4.3 mercrusier carb missfing and no power under load

[imknowasdave]

hey everyone i have a problem with a 1999 starcraft gt with a 4.3 merc. has no power under load and misfires and surges at higher rpm. need some advice so heres the long story of whats going on.

the boat ran great last October when the former owner took me out on it before i bought it. sat over the winter........... this season comes around take the boat out hitts 40mph at 3500-4grand and surges. i suspect fuel check pressure getting 9psi. to be on save side take out anti sphion valve, check vent tube, gas cap and all lines free and clear, chaged fuel filter, rebuild carb with full kit set choke and float level. same problem...... so i think im not getting enough fuel volume. swap pump off mydrag car holley blue 110gph with regulator at 7psi same problem and wired to 12volt direct to bypass any fuel cut off percautions. go to check compression find water in 1,3,5 remove heads fix vales replaced cracked exh mantifolds set timing at spec of 10deg purs like kitten at idle. take it out on water same problem. check ign rotor and cap look good pick up looks crapy, change pick up with a new improved one comes with a ground and all sealed up take it out same problem but worse cant get over 20mph and now poping through intake the marina i bought part from advised run 5deg of timing with the new fuels as well.

did some researcher found that the audio sensor system can cause a similar problem by cutting power to ign module so at the ignition module i cut and put a connector in line to bypass saftey system so it can not ground out. same problem

talk to 2 marinas out here they say that the ign module either works or it doesnt but it sounds like ign timing jumping around to me?????

i am an auto tech by trade new boating and it seems my luck follows me around headache after headache any advice would be great

 

[CobiaXL]

Re: 4.3 mercrusier carb missfing and no power under load

Fuel pressure should be 4-6 lbs,maybe 7 lb but that's pushing the needle to its limit.You say you have 9 lbs.,There Is your problem.I would image idle is pretty rough too.Your spark plugs show rich?

 

[imknowasdave]

Re: 4.3 mercrusier carb missfing and no power under load

sorry correction the pressure is 6 not 9 also is it possible someway the drive is limting the power? runs fine in Netural

 

[Bondo]

Re: 4.3 mercrusier carb missfing and no power under load

Your spark plugs show rich?

Ayuh,... I'm bettin' on white, or aluminum coated....

I think it's goin' Lean, 'n detonatin'....

also is it possible someway the drive is limting the power?

Naw, the outdrive woulda gernaded by now if it was outa oil...

Too big a prop will hold it back, but not cause carb backfirin'...

 

[imknowasdave]

Re: 4.3 mercrusier carb missfing and no power under load

prop has not been charged since hittin 60mph last year

 

[Don S]

Re: 4.3 mercrusier carb missfing and no power under load

Put the engines fuel pump system back together'
First do these tests to eliminate the fuel system to the engine. http://forums.iboats.com/showthread.php?t=167561
If that checks out, rebuild the carb. Hi speed side is probably plugged up, and look for water in the fuel bowl of the carb.

 

[imknowasdave]

Re: 4.3 mercrusier carb missfing and no power under load

had done this test to start with because i thought it was running out of fuel, carb has been rebuilt squeky clean and fuel tank has been drained refilled

 

[NHGuy]

Re: 4.3 mercrusier carb missfing and no power under load

Ignition wires? I thought mine were good & when I changed em I corrected a pop back through the carb. I used stainless core marine wires. And bonus they look cool and match the engine trim, RED.
Also recheck for water in your combustion chambers, if there's water there, check your new manifolds for water jacket leaks by drying them well, plugging the low end & filling the water jackets. Then wait & watch to see if any moisture shows in the exhaust tract.
If there's water in cyls also check your exhaust risers for water jacket failure. And make sure the manifold to riser gaskets are oriented correctly, they are arranged different ways for different cooling systems.

 

[imknowasdave]

Re: 4.3 mercrusier carb missfing and no power under load

will check the wires had bad exh mantifold rebuilt the heads and replace mantifolds really thought that was the problem was very mad after 800 didnt fix it

 

[90stingray]

Re: 4.3 mercrusier carb missfing and no power under load

Just to let you know... I had a 4.3 that would surge and backfire at 4000. I had done a lot of the same things as you... I went back thru the carb and problem is gone. Do like Don says... check out fuel system and then look at carb again.