How to troubleshoot vapor lock problems

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UVsaturated

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May 15, 2008
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Dayton, Ohio
How to troubleshoot vacuum lock problems

This is a simple guide to engine fuel mixture problems due to fuel tank vacuum lock. Vacuum lock occurs with the China Girl engine kits due to having too good of a seal on the tank gas cap. These caps are designed to vent fuel vapor when the air inside the tank expands, however they often fail to vent properly causing poor performance, lean out conditions, and engine stumbling due to fuel starvation in the float bowl.

Vacuum lock is a simple condition caused by the air trapped inside the fuel tank lowering in pressure from when the last time you put gas inside the tank and having a cap that does not vent. It is most common in cold conditions, and dramatic downward temperature changes. It also happens when you go from a full tank to an empty tank with a tight cap seal. During warm weather and stable air conditions, this should not be a problem. However, riding from a high to low altitude can also cause this, although I have no first hand experience with it due to my local geography.

The good news is it is a very simple fix. First though I want to discuss what symptoms you should look for. If you are sure of these following factors, and you have no other fuel problems with your engine, then you should pursue the solution to vacuum lock, which is simply opening the gas cap on your tank and closing it again, equalizing pressure. Here is what to look for first in ensuring you aren't having other conditions mimicking vacuum lock. Fuel leakage from the fuel line causing low fuel level is something that will cause similar symptoms. If the fuel line around the fittings are dry and not wet and you do not see fuel pooling underneath the carb, then this is not your problem. If you use the 'tickler' device which taps on the float to get it unstuck, and this does not alleviate your problem, then you know it could be vacuum lock. If your filters are clean and you have checked them, then it most probably is vacuum lock. Lastly, make sure the fuel shut off valve is open. Definitely this will make your engine lean out just like vacuum lock will. Now if all these other things have not solved the problem, open that gas cap and purge the air and reseal it. Give the fuel 15 seconds to fill the bowl back up and it should start right up. Oh, and one more thing; check the choke setting often and know which way is closed and which way is open. If the motor is warm, it should always be open. It only takes less than 30 seconds for the engine to warm sufficiently to open the choke.

Have fun riding.
 
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2door

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UV, I'm not ragging on you sir, just clearing up some facts for you so you'll have a better understanding of the principles involved.

I'm going to have to correct you on several points.
If the fuel tank cap doesn't vent, as it is designed to do, the fuel will not flow from the tank due to a 'decrease' in tank pressure. It's the same principle as putting your thumb over the end of a straw and lifting the straw out of a glass of liquid. The liquid will remain in the straw even when it is completely out of the glass. Try it. The fuel supply for a gravity feed system relys on atmospheric pressure, equalized between the outside and inside the tank. The term 'vapor lock' is not accurate in this case. True vapor lock is typically caused when a fuel line or another part of the fuel delivery system is overheated and the gasoline actually boils forming a vapor bubble which doesn't flow but stops the normal flow of fuel. This usually happens in times of higher ambient temperatures.

In the case of a non venting cap the fuel stops flowing because the outside or atmospheric pressure is higher than that inside the tank.
The fix is either replace the defective cap or drill a small (very small) hole in the cap somewhere that will allow the pressures to equalize but not allow fuel to exit. The caps are supposed to vent but some get through that do not have a vent as it was designed to have hence the problem you've encountered.
Removing or loosening the cap allows the pressure in the tank to increase to match atmospheric pressure and the fuel will then flow until enough fuel has left the tank to once again lower the pressure and the flow is stopped.

Fuel line leaks will not effect float level. The float level can be adjusted internally by bending the tangs that the float rests against to raise or lower the level of fuel in the bowl.

The 'tickler' or primer is there to push down or sink the float thereby allowing the bowl to overfill which will produce an overrich condition that sometimes, under certain conditions allow the engine to start easier. It's not to unstick the float.

