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1974 F100 390 Hard starting only when warm

 
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Old 03-14-2019, 07:23 AM
Brynjaminjones
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1974 F100 390 Hard starting only when warm

Hi everyone,
I've got a 1974 F100 with 390 that I'm slowly getting dialled in. Now that I'm using it regularly I'm noticing that it doesn't like starting when it's warm.

If it's cold it starts with two pumps and a flick of the key.
If it's hot, it starts with just a flick of the key.
If it's been driven then sat for about 30 minutes, it will have to crank for about 10 seconds before the revs slowly build and it starts and idles by itself.

It's not a huge problem, but it's a little embarrassing and is very irritating if I'm driving from store to store for example!

The choke seems to be pretty well adjusted - it will fully close when cold and fully open when warm.

Any idea where I should look?


 
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Old 03-14-2019, 11:58 AM
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Faulty starter suffering from heat soak I'd presume. I've had this problem with new remanufactured starters...start fine when cold & crank very slowly after a short drive.
 
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Old 03-14-2019, 12:23 PM
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The , it will have to crank for about 10 seconds before the revs slowly build and it starts and idles by itself has me asking? Is it turning over slower than when cold and hot or it just not catching and running till you hold the key longer?

Carb cars & trucks each have their own way they need to be started when cold, warm & hot.
It sounds like you found the cold & hot just need the warm. Try holding the gas pedal down 1/4 to half way when starting warm. It may take 1 pump and hold it half way down before trying to start.

It also would not hurt to see what the choke is doing when you have that hard starting. It could be all the way open and it may need to be almost closed?

Me I would check the choke but don't touch just yet and play with the pedal, thinking half way down to start, and go from there.
Dave - - - -
 
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Old 03-14-2019, 05:49 PM
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Thank you both for the helpful ideas. I've got couple of points to answer your questions:

The starter is strong and healthy. When it takes a while to start, it cranks at full speed the entire time so I don't think that's actually related to the issue, but I realise I hadn't written that in the original post!

If I remember correctly, the choke is usually in the almost closed position at this point, i.e. as closed as it can go without touching the gas pedal. I will check next time I'm out though that this is the case.

Does any of that help at all? You might well be right that I just need to learn the best way to handle it - I might try cranking with my foot on the gas a touch.
 
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Old 03-14-2019, 07:03 PM
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Vapor lock?
 
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Old 03-14-2019, 07:04 PM
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Have you checked the timing?
 
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Old 03-14-2019, 07:18 PM
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I've had this issue before, heat soak was boiling the gas in fuel line. Making sure my metal fuel line was spaced as far away from the block as possible helped.
 
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Old 03-14-2019, 11:46 PM
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Originally Posted by biasvoltage View Post
I've had this issue before, heat soak was boiling the gas in fuel line. Making sure my metal fuel line was spaced as far away from the block as possible helped.

Yup, Ill be doing that this weekend... Runs way to close to the header.
 
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Old 03-15-2019, 06:14 AM
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I'm not sure if it could be vapor lock, as it only happens after the truck has been sat for probably 20 minutes or so. If I start it before then it will start right up. It's seemingly only after it's cooled down a little. That said, I've never experienced vapor lock so I could be way wrong - I'm happy to be educated!

The timing is set to 10 degrees at idle, 35 degrees all in. Vacuum advance also works and is using ported vacuum.


 
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Old 03-15-2019, 07:56 AM
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Originally Posted by biasvoltage View Post
I've had this issue before, heat soak was boiling the gas in fuel line. Making sure my metal fuel line was spaced as far away from the block as possible helped.
This is not after a long run time to get the motor up to temp, it is warm not hot.
He also said it starts fine when hot, up to temp.

Like I said first check the choke when this happens. First with out hitting the gas then hit it to see if it closes. This way you have a base line what it is doing. Then when trying to start give it a little gas 1/4 to maybe 1/2 to see what that does.
Each carb motor is a little different on how it starts when cold, warm & hot you just need to find it.
Dave - - - -
 
  #11  
Old 03-19-2019, 06:07 AM
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I've still not had a chance to get the air cleaner off when it's warm, but I tried giving it 1/4" gas the other day after an hour or so and it started much better. I'll keep testing when I take it out next but it looks like it might just be a case of me learning what it likes.
Years of daily driving fuel injected cars means that a carb is a bit of a learning curve for me!
 

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