vapor lock

[jviator]

I have a 79 k20 with a brand new 350 in it but it still has the original rochester 4 barrel carberetor on it. today my truck ran for hours in the city in 100 degree weather. I was almost home and i stopped for a second to get parts. when I started my truck back up i put it in gear and rolled a couple of feet and it shut off. it did that a couple of more times and then it was able to get me home. I tried driving it again about a half hour later and it did the same thing. Ive been told it sounds like vapor lock. Can anybody help me figure this out and know how to fix it. I do live in denver colorado if that helps

['82 steppin]

Check if any fuel line touching the engine or any part of the engine I had similar problem the feed line from the mechanical fuel pump was touching the radiator hose and as soon the engine gets warm the engine will act up till I put a little piece if exhaust wrap around the feed hose and it solved it
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[tucsonjwt]

My 454 C20 in HotZona gets vapor lock even with everything I have tried. It is not so bad that the truck won't start when parked for 15-20 minutes after driving in hot weather, but it will start hard. The best advice I have heard is a spacer under the carb, but I had that and did not like all of the modifications in linkage and emissions stuff that I had with a spacer. If you don't care about that then the spacer is the way to go.

I wrapped the fuel line with insulating tape but I think most of the problem is the heat on the intake manifold - boiling gas out of the lines and carb.
With my truck, I depress the accelerator all the way to the floor and it always starts up and runs rough, until it gets enough gas to keep going, then it is fine. I had another 73 C20 454 which did the same thing.

It is possible that you also may have "heat soak" which causes the solenoid/starter not to engage when hot (heat causing excessive resistance) but this is a "hot" topic elsewhere on this forum (regarding the value of adding a remote solenoid - some posting here are vehemently opposed to it, but I do have a remote solenoid professionally installed on my truck, along with dual start batteries. )

My story on this "heat soak" topic:

I moved to Arizona from Michigan in march of 1982. My 73 C20 454 only had one 20 gallon gas tank and I was pulling a giant homemade 5000 lb. storage trailer (I still have it). So, I was lucky to make it from one exit with a gas station to another before the tank was empty(about every 100 miles or so.). Every time I stopped for gas and then tried to restart the truck it would not start (just a click, like a bad solenoid sound.) I had an isolator with an auxiliary battery for 12 volt power to the trailer. When I stopped for gas and could not restart the truck I ran jumper cables from the auxiliary battery to the primary start battery and the truck would start right up. I say this was heat soak on the solenoid which caused the truck not to start (and this was in the cold winter temperature until I made it to the sunbelt - still heat soak no start condition.) I think the extra amps of the auxiliary battery was enough to overcome the excessive resistance from heat on the solenoid (and associated wiring). Some suggest much bigger battery cables to the solenoid (like welder size wire.

So, I guess you could have an extra charged battery with you, or install 2 start batteries temporarily (very easy, even for the mechanically challenged like me) to test the "heat soak" issue.

Be advised that what I just stated is completely denied and vehemently opposed by some on this forum, so you will get disagreement on this topic. So, I might have lit a forum "firecracker" but I am just giving you my experience.

[Axle]

Did you notice if you belched a plume of black smoke when it restarted? If so, definately vapor lock. Fought that myself for a while in my car. A carb heat shield cured most of it. I still get a bit of vapour lock like symptoms if the car sat long enough for everything to get heat soaked.

If your not familiar, when you shut down, the carb gets heat soaked, the fuel vaporizes out of it, when you go to restart the engine, the float has dropped, the fuel pump dumps a load of cool fuel into a hot carb and it flash vaporizes, hence no start becuase you litterarly are sucking fumes. If you hold the throttle wide open, and dump enough fuel through it to cool things enough to get some liquid in there it'll eventually start. I used to do jobs in my car that had me sit at a customers site usually for 30-50 minutes, by far the worst was at the 35-40 minute mark. Putting in the heat sheild all but cured the problem.
You will also notice similar effects when conditions are just right if you have been doing hot weather city driving and accelerate to get to high speeds rapidly, you might get a major hesitation when you are accelerating, same sort of deal, as the fuel demand suddenly gets high and cool fuel comes pouring into the carb, the flash effect momentarily occurs, giving you that pause. Once you get enough fuel through it, the carb cools enough and you are back in buisness.

One thing you can do in the meantime, after a hot start, let it run for a couple minutes before you try to drive off. If you drive off immediately the carb has not had a chance to cool down a few degrees (air flow from the fan, plus cooler fuel from the tank) and you'll only manage to get as far as the fuel that is already in the bowl.

Alex.

[tucsonjwt]

x2 with your description of the problem.
What is the "carb heat shield" you are referring to? Is that a phenolic or other type spacer between the carb and intake manifold?

[Axle]

The heat sheild is a metal plate.
Manifold -> thick q-jet gasket -> shield -> another gasket -> carb.
(remove the staples first)

Easy enough to make if you don't want to buy one.

Alex.

[tucsonjwt]

It looks kinda thin. The spacers I have seen from Holley aftermarket, etc., are about 1/2" - 3/4", as I recall. It is hard for me to understand how you could get sufficient carb cooling unless the spacer got the carb off the intake quite a bit. The one I had was about 3/4" thick, aluminum - I think.

[Firebirdjones]

The heat sheilds are to stop "radiant" heat from rising up to the carb. To stop the actual contact heat where the carb is bolted down you can simply run 1/4" thick gaskets on either side of that heat shield to soak up the heat transfered from the intake manifold. They are available at any autoparts store. Stay away from the paper thin gaskets. These tricks will help to keep the carb cooler,,,,but you need to keep the fuel cool too.

