Mercruiser Carb troubleshooting

formula233

Well-Known Member
May 3, 2003
136
3
Marion MA
Hoping someone can throw some thoughts and ideas out there regarding some problems I'm having w my mercruiser carb.
Engine is a 5.7 Mercruiser 4V (stock carb, pretty sure it's a Weber 750 cfm). Engine was installed in the boat back in 2015 replacing a PCM TBI.
Problem I'm having is that the engine will run fantastic at hi idle leaving my river (1200 rpm), will jump on plane and scoot across the bay, but once I slowly pull it back and idle around for a bit she stumbles and is fuel starved when I go to bring her back up. If I power through it eventually she'll take off but in the 650-1200 range she falls flat on her face. I've played with the idle jets a few times and am kind of a loss.
I did tighten up on the mounting studs, the forward two were a bit loose but not too bad. It seemed to fix the problem for a couple trips but now it's returned. Mechanic seems to think it's somehow vapor-locking but I'm not sold on that.
Any Ideas would be greatly appreciated. The choke and all linkage are clean and free. I've religiously run fuel treatment through it from day one to keep it clean. I know it's getting time to probably break it down and rebuild it but trying to get through the rest of the season as it's more of a nuisance right now.
Thanks!
 

crhodes

Well-Known Member
Supporting Captain
Aug 26, 2003
2,731
91
Southport, NC
First things I'd check are 1) proper low speed idle adjustment and 2) potential faulty power valve (though not familiar with the Weber power valve setup). If it is only happening when you accelerate it could be a bad accelerator pump diaphragm. Does it do this wen cold? Is the Choke operating correctly. Will it run properly if you manually close the choke a little?
 

formula233

Well-Known Member
May 3, 2003
136
3
Marion MA
Thank you for the suggestions,
chrodes the choke seems to be operating correctly. This problem is intermittent however it's becoming more regular when the engine is at operating temp and I bring her down after a run at speed (3000-3400 rpm). After idling for a couple minutes when I bring her from 650-1200 or so she just boggs and stumbles. It very well could be an accelerator pump issue, however she'll continue to stumble through that lower rpm range, not just when I goose the throttle.
fluke37 I'm very intrigued by this idea as well...the 1409 is running close to $600 these days ugh...my biggest concern though is that the 1409 is a 600cfm carb while the stock weber on this mercruiser is supposedly a 750 cfm. Not sure why they put such a high flow carb on a 5.7 rated at 275-300 hp but I've already been through one engine on this boat and I'd rather not do anything that would get me into #3 sooner than later.
Interestingly enough all forums I've been searching put the 1409 for the small blocks and the 1410's for the big blocks. Also the edlebrock carbs seem to be the same casting as the webers/old carters. The rebuild kit for the sierra reman Mercruiser 750cfm weber etc is an edlebrock part number.
I appreciate the help.
 

Finnegan

Well-Known Member
Jul 12, 2022
51
6
Mine does a similar thing and I think it’s the accelerator pump diaphragm getting old and crusty. I would check that, it’s a relatively easy check.
 

formula233

Well-Known Member
May 3, 2003
136
3
Marion MA
Most likely I'm going to tear down the stock weber and clean/inspect and see what's worn out, clogged etc.
That being said does anyone think there would be a problem installing the Edelbrock 1409 as fluke37 mentioned? The internet is full of information just want to make sure I'm not going to jeopardize anything. going from a 750 cfm to a 600 cfm. Everything I've found says the 600's are for small blocks and the 750's are for the big blocks. Unsure why mercruiser decided to put the same carb on everything. S

Should the repair turn into a bag of $hit... which is how most things go for me when I'm in a rush lol, I may order the 1409 anyway.
 

Finnegan

Well-Known Member
Jul 12, 2022
51
6
I've been thinking about trying a 1409 on the PCM ford 351 in my boat. I have run the automotive version on street cars with good success.

The things I would look out for are any needed adaptations to throttle linkage that may need to be done (the throttle arm on the Edelbrock is different) and the fuel line connection (I don't *think* this is an issue but im not a marine carb expert). Other than that carburetors are pretty simple!

Down sizing in CFM may net you better performance and economy. I know I personally never run my boat at wide open throttle, and never really even get into the secondary's opening up. It's not uncommon to run a low rpm small block Chevy on a 650 cfm carb. (I had a modified 350+hp engine that ran a 650 happily)

I would call an engine builder who lives and breaths this stuff, or Edelbrock directly... They will be able to tell you what size carb to go with
 

formula233

Well-Known Member
May 3, 2003
136
3
Marion MA
So just to close the loop, I ended up purchasing the 1409 Edelbrock. Considering the Mercruiser supplied Weber is basically the same casting it was an easy swap, bolt patterns, throttle linkage, fuel line etc.
Other than adjusting the idle mixture screws and the idle speed, it was very straightforward. I wish I did this earlier this summer, I don't think this engine ran this good when new.
As I was taking the old Weber off, the accelerator pump definitely was operating correctly so I'm guessing either the idle jets are worn out or the metering rods etc. are stuck etc. Eventually I'll take it apart and investigate but if anyone has similar problems the 1409 did the trick. Thank you everyone for their help and advice.
 

Finnegan

Well-Known Member
Jul 12, 2022
51
6
So just to close the loop, I ended up purchasing the 1409 Edelbrock. Considering the Mercruiser supplied Weber is basically the same casting it was an easy swap, bolt patterns, throttle linkage, fuel line etc.
Other than adjusting the idle mixture screws and the idle speed, it was very straightforward. I wish I did this earlier this summer, I don't think this engine ran this good when new.
As I was taking the old Weber off, the accelerator pump definitely was operating correctly so I'm guessing either the idle jets are worn out or the metering rods etc. are stuck etc. Eventually I'll take it apart and investigate but if anyone has similar problems the 1409 did the trick. Thank you everyone for their help and advice.
Did you install a fuel pressure regulator? Or just go straight off the fuel pump? Some folks say a regulator is helpful, some say no need for one.
 


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