Vapor Lock?
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=1527
Printed Date: November-22-2024 at 4:03pm
Topic: Vapor Lock?
Posted By: Siveck
Subject: Vapor Lock?
Date Posted: April-25-2005 at 7:28pm
I've been having an issue with my '75 Nautique with the 351W that seems to be heat related. The boat runs very well for about 45 minutes or so and then it dies as if the fuel supply is shut off. I'm almost certain that the problem is that there is a vapor lock in the steel fuel line from the filter to the carb. The water temperature never goes above 180 degrees. After letting the line cool for a few minutes with the doghouse open, the boat cranks right up after a couple of squirts from the accelerator pump. It has been doing this for as long as I have had it and I just recently changed the entire ignition over, so I'm almost certain that it is fuel related. I'll verify the spark next time with a timing light when it happens. Has anyone else experienced this problem? I tried a piece of rubber hose, but had the same issue. Maybe the fuel is boiling in the bowl? Do I need a heat isolator for the carb? The last time that this happened was when I got up to around 4000 rpm for a few moments. It always seems to be starved for fuel at that rpm. I probably need a different power valve, but that is probably a different issue- and I hardly ever run the motor to 4000 anyway. Any ideas or experiences with vapor lock or boiling fuel?
Anthony
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Replies:
Posted By: Jim_In_Houston
Date Posted: April-25-2005 at 8:38pm
The only times I have had these exact symptoms were the result of a faulty coil or something in the ignition. I suppose a ballast resistor could be opening up. Could your tach be going to ground and be bringing down your ignition?
I rebuilt a carb 4 times once because I was sure I had a fuel problem. Even that turned out to be a hidden broken ignition wire. It would open and any movement would cause it to close and the problem would temporarily disappear.
The only vapor lock problems I have experienced came AFTER I turned off the engine. Fresh fuel pumped into the carb should be cool. The fuel will heat and vaporize after the engine is stopped and heat transfers into the fuel pump and fuel line.
Do you have an electric fuel pump?
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Happy owner of a '66 and a '68 Mustang
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Posted By: SS 201
Date Posted: April-26-2005 at 7:41am
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If its vapor just cut the line from the pump and insert a piece of rubber hose, This will stop vapor lock, if infact that is the problem.
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Posted By: nates78ski
Date Posted: April-26-2005 at 9:00am
You problem could be a plugged vent on the fuel tank. As thr fuel pump draws fuel out of the pump air must enter the tank so as not to create a vacuum in the tank. I've seen fuel vent tube that will let fuel flow out on fill ups but not work to let air in. Vapor lock requires enough heat to let the boil. Pretty tough to do on a marine engine. Also check fuel pressure and see if the fuel tank pickup tube is in good condition. If this does not solve let me know I can work you thru this one.
Don't buy any parts just yet.
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Nathan
http://correctcraftfan.com/diaries/details.asp?ID=1463&sort=&pagenum=3&yrstart=1978&yrend=1978" rel="nofollow - My '78 Ski Nautique
<a href="http://photobucket.com/Nates78ski" rel="nofoll
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Posted By: GottaSki
Date Posted: April-26-2005 at 9:21am
Additionally,
There could also be a collaping fuel (soft) line, or a clogged fuel inlet screen set within the dip tube's fittings
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"There is nothing, absolutely nothing, half so much worthwhile as messing around with boats...simply messing."
River Rat to Mole
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Posted By: David F
Date Posted: April-26-2005 at 12:13pm
Anthony:
Not much to add, and everyone is spot on, especially Jim_in_Houston. I tend to believe that vapor lock is nearly impossible on a marine engine running a cool 180 degrees. My initial thouhts would be hot ignition or electrical parts breaking down. Good luck.
Now, fuel delivery problems usually manifest themselves as stumbling above a certain RPM (not usually causing the engine to just suddenly quit). To eliminate a clogged vent idea, run the engine with the fuel cap loose (careful not to get water in). If the problem goes away, then you know the vent is clogged by mud daubers (Texas term for wasps that build nests out of mud) or something else.
Good luck.
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Posted By: Tim D
Date Posted: April-26-2005 at 1:25pm
I doubt it is vapor lock also. I think your coil might be breaking down,or if the timing is advanced too much, it will run, but after a few minutes above a certain rpms, it can advance itself and it will die.
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Tim D
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Posted By: kenny g
Date Posted: April-26-2005 at 5:02pm
a weak fuel pump diaphragm is another possibility.fuel pressure is important next time it shuts down on you pull the flame arrester & pump the throttle,see if it's getting a good stream or just a dribble of fuel in the carb throat.
other than that it does sound like a coil overheating.
