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Discussion Starter #1
Shop installed holley 600cfm with electric choke. Car starts fine cold and hot. After I've been driving and turn it off and try and start it 30-45 minutes later, takes a long time to turn over, when it does, it will stall, then I start it again, runs rough, i feather pedal until it smooths out, then I can drive it. The choke has been adjusted, and I even wired it to stay completely open with no change. I also bought some aluminum insulation made for the engine compartment and placed under carb against manifold to prevent the manifold heat from possibly boiling the fuel in the bowls, no difference. Shop continues to work on it, but obviously they are having difficulties figuring it out.

Any ideas would be appreciated.
 

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When you say "turn over" do you mean it cranks very slowly? If so that's a starter issue. Could be poor electric connections or the starter motor is heat soaked. The higher the temperature, the more resistance, less current flow and the slower cranking.
 

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Aluminum carb spacer? Rubber fuel lines? Incorrect ignition timing? Incorrect coolant mixture or level? Insufficient radiator cooling area, fan size or proximity to radiator surface or depth in shroud? Headers on engine?...
They all add heat that rises to the carb...
 

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First thing is to pull the sight plugs and check the float bowl level. Not every auto repair shop are carb specialists.

Next thing would be to look in the carb and see if its boiling over when sitting. You should see the fuel dripping out of the vent tubes. The problem is when you remove the air cleaner your cooling down the carb so its not a real world test.
Thousands of those Holley 600 carbs have been installed without boiling over. We had about a dozen F800s with Holleys on 429s. They got super hot under the hood after shutting them off and they never boiled the fuel out of those Holleys but they did have a factory phenolic spacer under them.
 

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First install.....a phenolic 1/2" separator between the carb base and manifold, ditch the aluminum stuff. Next, all of the above....
 

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I think a lot of people struggle with starting carbureted engines after driving fuel injected vehicles, I know I did for a while. I find what works for me is a cold engine start gets 2 full pumps of the gas pedal, one to set the choke and a second for enough gas to start. If it's been a week or more it takes 3 pumps. A hot start I do not touch the pedal, just turn the key until it starts. The one the OP is having trouble with and I think most people do the 30 min. to a couple hr. start I don't touch the pedal until I've cranked it a little, then just blip the pedal a bit. It's all a matter of the correct air/fuel mixture, If you mash the pedal to the floor every time you might be setting the choke when it doesn't need it not to mention too much fuel. I also set the choke so that the plate is just held open with throttle blades all the way open, This allows enough choke time when cold but still opens all the way when warm. This is with the Holley electric choke connected to the stator terminal on the alternator.
 

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I've had the same problem with my car. First, it had a 600cfm electric choke carb on an old school intake not much better than stock. It would get heat soak. I would have to feather the throttle for a minute or two after leaving hot. Kind of embarrassing getting out of parking spots at times :0 Then I added a 1 inch spacer, and that helped but not much. Now it has an air gap intake, same 1 inch spacer, and ceramic coated headers. I thought it would be better, but not much. Even with non-ethanol. I can see how lean it is when hot on my AFR guage. It's better, but still has the problem. Only other thing I have heard I could do would be to put one of those big plates under that carb.
 

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First install.....a phenolic 1/2" separator between the carb base and manifold, ditch the aluminum stuff. Next, all of the above....
That's how Ford avoided the problem when using a Holley on an aluminum intake on the BOSS 302. That spacer was 1/4", and even had a PCV connector. These are available new, and of course for much less you can get this from Summit for $29-
746266

Oh, and do you have headers? Heat soak of the starter from headers can cause the starter to crank slowly.
 

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Discussion Starter #9
Thanks for all your responses. Yes, I have headers and the starter cranks fine. All of this started when they removed a proform 750cfm racing carb, no choke, that was just dumping too much raw fuel into the engine. Also, 750 a bit large for the 351. So definitely carb and not starter problem(starter is wrapped with insulation). I would have thought that shielding the carb from the manifold would have solved the problem if it was a heat issue...
 

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Are you in Albuquerque by any chance? I might be able to help with the Holley. First thing..has it been tuned for this altitude...5000ft above sea level? PM me if you want...

sandybob
 

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Are you in Albuquerque by any chance? I might be able to help with the Holley. First thing..has it been tuned for this altitude...5000ft above sea level? PM me if you want...

sandybob
Yes sir, in Albuquerque. When I got home today after a cruise, I opened the hood and put 2 fans blowing directly under the carb. An hour later, she started right up without any problems. Going to try this a few more times to make sure it's a "heat issue". If so, going to tell the shop to put a phenolic plate under the carb.
Thanks for the offer and I'll certainly keep it in mind..
 

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I think the carb spacer will fix your problem.


