While searching the site for Vapor Lock I could not find any mention of the effect of high altitude. I have experienced what I think was “vapor lock” on my 240 that is running SUs with a manual pump (New in last 30 days). In both cases it was at altitudes of 5000’ and 4500’. In both cases shortly after spirited driving at higher RPM (4K-5K) and then slowing down to RPM of 2-3K. 

The engine would seem to run out of gas. Checking the filter showed fuel available. Fuel line from pump to carb was dry. 2 hours later and coasting down it started and ran fine for the next 150 miles home.

In 25 years of driving a 240 I have never experienced this (Vapor Lock) before.

Is it the extra engine heat by driving hard?

The fact boiling temperature is reduced at altitude.

Do electric pump cure this from happening?

If the fuel pump you replaced in the last 30 days is a Spectra or GMB brand, then your issue lies there. 

And 

https://www.classiczcars.com/discover/content-posted/?&before=1593194073&latest=1590678350

It is Spectra 241 87019  interesting because the name was covered by a white stick on label.

 I've experienced a high (5000'+) altitude misfire with SUs. The misfire was consistent all across the RPM range. It was temporarily solved by raising the nozzles 1/2 turn.

1 hour ago, Mark Maras said:

 I've experienced a high (5000'+) altitude misfire with SUs. The misfire was consistent all across the RPM range. It was temporarily solved by raising the nozzles 1/2 turn.

So mine was running awesome until I was near the 5000" foot mark and then it just ran out of gas at the carbs. When it did start it ran fine both times. It feels like it is the mix of hot motor bay (engine temp appeared fine on the block sensor) and the radiator. Both electric fans were running and my speed was pushing hard on 15, 25, and 35 MPH curved for about 3 miles just prior.

Of course it would be easy to adjust in a half turn (richer) in on those days.

Thoughts anybody? Electric pumps?

@zKars If the fuel pump you replaced in the last 30 days is a Spectra or GMB brand, then your issue lies there.

Where do your get a manual pump that is not one of those two?

I had a similar problem in Yellowstone with downdraft Webers, summer, 6500 ft elevation.  Put a Carter electronic fuel pump to replace the old one which I determined wasn't working and it solved the problem.

4 hours ago, 7tooZ said:

So mine was running awesome until I was near the 5000" foot mark and then it just ran out of gas at the carbs. When it did start it ran fine both times. It feels like it is the mix of hot motor bay (engine temp appeared fine on the block sensor) and the radiator. Both electric fans were running and my speed was pushing hard on 15, 25, and 35 MPH curved for about 3 miles just prior.

Of course it would be easy to adjust in a half turn (richer) in on those days.

Thoughts anybody? Electric pumps?

@zKars If the fuel pump you replaced in the last 30 days is a Spectra or GMB brand, then your issue lies there.

Where do your get a manual pump that is not one of those two?

Well currently you are out of luck short of going electric. The hot tip at the moment, assuming your diaphragm was the failure, is to put the top half (6 bolts) of your old OEM pump on top of the new Spectra body. This gives you the likely still good OEM check valves and a new diaphragm. 

About the only other way people are getting by with is using new OEM pumps for 510’s and such, sometimes available on ebay, as donors for the diaphragm and check valves. 

Edited July 28, 20204 yr by zKars

Do any of you experience with the Delphi or carter version fuel pumps?

I have used a Carter electric pump for years with no issues other than it is noisy. You only really hear it at idle. Once you go, the noise gets lost in the background. Be sure to add an oil pressure off switch so if you crash and the engine dies so does the fuel pump, FYI I live at 5,000 ft and drive up to 10,000 ft. and no issues.

Cheers, Mike

Edited July 28, 20204 yr by Pop's Z

The Carter and the Holley are noisy, not sure about any other types.  The great thing about electric is it works when your battery does, or doesn't.  It's easy to diagnose when the lights come on but you don't hear the buzzing in the back.  I've seen them mounted in the engine bay too, some of the noise is masked by the engine when its running.

 Many Z owners in the past reported the 1st gen. (circa 1980) Rx7s elec. fuel pumps were almost silent and were a great replacement in a Z. I've put many miles in an 80 RX7 and don't ever remember hearing the pump.

 Getting back to vapor lock, has anyone ever tried installing a Vapor Separator Fuel Filter under the hood?  One inlet and two outlets. One outlet to the carbs and one to the return line. Good up to 75 psi. They were OEM parts on many American cars back in the 1960s and 1970's.Wix 33040 is the one I looked at. I've yet to find one negative review when it comes to solving vapor lock.

The filter Mark is referring to is on the right.  Appears to be for horizontal mounting, as it is important to have the offset return line in the 12 o'clock position.  The left one looks like it is intended to be mounted vertically.   In & out are 5/16"  the return is 1/4".    Amazon,  about 7 bucks.

Thanks Mark,  I think you're on to something!