Hard starting 7.4 400EC

E

expressboy

New Member
Oct 16, 2010
92
pac north west
Boat Info
96 400ex
Engines
454
I am trying to figure out why when the boat sits for more than a couple weeks my port engine refuses to start. It cranks fine ... but will not fire. The stbd. engine starts fine. If I prime the carb ( it is a 1995 carbed motor ) wit a bit of raw fuel the beast will start up just fine. If I start her more often she starts right up. No amount of pumping the throttle makes any difference. The boat had a complete tune up about a year ago. Thoughts?
EXPRESSBOY
 
4 cans of seafoam and it will run like an efi... I was having same issues and wow what a difference... I now add 4 cans to eash tank each spring and it is smooth sailinf from there...

Jason
 
Both my engines are tough to start when cold. I have asked this question before and got all sorts of suggestions. The best thing I have come up with (on my own) is to crank a few short times, then go to the other engine and crank that a few times, wait a bit, then try again. Gets fuel to flow in to the carb.

If I'm not mistaken, your fuel pump is tied in to your raw water pump (where your impeller is). I have a mechanical fuel pump that's driven by the crank. Have you ever checked the fuel pump on that engine? After a certain year (not sure but early 90s), GM stopped manufacturing motors with a place for a mechanical fuel pump b/c they moved entirely to electric fuel pumps (for automotive). So Merc came up with this funky fuel pump/raw water pump mechanism which is at times problematic.
 
If you have troubleshot using the "Fuel, Air, Spark" rationale then consider below:

After chasing hard starting and occasional poor running issues on my 7.4, I replaced everything. You name it (plugs, wires, coil, fuel pump, ignition sensor, etc., etc.) I finally put on a new Edelbrock 1410 carb (with electric choke instead of the old "stove" choke).

Starts and runs way, way better. No guarantees, but look real hard at your carb.
 
Dave
I have to get the heads re-done on my stbd engine this winter. Since I have to take the whole thing apart anyway, I am probably going to also have the carb rebuilt - was thinking that may improve my situation. Tune in in April to find out!
 
If you don't have electric chokes, I highly recommend switching to them.
 
Dave
The mechanical choke works just fine. I checked it on the advice of others when I posted a similar question here about 5 years ago (or maybe it was the previous SR site >5yrs ago?) It's closed when cold and opens gradually as the engine (intake manifold) warms. Kinda of a amazingly simple mechanism.
 
Well if it performs as advertised that is good! Hope the rebuild does you some good. I have heard tale of a bulletin from merc recommending changing jets in the Weber 7.4 carbs....you got Webers?
 
Rochesters
 
Rochester's have screw at the bottom of the bowls that tend to leak down causing a no fuel situation until it pumps up again. They can be sealed up to correct.
 
I had the same issue on my '93 400EC. My starboard was always much harder to start than my port. I'd had the carbs rebuilt, plugs and plug wires changed, etc. That definitely made a difference but still one was always tougher than the other.

My procedure for started after a prolonged period of no use was:

Pump the throttles like crazy for 10-15 times.
Crank the engine for 30 seconds or so. Long, but not crazy long.
Let sit for a half a minute
Crank again while pumping the throttles.

It would always eventually start. The key seemed to be the step where I let it sit between cranking. Not sure why. It's like the fuel is evaporating or flow back out of the carb when idle.

Good luck.
 
Well... I do know it is a fuel issue ( starts immediately when primed with raw fuel dumped down the carb ) checking the fuel pump attached to the raw water pump is a good start.. The engines run great ( when they will both start ) I will look into the fuel pump
EXPRESSBOY
 
Before you do anything.... You diagnosed the problem correctly right off the bat. It's a fuel related issue. Additional fuel dumped into the carb fires the engine up, correct? Ok...

I think you have Rochester Quadrajet Carburetors (Quadrabog as some refer to it). Several issues can cause a fuel issue with them...

1. Like mentioned above, if you remove the bottom plate of the carb that contains the butterflys, you will see the fuel passages that are sealed with a welch plug of some sort. They leak over time. Mix up some JB weld and seal up the area. Most carb shops will do this as standard issue on a Quadrajet.

2. Cold start issues could also be a misadjusted choke. Next time the engine is cold, take off the flame arrestor, pump the throttle a couple times, and see if the choke gets set closed. If it does not close, adjust it.

3. On a cold engine, remove the flame arrestor, then stick a screw driver down the carb to open the choke up (if it's already closed). Now stand over the carb and have someone pump the throttle. Fuel should be squirted into the carb via the accelerator pump. The accelerator pump is just a plunger with a spring on it. You should see the rocker arm on the top flange of the carb body? That rocker is pushing down on the accelerator pump when the throttle is advanced. If no fuel is seen, either the fuel leaked out of the bowl (see fix #1 above), or the O-rings on the accelerator pump are not sealing correctly in order to pump the fuel.

4. While your troubles could also be a bad or lazy mechanical pump (diaphrams go bad), your description tells me that is runs fine everywhere but during cold startup. This would typically be the last thing I would replace.

I'm fairly confident what you need is a carb rebuild. If you're mechanically inclined, purchase a carb rebuild kit for $30-$50, spend two hours and rebuild the carb yourself.

Doug
 
Last edited:
Doug
I've heard horror stories about rebuilding carbs. Tedious, tiny parts, risky etc.? Myth or some truth? There's a good guy locally that gets around $250 for a rebuild who I was planning on giving a call to.
 
rondds said:
Doug
I've heard horror stories about rebuilding carbs. Tedious, tiny parts, risky etc.? Myth or some truth? There's a good guy locally that gets around $250 for a rebuild who I was planning on giving a call to.
Click to expand...

Yes, there are some small roll pins, jets, etc. I rebuilt my first one when I was 15 years old. $250 isn't bad if that includes the rebuild kit.

Doug
 
If you have the webers/carters/edelbrocks, the trick is to put electric fuel pumps on them. The carbs go dry after a week or two. When you put the electric fuel pumps on, you can turn the key, and listen to the fuel pump "pull down" when the float bowl is full, give the throttle a couple of quick pumps, and it'll fire right up like EFI.
 
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