Fuel System: High idle after hot start

[MrBulldops]

Ok. Need to pick your brains.

So, here's the deal: I have rebuilt and properly tuned the carburetor, replaced my plugs, fuel pump (mechanical), coil, and condenser within the last couple months. When I start the car cold, it fires up immediately and warms up smoothly, then runs at a smooth idle.

Except after I park for a bit (ten or twenty minutes) and restart the engine. It will fire up fairly easily, but idle high...really high, like I'm holding the throttle halfway to the floor. After a couple miles, it runs normally again.

I'm sure this is a heat soak issue at the pump. When I look at my fuel filter, after I shut the engine off, I can see it bubbling. I've fabricated an aluminum heat shield for the fuel pump, and isolated the carburetor with a 1/4" phenolic spacer already. I've been thinking of running the fuel through a radiator before and/or after the pump to try to help, but wonder if I should bother.

Anyone else have thoughts? Bearing in mind, we have alcoholic fuel here in AZ, and outside temps get pretty warm. The engine runs between the 1/4 & halfway mark on the gauge, and I've also disconnected the heat riser spring so it stays open. Also, I'm not interested in putting in an electric pump. There was one on the car when I bought it, and it was nothing but problematic.

Thanks in advance for your responses.

[RadioRoy]

Would you give specifics of the setup you have, please?.
What year?
V-8 or six?
Transmission type?
Carburetor brand/type/barrels?
Modifications/options/changes from stock?
Elevation/altitude in AZ?

I cannot see how heat soak could cause the engine to run way too fast. It's more likely that something is affecting the throttle linkage, but without more information, all we can do is guess/speculate.

Maybe the automatic choke is improperly adjusted.

[MrBulldops]

Would you give specifics of the setup you have, please?.
What year?
V-8 or six?
Transmission type?
Carburetor brand/type/barrels?
Modifications/options/changes from stock?
Elevation/altitude in AZ?

I cannot see how heat soak could cause the engine to run way too fast. It's more likely that something is affecting the throttle linkage, but without more information, all we can do is guess/speculate.

Maybe the automatic choke is improperly adjusted.

Stock 1960 Lark, 6-cylinder flat head, 3-speed manual, Carter AS carb, choke set one notch rich, but wouldn't matter since this is an issue restarting a hot engine. Cold starts are easy and smooth running.

I know this has to be heat related because if I pour cold water over the pump before restarting the engine, it idles fine. A trick I learned for easier restarts after filling up when it's 110 outside.

[64V-K7]

Ok. Need to pick your brains.

So, here's the deal: I have rebuilt and properly tuned the carburetor, replaced my plugs, fuel pump (mechanical), coil, and condenser within the last couple months. When I start the car cold, it fires up immediately and warms up smoothly, then runs at a smooth idle.

Except after I park for a bit (ten or twenty minutes) and restart the engine. It will fire up fairly easily, but idle high...really high, like I'm holding the throttle halfway to the floor. After a couple miles, it runs normally again.

I'm sure this is a heat soak issue at the pump. When I look at my fuel filter, after I shut the engine off, I can see it bubbling. I've fabricated an aluminum heat shield for the fuel pump, and isolated the carburetor with a 1/4" phenolic spacer already. I've been thinking of running the fuel through a radiator before and/or after the pump to try to help, but wonder if I should bother.

Anyone else have thoughts? Bearing in mind, we have alcoholic fuel here in AZ, and outside temps get pretty warm. The engine runs between the 1/4 & halfway mark on the gauge, and I've also disconnected the heat riser spring so it stays open. Also, I'm not interested in putting in an electric pump. There was one on the car when I bought it, and it was nothing but problematic.

Thanks in advance for your responses.

Type engine would help, but 1/4" phenolic spacer is not much help. To keep the heat away from the carb, the best outcome on a V8 would come from using a full 1" spacer, blocking off the intake cross over and removing the heat riser. To help with vapor lock, angle your fuel lines upward from the pump and even use heat reflective sheathing and an electric fuel pump. Your fast idle issue has to be something hanging up. If everything's hot, the choke linkage should not be involved, unless it's too tight.

[RadioRoy]

Stock 1960 Lark, 6-cylinder flat head, 3-speed manual, Carter AS carb, choke set one notch rich, but wouldn't matter since this is an issue restarting a hot engine. Cold starts are easy and smooth running.

I know this has to be heat related because if I pour cold water over the pump before restarting the engine, it idles fine. A trick I learned for easier restarts after filling up when it's 110 outside.

Pouring cold water over the fuel pump lowers the idle speed? Did I read that right? That'll take some figuring out.

Is any of the cold water getting on the carb? Do you do anything else to make it idle properly after a hot start?

[altair]

A lean mixture will cause the idle to increase, some where along the intake system there is a vacuum leak, check the intake manifold. When it is idling spray some water along the intake and see if it stumbles and check that the bolts are tight. In an aircraft there is a Vernier throttle to lean the mixture, as you lean the mixture the rpm will increase. A lean mixture can also cause overheating.

