Vapor Lock?

[Remeber Me]

My '99 has a tendency to act like it's starving for gas if I run hard then stop and sit at a light for a bit. I believe a friend had a vmax that did the same thing till he got it fixed. I forget what the problem was but it may have been something like vapor lock. Anyone ever heard of this and know the fix for it?

Hal

 

[Buster Hymen]

My '99 has a tendency to act like it's starving for gas if I run hard then stop and sit at a light for a bit. I believe a friend had a vmax that did the same thing till he got it fixed. I forget what the problem was but it may have been something like vapor lock. Anyone ever heard of this and know the fix for it?

Hal

Take your gas cap when this is happening and listen to see if it is sucking air when you remove it. If it is then you do have vapor lock. I can't remember the fix at the moment, but someone should chime in with the answer.

 

[Remeber Me]

I'll try that diagnostic, Buster. Thanks!

Anyone with a possible fix if it is vapor lock?

 

[maleko89]

Try removing the vent line including the tip over valve and clean them out. Tip the valve back and forth and make sure it opens/closes.

Mark
#1098

 

[Remeber Me]

Thanks. I'll try that this weeken.

Hal

 

[forestdaledave]

I have been a professional mechanic for 40 + years and I have never seen vapor lock. Vapor lock is when the fuel line gets so hot that the gas turns into a vapor. Fuel pump can't pump a vapor. As in old mechanical pumps and metal lines run too close to an very hot engine. You are not going to get vapor lock on a vmax because the electrical pump is located away from heat and sends the fuel to the carbs. As you use the fuel in the tank air has to replace the lost fuel or your tank will be drawing a vacumm. A fuel pressure gauge, a tee after the pump and a test ride will tell a lot.

 

[Remeber Me]

Wasn't vapor lock. It continues to do the same thing. I popped the gas cap off and it runs the same. I noticed through the clear fuel filter that the level never changes and the exhaust reeks of gas so I don't think it is a fuel issue. It also sounds about the same at every rev level and idles 300 rpm slower than normal. I sprayed carb cleaner around the carbs and the revs didn't change so I don't think I have a vaccum leak. I already changed plugs and that did nothing. Out the old plugs only one looked good, the rest were too black.

I think the next thing to check is for an electrical issue. It looked like rain today, plus the playoffs, so it will have to wait till next week. I'll pull plug wires but I think it will really stumble then. Any other ways to check for an electrical problem or did I miss something else to test first?

Hal

 

[Buster Hymen]

I have been a professional mechanic for 40 + years and I have never seen vapor lock. Vapor lock is when the fuel line gets so hot that the gas turns into a vapor. Fuel pump can't pump a vapor. As in old mechanical pumps and metal lines run too close to an very hot engine. You are not going to get vapor lock on a vmax because the electrical pump is located away from heat and sends the fuel to the carbs. As you use the fuel in the tank air has to replace the lost fuel or your tank will be drawing a vacumm. A fuel pressure gauge, a tee after the pump and a test ride will tell a lot.

OK, technically it may not be vapour lock but it acts the same as it and is a vacuum lock. I have personally seen it on 3 bikes.

 

[maxcruiser]

It sounds to me more likely a fuel/carb issue rather than electrical. Have the carbs been taken off to be cleaned, etc... 3 plugs were black and one ok is why I`m thinking carb related.

What`s the mileage of the bike and the last time it had a complete tune up. It maybe a good time to do a general overhaul.

I did one my 96 max when it had around 80,000 miles and found the following; 2 intake boots and 1 vboost boot had cracks, 3 electrical connections had corrosion, one spark plug wire was weak and the intake pressure hose was starting to crack. There was other small issues that I found when going through bike, from front to back. The bike ran like a new one after that.

:cheers: Mike

 

[Remeber Me]

The bike only has 8.7k, looks like new, and was running great then right after a quick run it started running like crap. I'll check the compression in all the cylinders in the unlikely event it cracked a ring or something.

Is there any way it could be bad gas? I had just put in gas 5 miles earlier. I looked in the tank and didn't see anything strange, the tank was shiny with absolutely no rust or anything.

