Can't seem to find a starting place on this

Port engine power loss at higher RPM

I have a 1985 Silverton SC with twin Crusaders. Took a trip to a local bar by boat only a 15 mile trek one way and the boat ran perfectly. On the way home, I had an issue where the boat slowed approx 5 knts. I noticed both engines temp went up about 10 degrees. There is a large amount of sea grass around so I did what I could to blow it out. Noticed a 10 degree drop in temp. Happened 1 more time on my way home and that was it.

Today, I went about 9-10 miles ran perfect, stopped at the fuel dock and put in some fuel. (This dock turns over a large amount of fuel),

Ran another 8 miles into the inlet and the ocean and turned around because the biting flies were so bad it just wasn't enjoyable. On the ride back I was running about 3200RPM with no issues and got back to the area of the fuel dock. From here its about 2 miles of no wake... ran fine.

As soon as I got to speed up again and roughly around 2800-3000RPM something wasn't feeling right and it sounded like the port engine was 'missing'. slight vibration was noted also.

At first I thought I may have damaged the prop or shaft but testing each engine individually yielded no problems up to 2200 RPM. At one point my port engine stalled while at 1400 RPM. I couldn't get it started again until I gave that EVIL anti siphoid valve a few taps. Fired right up, then I gave it a little throttle and as soon as it got to 2500RPM that feeling of 'missing' started again. I thought maybe I had gotten fishing line or wire wrapped up but wouldn't I notice this even at low speeds?

Anyway I got back to the dock, fire up the engine (port) and gave her some throttle. No strange sounds no missing sounds (a few days ago at first start it did sound like it may have missed - litle bit of smoke from exhaust, cleared up once at operating temp)

Nothing odd with starboard engine either. My plan is to start with the fuel filters, spark plugs and anti siphon pain in the neck. Because there was a slight vibration is why I'm really writing otherwise I would change the fuel and plugs before I wrote this. The vibration is was concerns me. It doesn't happen until you hit 2500-3000 RPM.

I'm hoping after filters and plugs are changed it wont give me any hassles. Just wanted to know if anyone had a similar problem with something other than the fuel and plugs being an issue. What about timing? I would notice that at low RPMs also right?

Ugh <bash head in wall>

-Justin

First thing I'd do is go over the side and check your props.

Jeff

props are fine. everything underwater looks in order. I'm going to start with the spark plugs and fuel filter and stuff of that nature.

Where can I find replacement anti siphon valves? Are they standard sizes?

The valves are NPT threaded, most I've seen are 1/4". If you don't need them, you can remove them.

You may wanna check the advance mechanism, inside the distributor if it still the original one.

I'm not sure about the distributor. I changed all the plugs today and went for a test run. everything ran smooth and I actually it felt as if I had more horsepower and it ran much smoother..

I'm still thinking there may be an issue but I did note on the starboard engine one plug was very very loose which may have been what caused the problem to begin with.

I gaped all plugs accordingly and I even checked the gap on the old plugs and they were all .010 bigger than what is spec'd for the engine.

What kind of problems if any would be noticed with too big of a gap or really nothing?

With a conventional, non-high energy ignition, too much gap can cause some power loss at high power settings. +.010 won't cause big rpm loss, in an otherwise heathy engine, though. But, a very loose plug can't help either.

A loose plug will overheat itself and cause preignition. That might have been one of your problems.

Jeff

I had a similar problem and it was caused by dirty distributor points under the cap.