Mercruiser twin inboards backfire/stumble

[...bill]

Update. Removed the carbs (Weber 4bbl). Both severely full of flaked crud of whatever that stuff was in the filter. The crud consolidated into larger particles in the fuel bowls. Bowls were at least 20% full of this junk. This stuff worked its way into the jets and was clearly limiting fuel flow. On one carb, one of the secondary jet passages is so clogged internally that I'm not certain I'll be able to save the carb body. Because...
THERE WAS WATER in there too. With resulting corrosion and oxides making a mess of things.
How did these engines run at all? Obviously, this has been going on for a long time.
Nonetheless, one carb cleaned up well. Thoroughly cleaned and installed the rebuild kit and set the floats, etc.
Second carb, as mentioned, has oxide clogged secondary fuel passage so solvent has no effect. High pressure air ain't clearing it either. 100% stopped up.

So, bunch of questions:
1. Any tricks to un-gunk the internal passages? Jet is also hopelessly stuck in that secondary circuit. Tried some penetrating oil overnight, no luck.
2. If I have to replace the carb, can I buy an Edelbrock carb and jet it the same as the original carb? What are alternatives?
3. Yes, know we'll have to drain/clean tanks. In the meantime/short term, thinking adding adding fuel treatment for any water present. What's good to use?
4. Have been using the stock Mercruiser fuel filter. Obviously, not good enough. Is there a better filter that is a direct replacement? Also thinking of adding another filter in series.
5. What could the fine dirt material be that's getting into and past the filters? Some precipitate from the fuel? Bad luck load of dirt? Deteriorating fuel lines?

Thanks to the folks here that got me pointed in the right direction.
...bill

 

[Scott Danforth]

thin piece of wire. I use a section of stranded 10 gauge copper, peal off the insullation, and destrand it. I then use the wire to prod the passages, twisting in my fingers as needed. WD 40 works to loosten sediment up a bit.

as a last resort, you can use a pin drill https://www.amazon.com/CML-Supply-Micro-Drill-Chuck/dp/B001RJE3X8. however only as a last resort.

the fine dirt could be sediment from where you get fuel, or if it has a green tinge to it, it could be the ethanol trying to turn back into corn. my first suggestion is finding a different fuel stop. second suggestion is let the marina know that they are pumping sediment into customer tanks.

as far as fuel water filters. get a 3 micron vs the standard 10 micron. if water made it past the filter, change it more often.

 

[Bondo]

2. If I have to replace the carb, can I buy an Edelbrock carb and jet it the same as the original carb? What are alternatives?
3. Yes, know we'll have to drain/clean tanks. In the meantime/short term, thinking adding adding fuel treatment for any water present. What's good to use?
4. Have been using the stock Mercruiser fuel filter. Obviously, not good enough. Is there a better filter that is a direct replacement? Also thinking of adding another filter in series.

Ayuh,.... Welcome Aboard,.... The Edlebrock 1409 IS the alternative replacement,... a carbon copy of what's there now,....

In the meantime, short term, run it from on-deck portable fuel tanks,....

Why would ya rebuild or replace a carb, then pump the same crap fuel into it,....
Ya need to clean the tank, 'n the fuel in it,....
A fuel filter can only do so much,....

While on filters, the standard Merc filter is just Fine,...
I use Napa 3225 or 3226 which is atad shorter,....
Any fuel filter needs to be Changed to work,....
Once full of water, it's gonna pass water, regardless the micron ratin',...
As for duoblin' up the filters,...
That just doubles the amount of places to have a vacuum leak in the fuel system, doubles the price of swappin' filters, as the 1st will catch most of the water, 'n crud, but ya gotta change the 2nd one, 'cause ya don't know what got past the 1st one,...

'n lastly,.... Whenever ya change a fuel filter, inspectin' the Contents of the old one is a Valuable diagnostic tool,....

I use a qt. freezer baggie to change filters,.... stops spills, 'n makes inspection, as well as clean up much easier,...

 

[NHGuy]

If the jets won't turn use an impact screwdriver. It's ok to ruin the jets, they are cheap. $6.95 per pair at Summit Racing equip't.
I'd say that gunked carb (or both) are worth saving if possible. I'd see how much it would cost to buy one of those ultrasonic cleaners. If anything will save the carb body it's that.
Don't use carburetor dip, just use Dawn for the final wash.

 

[...bill]

Local carb guy came to the rescue. Drilled out the secondary fuel passage machining plug and re-bored the secondary internal fuel passage. He also fixed the jet. Very reasonable. Carb is saved.

Learned the standard Mercruiser filter is 25 microns. That's what I ripped open. Will switch to 10 micron, use a different fuel treatment, and change filters often until tanks gets clean.
Carb guy suggested removing the carbs every off season. Says even ethanol free fuel is troublesome these days.

anxious to get these reinstalled.

Thanks for the help here.

 

[Scott Danforth]

probably not a bad idea to pull the fuel senders and look in the tank to see if you have a bunch of the crud in there as well.

 

[...bill]

Scott:
Indeed, we ran an LED boroscope into the tanks through the fill hose to take a look all the way in to the bottom. These are steel tanks and we saw no sign of corrosion or sediment in either tank. Still has the heavy zinc galvanized spangle everywhere.
Also took fuel samples (~ 1 qt each) from the very bottom of both tanks. All we got was nice clean fuel and one or 2 tiny water droplets.

Filter change at beginning of the 2016 boating season showed no evidence of water in the filters (carefully poured out fuel and sectioned open the can).
Still, no denying the filters had lots of fine dusty matter. Carbs had both this crud and some water, but no evidence in the tanks. It could be we just didn't see it.

I'm thinking carbs were compromised a while ago.

Here's our plan:
1. Inspect and replace fuel filters. Only 20 hours them. Make assessment for water/crud. Switch to 10 micron filters.
If we see little to no crud/water, we'll proceed to run again. If gunked, will have no option but to clean tanks.
2. Reinstall carbs. reset idle mixture, etc.
3. Sea trial

Also, a question for the experts. Fuel lines are somewhat old. What are symptoms of fuel lines breaking down? There are no leaks, but what happens?
Do they precipitate crud?

Thanks,
...bill

 

[...bill]

Followup on this old thread...
Issue was resolved and discussed in a later topic here:
http://forums.iboats.com/forum/engi...cruiser-5-7l-stumbles-backfires-after-40-mins

...bill