Vapor Lock

Neighbor has a 641 and having trouble vapor locking. Let set and cool off then runs OK until it warms up again. Any ideals? Thanks

Not running after running for a while can have multiple possible causes. When it is having the problem has he checked for good spark? Has he tried loosening the gas cap to see if it is a venting problem?

Well, since it does not have a fuel pump to pull down below atmospheric pressure, it is not technically "vapor locking", but maybe the fuel is boiling? Is it that "warm"? A weak spark will exhibit the "runs cold but not hot" symptom. too. Need to find out which is missing when it stops.....fuel or spark. Just warming the copper windings of a coil from room temp to full operating temp results in a spark energy drop from 100% to 56%, so if starting out weak, it stops being strong enough to run when warm.

Does it still have the factory fuel line and heat shield?? If either one is not factory that could cause the problem big time due to gas boiling in the line or tank or both or pressure build up in the tank due to a clogged vent

Gas cap vent may have gotten stopped up,
Insure your gas cap vent is open before you spend a lot of time and money on this problem.

Some one could have changed fuel tank caps to a non vented cap, insects could have plugged vent over winter.

Do you have an inline fuel filter that was added, not factory installed, if so replace it, be sure to use a metal filter.

Also, check your air filter breather tower, mice could have built a nest inside stopping it up, cutting air flow.

At this point, everything is on the table.

At one time I had a coil go bad, would run a while, coil would get hot, engine stop, when it cooled off, engine would run again.

Then go from there,

I messed up and installed an inline paper fuel filter on my Jubilee. When fuel temp got to around 145 you could see an air bubble begin to fill up the see through paper filter. It wasn't long after, carb ran out of fuel. When gas cooled, you could see it start to flow again. So do you have an in line filter?

I had the same problem on my 8n ford. I took the plug out of the carb and no gas. let it sit for a spell, she fired right up and ran the it would do it again. same results with the plug so after I installed a new carb kit it it fixed the problem. Back in the 50s on some ford cars you would have that problem We used to wrap the fuel line with tin foil and that would fix it. Unless your fuel line is real close to the motor I doubt if it is vapor locking. norm

Like JMOR said, without a fuel pump it cannot vapor lock.

That said, you can have issues with fuel boiling in the tank and/or fuel line with modern gasoline.

Heat shield MUST be in place.

Avoid gasoline with ethanol if possible.

Are you CERTAIN that it is not the ignition switch?

Dean

(quoted from post at 23:25:59 06/09/15) Like JMOR said, without a fuel pump it cannot vapor lock.

That said, you can have issues with fuel boiling in the tank and/or fuel line with modern gasoline.

Heat shield MUST be in place.

Avoid gasoline with ethanol if possible.

Are you CERTAIN that it is not the ignition switch?

Dean

Click to expand...

es, Dean, every time I see that term used in reference to an old gravity fed tractor, the hair on the back of my neck stands straight up! :x

Check to see if fuel line is too close to the exhaust manifold. Wrap fuel line with tin foil. Put some fiberglass insulation between heat sheild and fuel tank.

(quoted from post at 05:08:56 06/10/15)

(quoted from post at 23:25:59 06/09/15) Like JMOR said, without a fuel pump it cannot vapor lock.

That said, you can have issues with fuel boiling in the tank and/or fuel line with modern gasoline.

Heat shield MUST be in place.

Avoid gasoline with ethanol if possible.

Are you CERTAIN that it is not the ignition switch?

Dean

Click to expand...

es, Dean, every time I see that term used in reference to an old gravity fed tractor, the hair on the back of my neck stands straight up! :x

Click to expand...

Yes, suddenly gravity takes a holiday and liquid floats on top of gas.

Make sure you have the needle and seat installed from the later 172 engine. The seat has quite a bit larger hole than the one for an older 134

Guys, I dont get this statement... vapor lock is when you get bubble of air in the fuel line. ie vapor has locked the line. so either way it happens, its a vapor a lock... So what am I missing here? Boiling gas forms a bubble of air and "vapor locks" the fuel flow. A fuel pump will actually help stop the vapor lock by pressuring the fuel line and thus greatly increasing the boiling point of the gas. So the addition of fuel pumps and placement of the fuel pumps lower on the engine meant that they got cooler fuel on the suction side and put out pressurized fuel on the hot side. Later in 50s pumps went from 3 to 7 lbs to as high as 13 pounds to even improve this, but the floats on the carbs had to be improved for this to happen. But a vapor lock is a vapor lock no matter where and how it happens???? Design of fuel lines so that there was no HIGH point to trap the vapors also improved this so that bubble would not be trapped in the high spot and the flow would naturally bleed out the line. So fuel line routing and design also helped along with the carbs being move to the TOP of the engine. The early up draft carbs by the low location meant the bubble was guaranteed to be trapped up higher unless the line was routed correctly to "bleed" itself back to the fuel tank and give the bubble an unrestricted path to self bleed back.

And thoughts here

Bill, a bubble in the line won't stop flow in a gravity feed system. The weight/pressure head will push the fuel past the bubble, compressing it if need be or pushing it or part of it out the line. Get a bottle, length of clear tubing & play with it.

I'm with JMOR and the others on this. I don't see how a gravity fed system can vapor lock. The vapor will rise, the liquid will sink.

About 12yrs ago I was a member of this forum under another name that I can't remember, thus this new name. And I can remember people arguing with me over this topic. It would always seem to be the topic of the month once in a while.

You may have a problem, but it's not vapor lock. Good luck with it.

Gil

I would agree untill heat is involved and the bubble has enough pressure to displace the liquid. and this pressure trapped in a high spot in the line will stop all flow both directions per your drawing.

Gravity only has so much pressure to push the fuel down.. And a pressure bubble of expanding gas due to heat can and will find a trap or high spot in the line if it exists. one lb of pressure can stop the vertical weight of two feet of water. A rough formula we used to pressurize underground cables to keep water out. We tried to keep 6 to 9 lbs on all underground paper insulated telephone cables so that we could keep water out. 6 lbs would protect a cable in a underground conduit run up to 12 feet deep. Most cables were never more than 10 foot due to 3 foot of cover and a 7 foot tall manhole. Again a pressure bubble in a high spot will sit there and stop all flow on a gravity system if I am thinking correctly... And if the source of heat is at or near the high spot, you keep the pressure generating and thus keep the block.... and therefore you expirence vapor lock... until the heat is remove or you force the bubble out with a greater pressure from either side.

And a fuel pump working properly will ad that pressure to one side, if its located away from the heat source.

In the telephone example, you had a closed tube (cable) to which you applied pressure from an external source. In the gravity feed system/tractor, the only pressure your bubble can sustain is that of the column of liquid above it. Heat it up & it is still the same pressure, just a larger volume of bubble.