Vaporlock revisited

[Phils]

One recurring problem with my PT (and others) has been vaporlock on hot days during extended use. I wrapped the stock fuel hose with "insulating hose" (larger diameter hose slit and wrapped around the original hose). That helped but didn't eliminate the problem, just made it take longer to happen. When it does happen I can see air bubbles in the clear fuel filter so I'm sure it's a vaporlock situation.

Late last year I replaced the cheap small plastic tubing that PT uses for fuel line on the 422 with standard automobile fuel hose, including a larger clear fuel filter. I thought it eliminated the problem, but, again, it just made it take somewhat longer to happen (or so it seemed).

There was mention made in another thread that maybe my Supertrap muffler was heating the engine tub and contributing to the problem. I've done measurements with an infrared thermometer and find the heat in the tub evenly hot, not especially hotter in the muffler area.

The past few weeks I've noticed that the vaporlock problem on mine has returned (did it ever go away?) but is more a function of time vs heat, as it's not been that warm around here. It could have ALWAYS been a time function vs heat for all I know.

Knowing that the outlet from the tank probably had pieces of sealant plugging it up again I've just been waiting for the tank to get low enough to pull the elbow without draining the tank. I did that yesterday and found (again) little chunks of white sealer that I removed. Great! Problem fixed again!

An hour later the PT is shutting down due to vapor lock again. Are there even MORE chunks of crud in the tank outlet? I haven't added any more gas so it'd be easy enough to pull the two bolts, tilt the tank up and check the elbow.

I'm pondering my next action while concurrently waiting for the machine to cool down and I hear the tiny but continuous sound of air venting. It's coming from around the gas tank cap. When I remove the cap there's the telltale "whoosh" of a pressure differential being equalized. And then the PT starts and runs fine.

Gasoline "boils" in a vacuum much easier than at atmospheric pressure. Have I been fighting something so simple as a fuel cap that doesn't vent properly?

I think that I'll be choosing a cap with a 1 or 2 pound pressure relief. I suspect this cap has a much higher setting. It'll be awhile until I can get to the parts store so in the meantime whenever I pause during some chore (stump grinding, mowing, etc.) I'll equalize tank vacuum by a quick twist of the fuel cap. Maybe I can finally say "goodbye" to vaporlock?

Phil

 

[KentT]

IMO, your gas tank cap shouldn't have ANY pressure relief setting. In fact, it should never build pressure OR vacuum inside it. What may be happening is that it builds vacuum over time, as the level of the gas drops in the tank as it's being consumed -- not pressure buildup from heat-- and the gravity feed to the fuel pump (if there is one -- many/most small engines don't have them) can't offset this partial vacuum...

I've had this happen on old Simplicity tractors, which have a "weep hole" in the gas cap. When that gets clogged, the engine would starve for gas...

I don't know how these PT caps work since I've not encountered your problem.

 

[Gravy]

Phils,

I think you may have figured it out. I sure hope so, because my 425 seems to be doing the exact same thing. It happened again just this morning. About 30 minutes after I started plowing snow, it sputtered and died. I had to let it sit for a few minutes before it would run at full throttle again.

I have previously always blamed the problem on heat, but I really don't think that was the problem today.

Next time it does it, I'll pop the gas cap and see what happens. If that fixes it, I'll make sure the tank stays vented.

Thanks for the info!

Dave

 

[J_J]

I had a similar situation on my U-Haul trencher. Engine would run for a while, and shut down, like fuel starvation. Cleaned the carb, new filter, new line, it still ran awhile and shut off. One day I cranked it up with the fuel cap off, and it continued to run. Checked the cap for the tiny vent hole, and it had rusted over. Bought a new cap and now works fine.

You just might have found your problem.

 

[pajoube]

I ended up drilling the vent hole just a tad larger on my cap.
So far it has been fine.
PJ

 

[SnowRidge]

Anytime a gasoline engine starts dying after it has been running awhile, I suspect the gas tank venting system (in this case, simply the cap). It is usually the easiest thing to check, and it is often the culprit.

 

[ksmmoto]

When I did small engine repair, I always filled the gas tank, checked for spark and checked the gas cap vent before anything else. Big problem here in Michigan with mud wasps, they LOVE to plug small holes.

I have had the mud wasps plug gas cap vents as well as carb vent lines on my equipment and on customers. They also have plugged my pressure washer so I am forced to cap all the fittings when not in use.

ksmmoto

 

[Phils]

Well, DANG!

I jammed the gas cap relief open, now neither pressure or vacuum can be in the gas tank.

After an hour or so of tractoriing, the PT still vaporlocked and quit running. I was really hoping this tank vacuum was the problem.

My next idea is to install an electric fuel pump. I'll also re-route the fuel line and eliminate the Robin mechanical pump. Electric pumps were the only way I was able to completely eliminate vaporlock on a Jeep I had for decades. Worth a try at this point. No irreversable damage done if it doesn't solve the problem. I'll keep you all posted and probably take some photos of the install. Got to get it done before mowing season gets here.

Phil

 

[toolz_not_toyz]

That's weird. Although it's only been up to a high of maybe 60F here for the past several days, I've been running the PT for at least four hours at a time digging and moving large bucketfulls (using my 4n1) of dirt without any problems. All I've had to do is keep on top of lubrication as the lift arms started to moan at the end of day 2 or 3. I have the Robin engine too and love it.

 

[J_J]

Have you tried a fuel pressure gauge hooked to a tee in the fuel line. The diaphragm in the mechanical fuel pump may have a hole in it. An electric fuel pump should help. Make sure you get one for low pressure. Is the fuel solenoid shut off free of trash?