Vapor lock?

[cwlindstrom]

I own a 79 GS750L I can ride it all day long as long as I keep it below 70MHP. If I push it above 70MPH for more than 10 min it starts to act as if it I am running out of fuel and then dies. If I let it sit for 5-10 min it starts again but onlt when fully choked and then it takes a few minutes to warm up. Seems like some kind of vapor lock problem. Has anyone run into this before?

[chuckycheese]

gas cap

When that happens, you might want to immediately open your gas cap to see if it sucks in a bunch of air. If that's the problem, you can't miss it...it will be really obvious. It certainly sounds as though that could possibly be it. It's probably not...but it's easy to check. Good luck and let us know!:-D

[aosmith]

what oil are you running? have you ohmed out your stator

[cwlindstrom]

Thanks for the replies. I will try the gas cap and the Stator. I am heading out for a week long cruise in the morning. I'll let you know what happens.

[tfb]

If you have an inline fuel filter installed, THROW IT AWAY. They are common culprits for the dreaded vapour lock.

[twistedwankel]

I own a 79 GS750L I can ride it all day long as long as I keep it below 70MHP. If I push it above 70MPH for more than 10 min it starts to act as if it I am running out of fuel and then dies. If I let it sit for 5-10 min it starts again but onlt when fully choked and then it takes a few minutes to warm up. Seems like some kind of vapor lock problem. Has anyone run into this before?

Assuming you don't have an inline gas filter that got dirty I would guess that you have a significantly plugged screen on your petcock inlet. You might put it on Reserve which would open up the lower screen intake and try your 70MPH thing again. Either way as old as your bike is I would recommend pulling off the petcock and cleaning those screens off. While at it rinse out the tank with clean gas.

As has been mentioned repeatedly on this site. You cannot get vapor lock on a gravity feed system.

You are running the carb bowls dry at the higher speed is all it sounds like. Eventually you will go slower and slower and slower...you get it.

Ethanol added gas will strip the rust and goop off the tank walls too.

Good luck

[chef1366]

Mine did the same thing when the pecock was bad. Just get a new one if the tank has good venting and you don't have an auto style fuel filter. Also your vent lines between carbs 1&2 and 3&4 cannot be exposed to wind or you will experience fuel starvation. These lines either need to go away (if running pods) or run in between the frame and the airbox.

[tfb]

You cannot get vapor lock on a gravity feed system.

Twistedwankel, please explain.

Because what else should we call it when a bubble trapped in the inline air filter expands due to the heat from the engine, and mostly fills the filter body, so blocking the fuel flow? "Vapour lock" is a logical term for this, I would have thought: a bubble of fuel vapour is locking the fuel flow.

Semantics aside, and as many on this site will agree, if you've got an inline fuel filter, throw it away! The fine nylon gauze filter inside the tank does a brilliant job, so there's no need for more troublesome clutter along the fuel line.

[Killer2600]

Twistedwankel, please explain.

Because what else should we call it when a bubble trapped in the inline air filter expands due to the heat from the engine, and mostly fills the filter body, so blocking the fuel flow? "Vapour lock" is a logical term for this, I would have thought: a bubble of fuel vapour is locking the fuel flow.

Semantics aside, and as many on this site will agree, if you've got an inline fuel filter, throw it away! The fine nylon gauze filter inside the tank does a brilliant job, so there's no need for more troublesome clutter along the fuel line.

That vapor bubble won't block fuel flow in a gravity feed system, the only vapor lock that we can get is from a tank vent restriction which prevents fuel from leaving the tank.

[twistedwankel]

Twistedwankel, please explain.

Because what else should we call it when a bubble trapped in the inline air filter expands due to the heat from the engine, and mostly fills the filter body, so blocking the fuel flow? "Vapour lock" is a logical term for this, I would have thought: a bubble of fuel vapour is locking the fuel flow.

I would call that a "plugged filter". The gas in the line beyond that blockage would slowly drip into the carb bowl which is vented to atmosphere thus emptying it of all "heavy liquid". Call it a "bubble" if you please or a "vapour lock" if you please but it is a "physical blockage" in this case caused by filth on the surface of a filter (not by temperature and vapor pressures).8-[

[jimcor]

Mine did the same thing when the pecock was bad. Just get a new one .

Same story as my bike. Petcock. As many suggest rebuilding is usually a futile effort. Rebuild kits have wrong and missing parts. Spend the 40~$70 and by a new genuine Suzuki part. Many claim they last about 20 years. Mine needs a new one again and the current one has only been on the bike for 12 years.

[tfb]

That vapor bubble won't block fuel flow in a gravity feed system

Well, in my experience -- and the experience of many others on this forum -- it does block the fuel flow. Maybe not absolutely, but it will restrict it to the point where you get the symptoms which cwlindstrom describes. It's happened on my Katana and on a GPz900 I used to have. Each time, it was ditching the filter which fixed the problem.

In fact, maybe if we didn't have a gravity feed system on our bikes, but had a fuel pump helping push the fuel through, then the inline filters wouldn't give us the grief that they so often do with (and I'm going to persist in calling it this!) vapour lock.

Debate aside, all I'm really saying is that if cwlindstrom has an inline filter installed, there's a very good chance it's the problem. Therefore, throw it away!

[twistedwankel]

In fact, maybe if we didn't have a gravity feed system on our bikes, but had a fuel pump helping push the fuel through, then the inline filters wouldn't give us the grief that they so often do with (and I'm going to persist in calling it this!) vapour lock.

So you take a soda straw (tube) in a glass of any liquid. Put your finger over the end and lift it out of the glass. The liquid remains magically suspended within the tube.

[tfb]

That's not confusing! A drinking straw is just narrow enough for capillary action to hold the liquid in the tube -- but that's not a force that's going to apply to something as wide as a fuel line.