The altitude really has no effect on the engine except that at higher elevations above sea level the performance is lessened somewhat due to the decrease in atmospheric pressure and there is less air density entering the carburetor/engine. An engine must be tuned, fuel/air mix leaner, for higher elevations to achieve the proper burn. At lower altitudes down to sea level the air density is higher so you can mix more fuel with it, or go richer on the fuel/air mix to get that effecient burn we all want.


Tom
 

UVsaturated

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May 15, 2008
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I do understand it just fine, but may not have used the same terms and expressions as you have. When the air is cold, as I have been using it in 15F weather the cap is prone to not vent for whatever reason. I know drilling a hole in the cap can solve this, but can also leak fuel if the bike is upset. It takes 2 seconds to open and purge the tank versus 10 minutes drilling a hole in your cap.

Altitude differences has a huge effect on the engine. 10,000 ft will reduce the rated horsepower of an engine by almost half of one jetted for sea level. If your fuel line leaks enough that gasoline is pooling on the top of the case, chances are at WOT your bowl will run dry considering all things.

The tickler does have a purpose and overfilling of the bowl serves no purpose. It doesn't need an accelerator pump or a primer. The choke works fine in sub freezing temps. The tickler is used when debri gets stuck in the needle and seat valve and this is mainly for unsticking this valve. Are you saying you want to flood the engine, too rich by overflowing the bowl? That is absurd when you have a choke which makes a rich mixture by reducing the airflow.

Let me correct you. The tickler is NOT a primer. It does not pump additional fuel into the carb. If you overfill the bowl in this way you run the high risk of fouling out your spark plug. Bad advice.

I'm not trying to dog you either. You do have one point but I should have rephrased it. Fuel leaks do not in fact change the float level, but the fuel level. Just a minor point of incorrect wording on my part. The fuel level will run low at WOT if the leak is large enough. Also, vapor 'lock' may be the incorrect term and maybe I should have used the term 'hydraulic locking'. The air in the top half of the tank is under a vacuum due to pressure differential between it and outside pressure and a cap that cannot vent. Thanks.
 

biknut

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Sep 28, 2010
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I think you're talking about 2 different things. Gas tank venting is one problem we're all very aware of. As you know the vent hole is in the gas cap. A lot them don't have one. In that case your bike will only run about 50 yards before the motor dies. The fix is to drill a hole in the cap, or get a new cap that has a vent hole. After that it's always going to vent no matter the temperature, or altitude changes. There should never be a need to burp the tank after that. There's no difference between a properly vented tank, and a tank without a gas cap, or a loose cap.

Vapor lock is something totally different. It's caused by heat. I've noticed NT carbs seem to be particularly susceptible to it. When you turn off a hot motor, the gas in the float bowl seems to overheat. Exactly what happens after that I'm not sure, but somehow it interferes with the float bowl filling properly, and causes the motor to run like it's starving for fuel for a while till it corrects itself. Loosening your gas cap when it's having that problem has no effect on it.

I think maybe the tickler is supposed to help speed that process up.
 

2door

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I tried. Whatever you want to believe sir. If it works for you that's the way it should be.
Sorry if I ruffled any feathers. It wasn't my intention.

Tom
 

UVsaturated

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No problem, as I said, I have used the incorrect term. Thanks for pointing that out. My cap doesn't vent and it caused the fuel bowl to dry up. I have changed the original post to 'vacuum lock/ing' as this is a more correct term.
 

Bikeguy Joe

Godfather of Motorized Bicycles
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I tried. Whatever you want to believe sir. If it works for you that's the way it should be.
Sorry if I ruffled any feathers. It wasn't my intention.

Tom
I believe you were right...and that works for me. :)

Also, you didn't ruffle my feathers by making proper corrections.
 

Bikeguy Joe

Godfather of Motorized Bicycles
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Finally- a tickler is not for getting crud unstuck from the float seat, it's to (lightly) flood the carb for easier/quicker start up.
 

