The Dreaded Vapor Lock

[Jeff G 78]

15 minutes ago, Zed Head said:

So, considering your last post about deadheading the metal rail, it seems that the big difference might be the rubber hose versus metal lines.  Maybe the key is fuel temperature before it gets to the bowls.  The cooler the better.

We've talked about fuel differences in the various EFI heat soak threads.  Fuel quality varies across the country.  Might explain the AL versus MI difference.  Could be a winter blend thing also. 

Jeff G were you running aviation or race fuel, or pump fuel?

Sorry, I forgot to say WHY it worked.  The steel rail is bolted directly to the head, which is only a few inches from the exhaust ports.  The heat was trasferring through the steel bracket directly into the rail where it would heat the fuel.  By going all rubber, the only heat was radiant which is much easier to combat.  As for fuel, we ran 93 pump gas.  At a typical race, we'd go through 7 gallons per hour or 150+ gallons in a 24 hour race.  We'd send somebody to the nearest gas station every four hours to buy 6 jugs of fuel.  Some tracks had close-by stations but most were quite a ways away.

I think we always had summer blend.  The latest race we did was late October.  Most were mid-summer.

Edited June 10, 2022 by Jeff G 78

[VaCat33]

A few comments in response to questions/comments.

Fan shroud:YES

Test conducted sitting in Sun, hood closed on 90+ degree day.

No I did not try adding choke as engine stumbled but I will.

my floats/needles were installed and set by Mike at Banzai Motorworks.

My car is “bone stock” and I would like to keep it that way.

When I next get a chance I intend to add fuel pressure gauge to monitor fuel pressure. Might try temporary insulation in various locations. also plan a test on a 75 degree day, see if there is any difference.

Thanks

Jim

 

[Zed Head]

Unbolting the fuel rail would remove the heat path that Jeff G described.  Insulation won't help that.  Conduction, convection, radiation.

[Jeff G 78]

2 minutes ago, Zed Head said:

Unbolting the fuel rail would remove the heat path that Jeff G described.  Insulation won't help that.  Conduction, convection, radiation.

Agreed and it's a 10 minute job that is completely reversible.  I came up with the idea for this mod and did the install during a pit stop in a race.  It took less than 2 minutes and the car was back on track.  It was the first time in years that the car ran right.  We completed the last 8 hours of the race and climbed from 36th place to 9th place after the fix.

[VaCat33]

When I get back to testing I will give unbolting the fuel rail a try. Should be fine just idling in my driveway. Will make sure it is not touching anyplace where heat can be conducted. Looking forward to results.

what confuses me…is why/how are the physics different now than in 72. This (vapor lock caused by conduction of heat through fuel rail brackets) was not a problem back in 72 and as I mentioned I owned a 72 in 72 and never had this problem. In later years vapor lock became a problem. Is modern gas that much different Re boiling point.

Thanks

Jim

[Jeff G 78]

14 minutes ago, VaCat33 said:

Is modern gas that much different Re boiling point.

Yes

[87mj]

I had a '73 back in '79 as my first car.  After it heated up, it would vapor lock if I clutched/downshifted during a turn. it was recommended that I get the carbs (flat tops) rebuilt. I did but no change. It would idle all day long. But if I drove it and clutched it, it would die.  

After I got the car hot enough that I knew it would die, I disconnected the fuel supply and return from the fuel rail and connected long hoses to both ends of the rail.  On the other end, I put the hoses in a glass jar of gas.  When the vapor lock occurred, it didn't draw gas from the jar. For my POS '73, that eliminated the tank as the culprit (but not the rail). It didn't draw gas from the can. Then eventually it magically started drawing fuel. I never figured it out. 

About 5 years later, I bought a '72 and never had any trouble with it.  I always wondered if the design of the '73 fuel rail had something to do with it. Reading about vapor lock really frustrates me to this day because I remember the pain I went through.

In hindsight, I know I should have replaced the carbs on the '73 instead getting them rebuilt but I was stupid and listened to the datsun dealer. 

 

Have you tried something so simple as to use alcohol free gas?  In my area, we have something called "Rec gas".  I think it is 91 octane. We have many small lakes in the area and boats dont like alcohol in the gas since according to marinas, it can introduce moisture to the lines.  We also have racing fuel too but it but using that would be illegal.

[VaCat33]

No ethanol free gas less than an hour away. I am considering making a run, even if only for test purposes.

[siteunseen]

They have marine stabilizers that get rid of ethanol or av-gas. If you do get e free fuel you can run the feed hose of the mechanical pump straight out of a gas can between the bumper and radiator. I did that to confirm a nasty tank on one I bought. Drove it around the neighborhood for a few days, very slowly.

 

[w3wilkes]

Questions... 1-How did you mount the hoses for bypassing the metal fuel rail? 2- Rather than dead head... Why not a short piece of piping in the hose on the return side (after the carbs) that had the correct restrictor orifice size that Datsun crimped in the end of the return line of the metal fuel rail to even out the fuel pressure?

[Jeff G 78]

11 hours ago, w3wilkes said:

Questions... 1-How did you mount the hoses for bypassing the metal fuel rail? 2- Rather than dead head... Why not a short piece of piping in the hose on the return side (after the carbs) that had the correct restrictor orifice size that Datsun crimped in the end of the return line of the metal fuel rail to even out the fuel pressure?

I didn't mount the hoses when I deleted the rail.  I might have used a zip tie or two so they wouldn't flop around, but they are not that long or heavy, so they stay in place pretty well.

After running the original rail with an electric pump, I saw no benefit in having a return.  It actually ran better dead headed, so when I went to rubber, there was no point in rigging up a return.  The rail has a loop built-in to make the flow and pressure equal to both carbs when there is a return.  The hoses don't have that, so any return would likely make the flow uneven between the two carbs.  The car ran perfectly with no return, so why try to fix what isn't broken?

[cgsheen1]

It's a drag working in the heat (at least here in Phoenix it's "a dry heat"...) but first thing I would have done was pop the top off a float bowl and see if there's any fuel in there. Well, maybe I'd pull the fuel line off the rail and run a 3 or 4 foot piece of hose off it into a fuel can to verify that the pump was pushing fuel first, then check the float bowl. Gotta check when you're having the problem though.

Carbs (I thank goodness every day that I have none) don't run off the pump, or the rail, or the fuel lines. They run off the float bowls. If there's no fuel in there it's somebody else's fault... Figure that out. If  there IS fuel in there and the engine doesn't run then it has to do with delivery from the bowl to the carb - bowl outlet, weird little hose, jet, needle...

Personally I'd lower the jet (L-series seem to hate idling lean) - you accomplish the same thing pulling the choke - because I'd bet that your "altitude/temperature correction" is off. My second bet, if the jet/needle setting is correct, that there's either a restriction aggravated by temperature or your fuel pump / fuel system is affected negatively by increased temperature. I also wouldn't be surprised if your float valves stuck intermittently (cooler fuel (more dense) will push them back open but hot fuel (less dense) and/or vapor will not). Or, maybe your carbs aren't set up and sync'd properly at all...

(And I definitely WON'T say that I have a 240Z in the shop right now in the 100+ (113 the other day) degree heat (L28, SU round tops, electric fuel pump, MSA header with a stock heat shield (for an N33 intake), no shroud, alum radiator, stock fuel rail (that is not bolted to the head however), and also - no coolant to intake manifolds...) that doesn't have any "vapor lock" issues. I didn't make it that way - but someone did. Nope, I'm not going to say that...)

Edited June 14, 2022 by cgsheen1