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kenny g
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Posted By: Siveck
Date Posted: April-28-2005 at 1:55pm
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Thanks everyone for the replies. I don't have an electric fuel pump, just the standard mechanical one. I tried replacing the steel line with rubber hose, but that didn't change anything. As many of you said, it's probably not vapor lock due to the fact that the fuel is flowing when it happens. I really think that the issue is heat related due to the fact that it happens consistently after running the boat for 45 minutes or so, and definitely when I open the rpms up for a short time. I could make it happen by riding around the lake for 45 minutes and then pushing the motor to 4k for 10 seconds or less. It happens every time like that. Even though it seems like a coil issue, it can't be because I just replaced the entire ignition and distributor and the problem is the same as before. I now have a $500 distributor with gm HEI that has a simple 2 wire hookup. After reading some other posts, I think that my problem might possibly be a resistor of some sort under the plate on the back of the motor where the voltage regulator and switch are located. Even though I did remove a ballast resistor near the coil when I converted to HEI, I might still have another one under the plate if that's what that is back there. I don't know why my boat would have had 2 ballast resistors, but it may be possible due to the fact that someone had converted to mallory electric eye ignition from points. I need to check and see if I get 12 v at the distributor. That resistor could be heating up and cutting power. That's my theory for today. I'll check it this weekend. Does anyone agree with me on this theory? Or does anyone know if in fact that is a ballast resistor under the plate at the back of the motor?
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Posted By: kenny g
Date Posted: April-28-2005 at 2:16pm
i'll leave the ballest resister & electronic ignition to someone who knows.
i do know new does not = good..and the mechanical fuel pump is what i was refering to.
i had a 1973 ford LTD 351 modified.10 miles from home,stomp on the gas to get on the highway and it would just die,would'nt restart till it cooled down.after the 3rd day late to work i changed the fuel pump.fixed !
i was still late to work but thats another story.
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kenny g
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Posted By: Guests
Date Posted: April-29-2005 at 2:50am
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If the boat already had a Mallory unit most likely a ylu554cv model you really wasted your money on the HEI unit, not that the HEI unit is bad or anything, Try running hard or over 4k just after its warmed up say after 10-15 minutes and see if you have the same problem and if so I would look at the fuel system like low fuel pressure you should have atleast 5 lbs of pressure, if the pressure is good then you may need to rebuild the carb or replace the secondary diaphram or power valve the power valve should be 6.5.
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Posted By: JEFF KOSTIS
Date Posted: May-01-2005 at 2:06am
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I agree with kenny g. His idea checks to see if theres fuel in the bowl. There just might not be enough fuel being delivered to the bowl when you stomp on it and the bowl is going dry. You can stick a wire in the overflow went of the carb and measure the fuel level in the bowl after it died. If it turns out to be a possible ignition problem, see if the tach reads when your trying to restart it and it doesnt. If it does, that means more than likely the electronic module (that replaced the points), is working. start her up, and take a hairdryer, and heat up the coil, etc., and see if she dies.
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Posted By: Siveck
Date Posted: May-04-2005 at 10:02pm
Thanks to everyone for the replies. I think that I found the problem. The little resistor under the plate on the back of the intake was in fact wired in with the ignition wiring. I was only getting 9 volts to my coil with a cold motor and probably less with a warmed up motor. I bypassed the resistor and I now get full battery voltage at the coil. A test run at the lake showed no problems at all. The conclusion is this:
Apparently the original ignition resistor is indeed the one under the plate on the back of the intake. It was used with the original points to keep them from burning up. The previous owner converted the system to electronic ignition with a Mallory setup with an electric eye. Yet another resistor was added with that setup because the electric eye doesn't like full battery voltage either. Apparently the previous owner did not know of the original resistor and felt the need to add another one. So, I had two of them. I took all that Mallory and Prestolite junk off when I installed my HEI setup. I removed the new resistor, but not the old one. The old one kept cutting my voltage when the motor heated up. The HEI distributor from performancedistributors.com is a very nice unit. Yes, it is pricey at nearly $500, but I will never have to worry about all that points, electric eye, external coil, spagetti wiring, and resistor junk. The unit is all one piece with two, yes only two wires to hook up. One for 12v and one for the tach. It is basically a GM style distributor with the coil in the cap. It has a powerful coil that allows me to open up the gap on my plugs to .055 or so which cuts down on plug fouling. The timing has a much smoother advance curve unlike the original Prestolite or Mallory or whatever that junk is. It is well worth the money in my opinion.
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