Vapor lock questions for the hotter climate guys

[jalexquijano]

Well i tested the car last night after setting the idle mixture Wheel underneath the carbs to 3 turns clockwise and new NGK BP6ES plugs set to .031 gap. Once i crank the engine the accelaration felt very poor and did not became stable until 5 mins of warm up, when the needle of the temperature gauge sat on the middle. idle rpm was 900 and mantain stable. I did think the car lacked a Little bit of power. Came back and open the Hood after 20 mins of driving and all plugs showed as attached. Should i give it more gas at the idle mixture knob? maybe turning one flat or am i okay?

Edited October 27, 2016 by jalexquijano

[Jarvo2]

Overall looks like an ok plug to me, maybe a little on the leaner side, but a lot of that has to do with the quality of fuel being used.  What do you get in Panama?  Is it leaded or unleaded?  Is there any ethanol in it?  Whats the octane?  Are there any additives to the fuel in your country?

[Stanley]

Regarding the two line-three line issue, the third line is just a vent for the expansion chamber (hope I got the right name for it) that goes to the flow guide valve; don't see how it would make a difference.

Regarding the "three reasons" for "percolation", in the modification manual that was posted, reason 1 makes sense, the other two seem a little iffy. However, that information applies to the flat top carbs used in '73 and '74. I'm not familiar with how those carbs work so maybe it's correct for them.

My fuel pressure gauge (hooked to a tee between the mechanical pump and the custom fuel rail) seems to back up the percolation thing. On hot days, a few minutes after shutting off the engine, I've noticed the fuel pressure climbing slowly, past the usual 3.9 psi, to about 5 psi or higher. Then it drops suddenly. If heated gas vaporized and built up enough pressure to open the float valve, like the book says, that could explain it. I've also smelled gas after a hot start. If the float bowl is full when I shut off the engine, and the percolation effect pushes more gas in, that would explain it. My fuel rail is more isolated from the engine than the stock one, but maybe not enough.

It seems like having a return line (I don't) would help the situation, but maybe not. Maybe no fuel return line would raise fuel pressure slightly, that might help prevent vapor lock (not percolation). Also, they already had return lines on the cars discussed in the modification manual, so apparently that didn't solve the problem. 

If the main cause of heat in the fuel lines is due to conduction from the bolted attachments to the engine, then it seems like insulating the lines would help keep the heat in.

[Captain Obvious]

With a stock system, the orifice on the return line should bleed off all pressure in the rail as soon as the fuel pump stops pumping. I guess it's possible for the fuel in the rail to become superheated and then all of a sudden "burst" into a boil during engine off heat soak? Maybe that generates enough of a pressure impulse to force fuel past the needle valves before the orifice bleeds the pressure off? Maybe some liquid "hammering" as bubbles alternate with liquid trying to get through the orifice rapidly?

 

The second reason (as Zkars suggested)  is exactly what the needle valve is supposed to do... Regulate the fuel level in the bowl.

 

And as for the third reason, I would be hard pressed to be convinced that the vent tube cannot dissipate vapor pressure quickly enough such that pressure would build up in the bowl.

 

I'll offer up a fourth idea... My understanding of percolation is just like the old school coffee pot. You boil the liquid and the frothing emulsion of bubbles and liquid not only takes up more volume than plain liquid, but the frothing can also temporarily block off the vent tube hole with a slug of liquid making the jet hole in the venturi an equally attractive temporary exit during the boil.

[Captain Obvious]

Been a long time since I've looked at a carbed fuel rail... Are there insulating plastic bits to act as a thermal break between the head and the rail?

[jalexquijano]

so how do i avoid the sudden percolation caused in traffic jams due to heat? i already installed some heat shield extensions isolating the 3 screw su carb round top fuel bowls.

[Mark Maras]

12 hours ago, jalexquijano said:

so how do i avoid the sudden percolation caused in traffic jams due to heat? i already installed some heat shield extensions isolating the 3 screw su carb round top fuel bowls.

As an experiment, I'd re-direct the fuel lines with a temporary fuel rated hose bypassing the original steel fuel rail. Do it cheap and easy. Leave the old fuel rail in place until you decide if it was part of the problem. If the experiment works, replace the temp line with a permanent line.

 

[ta240]

16 hours ago, Captain Obvious said:

Been a long time since I've looked at a carbed fuel rail... Are there insulating plastic bits to act as a thermal break between the head and the rail?

There are thin pieces of plastic, I don't think they do too much to stop the heat though.   When I added insulation to the fuel rail it seemed to make mine worse which went with the thinking that the heat is largely coming from the block.  At one point I thought of trying something like cutting out  a section of the supports and replacing them with exhaust hanger rubber clamped on both sides.

