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Thread: High idle after hot start

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    High idle after hot start

    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.

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    President Member RadioRoy's Avatar
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    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.
    Last edited by RadioRoy; 10-24-2017 at 09:44 PM.
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  3. #3
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    Quote Originally Posted by RadioRoy View Post
    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.

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    Quote Originally Posted by MrBulldops View Post
    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.

  5. #5
    President Member RadioRoy's Avatar
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    Quote Originally Posted by MrBulldops View Post
    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?
    RadioRoy, specializing in AM/FM conversions with auxiliary inputs for iPod/satellite/CD player. In the old car radio business since 1985.

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    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.

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    Quote Originally Posted by RadioRoy View Post
    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.

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    Quote Originally Posted by altair View Post
    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.

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    Silver Hawk Member jclary's Avatar
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    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.
    John Clary
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    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
    PackardV8

  11. #11
    President Member RadioRoy's Avatar
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    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.
    Last edited by RadioRoy; 10-25-2017 at 04:41 PM.
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    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

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    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.

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

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    Golden Hawk Member StudeRich's Avatar
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    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".
    StudeRich
    Second Generation Stude Driver,
    Proud '54 Starliner Owner




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    Quote Originally Posted by StudeRich View Post
    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".
    Not sure what difference the heat riser would make in a hot engine bay when it's 90+ degrees outside, but it's a thought. I can try reconnecting the spring and seeing what happens if the new gasket doesn't have any effect. A pressure regulator is something I might look into as well.

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    Quote Originally Posted by az64stude View Post
    Hi can you post pictures of carb setup and fuel pump?
    It's the stock setup for a 1960 Lark VI: 170ci flat-head six, Carter AS carburetor and a mechanical fuel pump. I can try adding photos once I've got it all back together.

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    Does it have electric wipers and no vacuum boost fuel pump? The only way I can figure cooling the pump could make a difference would be a vacuum leak at pump when pumps hot.

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    Quote Originally Posted by kxet View Post
    Does it have electric wipers and no vacuum boost fuel pump? The only way I can figure cooling the pump could make a difference would be a vacuum leak at pump when pumps hot.
    There's nothing on the car the runs on vacuum; manual brakes, manual steering, electric wipers. The pump is the normal cam actuated variety. But as I said, I traced a vacuum leak at the manifold gasket so we'll have to wait and see if the new gasket corrects the issue. I won't have it put back together for a few days... Just waiting on a second valve chamber baffle.

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    Since the OP said the carb had been rebuilt, I would concentrate there. These old carbs can be finicky. Linkages wear and come out of adjustment, and that's true if the rebuild wasn't done right. I think the guys suggesting the choke fast idle are on the right track. The fast idle sets the throttle plate open a tad to help with a cold start. It could be the fast idle cam and or linkage is binding. So after driving a bit, the throttle plate is staying open. That would give high rpm's on a hot start. My two Honest Abe's.

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    If you have the intake manifold removed check it for flat with a straight edge, it may need to be resurfaced.

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    This is one I would like to see in person. My first thought when reading the initial post was the linkage binding, or the fast idle cam coming back up some way.
    The real head scratcher here is pouring cold water on the fuel pump apparently solves the problem. Running the different analyses through my little pea brain is giving me a headache.

    Any chance that just opening the hood results in normal idle on re-start? Even assuming Jack's posit that the fuel in the bowl is heated enough to raise the float level, one would expect that increase in fuel supply to lug the engine down without a corresponding increase in air flow some where. I am really doubting there are two separate causes happening simultaneously to create more fuel and more air. I also can't imagine (unless the fuel supply is already way over rich) that a mysterious vacuum leak is making it idle fast on hot re-start. Most vac leaks get better as the parts expand under heat, rather than worse. Besides, if a vac leak is causing this, how would you explain cold water on the fuel pump curing the problem?

    Did it do the same thing when the electric pump was on there? I have electric fuel pumps on three vehicles with carbs, and have not issues with either of them. Using a fluid cooler of some sort may solve this problem, but you won't know without installing one.

  23. #23
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    Next chance you get look closely at your carb linkage with hot engine and no water on pump.especially the fast idle cam. when installing manifold gasket a THIN skin of ultra copper silicone will help the seal and wont burn out.if this problem goes away by it self i will tear my hair out! Luck Doofus

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    Lynn, when an engine is operating at a given temperature say 170 - 190 range and is shut off the block temperature will rise substantially possibly to boiling point for a short time and then start to cool back down again. If the intake manifold does have some irregularities and is prone to leaking while at this increased temperature and the engine is restarted while still hot a slight vacuum leak could develop resulting in a temporary lean mixture and an increase in idle speed. I have half dozen manifolds in my supply, intake and exhaust and all of them were not flat and required resurfacing, some were .003 - .005 and the worst was .022. With the age of the engines and the number of times these manifolds have been on and off, the probabilities of a non true surface is relatively high and hence vacuum leaks become an inherent issue.

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    Ok, replaced the manifold and exhaust flange gaskets, verified heat riser works, installed valve chamber baffles, new inspection cover gaskets and filter element for crankcase breather. I also double checked the carburetor adjustments for fast idle and unloader and the float.

    So...

    The engine now runs much more smoothly, and the high idle for hot starts is corrected. Now I've got cold starting problems:

    It took me fifteen minutes of trying to get my car started this morning and it's only 60 degrees Fahrenheit. The choke is set to the leanest mark, but I still get flooding if I start per the manual (completely press the accelerator once and then turn the engine over). Even if I hold the throttle open, to disable the choke, the engine takes forever to start, and runs rough for about twenty seconds before clearing up. If I take the air filter off, and adjust the choke plate manually, I can hold it completely open and there is no change in the idle... Not sure what to make of that. I would think I could just not use the choke ever then, and simply set the "choke" to get fast idle on startup, but that just makes me think the jetting is way WAY off.

    To eliminate the variables:
    New plugs
    New cables
    New coil
    New condenser
    New battery

  26. #26
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    Quote Originally Posted by MrBulldops View Post
    The engine now runs much more smoothly, and the high idle for hot starts is corrected. Now I've got cold starting problems . . . .but that just makes me think the jetting is way WAY off.
    If the engine runs smoothly and pulls fine, it's unlikely jetting is the cold starting problem. Jetting is the least likely cause of the problem. It's rare to find a 1-bbl where jets have been changed. Your suggestion to work with the choke adjustments is the best science. Once that's as it should be, then if curious, have someone with a wideband O2 sensor and meter tell you what your idle and cruse air/fuel ratios are.

    To eliminate the variables:
    New plugs
    New cables
    New coil
    New condenser
    New battery
    Excellent procedure. The majority of carburetor problems are cured by a completely new ignition system. The Champion is especially prone to plug wire problems inside the metal loom which holds all the plug wires tight together.

    jack vines
    PackardV8

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