Vaporlock revisited

BobRip · Feb 13, 2006

I tried to buy a rebuild kit for the fuel pump in January and found that they have an upgraded fule pump. I believe that it has higher pressure. You might try this before you try the electric. I am leary of the electric. In automotive fuel injection systems, they sense rotation and when the engine stops the fuel pump stops in 1 to 2 seconds. If you add a fuel pump you would not have this. A fuel leak could become pretty bad, shutdown the engine, but keep the pump going.

Bob Rip

FOURTEEN · Feb 13, 2006

( My next idea is to install an electric fuel pump. )

 Find a small, low pressure pump. Your PT won't need much help.

Locate it near the tank to push the gas, rather than near the carb to pull the gas.

Make the fuel line as short as possible.

You can power the pump off the cover fan supply.

PS: Is this a Robin engine problem?

J_J · Feb 13, 2006

When the engine quit running, did you ever take the bowl off the carb to see if any fuel was in the bowl? If your engine has the fuel shutoff solenoid, did you check to see if you still had 12v on the solenoid to hold it open.

Phils · Feb 13, 2006

It's a Robin problem, peculiar to PTs only I'm guessing. That engine bay gets pretty toasty!

I'm planning on mounting the pump (12V diaphram type) as high and far forward on the left inside of the tub as I can make fit. I'll have an inline filter between the tank and the pump, which should be a short run. I'll have the pump output hose go back to the engine just below the top lip in the tub, and about where the Robin fuel pump was I'll have the hose travel up and follow the original path from there. Except for the one foot from the tank to the pump, the rest of the fuel line will be under pressure which should solve the vaporlock problem. I suspect that increasing the power of the mechanical pump wouldn't help as the hose is still travelling under suction through the hottest part of the machine.

Thanks for the tip on power.... that may be easier than tying into the "run" circuit.

Phil

Phils · Feb 13, 2006

Hey, JJ...

I've never opened the carb. When it starts to vaporlock, I can hear/feel power diminishing. Then, after it's stopped, I can tilt the clear fuel filter and see bubbles coming from the carb side, indicating that the hose is full of vapor. And it's dang hot in there and coupled with the junk gas available in California, this problem isn't totally surprising.

Phil

SnowRidge · Feb 13, 2006

I don't know if this will help, but I just remembered that sometimes carbs have a heat isolator installed between themselves and the manifold specifically to prevent overheating the fuel. They are really just specialized gaskets. Any chance there is supposed to be one on the Robin, and it somehow got left off, or the wrong one got installed at the factory?

BobRip · Feb 13, 2006

Somewhere in the history of cars (around 1980) they started putting return lines on fuel pumps. They put a second small outlet on the fuel pump which carried some of the fuel back to the tank through a separate line. This kept cold gasoline (from the tank) recirculating into and back out of the fuel pump and eliminated vapor lock. It takes a little extra flow, but it might work on the PT. Put a T on the pump outlet and run a very small line with a restriction back to the tank. I have not tried this since my machine does not have this problem.

Bob Rip

Joe1 · Feb 14, 2006

That started in the 180's because the fuel pumps were for fuel injected cars and the pumps are in the tank. These pumps are of a much higher pressure (I think around 40 psi) than those for engines with carbs (4 - 6 PSI). Hopefully he has matched the pump to the aplication.

SnowRidge · Feb 14, 2006

Another thought: It may be the muffler(s) causing the problem. I replaced my PT built muffler with a stock Kohler muffler (same as came on Moss Road's machine) because I was concerned that excess back pressure was causing the engine to run too hot.

The PT mufflers appear to be hand built, probably with the aid of relatively crude jigs. On mine, it looked like the weldor had been sloppy and welded it up when the pieces weren't all aligned quite correctly, resulting in increased back pressure and extra heat. Since I replaced it, run-on has been eliminated, which tells me it was running too hot.

It looks like PT mufflers are of variable quality and probably with varying degrees of back pressure. You may have one with excess back pressure. That in itself would cause the engine to run hotter, perhaps enough so to cause vapor lock problems. If that is indeed the case, adding the SuperTrap would likely exacerbate the problem.

It is even possible that California's unique gasoline blend may contain more volatile materials than elsewhere, further contributing to the situation.

As I said, just a thought.

toolz_not_toyz · Feb 14, 2006

(
On mine, it looked like the weldor had been sloppy and welded it up when the pieces weren't all aligned quite correctly...)

Sort of reminds me of the way the welds look on the ROPS corners and how the canopy is welded ot the ROPS. Looks like the work was done by a six year old. /forums/images/graemlins/frown.gif

Back to the engine. To be clear, this is NOT a problem that affects all of the Robin engines. IIRC, this is the only instance reported here. Mine certainly doesn't have this problem as I pointed out. So at most, it might just be a particular production run. Jack might be able to comment on that.

If it were me, I would NOT install an electric pump. That's treating the symptom and not the problem. But worse, as was pointed out, if the engine stops running, what's going to stop the pump?