3.7 Misfire

I have a 89 3.7 mercruiser that has recently developed a miss. On my last outing it blew out the exhaust bellow on the outdrive and developed a miss that got worse as we returned to the dock (about an hour or so) I have since replaced the bellow, new points, cond, rotor button, cap, wires, plugs, and coil, no change. Did a compression test with #1 @ 150 and 2,3, and 4 @ 135, reinstalled plugs and ran for about a half hour, mostly at idle 750 RPM and about 10 min. at 1500 RPM. Hard to tell but did seem to be smoking a little gray and sparatically. I pulled the plugs again and #1 and #4 looked like I pulled them out of the package, #2 and #3 were black soot on one side. Any ideas as to my problem? I have added pics of the plug differences, two clean and 2 not.

Re: 3.7 Misfire

Sounds like fuel is not getting to outer cylinders. Bad carb imbalance?

Re: 3.7 Misfire

mpdive said:

Sounds like fuel is not getting to outer cylinders. Bad carb imbalance?

Click to expand...

Or water getting in with the fuel (inside the manifold???). I would love to have seen the old plugs.... They will tell a more complete story than the new ones.

Re: 3.7 Misfire

It has a split cooling system if that makes any difference, and I have the old plugs I can take some pics today but don't know the order, should I have the carb rebuilt or get a new one? I was thinking about hooking up a gas can to the fuel pump and see if that made any difference.

Re: 3.7 Misfire

Have pics of old plugs, same issue, two cleaner than other two

Re: 3.7 Misfire

Here is a little history: First time boat owner, purchased about 6 months ago, fresh water ohio boat that had been in storage for a couple years, drained and changed all fluids including gas, refilled with 70 gals, have been out 8 times now averaging about 20 miles per outing, have a little less than half a tank left. I changed the water seperator filter, but no other fuel filter inline. Looking at it today there may be a filter in the carb, not sure. My carb is the two barrel, but I do have another engine with the 4 barrel and could change them out relatively easily. Of course not sure of the condition.

Re: 3.7 Misfire

Another note: When I drained the tank I hooked up a electric fuel pump to the line and pumped it dry so I am sure there was still fuel in the tank. The fuel did not seem bad and I did use it in my truck and mower with no problems. My question is could there still be water in the tank and because I am getting down on fuel it is causing this problem?

Re: 3.7 Misfire

Try pulling the distributor wires one at a time while the engine is idling to see if you have a weak or dead cylinder. Ypu have two filters in the line. One at thr bottom of the flel pump and one behind the large nut at the carb inlet.

Re: 3.7 Misfire

Thank you, I will give that a try and I will locate the filters and change them out aswell. I am leaning towards the fuel being an issue so I am going to hook up a gas can and see if that makes any difference and if not I see that water does run through my intake so there may be intrusion there and I do have the complete 4bbl setup I can try, truthfully I am grabbing at straws here.

Re: 3.7 Misfire

Issue solved, turned out to be water in the fuel, changed the seperator filter and she is running like new again, hope to take it out this weekend. Thanks for all the help.