Starting Problems w/ Merc. 7.4L Carb. Engine
10-16-2002, 09:22 PM
#11
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OBX, I have a 93 Powerquest with a 454 mag that I bought over the summer. I have the exact same problem that you have described! I placed the same question on the Powerquest side of this forum and was told of possible fuel percolation. I called a local mechanic and after explaining to him what I had he said the same thing! Apparently the early 90’s big blocks had this problem. It’s fine if you’re running the boat every weekend but if she sits past the 2 week mark, what a bear to start. He recommended installing an electric fuel pump. He also said that a heat shield or some type of spacer between the carb and intake might take care of the problem. I myself opted for the fuel pump. It should be in this week and I’d let know how it work’s other than I’m in Michigan and just had her winterized today to put away. But I’ll be sure to post the results this spring.
10-16-2002, 10:21 PM
#12
Toxic FORMULA
Platinum Member
PQC,
Fuel perculation happens on a hot engine after its been running just like perking coffee , hence the name. No such problem on a cold engine thats been sitting.
Judging by the fact the boats got 90 hours in three years , my bet is it sits quite a bit
My bet is fuel is evaporating from the carb and perhaps draining down from the fuel line going to the carb. It won't leave the carb and drain back because the fuel inlet is at the top of the carb.
I have two 330's and if I start them every two to three days , they fire right off
If they go a week...it's a couple of pumps and a couple of cranks. Miss a weekend due to rain and pump crank pump crank pump crank
.Rest of the day the starter no sooner engages and they fire. The electric fuel pump will probably solve the problem
Fuel perculation happens on a hot engine after its been running just like perking coffee , hence the name. No such problem on a cold engine thats been sitting.
Judging by the fact the boats got 90 hours in three years , my bet is it sits quite a bit
My bet is fuel is evaporating from the carb and perhaps draining down from the fuel line going to the carb. It won't leave the carb and drain back because the fuel inlet is at the top of the carb.
I have two 330's and if I start them every two to three days , they fire right off
If they go a week...it's a couple of pumps and a couple of cranks. Miss a weekend due to rain and pump crank pump crank pump crank .Rest of the day the starter no sooner engages and they fire. The electric fuel pump will probably solve the problem
10-16-2002, 10:25 PM
#13
IF it starts hard on friday but not on saturday and sunday mornings than the problem is fuel percolation. When the carb sits all week long the fuel boils out of the carb. look into the top of the carb with the choke open and have someone pump the throttle without starting the engine and see if you are getting gas spray out of the enrichment jets. if you are not ( This is what I think you will find) then the carb is boiling dry during the week on the trailer.
10-17-2002, 08:28 AM
#14
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I love all the speculative crap people post here. You're never going to troubleshoot a problem from other peoples "fixes" to problems that may or may not be exactly like your situation. So think about the problem logically.
An engine with spark at approximately the right time, atomized fuel in the chamber, and some initial motion from the starter has to run - it has no choice in the matter. So, that said:
A) The engine runs so lets assume you have consistent spark - although - if your battery is weak, you may not have efficient spark for a cold engine.
B) The engine turns over, so it's not the starter.
C) That leaves fuel...
C), 1. Is there fuel in the float bowl at the time that then engine won't start? If so, there's an obstruction between the float bowl and the venturi or the mixture is incorrect for a cold engine, check/adjust the choke to operate properly.
C),2. If there's insufficient or no fuel in the float bowl then there's a problem upstream of the float bowl, either the fuel pump, filter, line, or tank. The engine runs once started so the fuel pump is probably OK. Sidebar - fuel pumps don't choose to work when warm and not work when cold. Extreme temperatures ignored for the moment.
Following the fuel pump logic - why would a fuel pump work when run recently and not work when not having been run recently?
A) It's pissed off for being neglected.
B) It's not staying primed, fuel is draining away from the diaphram.
C) The pump is not priming quickly upon being activated.
Hmm, B) and C) sound fairly likely, how can they be confirmed?
After not using the boat for a while disconnect the fuel line at the pump inlet. Is there a full pipe? Can you pull and hold a vaccuum on the line from the tank with the tank full? A Mityvac can be used to check this. what about with the tank near empty? Connect the line and disconnect the exit side, does fuel flow immediately or does in not have sufficient flow to fill the float bowl rapidly enough to start when you expect it to?
By breaking each system down into their component parts and evaluating if each component is doing it's job, you will find the problem.
I had a similar starting problem. It baffled even Holley's technical staff because according to what was happening at the carburetor, it was an impossible set of circumstances. Turns out there was a leak internal to the fuel tank at the junction of the pickup tube and the fitting at the top of the tank. Two things would happen 1) after sitting for a week or so, fuel would drain back away from the pump (fairly normal) but since the pump had to suck air through the leak as well as fuel through the pickup tube, the pump would not prime. Fuel pumps pump fuel very well, air not very well. 2) the engine idled poorly because of the airated fuel mixture effectively pressurizing the float bowls and throwing the idle adjustments all out of whack. With a full tank (the leak covered with fuel), everything worked great - until the fuel level dropped.
Again, sit back and look at each component. What is it supposed to do? How can you check it to see if each component is doing it's job?
An engine with spark at approximately the right time, atomized fuel in the chamber, and some initial motion from the starter has to run - it has no choice in the matter. So, that said:
A) The engine runs so lets assume you have consistent spark - although - if your battery is weak, you may not have efficient spark for a cold engine.
B) The engine turns over, so it's not the starter.
C) That leaves fuel...
C), 1. Is there fuel in the float bowl at the time that then engine won't start? If so, there's an obstruction between the float bowl and the venturi or the mixture is incorrect for a cold engine, check/adjust the choke to operate properly.
