Looking for help diagnosing this problem. Boat ran fine yesterday right up until I was about to head to the ramp. Started it up, hit the gas, and it started stuttering and there was a loss of power. Probably should have shut it down then, but it was late and doubt anyone would see us waving for a tow. I putted (pretty much idling) over to where my buddies boat was 400 yards away, and let him tow me in. He was hanging out in my boat. I was having a steam issue prior to this, but it wasn't steaming at all, until shortly before this happened. I was getting white smoke, or excessive steam (couldn't tell), and the water out the exhaust was too hot to touch. The elbow was too hot to keep your hand on. I believe the manfiold/riser/elbows to be original and the boat is a '91.

I pulled the plugs and several if not all had an black residue on them...some more noticeably wet. not water, but either carbon & gas mix, or oil. When I pulled the plugs I did notice a gas smell.

Here are the results from the compression check::

8,7 - 145, 145
6,5 - 140, 147
4,3 - 135, 150
2,1 - 145, 145

My manual suggests 150 psi for the 454 mag. 135 seems a little low, but it's within 10 percent of the highest reading.

I gave the exhaust manifolds/risers/elbows a look over. Noticed rust on the outside near where it attaches to the motor. No noticable cracks. Didn't tear anything down. I was going to pull the oil, but couldn't find a dipstick oil extractor at any auto store nearby. I'll call some marine places around here tomorrow.

Could the Manifold/riser/elbow be clogged causing the gas/carbon mix at the plug??? Could this fouling of plugs then have caused a misfire results in running on 6-7 cylinders causing the stumbling issue I experienced?

I posted this over at checkmate-boats, but looking for all the help I can get.

Thanks.

did you over heat at any point? Is your choke stuck closed? Do you have soot on the transom?

Risers are most likely clogged from rust/scale and need to be replaced. You can soak them in muratic acid to clean out the rust but with thier age replacement would be wise. The stumble could very well have been water getting in to the cylinders from aged exhaust as well. This usually precedes an exhuast valve head popping off due to getting brittle from cold water spashing on it from leaking exhaust risers. Were the wet plugs in the center of the engine 3,4,5 or 6?

mr3dman: The boat never overheated, it was rock steady at 160 degrees. I don't think the choke is stuck closed, but if you tell me how I'll double check. It's been a little finicky to start...but nothing overly difficult. Usually takes a start or two and runs at low idle till it warms up. If you're asking if soot on the inside of the transom...no, neither on the outside. Before changing the exhaust tips I did notice they had a soot in them (had a internal flapper)

Blue Thunder - The plugs were dark in color. They all seemed similar, with a couple having more of a carbonized paste on them....if that makes sense. It was black in color and the consitency of a little less than shoe polish. Not a lot, but I could wipe it on my finger off the threads. I can post a pic. At first I thought this was oil....maybe it's carbon/gas mix....or carbon/water mix??

Do the compression numbers look ok?

Popped off the manifolds tonight. Looking at them I assume they are original which makes them about 21 years old. Posting some pics to get your thoughts.



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I would be looking at ignition for hard start and stuttering issues.....

Full Force, I agree with your post which is why I pulled the plugs. They were definitely carboned up, which made me think it was causing a misfire and I was running on 6-7 cylinders. I'm going to double check for spark at each plug tonight. What else should I look at??

I been having issues with my thunderbolt system on one engine it was the coil voltage was low and that was in the coil, the other seems to be a P/U in distributor as I did the coil and control box on that, I plan to do P/U this wekeend...

I am about to ditch the thunderbolt and go aftermarket... sick of issues due to older parts and new are insane expensive..

I can't tell in the pic, are there any water trails going down from the riser gasket surface towards the exhaust valves? The riser gasket was clearly leaking outwardly.

The water wont cause a miss, just break valves, I bet damn near anything its ignition...