5.7 mercruiser surging

[rooferdave]

ok here is my other issue, (see 5.7 overheating) the other engine on the boat(starboard) will surge, as in running out on the water would go from 3500 to 3400 up and down, then get worse and worse 3500-3000, then will not even get as high as 3000 or then 2000, as long as I am not demanding fuel it will run stable, (although I end up at 1200 rpm) and at that point to thottle right up will cause it to stall.
first time this happened I replaced the in carb filter (metal/ rusted) water separator (cannister type)0 it was full of water and crap) and ran great, not sure of duration of test trip.

Happened again a few days later and I shut engine down, removed cannister drained it in an enviromentall responsible manner, reinstalled and problem solved, I figured water, but only on starboard engine!
-enter the e-10 debate
both tanks were treated with gas treatment and metle hydrate added, they had about 1/8 tank each of fuel to start. starboard tank filled with shell ultra (no ethenal.... port tank filled with regular gas (e-10) I was trying to see if a tank with e-10 would react different to reg gas,
The tank with e-10had no probs for the entire season, the gas with no e-10 still had the problem, (surging, stop, drain separater problem goes away)
-next I shut one gas tank off and ran both engines off the one tank with no ethanol, starboard still acts up and port is fine..did this for about a month with same results.....swicthed to the other tank and same thing.
The last time I went out I was testing the other engine for its overheating problem I ended out in the lake a lot further than intended and on the way back in the surging started again ( keep in mind we had been out WOT for about 30 mins no probs before turn around to go back to club) Sooo I shut down drain seperator and...no dice same problem(uh oh) maybe separtor clogged, have a new one onboard, swap it out and it seems to be ok but goes bad quickly. throttle waaaay back and got in safely
-so far we know it is not
-a gas quality problem
-gas tank or pick up problem
-a water issue
-a timing issue (put a timig gun on it both at the dock and on the water)
- a filter issue
-just for laughs compression is 215 across the board

The only thing I am left with is carb icing???? Perhaps it is running too lean and thats doing this???

Anyhow if this is read in conjuction with the overheating thread ( and perhaps a too rich scenario) I am going to switch carbs over and see.

sorry for the epic posts but the more info the better and you can all see I have been pretty thorough (I think)

suggestions?

[koviak]

Going thru the same issue with mine now. I believe it may be distributor related.
Do you get popping thru the carb? New distributors will be arriving today.
I also did the fuel system checks with no change.

[rooferdave]

no popping through the carb

[captainmaniac]

You didn't mention the in-line fuel filter - have you tried checking it or swapped it out yet? You also didn't say if you found any water or grit / sand / rust in the seperator or paper element in the carb.

Any valves in the fuel line to that engine not fully open (leading to starvation or vacuum in the system)?

Other thoughts : sticking float in the carb, or to koviak's comment re distributor, is the advance mechanism moving freely or possibly binding? I had the opposite problem a few years back (couldn't get over about 2400 rpm under load) and problem was no advance. If yours is advancing, but not retarding when you come off the throttle, might see what you are seeing...

[rooferdave]

1)suggestion...
You didn't mention the in-line fuel filter - have you tried checking it or swapped it out yet? You also didn't say if you found any water or grit / sand / rust in the seperator or paper element in the carb.

-I put this in the first paragraph, sorry it was probably buried with my epic post
"first time this happened I replaced the in carb filter (metal/ rusted) water separator (cannister type)0 it was full of water and crap) and ran great, not sure of duration of test trip"


2)question
"Any valves in the fuel line to that engine not fully open (leading to starvation or vacuum in the system)?"

-no but I will check again,


3)suggestion
Other thoughts : sticking float in the carb, or to koviak's comment re distributor, is the advance mechanism moving freely or possibly binding? I had the opposite problem a few years back (couldn't get over about 2400 rpm under load) and problem was no advance. If yours is advancing, but not retarding when you come off the throttle, might see what you are seeing...

-I will check thanks for the input! will report back on results in the next day or so

[sluggo]

I had this problem, and ran it back to that carb filter you said you changed. I still think it is a water issue. But I also found that my in line water seperator was broken.(the one between the gas tank and the engine) I thought it was fine, as I replaced the element and all looked good. but what I found was something that looked like a spool of thread (which is supposed to be in there) on one side was missing and the other side was broken. I think, off the top of my head, something about Dahl 66. Check those things and see what you have and then compare to what the diagram looks like. It helped me, alot with what sounds like the same issue.

[Big D]

Hook up a fuel pressure gage at the carb and see what you get when the symptom shows up again. I also like hooking up a vacuum gage any time I'm looking at a runability issue. They can tell you a lot. On the Merc, you should be able to hook up directly to the intake manifold behind the carb.

[Big D]

Check your site tube from fuel pump to carb to see if it's dumping fuel from a ruptured diaphram.