[MrBulldops]

Pouring cold water over the fuel pump lowers the idle speed? Did I read that right? That'll take some figuring out.

Is any of the cold water getting on the carb? Do you do anything else to make it idle properly after a hot start?

The pump is at the bottom of the engine block and the carburetor is at the top, so no. Also, pouring cold water on the pump doesn't make it idle lower, it makes it idle normal. When I start the engine hot, the idle sounds like 2500rpm. Normal idle is sub-1000 (no tach, so I set it by ear). The only thing I have to do is pour water on the pump. Otherwise, it fixes itself by driving normally: high idle in the parking lot, then drive a mile or two at 35-45mph, stop at a red light, idle is normal.

[MrBulldops]

A lean mixture will cause the idle to increase, some where along the intake system there is a vacuum leak, check the intake manifold. When it is idling spray some water along the intake and see if it stumbles and check that the bolts are tight. In an aircraft there is a Vernier throttle to lean the mixture, as you lean the mixture the rpm will increase. A lean mixture can also cause overheating.

Haven't had overheating issues, and this is my daily driver. I used an IR temp gauge and the engine runs between 160 and 180 and the gauge in the car reads 1/4 on cooler days and 1/2 on warmer days. It'll climb to 3/4 going up a hill on a warm day, but my coolant has never boiled over and I don't have any other running issues. I'll try the trick with the water and get back to you.

[jclary]

On a couple of my engines, there seems to be a "tight spot" on the linkage where the idle does not return all the way to the idle screw set point. However, a little quick tap on the gas pedal usually sends it home and the idle speed drops immediately. On my V8 Lark, when I was using it regularly, I put a stronger return spring on the accelerator linkage, and lubricated everything to cure the problem. However, the spring was so strong that it fatigued my foot on long trips. I went back to the weaker spring and found that the lubrication alone had cure the problem.

On my '55 truck, I still have the tight spot in the linkage. But, the linkage, on that 62 year old vehicle, includes a hand choke and manual throttle cable. When the idle hangs a little high, it is not so much that a quick tap on the pedal won't cure. Actually, on that engine, I use the throttle to set my idle speed depending on traffic & weather conditions. I have found that on very hot days, the idle can be too low in traffic and cause the engine to run on the hot side because the fan is running too slow for good air circulation.

I wonder on your 60 Lark, if some how the choke "fast idle" linkage is getting some kind of interference causing it to advance idle speed. That kinda goes against conventional thinking because you would think that condition would occur only at start-up on a cold engine. Might be worth investigating anyway. You might want to get it warmed up real good, park it in your drive way, remove the air cleaner so you can observe, and use a remote start switch, so you could observe the carburetor linkage on a hot start. If it "fast idles" with the linkage in it's correct position, then you will at least know it is not the linkage.

[PackardV8]

If we ignore all other possibilities and say the high idle is cured by cold water on the fuel pump, then maybe the high idle is caused by heat boiling the fuel and raising the float level to where it overflows into the manifold. If a remote electric pump is out, then a pressure return line, such as used on the supercharged cars might be a fix.

jack vines

[RadioRoy]

Has anyone ever seen a situation where cooling the fuel pump changes the idle speed?

I'm still thinking something is going on with the throttle or choke linkage. It would be nice to know if tapping the accelerator pedal returns the idle to normal.

Also, what happens if you start the car hot, get the high idle, then pour cold water on the fuel pump without touching the accelerator pedal? That's a change in the order of the way it's happening now, but would be an interesting experiment.

[Ron Strasser]

Mrbulldrops
If as Jack and others believe, the fuel boiling in the pump is causing the carburetor to leak fuel into the intake manifold; where is the additional air coming from to burn the fuel. Either there is a vacuum leak due to the heat soak of the engine or the heat is causing the throttle linkage or plate to come off of its idle setting. As Jack said a return line to the tank, as is done on the Avanti might solve your problem. Or maybe not if heat is causing the throttle plate to open off its idle setting.
Ron

[MrBulldops]

Checked adjustment of linkage and lubricated with light engine oil: no noticable effect

Replaced throttle return spring with new, dual-spring: stabilized throttle speed after deceleration and smoothed throttle response, but no effect on high idle when starting hot

Replaced carburetor mounting gaskets with new ones. Sprayed brake cleaner around area of carburetor and manifold and found the idle climbs slightly near center intake channel. I've removed the manifold and purchased new gaskets and hardware (brass nuts, star washers, anti-seize), but have to hold off putting everything back together. Unrelated to this, I've been burning a little oil when starting the engine cold and, while the manifold was off, I decided to open up the side covers and take a look. Turns out my valve chamber baffles are MIA. Sourced replacements, but I'm waiting for them to arrive. After I get it all back together, I'll let you know if the new gaskets correct the hot start problem.

[az64stude]

Hi can you post pictures of carb setup and fuel pump?

[StudeRich]

It sounds to me like you need a Fuel Pressure regulator in line to cure the excessive pressure when hot.

The High Idle could be because you disconnected the heat riser preventing the Choke and fast idle from "pulling off".