Sunoco fuel Article. You can try a different octane fuel and see if that helps? If I'm reading this correctly It sounds like a lower octane fuel boils off easier.


Octane Stability: High Octane vs Low Octane Fuels

Octane is the most talked about property when it comes to gasoline. It’s no surprise because selecting octane of gasoline is the only choice the consumer has, except what station to buy it from. In this article we will focus on octane stability of 87 and 93 octane pump fuels versus racing fuels and what you can do to prevent octane loss in your fuel.


It is necessary to briefly review what octane is in order to dive deeper into stability issues. At fueling stations in the United States, octane is typically displayed as a number, 87 through 93. This number is known as Anti-Knock Index (AKI). AKI is the average of Research octane number (RON) and Motor octane number (MON)(Totten). Essentially, these numbers provide a scale to measure how much heat and pressure can be put on the fuel before it spontaneously combusts. Spontaneous combustion is a source of engine knock which can quickly damage an engine. Octane ratings are important because different engines expose fuel to different amounts of pressure and heat. Engines must use the proper octane fuel to avoid knock and provide reliable operation.


A key aspect of gasoline stability is vapor pressure. This is determined by how much pressure builds up inside a sealed fuel container when the fuel is heated to 100°F. A higher vapor pressure suggests a higher concentration of low boiling point hydrocarbons that vaporize under 100°F. Pump fuels with high (12 pounds/square inch, psi) vapor pressures are used in cold weather to prevent engine starting issues due to low temperatures. Pump fuels are limited to 7.8-9psi maximum in warm weather depending on county and state (www.epa.gov). If stored in a vapor tight container the vapor pressure can be maintained for long periods of time. Fuel exposed to the atmosphere can lose light components within a couple of days. Over time as vapor pressure decreases the fuel can become stale. Stale fuel doesn’t evaporate as easily and can cause rough engine idle and hard starting. Butane is a volatile gasoline component used to tailor vapor pressure in accordance with seasonal needs. Cold weather fuel has higher concentrations of butane. Butane has a high blending octane value which helps manufacturers hit their octane targets. The main downfall of butane is that it boils at 32°F. If the fuel tank is vented to atmosphere the butane can start to evaporate out unless the daily temperatures are below freezing. This makes cold weather fuel more susceptible to vapor pressure loss and octane decreases.


87 octane fuels tend to be less refined and contain more unstable hydrocarbons. As the months pass during storage these unstable components react to form gums, varnishes and lower octane hydrocarbons. As a result the octane can decrease within months for 87 octane fuels, especially when stored under less than ideal conditions. 93 octane fuels are more refined and contain more stable hydrocarbons. These stable hydrocarbons can last 2-3 times longer than 87 octane fuel. Even in proper storage 87 octane gas can start to degrade in 3 months, 93 octane fuel should last closer to 9 months before degradation is noticeable. Keep in mind that 93 octane fuels are still susceptible to octane loss and vapor pressure decreases due to butane evaporation.


Octane stability in racing fuels is much different because fuel quality is valued more than production cost, unlike the pump gas industry where cost drives the majority of refining decisions. A large part of any quality race fuel is consistency. Race fuels are designed to be high in octane to allow for increased compression ratios and boost levels. In order to achieve high octane and consistent composition, pure chemical components are mixed with highly refined gasoline. The components used in Sunoco race fuels are very stable and can retain octane in excess of 2 years when properly stored. We have test results confirming octane stability in our unleaded, leaded, ethanol-free and ethanol fuels. The butane vapor pressure issue is addressed with the use of chemical components that boil around 80°F. The higher boiling point means vapor pressure decreases won’t be as common until the fuel is exposed to temperatures above 80°F.


Some high octane unleaded fuels, 260 GT Plus, and octane boosters contain the additive MMT. MMT is a very effective octane booster and doesn’t harm oxygen sensors or catalytic converters so, it is ideal for modern vehicles. Please note this additive is degraded by sunlight and can lose all octane boosting properties within minutes of exposure (L.Ter Haar). Degraded MMT will settle to the bottom of the container as a rust colored material that can clog fuel lines and filters. Extra care needs to be used when storing and handling MMT fuels in order to minimize contact with sunlight. The additive is stable in gasoline as long as no UV light hits the fuel. Sunoco 260 GT Plus is our only race fuel that contains MMT.


Octane stability is greatly affected by the storage conditions. Proper storage can preserve octane for years but improper storage can reduce octane and degrade fuel within weeks. That is why we provide proper storage information on our website. Click here to view our storage recommendations. Sunoco Race Fuels





Special thanks goes to Minnesota State University Mankato, Automotive Engineering Technology students for suggesting the topic of this article.


Author:


Zachary Santner

Technical Specialist
Sunoco Race Fuels
 
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