C),2. If there's insufficient or no fuel in the float bowl then there's a problem upstream of the float bowl, either the fuel pump, filter, line, or tank. The engine runs once started so the fuel pump is probably OK. Sidebar - fuel pumps don't choose to work when warm and not work when cold. Extreme temperatures ignored for the moment.
Following the fuel pump logic - why would a fuel pump work when run recently and not work when not having been run recently?
A) It's pissed off for being neglected.
B) It's not staying primed, fuel is draining away from the diaphram.
C) The pump is not priming quickly upon being activated.
Hmm, B) and C) sound fairly likely, how can they be confirmed?
After not using the boat for a while disconnect the fuel line at the pump inlet. Is there a full pipe? Can you pull and hold a vaccuum on the line from the tank with the tank full? A Mityvac can be used to check this. what about with the tank near empty? Connect the line and disconnect the exit side, does fuel flow immediately or does in not have sufficient flow to fill the float bowl rapidly enough to start when you expect it to?
By breaking each system down into their component parts and evaluating if each component is doing it's job, you will find the problem.
I had a similar starting problem. It baffled even Holley's technical staff because according to what was happening at the carburetor, it was an impossible set of circumstances. Turns out there was a leak internal to the fuel tank at the junction of the pickup tube and the fitting at the top of the tank. Two things would happen 1) after sitting for a week or so, fuel would drain back away from the pump (fairly normal) but since the pump had to suck air through the leak as well as fuel through the pickup tube, the pump would not prime. Fuel pumps pump fuel very well, air not very well. 2) the engine idled poorly because of the airated fuel mixture effectively pressurizing the float bowls and throwing the idle adjustments all out of whack. With a full tank (the leak covered with fuel), everything worked great - until the fuel level dropped.
Again, sit back and look at each component. What is it supposed to do? How can you check it to see if each component is doing it's job?
10-17-2002, 11:14 AM
#15
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There is a long running problem with the Weber carbs being hard to start. When they sit for a period of time the fuel drains, evaporates or whatever from the float bowls. There was extensive discussion on this forum a year or 2 ago on this problem. There have been a lot of fixes tried, but I'm not aware of any that really work. The Edelbrock carbs which are identical don't appear to have that problem. I fought it on an old 454 I had and never could resolve it. The easiest solution would be the electric fuel pump or replace the carb. There is a Merc service bulletin that talks about starting problems with this carb, but it doesn't address the no fuel issue. If you pull your flame arrestor when it won't start and pump the throttle you'll most likely see that there is no fuel being pumped.
10-18-2002, 01:47 PM
#16
I have the exact same problem with a pair of twin 454's with Edelbrock 750 electric choke carbs.
I pump 10 times while cranking the engine, then go to the other engine and do the same thing (allowing starter to cool). After about 3 times each the engines will fire off and I have no further problems starting.
If it sets for 1 week without starting..I can bank on this scenario, couple days, no problems. I just fire it up.
As far as the chokes go? Mine are full closed during cold starts.
Yes the fuel bowls are low. I've pulled the tops of the carb off and they are 1/4 full, and floats are bottomed out.
Why? I don't know yet, The jury seems to still be out on this one.
I pump 10 times while cranking the engine, then go to the other engine and do the same thing (allowing starter to cool). After about 3 times each the engines will fire off and I have no further problems starting.
If it sets for 1 week without starting..I can bank on this scenario, couple days, no problems. I just fire it up.
As far as the chokes go? Mine are full closed during cold starts.
Yes the fuel bowls are low. I've pulled the tops of the carb off and they are 1/4 full, and floats are bottomed out.
Why? I don't know yet, The jury seems to still be out on this one.
10-18-2002, 07:25 PM
#17
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My solution on my '93 7.4L with a Weber is this;
Pump the throttle one time to set the choke.
Crank the engine while counting slowly to 15. Stop Cranking.
Pump the throttle three times.
Crack the throttle just off idle.
Wait 10 to 15 seconds for raw gas that was just pumped into intake manifold to vaporize.
Turn the key. It usually fires right up. It sometimes takes a little finesse on the throttle to keep it running just after it starts.
I know this sounds like a big hassle but, it works. Like everyone else has said, you only need to do this if it sets for more than a few days.
Pump the throttle one time to set the choke.
Crank the engine while counting slowly to 15. Stop Cranking.
Pump the throttle three times.
Crack the throttle just off idle.
Wait 10 to 15 seconds for raw gas that was just pumped into intake manifold to vaporize.
Turn the key. It usually fires right up. It sometimes takes a little finesse on the throttle to keep it running just after it starts.
I know this sounds like a big hassle but, it works. Like everyone else has said, you only need to do this if it sets for more than a few days.
10-18-2002, 10:47 PM
#19
Toxic FORMULA
Platinum Member
Maybe I'm stupid , but lets see.. a COLD engine sits for a week and the fuel boils off
. The laws of physic say that would happen when you shut off a HOT engine and wait for 1/2 to 3/4 hour and TRY to restart it. This allows the heat to transfer to the gas ...causing to to become hot and boil or perculate. This can be more of a problem in cars and trucks since the underhood temps are way above boat temps. My kid has a 496 with two of these carbs on it and it does the same thing. Leave it sit in the garage a week or two and it a bear to start. Hottest day of the year it will start over and over all day long.
Sorry it's evaporating or draining back.
. The laws of physic say that would happen when you shut off a HOT engine and wait for 1/2 to 3/4 hour and TRY to restart it. This allows the heat to transfer to the gas ...causing to to become hot and boil or perculate. This can be more of a problem in cars and trucks since the underhood temps are way above boat temps. My kid has a 496 with two of these carbs on it and it does the same thing. Leave it sit in the garage a week or two and it a bear to start. Hottest day of the year it will start over and over all day long.Sorry it's evaporating or draining back.
