Starting Problems w/ Merc. 7.4L Carb. Engine
 

I have a 1994 24' Baja Outlaw with its original Mercruiser 7.4L 300 hp carburetor engine. I bought this boat used 3 years ago with 149 hours on it. (It now has 239 hours.)

In the 3 years I've owned it, it's always been very difficult to start when the engine is cold. It usually takes 15 to 20 cranks of the engine coupled with pumping the throttle about 2 dozen times before it will start. (And sometimes the amount of throttle pumping doesn't even seem to make a difference.) Once started, it runs great, and subsequent warm starts during a day of boating are NO trouble (usually requiring no throttle pumping at all).

My mechanic has not been able to help with the starting problem. He has confirmed the gasoline is not leaking down from the carburetor. He has replaced both the fuel pump and the anit-siphon valve. Neither has helped to improve the starting.

The engine hasn't failed to start for me, yet (knock on wood), but I'm probably killing my starter in the process. Does anyone have any idea of what might be wrong and what I can try next?

Thanks!
Mike

i assume you have a quadrajet with electric choke? the choke is closed when cold and is still hard to start? you could try a couple pumps of the throttle after you prop the choke butterfly wide open by the linkage with a tie wrap or electrical tape, and open your throttle up at least half way while starting. you should be able to see into the intake when you look down the carb throat with a flashlight before you start the engine. if it fires right off you will obviously have to pull back on the throttle quickly, but chances are your leaking fuel and your in need of a rebuild. or your idle adjustment screws were overtightened at some point in time and may have broken a tip and your in need of a rebuild. good luck.

Thanks for your reply, excaleagle42.

My mechanic did take the carburetor off of my boat, put it on his workbench, and left it there for a day to check for gasoline leaking down. I wasn't there, obviously, but he did say there was no leaking.

Regarding the idle adjustment screws, is it possible they were overtightened and something broke yet it could still be idling okay (because it is)?

Is it easy to tell if I do have the quadrajet with electric choke carb.?

Thanks!
Mike

Mike, I had a '93 w/454-300 also. It did exactly the same thing yours does, except after it was started, it would start every consequtively(sp?) day. if a week or more were in between starts back to the same problem.

My fix was to install a new 454-450hp & pro-charger, never a problem after that!!! :D

Seriously, install an electric fuel pump and regulator and problem will be solved.:)

No a 1994 has the Weber (edelbroc carter) carb but it does have an electric choke. You can ajust them fairly easy I did this and it made my 95 330 hp 7.4L bravo start much easer when it had been seating. Charlie

This should be a simple problem to fix. But, you need to see the boat to fix it properly. Probably a choke problem. Sounds to me like you need a new mechanic. This should not be hard to diagnose if you know what you are looking at.. Your idle mixture adjuster screws have nothing to do with a hard cold start problem. These screws only control the idle mixture.

obx
webers have two floats in them. i would check them and play with the choke adjustment like ragtop said. i like blown formulas first suggestion best though.:D

Your idle mixture adjuster screws have nothing to do with a hard cold start problem. These screws only control the idle mixture.

checkmate, this is right from the edelbrock carb manual.
"Fuel is drawn through the Idle System by the intake manifold vacuum that is communicated at the Idle Screw Port and Transfer Slot. The emulisified air is then drawn through the restriction, a drilled passage that serves to incresae the velocity of the air and fuel and promote better mixing. The Idle Screw Port is a variable discharge restriction that is adjusted by the engine tuner to achieve the desired A/F ratio at engine idle.

excaleagle42;

I am not quite sure as to whether you are disagreeing with me or maybe I am not reading your reply to my comment about the idle mixture screws correctly. But, didn't Edelbrock say the same thing that I said?? They just wrote more extensively that the idle mixture screws adjust idle mixture, which comes after the initial start-up.

How about "no accelerator pump shot" did you check to make sure the accerator pump is squirting gas when you pump the throttle after a sit? Just a thought.

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.

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:D 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:rolleyes: .Rest of the day the starter no sooner engages and they fire. The electric fuel pump will probably solve the problem:D

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.

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?

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.

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.

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.

JOHNJAN--THIS SPECULATIVE CRAP is fact. The carb IS boiling dry. Maybe you need to do a little more research before you discount others ideas. KEEP AN OPEN MIND.:)

Maybe I'm stupid , but lets see.. a COLD engine sits for a week and the fuel boils off:confused: :confused: . 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.

MOPOWER--- evaporation is boiling. If the temperature in the engine compartment goes above the vapor point of gasoline the gas will evaporate (boil off)

Tink,
OK , you win ,it's boiling , evaporating , perculating , whatever.
Gas is very volitile and evaporates quickly. My bet is the carb. is almost dry.
I didn't go out last weekend and not today either:mad: damn weather. Maybe tomorrow:rolleyes: , and my quadrajets will do the same thing:( crank , pump , crank , pump , crank ,cough , cough , sputter , crank , pump , VARRROOOOOOM:D , and the chokes are working. Once they start , they run fine , no stumble , no hesitation.
Until the day comes we all have FI , I think it's just a fact of life:rolleyes:

It makes sense that a vented fuwl bowl will eventually allow the gas to evaporate. But the wierd part is, my old holleys dont do that at all. I can go a month and bang, they start right away. I wonder if the large accelerator pumps I have on them keeps fuel in them?

--
 

Formula31--- I agree my Q-Jets did just what his are doing. I changed over to HOLLEY HP 950's and I no longer have the problem.

Your both right there;)
My last boat with a 540 and a Dominator could sit for two weeks and 1 pump in the parking lot while I turned the blower on and when I got it in the water it started like it was warm:cool

PS That also had 50 cc pumps on it. Primary and secondary