Help! Lost 3 mph
 

Today I lost 3 mph and 300-400 rpms each motor. This happened between one ride and the next. All gauges read normal including trim, except for the speed and lost WOT RPM's. Motors sound perfect at all speeds, no stumble, no miss. Visually examined props, drives, hull, tabs, all perfect. Boat can be felt to bog slightly and then loosen up again. I can't figure it out. By the way my boat is a 1997 31 Sonic with 454 Mag carbs, 200 hours. Also every once in a while at WOT a twitch could be felt in the steering ( standard merc power steering, not external ) . It feels like the boat is dragging something. Any ideas???:confused:

I think I will check the fuel filters and the gas in them, but I don't really have any solid ideas. Someone on this board always has the answer to anything, hopefully someone can help me.

I'm experiencing the same thing. I've checked the fuel filter, seperator, next is the fuel pump. Beyond these..I'm open for suggestions too.

I would try the easy stuff first, fuel filter& pressure, maybe check the spark plugs, how are they coloring{rich, oil fouled, lean} todays gas can be a problem sometimes, take a gas sample in a glass jar, let it sit for a while, do you see any water or trash in it, let us know what you find, good luck. Mark

Since the problem is the same with both motors at the same time I would think it has to be fuel related. Bad fuel filters could also be part of the problem if both engines sucked up something out of the tank. Chang the filters and put in a bunch of dry gas and some premium fuel.

Sounds like something isn't reading in corelation with the back of the boat ie; drives and/or trim tabs check with straight edge in relation to bottom and indicators.....next would be something fuel or electrical related.....try that and get back to me......

Sean

Thanks guys, I'm heading out to the garage to mess with it now. Both engines down on power is confusing me, as they feed off seperate fuel tanks. But I of course fuel them at the same place at the same time. I'll post any results as soon as I have some.

Sean, I agree with your guess, the trim for drives and tabs I checked as soon as I got it out of the water last night, including trimming the drives to their top speed position. I couldn't find a fault. I have solid hub props, so its not a spun hub.:confused:

I have an idea bring it to World Class Marine in Jersey....they can diagnose it and you will still have your shirt left when you're done....they'll even buy you lunch......Sean

856 498-9606

Check spark plugs for corrosion. Is it more or less humid/hot than earlier in the season? What condition is distributor in?

I had same problem on my 358 this summer. I cleaned my air filters and changed the fuel filters. I gained most of the speed back. I will change the plugs and probabley get it all back.:)

I pulled off the fuel filters and found a little water in the stbd one. I will get new and test the boat tomorrow. I don't have the tools at home to cut open the filters but I'll bring it home tomorrow. I have K&N spark arrestors, I just serviced the filters a couple weeks ago. I will check the plugs, they are 2 year old GM Rapidfires, may be time for new. The heat and humidity are lower now than prev. I don't think its the ignition, but now that I think of it , it is time for new caps and rotors (original)!! Thanks for the help everyone!

I bought the tune up parts but the thunderstorm here is going to delay installation until tomorrow. I'm very frustrated!!:(

I'm sure that it's something simple. How about the oil level . Could you have overfilled by a quart or two by mistake. Also don't be fooled by the fact that both engines are down. A Bad valve spring on one motor will drag down both motors at top end for example. Check that both motors have the same RPM's through out the Rpm range with the same throttle position. Otto

YEE HAH! I found them again!
 

I got my RPM's and speed back yesterday. As a matter of fact , in the cold am (60) I was bouncing off the rev limiter and had to pull back. I like that much better than under revving!!! I had restricted fuel filters, only 31 hrs old. I also pulled out the 80 hr old Rapid Fire spark plugs (platinum, supposed to make power and last 200 years) that looked like crap and cost $4.50 each, and made no discernable running change and replaced them with $1.50 each NGK's that are working great. Also I replaced the caps and rotors because I had forgotten about them :eek: . Thanks everyone for the help, I was thrown for a loop because the way the boat felt when it was acting up was very strange and confusing, not like it just needed a tune up. Its really nice to have everyone help by looking at it from a different angle and share their experiences.
THANKS, Chris

Great, glad it was something simple, enjoy that boat, I think I am going to replace my fuel filter it has 45 hours on it, still runs good but it is time:cool:

I was going to be a smart ass and tell you to push harder on the sticks.............glad you found you're problem. And you also reminded alot of us to change our fuel filters!! Going to do that now!!!!;)

Ultra trick spark plugs are just gimmicks.

NGK/ND/AC/Motorcraft/Champion/Bosch - all make fine plugs in the "normal" category.

I HAVE heard not to use the "small tip" platinum plugs in many applications (namely boats).

A plug with 6,200 separate electrodes, each with a scalloped edge and a groove up the channel that will pick up 3 cabel tv channels is still just a spark plug.


If a plug is the correct heat range, is gapped for the application, and is clean and being fired by an ingnition system that is working properly, thne there is not much to gain with a gimmick plug.

Save your money and just keep spares handy. I change plugs every spring after a run to burn off the motor fog. They always look like they are new.

3 Cable channels........Where can I get them!!!!:p