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Sharing story - Solving issue of starving for fuel after running for 10 minutes

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There is no heat shield. The carbs were converted from 2 to 3 in 1969, and the cam and headers were changed. I can't see how there would be any room for a heat shield with the rear two bowls being so close to the manifold. Since I'm sure the bowls have always been there, I'm also sure that different fuels must have different boiling points. I just topped off the tank with 93 octane from Sunoco, and drove around for 20 minutes without a sputter. I think I've identified the problem; now it's just a matter of dialing in which fuel won't boil on me.

Recap on the symptoms (which now all fit together):
1. Car runs like a champ when cold.
2. After 7-10 minutes, it starts sputtering, hesitating, and losing power. It does NOT recover after driving for a while.
3. After a couple of short stops the sputter comes on soon there after (also comes on even if I don't stop and turn off the car for a minute).
4. After the car cools down for 15 minutes or so, it runs fine again.
5. I can hear the fuel pump working extra hard and pumping fast around the same time the sputtering occurs.
6. During a normal sputtering episode, I quickly removed all 3 float bowl lids and could visibly see bubbles coming from the bottom of the two rear bowls, which are located within 1/8" from the exhaust manifold. There is no heat shield installed (and has not been for 49 years.)
7. The fuel tank is clean.
8. New plugs, wires, coil, condenser, electronic ignition, OD dash switch, fuel filters, bowl seats, and fuel pump did NOT fix the problem. (at least I have a nice list going of what is working well!)
9. I have been running 100% gasoline, which is 87 octane and contains no ethanol. I have also been using lead additive.
10. After topping off the tank with pump 93 Sunoco, the car ran fine for 20 minutes and never started sputtering. After pulling into the garage and loosening the fuel line between the carbs, vaporized fuel escaped, indicating the fuel was starting to boil again.

I think I've about got it figured out if I can continue to validate that normal pump 93 fuel allows it to run okay. More testing to come!

I genuinely appreciate everyone's input. I hope others can learn from my frustrating ordeal!
 
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Best picture I could find with a quick search, but looks legit. Compare the number of, and position of the spacers (phenolic insulators). Without the required amount, the linkage will bind on the heat shield, so make sure to check for full range of travel.

P1010112.jpg
 
Yep - I'm definitely missing the heat shield. It looks like those spacers would put the carb bowls a good 1/2" further away from the header pipe as well.
 
UPDATE:
I haven't sorted anything out for a heat shield yet, but switching from 100% gas to super unleaded pump gas has certainly addressed my sputtering issue. It is amazing that the 100% gas has a significantly lower boiling point to where it was boiling in the float bowl so consistently.

The new electronic ignition is velvet smooth. Car is really running well now. :eagerness:
 
Hi and thank you for keeping us informed as the more information gathered on this type of uncommon issue, the easier it will be to apply logic-with-information in addressing the next...and there will definitely be a next.

Because of my higher compression and more aggressive ignition advance, I have always used and needed to use Hi test fuel to eliminate pre-ignition. Previously, leaded high test pump fuels had Octane rating as high as 103 and, although pricy, allowed for a reasonably higher ignition advance that helped my engine be very responsive. Todays unleaded high test pump fuels start at 93 Octane and higher octane fuels are much more difficult to come by. Although I was unaware of the higher boiling point of these fuels, I still use unleaded 93 Octane rated fuel (as a standard 12 degree advance) and am wondering if your sputtering is as much an issue of vapor or fuel gasification lock-up as well as pre-detonation. Since you are using 3 HD8s, and I think you mentioned a more aggressive cam, what have you set your dynamic ignition timing at?

All the best,
Ray(64BJ8P1)
 
I have the installation spec sheet from when the cam was installed in 1968. It said to set it at 6 degrees, or (1-1.25") before the center mark. It starts and runs well at this point. I tried advancing it several degrees and it seemed to run slightly better, but it was causing starting problems - the starter didn't want to crank it over. So I put it back to 6 degrees and it starts right up now.
 
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