Hal

 

[Robbarrie]

Sounds dumb but did you check (to read) the plugs ? as you checked the compression. If it was compression, you would have a hard time starting and keeping it going - for sure. Now is it only while your on the throttle or is it running like crap all the time now ? What RPM range ? Is the fuel filter ok ? What about the coils or timming. You need to narrow it down more if you can. Anything come undone or maybe cracked or broken.....it happens. Do you suspect, an electrical fault or fuel and carb issues or is there anything else that you can think of or that you noticed ?

 

[Remeber Me]

It just started doing it after a quick run. It never hiccuped or anything on the pass, just a normal quick run up the gears then slowed down for a light. Turned around at the light and it was immediately apparent it didn't want to go that well. Plugs were swapped with new of the same after the problem started.

It effects it at all rpm ranges. Even makes it idle a little lower than normal.

Timing? I'll have to check that. I was thinking it could be a coil but that's always hard to do. I might just drain this tank of gas and put in new stuff because it's just too suspicious.

Hal

 

[Remeber Me]

Finally got motivated to diagnose some more on the bike. I've checked the fuel pump/filter and spark and checked the battery. The spark seems a little weak. Very thin spark but it does happen on all 4 plugs. I just don't get why it would run so poorly while getting plenty of gas and spark.

Anyone know of a good shop in or around San Jose, CA?

 

[shawn kloker]

Check the fuses good.I had a turn signal short out and it blew a fuse and the bike ran like you are describing.

 

[Remeber Me]

Funny but I was at a turn light right before it started running like crap. Was it a turn signal fuse?

 

[cutlass79500]

My '99 has a tendency to act like it's starving for gas if I run hard then stop and sit at a light for a bit. I believe a friend had a vmax that did the same thing till he got it fixed. I forget what the problem was but it may have been something like vapor lock. Anyone ever heard of this and know the fix for it?

Hal

is this a reaccuring problem or did it just start. if it happens all the time floats could be a little high with the extra heat and if the fan is running might be spilling over a little .usually you can tell by shifting it really hard it will nose dive for a sec. i went thru stale gas about a month ago mine didnt idle right and seemed like it was starving for fuel would even pop back thru the carbs.i know its impossible but a vac guage would tell alout hopefully a valve isnt to tight or sticking .just be carefull. if it gets worse or starts smelling like gas all the time you dont want it to hydrolic when you start it very bad thing tends to bend rods and other things best of luck

 

[Remeber Me]

It just seems the problem is getting worse and worse. Now, it idles around 500, can't rev, and when driven slowly you get about 50 feet before it's really starving for fuel. I went half a block and it died so I turned the key off then back on and the fuel pump had to run for a few seconds to fill back up the carbs. I think the floats are ok because otherwise the pump would not have run. I kind of think it's the pump again though it does fill the bowls when turned on. Nothing is obviously wrong at all. It's like the only time the pump works is when the bike is first turned on. Anyone have a spare pump?

 

[gleno]

First thought is a faulty fuel pump relay or fuel level sensor which feeds the relay.

The fuel pump is working at least at ignition on so fuel may not be the problem.

Second thought is a thin weak spark seems to be the common theme here.

You should have a fat spark.

If you pull the plugs when this happens and they are wet or sooty, you probably should start with electronics testing.

I had a problem very much like this with very similar history. Turned out to be the voltage regulator going bad.

Is your battery getting a full charge when the motor is running? or gradually discharging over a few weeks or days? Does your headlight go duller at idle and brighten when the RPM's are increased?

This was my earliest symptom. I thought I had a bad battery or ground connections but they were checked and no real problems found. That didn't really make sense to me though because like you, the onset was pretty acute.

The regulator finally shorted completely on a WOT sprint.

If all four leads give a thin thready spark that would clinch it in my mind.

Work back down the lines and check regulator voltage output. If it is under spec you may also want to check the power and ground connections as well as the three AC inputs from the alternator to make sure the problem is the regulator and not the alternator.

Hope this is helpful.

 

[Remeber Me]

Even though it seems like a gas problem that thin spark is very odd. I can see how a bad regulator would cause both a low spark and a weak pump. The voltage regulator is easy enough to check/replace so I'll do that next.

So I realized the obvious test for the regulator/alt is just checking the charging voltage and it's around 13.5 which means the regulator is a little low. I'll probably just swap that out.

Hal

 

[COP RUNNER]

what ever happened ? ( to fix the bike ) ?