I went over this recently on here. Vapor lock is a continuing problem as the gas quality continues to get worse. The added Ethanol reduces the flash point and is much more susceptable to vapor lock, and your government wants to add more so be aware,,,also low octane fuel is also more susceptable to vapor lock due to it's inherant lower flash point. You mix the 2 together and you have classic vapor lock waiting to happen.

First way to fight it is to run a higher octane rating. Everyone wants the cheap stuff,,,,I know. But in the hotter summer months it can be an issue. It's especially noticable with winter grade gas and the anti-freeze additive packages in the fuel is more vapor lock prone,,,,so when you get those warm winter days,,,,or the stations simply haven't switched over to the summer blends before the weather warms up,,,you get vapor lock.

Next thing you need to check is to make sure you still have the correct mechanical fuel pump installed that has your 3rd return line. Make sure that is hooked up and working. That return line takes your access fuel that isn't used during closed needle and seat operation and continually pumps it back to your gas tank. This continual flow keeps the fuel moving so it isn't sitting stagnent in the fuel line and collecting heat. Every GM car and truck had this by 1971 and it's become an invaluable idea with todays crappy gas.

With this working properly,,,the only fuel that will sit stagnent in the fuel line will be the line from the pump to the carb. Pay particular attention to this line and keep it away from the water pump and engine block (at least an inch is good enough). If it is touching anything,,,,fix it. You can also insulate this line if you wish. Either small diameter foam from the plumbing section, heat tape, what ever appeals to you,,,but I find that if I take all the other necessary precautions,,,I don't need to insulate the fuel line.

After that,,,thick gaskets under the carb and a heat shield to finish it off will help. I also prefer to block off the heat risers in the intake manifold which cuts WAY down on the amount of heat running through the intake. If you live in at least a semi warm climate,,,heat crossover isn't really needed.

Stay away from anything chrome in your fuel system,,,chrome carbs, chrome fuel pumps,,,chrome fuel lines etc.....Chrome attracts and traps the heat and doesn't allow the heat to disipate. Looks pretty, but can cause headaches on a fuel system.

[Axle]

Originally Posted by tucsonjwt

It looks kinda thin. The spacers I have seen from Holley aftermarket, etc., are about 1/2" - 3/4", as I recall. It is hard for me to understand how you could get sufficient carb cooling unless the spacer got the carb off the intake quite a bit. The one I had was about 3/4" thick, aluminum - I think.

What you are describing is a riser, its function is to raise the carb, and lift it away from the splitter in the base of a dual plane manifold for high rpm operation. (theory being that at high rpm, having the carb spaced higher up will get you a pseudo single plane manifold type of operation. Being metal, it will just conduct heat to the carb.

You use a stock carb gasket on the bottom (the 1/4" thick one--which itself is that thick to provide heat isolation from conduction) then the sheet metal shield (for radiant heat as Firebirdjones explains), and another gasket (to seal the carb). Wether you use the skinny gasket or another thick one is personal preference.

Alex.

[INSIDIOUS '86]

Stuff a cut up beer can slice under the intake between the head and the intake were the manifold heat riser is and get rid of a lot of heat that way. Then your thermostat should be 180* at most. A 165 prob be good for summer.
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[spenceman72]

Originally Posted by Firebirdjones

After that,,,thick gaskets under the carb and a heat shield to finish it off will help. I also prefer to block off the heat risers in the intake manifold which cuts WAY down on the amount of heat running through the intake. If you live in at least a semi warm climate,,,heat crossover isn't really needed.
.

Firebird-great info, this thread describes an issue I've been having with 'hot/warm starts', and your advice summarizes what I've been learning on this topic very well. I'm going to use the thick gasket/heatshield on the carb (any reason not to use one of the 1" carb spacers for Edelbrock spacers versus the thicker gaskets..or is that basically the same thing?), insulate the fuel line going from the pump to the carb (it runs very very close to the engine), and insulate the starter (it is basically touching the headers). It seems that I'll have to remove the headers to get to the starter, so maybe I should paint those with heat resistant paint as well?
Anyway-my question. You mention blocking off the heat risers in the intake manifold and after much searching I can't seem to find where these heat risers are? (I changed oil for the first time 3 weeks ago ). I have a Edelbrock manifold if that helps...I believe it/they are on the passenger side..but I can't find a picture online to reference when I'm staring into my engine bay
Thanks!

[Firebirdjones]

Just using the thick gaskets under the carb always worked like a charm for me. The only time I'll run a 1 inch spacer under the carb is if there is a power increase shown on the dyno with it, and/or if I even have the hood clearance to get away with it.

The heat cross over is the exhaust passage that runs through the intake manifold from one cylinder head to the other. It's meant to heat the choke stove primarily, and also quickly warm up the fuel charge for better cold weather driveability. If you run an electric choke, then the heat crossover will only serve the second purpose I mentioned.

There are intake gaskets made that completely block this passage, or you can partially block this passage with a smaller hole to cut down on the amount of heat. There are stainless inserts that can be placed in there as well. Alot of your aftermarket performance intakes and cylinder heads don't even utilize this passage. Cooler intake means denser fuel charge and more HP.

However living in a cooler climate may or may not cause cold drivability issues, stumbles, and the like when cold.

I keep them all completely blocked in my cars.