UVsaturated

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May 15, 2008
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Finally- a tickler is not for getting crud unstuck from the float seat, it's to (lightly) flood the carb for easier/quicker start up.
How long do you feel it takes for the fuel bowl to fill up? It sounds like it was invented then for a very impatient person, because mine fills rather quickly. Have you ever had a needle and seat valve stick, whether it be from dirt or just the nature of the valve? In either case, that is exactly what it will do, but I see no reason to need a 'tickler' if your fuel delivery is acting normal. From the time I open my fuel valve to the time I have pedaled fast enough to release the clutch, my bowl has fuel. How much quicker need it be I wonder?
 

Bikeguy Joe

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It's not for impatient people to fill the float bowl, it's to "overfill" the bowl slightly to flood the carb and cause an over rich condition for easie(r) starting.

If you have ever had an older British or Italian bike, you'd know how helpful this is.
 

UVsaturated

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May 15, 2008
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It's not for impatient people to fill the float bowl, it's to "overfill" the bowl slightly to flood the carb and cause an over rich condition for easie(r) starting.

If you have ever had an older British or Italian bike, you'd know how helpful this is.
Well, you're right, I've never owned a British or Italian bike and cannot see the reason to do this considering you run the risk of fouling out your spark plug. This is entirely what the choke is for, and have never had any problems starting this engine using the choke in 15 degree weather. So obviously then, if your theory is correct, then it is for starting your engine when it is colder than 15 degrees? I'm just trying to find a trail of logic on this one. This is like someone dumping gas down the carb of an engine to get it started, yet you only do this when there is no fuel in the carb. Do you personally encounter this often? If so, why is your fuel bowl so empty when you start it?
 

Bikeguy Joe

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lol. Yes, exactly. Two strokes LIKE a bit of extra fuel, as do some four strokes.
The choke does well enrichening the engine. BTW- what do you think an "enrichening circuit" does? It's not a choke, but a way of dumping raw gas into the intake.
 

Bikeguy Joe

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Finally, it's not that easy to "foul a plug" with raw gas in an engine with good spark and compression.

In fact, more than once I've removed a plug, poured gasoline right in the plug hole, and gotten a stubborn engine to fire. I just did this with a Ford tractor that defied starting any other way. I have done it with two strokes and four strokes.
 

UVsaturated

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Well this is the same as pumping your gas pedal using an accelerator pump on a car, but I've owned cars that would not start without using this method. I don't have any trouble starting my engine just using the choke. It starts extremely well without using the tickler. So again, I ask you, does your engine need this? Do you have trouble starting your engine with just the choke?

Also, tell me one more thing. If you do get debri lodged in the needle and seat valve (forgive me, I am only running the factory filter and not a second inline filter) then the valve will stick, either open or partially restricted. The tickler can be used to dislodge the valve. It is a multipurpose device. I don't think you or I can clearly say its absolute intention without hearing from the guy who designed it. I've written the Flying Horse Engine factory several times so far about different design features without any answers from them. They aren't much help unless you want to buy a bunch of their kits.
 

Bikeguy Joe

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Yes, now your getting the concept!
I have had bikes that liked it, and some that didn't need it.
I had one that ran fine, but liked a bit of choke if you left it for 5 minutes.

Originally, the tickler was for "flooding the engine". It was not designed to dislodge debris, as they don't design for stuff that's "not supposed to happen". :)
 

UVsaturated

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May 15, 2008
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Why not put a primer on it instead? Makes more sense. The tickler device is akin to beating on the side of your computer because it is slow or something.

One problem with the HT motor is you don't get a good spark unless you have a good CDI, plug wire, and magneto. The advertised compression is 6:1 according to the manufacturer and many people have mentioned a plug gap of around .010 less than the factory spec because of it. I don't doubt a Ford tractor would have a good ignition system though being that it uses a conventional charging system, battery, coil, points, etc., especially if it has been converted to 12V.

The ignition systems on these bikes are somewhat lacking. It would be nice to see the white wire go towards charging a battery, but I am not sure it has enough power to do much of anything.