Vapor Lock Issue

[510_dreamin]

Looking for some guidance on addressing what I think is a vapor lock issue in this California heat wave. Over the weekend, I was driving my 510 home and it was about 95 degrees out and got stuck in traffic. It has electric fans and a great radiator but as I sat in traffic it started to get a bit hot and started to sputter a bit and I had to start giving it some revs to keep it fueled. Luckily I was able to get it home and once traffic broke it cooled down considerably. I drove it last night in much cooler weather with no issues but a slight knock with my foot on the floor. That’s new after the fuel issue this weekend but didn’t seek to affect drivability. 

 

Any suggestions? Should I switch to stainless fuel lines? Insulate the ones I have? Fuel pump maybe?

[Draker]

Do you have a return line? That could help, but not too common. Some 510s came stock with it. You could also try to insulate your fuel lines that run above the  intake and exhaust. 

 

I've had the same issues before on hot days..  I insulated mine which helped, but it still happens at times.

[510_dreamin]

No return line.

 

What did you insulate with? 

[wayno]

I had the same issue with my dual SUs, on the freeway I was just fine except slight knocking sometimes, but when I exited the freeway and came to a stop sign, after sitting in line it would start sputtering when I touched the pedal until I got the truck moving and in 3rd gear, enough cooler air got in the engine compartment and it would cool things down and the issue went away.

I did a couple things to fix this issue, first thing I did was get rid of the modified 510 air filter housing assembly, this made this kind of tunnel under the carbs that came to a dead end at the firewall, it held the heat in, the next thing I did was use a large wiring loom plastic cover from a modern vehicle on the gas line from the fuel filter to the carbs themselves.

I have had a return line all along, but it still boiled the fuel in the bowls.

Insulate your gas line/lines, and if you can put in a return line which is not that hard to do, what I did in my 520 was I made my own fuel pressure regulator, I did that by welding the end of a metal fuel line shut and then I drilled a 1/16th inch hole in the end I welded shut, I put a tee in the fuel line right next to where the fuel line split to go to each SU carb and I put my fuel pressure regulator in the line returning to the tank, then since my tank didn't have a fuel return barb in the tank I put a tee in the vent line in the fuel filler and used that as my return to the tank.

Edited August 8, 2019 by wayno

[datzenmike]

I thought only the L20Bs had the return, but??? You could retro fit an L20B return onto the earlier L16/18s but you would need to run a new line back into the tank. The return assy has a small opening or restriction that vents gas back into the return line to the tank. It's very small so the pump will build pressure on the carburetor side or gas would just take the easy way back to the tank. In use cooler gas from the tank is passed by the carb and warmed gad near the carb returned to the tank. Simple but effective.

 

Most L series engines have a heat shield that bolts onto the same studs as the carburetor mounts on. Make sure that it is there or get one or make one from sheet metal.

 

There should be a 1/4" plastic heat insulating spacer between carb base and the heat shield on the intake.

 

If you have the stock air filter add a hose from the snorkel  to in front of the radiator for cooler air than that under the hood.

 

Insulating the fuel line is also a great idea. 

 

Paint the underside of the intake with silver paint to reflect heat. Doing the exhaust manifold will also help. It's the small things that add up. If you have a header, paint silver or get header wrap.

 

Look at the front base of the carburetor on the valve cover side of the intake. There should be a fitting with a hose to a metal pipe. This pipe should in turn join another metal pipe from the thermostat housing and then travel down the front of the engine to the lowere rad hose return fitting . This MUST be connected and working. It allows coolant from the head to pass through the intake runners and in very hot conditions it 'cools' the under side of the intake by drawing away radiant heat from the exhaust manifold just an inch away. In very cold conditions it 'warms' the intake for better running. 

[510_dreamin]

I should have clarified I am running an L20b with dual Webers. 

 

I jet coated the header when the motor was put together to help with the heat but it still gets pretty warm in there especially when it’s hot outside

 

Not sure how I would go about adding the return line with the Weber set up

[banzai510(hainz)]

Your boiliing  the gas this is common.

 

wait till winter or drive on Sundays with no traffic.

 

Mine does this also  4omm Mikunis

 

 

some folks put in a cool can with dry ice  which is a can with a coil fule line and the ice will cool the gas before it goes in the carbs.

 water injection  , or really have tiny ports that suck in water to cool the piston off that works off vaccum .water tank vented

 

 

Edited August 8, 2019 by banzai510(hainz)

[wayno]

I guess I didn't insulate the hoses going to each carb, but I did from the fuel filter to the tee.

See the first tee with the wiring insulation in the photo below, see the other tee just above it in the photo, that tee is the one I welded the end of the tee shut and then drilled a 1/16th inch hole in the end(the end going over the valve cover), that tee is my fuel pressure regulator, my engine runs great, but I am using a stock Nissan 720 electric fuel pump also back at the tank.

This is the modified tee.

See the small hole.

 

You can see the return line going over the valve cover back to the line going to the tank.

 

Edited August 8, 2019 by wayno

[510_dreamin]

^^ thanks!

[yenpit]

If you are going to shop for a dual Weber/Mikuni heat shield, keep in mind that although the dual carb throat spacings are the same (one carb, dual throat) between Weber, Mikuni, SK, Dellorto etc, the spacings IN BETWEEN the pair of dual carbs varied between the INTAKE MANIFOLD manufacturers!  T3 made one heatshield a few years ago & came to find out that there were different spacings between the pair of carbs................problem was that I think the heatshield mounted between the carbs & the intake, so it fit only ONE version intake manifold, didn't work on all versions!  We build our own heatshields on our vintage race Z's, mounting on the air filter face, down & then BACK under the carbs.  Just remember to "support" the heatshield at the opposite end & it's a good idea to bend a "lip" on the sides for rigidity!  Hope that helps!

 

TJ

[510_dreamin]

Now having an issue with heat fluctuations. Stays pretty cool then when idling like at a drive thru or a light it slowly creeps up and gets hot which then leads to the vapor lock issue. I’m thinking it could be a thermostat and/or radiator cap issue?

What temp thermostat would you run on a bored L20 with dual Webers? Also my radiator cap on the aluminum radiator is a 1.1 bar maybe a 1.3 bar (18psi) may be necessary?

[datzenmike]

I would run the stock 180/185F. If the cooling system is working the thermostat keeps the temperature at a relatively constant temperature. If there is a lack of ait to take the heat away or a lack of flow through the rad to transfer the heat the temperature will go up.

 

Cooling comes down to air or coolant flow to transfer heat. Plugged rad... bad coolant flow. Bugs/dirt/damage on the outside in the fins.... bad air flow.

[distributorguy]

Heat can also come from a closing point gap that retards engine timing.  Or a vacuum leak.  Or a weak fuel pump.  Or bent float tabs in Webers.