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43 LX engine surges at cruise

D

ddavis3459

New member
"I have a 96 290 Searay with t

"I have a 96 290 Searay with twin 4.3 engines, with 4BBL Webber Carbs. The Port engine surges at cruise and the RPM varies by two or three hundred. The Starboard engine has now begun to backfire when the throttle is advanced to go on plane for the first time of the day. After that no backfire.

The Surging by the Port engine continues all day.

What I have done so far:
1. Rebuilt Carbs
2. Replaced Fuel pump
3. Replaced Fuel filters (more than once)
4. Checked Ignition timing.
5. Checked fuel tank vent, clear.

Other significant facts:
1. We had 10% Ethanol last summer and up until early this spring. Since we have used Straight Gas. I have run about 175 gallons of gas so far.
2. Both engines have been replaced Port 30 hours,
Starboard, 250 hours.
3. I used a outboard fuel tank and ran the surging
engine (port) with the boat fuel system
bypassed. It seemed like the surging
did not occur then, but the water was
rough that day. Somewhat hard to tell.
4. I can smell gas when near the fuel tank vent.
(the thru-hull)
After the outboard tank experiment, I replaced the fuel pump.

None of the above items corrected the surging.


My Theories:

Issue with anti-siphon valve.
Carb float level issue."
 
"Having used 10% ethanol for a

"Having used 10% ethanol for a season I am inclined to think that water is in the fuel tank.
The antisiphon valve may have been corroded by the water or age.

Have you used a water dispersant in the tank? Stabil makes a treatment just for ethanol fuel. Get enough Stabil for ethanol fuel to treat the fuel in the tank. Figure a way to rock the boat in order to blend the Stabil w/th gas.

If there are fuel/water separator filters on the engines, dump their contents into a jar to check for water. Pour 1/2 can of SEAFOAM into each filter and run the engines at idle for 10 mins. Let the boat sit overnight so the Seafoam can absorb any water in the fuel bowls and dissolve any fuel sludge.

Try the engines the next day."
 
"Guy, Thanks for your help.

"Guy, Thanks for your help.

The marina now serves Pure Gasoline. I ran all but four or five inches in a 130 gallon tank of Ethanol lased fuel out before filling the tank with Pure gas. I have run 20-25 hours since then maybe 160 to 180 gallons of Pure gas.

I always used the Startron Blue additive with the Ethanol fuel and added a dose just before we got pure gas.

I am not familiar with Seafoam, but I will give that a try.

Today I removed the anti-siphon valves, they were fine.

I set the ignition timing at 8 degrees before TDC at idle. I did not check the full advance.

Could the ignition coil have intermittent problems?

Today the #2 spark plug looked to have some rust on it. I noticed the riser gasket and ex manifold have a few rust tracks, could water in the manifold cause the surging?

I will take the riser and manifold off and inspect them for leaks.

I have heard of ruber fuel lines having deteriorated from using ethanol gas."
 
david sea foam is in a sence a

david sea foam is in a sence a very well recomended cleaner that is highly recomended by all who have used it. i was also gonna ask if you have checked/changed the plugs lately? and its fact not fiction about ethanol and rubber lines
 
"Where can I buy Sea Foam?
T

"Where can I buy Sea Foam?
The plugs have 70 hours on them. I checked all six and the look good.
This is a 1996 yr model, I don't think they used ethanol approved rubber lines then. Maybe I should take the line off and inspect it."
 
""Could the ignition coil

""Could the ignition coil have intermittent problems?"

Yes; most people comment that the coil failure occurred above 2000 RPM. Inspect the wires for corrosion and clean them. Inspect the tower where the coil output wire is connected. Coil output wire can also fail sporadically.

"I did not check the full advance." I would.

"Today the #2 spark plug looked to have some rust on it. I noticed the riser gasket and ex manifold have a few rust tracks, could water in the manifold cause the surging?"

Yes; but you said the plugs looked good. What color? Clean & new, off white, tan, brown, black, carbon coated? If they looked clean & new then suspect water infiltration.

"Where can I buy Sea Foam?" Most marinas, repair centers, auto parts stores.

Inspect the rubber fuel lines for internal swelling narrowing the passage.

Inspect the plug wires for cross firing; rubbibg together w/carbon streaks. Inpect the caps and rotors for wear, carbon tracks & cracks in the cap. Remove the rotors and inspect the optic triggers for rust buildup on it. There are fine wires on them that will corrode and fail."
 
"Guy,
The plugs were tan. The

"Guy,
The plugs were tan. The insulator is mostly white with a vertical stripe of a rust like color. They do look pretty clean on the electrode though. Maybe a little lean mixture.

I have inspected the distributor, cap, rotor and cleaned all. No carbon tracks or cracks. This engine has a Prestolite ignition and I intend to replace the distributor with the original Thunderbolt IV ignition system.

I bought this engine in 2005 to replace the original one. It is a Marine Power Engine package, which came with aftermarket Carb (Holley), Dist. and Fuel pump. I changed the Carb back to the Webber and replaced the fuel pump. When I change the ignition system it will be back to original components. This will require a new ignition coil.

I will proceed with your suggestions."
 
Hey fellas! Some of you profe

Hey fellas! Some of you professional 24/7 boat mechanics jump in anytime; please?

...a vertical stripe of a rust like color: is it on all of the plugs? Rusty moisture dripping down on the insulator? Look above them for rust. If it was a black stripe then it would be spark leakage thru the plug insulator.

...plugs were tan: perfect; carb is working fine

...has a Prestolite ignition: plugs say it has been working. There should be a test procedure on the web for it.
 
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