do you have to wait for the vehicle to cool down before it'll start/run?
Heat from the intake manifold can do it. Both heating the line and the carb. Take a look at what base gasket and/or insulators are under your carb. Some tossed out and put a real thin gasket on their cars which conducts heat to the carb better. A thicker gasket can help a lot. I'm not sure what's right for your yr. A shield between the manifold and the steel line can help (or a foil-insulated sock over the line). But since you switched back to a hose, maybe it's not necessary now. What temp is your engine running at?
Heat from the intake manifold can do it. Both heating the line and the carb. Take a look at what base gasket and/or insulators are under your carb. Some tossed out and put a real thin gasket on their cars which conducts heat to the carb better. A thicker gasket can help a lot. I'm not sure what's right for your yr. A shield between the manifold and the steel line can help (or a foil-insulated sock over the line). But since you switched back to a hose, maybe it's not necessary now. What temp is your engine running at?
Melting Slicks
Joined: May 2006
Posts: 2,037
Likes: 36
From: The Corner of Moncks
St. Jude Donor '07
I'm having a similar issue with my '68 convertible, 327/300 slushbox. I've thought about a 1/4" gasket but I'm concerned that the choke rod length will become a problem. Engine appears to run at a normal temp, I think it just boils the fuel away while sitting. I've also noticed that my fuel pump suction hose has a bit of a kink in it. (I'm going to tackle that tomorrow)
Thread Starter
Intermediate
Joined: Apr 2020
Posts: 34
Likes: 6
From: Tennessee
I don't have to wait for it to cool down but it's real hard to start. When it finally does catch, it runs very rough for about 30 seconds then smooths out and runs perfectly. I guess the best way to describe it when it starts is lobbing, almost stalling. Sound awful and it's embarrassing too...lol
Temp. is in the normal range.
Temp. is in the normal range.
Melting Slicks
Joined: May 2006
Posts: 2,037
Likes: 36
From: The Corner of Moncks
St. Jude Donor '07
I don't have to wait for it to cool down but it's real hard to start. When it finally does catch, it runs very rough for about 30 seconds then smooths out and runs perfectly. I guess the best way to describe it when it starts is lobbing, almost stalling. Sound awful and it's embarrassing too...lol
Temp. is in the normal range.
Temp. is in the normal range.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,240
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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
Its not only boiling and evaporating but it can be boiling and overflowing into your motor which floods it. I was carrying some starter fluid with me to help out. You can also pop the hood to help the heat escape but the heat inside the cast iron of the motor will still boil it. The stock cast iron intake will take the 1/4 inch think gasket and it has the heat crossover that heats the bottom of the intake which will find its way to the carb. You can get gaskets that block that passage which will help. If you have an after market intake you can the tallest phenolic spacer that you can fit under the carb and not hit the hood. That helped with my issues a lot but I had a L88 hood and lots of clearance
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Drifting
Joined: Nov 2018
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From: USA
Advanced
Joined: Apr 2020
Posts: 80
Likes: 14
From: Ont, Canada
Correct ( thin ) gasket for 1968 quadrajet and QuadraJet stainless steel heat shield. The gasket goes on the intake manifold first with the heat shield above it. You can see the gasket has an exhaust crossover at the front of the carb for cold climate warm up.
This crossover in the front of the intake manifold is visible with carb removed. In warm climate no winter driving you can block this passage by drilling and tapping the passage at each end and screwing in a plug. Don't remember the size , approximately 3/8".
In 1969/1970 this was eliminated and they went to 1/4" gasket as above on Chevrolet Quadrajet.
Most issues as described in the original post were caused by a leaking inlet needle and seat or incorrect float level. Sometimes the needle and seat just have dirt or whatever stuck to to it. I have found other issues causing this such as a bad sinking float or a completing manipulated interior float bowl by a hacker.
If the carburetor is properly rebuilt / restored heat is seldom if ever the issue.
Of course fuel pump fuel pressure could be to high and should be tested, this goes with any carb.
Pro
Joined: Nov 2019
Posts: 617
Likes: 56
From: Dover, Delaware
Good post/pic.................Looks exactly like the one one Lars send back when he rebuilt/reengineered my 78's QJet..
Only prob I have is having to do multiple pedal pumps in order to refill the bowl and then it fires right up and runs like it did in 78..
Lars explanation is todays fuel problems with 10% ethanol gasoline...I live in central Delaware FWIW!!!
JJ78
Only prob I have is having to do multiple pedal pumps in order to refill the bowl and then it fires right up and runs like it did in 78..
Lars explanation is todays fuel problems with 10% ethanol gasoline...I live in central Delaware FWIW!!!
JJ78