NEED HELP Marine Power 340 running like crap

[adamsjstt]

Before I ever put the boat in the water I checked the plugs, cap, and rotor. Plugs seemed to be a little black put all where consistant. Cap and rotor had a lot of corrosion so I cleaned them up and reinstalled. First time on the water engine sputtered a little at take off just after plain then ran fine, the rest of the day it continued but didn't think a lot about it. Second time out same situation so I replaced cap and rotor. Yesterday (third time out) its worse. I had to winterize the engine and in doing so the bolt that connects the electrical connector to the knock sensor on the port side stripped out and I didn't have time to replace it so the connector spins on the sensor. Could this be a issue? What does the knock sensor do? After getting home I pulled the plugs, 6 of them are lightly tan and to of them are still a little black and one was wet with gas. I also did a compression test 6 of them are at 120 and the other two (same two that the plugs are a litttle black) had 145 and 150. Any suggestions ?????
Boat has 430 hrs, I changed the fuel filter and removed all the old gas before the first trip

[mls6722]

I would not worry about the knock sensor until you get the mis-fire issue worked. It sounds like you definitely have 1 or 2 plus not firing. Easiest thing is to shotgun the plugs and wires. It's probably the plugs, hopefully fouled by the bad cap and rotor.

[talltigeguy]

I am no mechanic, but shouldn't the compression test show all cylinders within a 10% margin of each other? I do not think that is the current problem, but I thought that spells doom for the engine down the road.

[Surf406]

check the paddle wheel on the bottom of the boat as well if it is cracked or broke it will send a bad signal to the engine and usually puts it in lymp mode.

[dom w. forte]

Who did the compression check and how was it done, example all plugs out, throttle wide open ?

[adamsjstt]

Who did the compression check and how was it done, example all plugs out, throttle wide open ?

I did the test, I have a compression tester I use on my dirt bikes. I pulled one plug at a time screwed in the compression gauge with the brass extension that is about 2 to 2 1/2 inchs long (except for 1 cylinder I had to screw the tester in without the extension because it hit the exhaust maanifold) unplugged the coil and removed the coil wire and cranked the engine for about 4 seconds to be sure it came around to intake stroke, no throttle and reinstalled the plug before testing next cylinder

[boatwakes]

Your compression test technique was fine. 120 is fine, 145 and 150 are fine, kinda odd but fine. If the problem persists after all the spark plugs and the cap and rotor have been replaced and the plug wires have been replaced (firing order is 1-8-4-3-6-5-7-2) then there are several other places in the ignition or fuel systems it could be: distributor ignition sensor, coil, stuck fuel injector, too high or low fuel pressure, etc...
Does it only do it at idle, at speed or when you're applying throttle to get on plane? that part wasn't entirely clear so I'm asking to clarify. If it's just when you're first applying throttle, the most likely culprit is the TPS or throttle position sensor. If it's at idle, the IAC or idle air control valve is a likely culprit, and so on.
However, I suggest that you find a mechanic who has a scan tool to check the computer for codes so you're not chasing ghosts. Almost every sensor on those engines runs about $75 and a code scan at around $85 could save you some trouble. Good luck.

[adamsjstt]

The last trip out I let it warm up for 5 min, seem to idle a little rough, throttled up slowly to around 20mph and could tell there was still a problem, sputtered a couple times, applied more throttle to around 30 mph, no sputter but it seemed like the exhaust was plugged up. Like a muffled sound. Then the engine seemed to clear up, sounded better and reved up a little so I was like yea there we go then it reved even higher and sounded even better, that lasted about 4 seconds. Because it happened twice I tend to believe its 2 cylinders. When I was cruising around about 15mph the engine would surge (rev up) and it seemed to sound better but that only happened a couiple times and each time it lasted about a second.

[adamsjstt]

Seems to idle rough. Takes about 4 seconds of the starter turning before it fires. Getting up to plain takes a while and the exhaust sounds like its plugged up, once on plain it holds speed with no sputter just seems like the exhaust is restricted and at WOT it was going about 35mph at 4000 rpm

[dom w. forte]

You will not get accurate readings doing a plug at a time as well as with throttle closed this will not allow enough air in to be compressed, this could account for different readings.

[dom w. forte]

Suggest a complete fuel system evaluation, pressure and volume, unplug one pump at a time and listen to them run, sounds like you might be looseing the low pressure pump.

[adamsjstt]

You will not get accurate readings doing a plug at a time as well as with throttle closed this will not allow enough air in to be compressed, this could account for different readings.

So I should remove all the plugs and set the throttle at wide open? How long should I let it crank

[robert theisen]

So I should remove all the plugs and set the throttle at wide open? How long should I let it crank

3 to 4 seconds per cylinder is enough.

[adamsjstt]

Going to the boat to do some more tests. How do I check the plug wires?

[NICKYPOO]

This really sounds to me like old gas chunks in the line somewhere. Have you pulled all the fuel lines and cleaned them out? Especially the line from the tank? You say you drained the old gas. Did said gas sit for an extended amount of time with no stabilizer? When you drain a tank is when the floaties can get sucked up into the system.

It would be a real pisser to find out it was something simple after checking all the hard stuff first.