32 minutes ago, Mark Maras said:

As an experiment, I'd re-direct the fuel lines with a temporary fuel rated hose bypassing the original steel fuel rail. Do it cheap and easy. Leave the old fuel rail in place until you decide if it was part of the problem. If the experiment works, replace the temp line with a permanent line.

That is what I did.  And what has been recommended over and over in this thread.
Prior to doing the fuel line change I added a fan shroud, insulation on the rail, an electric fuel pump at the tank and none did anything.  So far with the new fuel line it has been a lot better.  I haven't driven it in traffic on a really hot day yet but it is an improvement from the driving I have done.
Personally I think the engine looks better without the mechanical fuel pump and fuel rail on it.

[Namerow]

20 hours ago, Mark Maras said:

As an experiment, I'd re-direct the fuel lines with a temporary fuel rated hose bypassing the original steel fuel rail. Do it cheap and easy. Leave the old fuel rail in place until you decide if it was part of the problem. If the experiment works, replace the temp line with a permanent line.

 

Agree with Mark on this easy-to-do-and-reversible experiment.

That said, I continue to believe that the evidence points to the cause of the problem being heat conducted into the fuel in the delivery/return lines by way of the fuel rail assy's mounting straps (which, don't forget, connect to the cylinder head -- arguably the second-hottest piece of metal under the hood).  The problem certainly doesn't seem to be convectional heat transfer caused by hot underhood air temps.  If that were the case, the factory 'fix' of adding insulation along the steel lines should have cured the issue (and it apparently didn't).  I don't think that the success of Jeff's strategy of replacing the OE steel with rubber hose comes from the better insulation qualities of the rubber walls against convectional heat transfer from the air.  Instead, I think it succeeded because either the rubber hose or Jeff's hose mounting scheme -- or a combination of the two factors -- provided a better barrier against heat being conducted from hot engine parts into the fuel in the lines.

BTW, I find it interesting the the E-Type Jag's (fitted with triple SU's and probably generating equally-high underhood temps) didn't seem to suffer from fuel starvation problems -- even though they were notorious for high coolant temps and overheating.  Anybody know how the E-Jag's fuel lines are mounted?

So, JAlex:   In the absence of a bulletproof technical diagnosis (we still don't have one, after 8 pages of input), you're just going to have to experiment in a methodical way until you/we find either the cause or a solution (preferably both).   Try Mark's suggestion and see what happens.  Then report back to us here with your findings.  Until you do that, you're just going to get more conjecture (mine included) about possible causes and possible cures. 

Note that there is no over-the-counter 'kit' that you can buy to follow Mark's suggestion, so don't ask us for parts numbers or suppliers' names.  Here's what I would do.  Buy a couple of lengths of automotive-spec fuel line (1/4" for the supply hose, 3/16" for the return hose -- buy 'traditional' low-pressure rather than 'EFI' high-pressure hose).  Also buy a length of 1/2"-width steel 'strapping' (plumbing supply shop -- comes in a roll).   Use the existing steel fuel lines to provide mounting points for the new hoses.  Use automotive hose clamps (screw-type) to mount the hoses to the steel lines.  You'll need T-fittings to split the delivery hose into two paths (one for the front, one for the rear carb).  Same goes for the return hose.  Be tidy with the hose mountings and routings. Keep the hoses away from moving parts (like the fan and the throttle linkage) and off of hot parts (like the exhaust manifold).  Be sure that the clamps at the fuel pump, carburetor, and 'T' connections are tight.  Be sure that the new hoses won't snag the throttle linkage. 

Once you've got your new 'experimental' fuel hose set-up in place, post a few pictures here so the members can warn you if you've done anything wrong.  If you get the 'all clear', the next step will be starting the engine and doing final checks before you try it out on the road.

IMPORTANT:  If you are in any way concerned about your ability to do this type of work safely and successfully on your own, do not attempt it.  Instead, get a licensed mechanic to do it for you.

[siteunseen]

You have 175F to 200F running through your carburateurs.  Fuel boils anywhere from 100F to 400F.

This is helpful for carburateurs in cold climates but if the thermostat doesn't close it could bring unnecessary heat into the carbs.  There's mention of it in the '72 FSM, according to this reply.  " The manual also states that if the thermostat fails to close in hot weather, carb percolation may occur."

 Z Therapy says plug it, I've plugged mine as well as Beer Man Pete (I followed his instructions).  I'm going to borrow Namerows disclaimer:

IMPORTANT:  If you are in any way concerned about your ability to do this type of work safely and successfully on your own, do not attempt it.  Instead, get a licensed mechanic to do it for you.

 

[Zed Head]

We beat the heck of of the thermostat question about 3 or 4 months ago.  It's probably in one of the 7 previous pages. 

[Patcon]

This thread makes my tongue bleed...