When cruising rpm drops 400 then picks back up

[Mr. Demeanor]

2002 200hp carbed Merc saltwater<br />I'm chasing a gremlin. Today while cruising around 4k rpm, the rpm will intermittently drop 4-500rpm for 5-10 seconds then pick back up. I think I may be dropping a cylinder. Carbs have been completely gon through so I am guessing at a bad coil. I pulled one plug wire at a time and they are all working at idle on the hose. I also put a timing light on every wire and observed the strope which appeared steady. Would dropping a cylinder lose bout 4-500rpm?<br />I have a new coil I will be trying but I am open to any other suggestions.<br /><br /><br />Any sugestions appreciated.

 

[JB]

Re: When cruising rpm drops 400 then picks back up

Water in the fuel can cause the same thing, Mr Demeanor, as can debris in a carb.<br /><br />Welcome to iboats.

 

[MercFan]

Re: When cruising rpm drops 400 then picks back up

If the dropping is consistant I'd do the following...<br />Take the boat out with all plugs connected. Let it drop once or twice. Stop. Remove a plug wire. Ride it again and see if the drop still happens. Rinse, repeat. At some point the drop won't occur and you'll be running permanently less 1 cylinder and the drop won't happen. If the drop still happens after you tried every plug wire it means your spark is fine.<br /><br />But I'd also suspect fuel as JB suggested. So do this test after you completely drained your tank. Water collects below fuel, which is where your fuel pickup is in the tank.<br /><br />Debris in the tank can have the same effect aswell.

 

[Mr. Demeanor]

Re: When cruising rpm drops 400 then picks back up

I have the seperator with the clear drainable bottom. I do get a little water in there from time to time. I just bought this boat in December and it was geting water in the seperator more often then now. I switched to the clear seperator so I can monitor it. The tank is full right now. Clear filter in the line is also clean.<br />Is it safe to use a water removing additive in the tank on a 2stroke ie. alcohol?<br />The carbs are spotless. Every passage cleaned and checked. I am confident it is not in the carbs. Plugs are uniform and tan. I redid the link and sync today and made some minor adjustments. Also ran the motor on the hose and wetted it down hoping to find a problem. I bought one new coil. I figure its a good item to have either way.<br />Going fishing Friday so we will see.

 

[waterinthefuel]

Re: When cruising rpm drops 400 then picks back up

JB <br />Member # 1031 posted March 07, 2006 10:00 PM<br />--------------------------------------------------------------------------------<br /> Water in the fuel can cause the same thing, Mr Demeanor, as can debris in a carb.<br /><br />Welcome to iboats. <br /> <br /><br />Hey man, don't blame me, I didn't do anything to his engine. I don't even know him!

 

[Mr. Demeanor]

Re: When cruising rpm drops 400 then picks back up

I think what I have hear is a load vs. rpm spot where the engine just isnt happy. I have checked everything and it is difficult to recreate the problem. A slight bump or decrease of throttle and it clears up. Even changing trim ie. load makes it go away. I would guess that at about 4k rpm the engine is transitioning form midrange to high speed jetting circuits. I think what is happening is that the engine just isnt happy right there. The engine was just rebult and is probably a little down on power. Compression is still down about 5psi from where ut was prior to rebuild. May still be a little extra oil in the tank. We went one jet size up on the cylinder that is oversize just to be safe during the break in. AFter the first extended high speed run yesterday I pulled the plug and it was very dark. Time to put that one back to stock. Otherwise, the engine is running great. Thanks for all your inputs.

 

[gss036]

Re: When cruising rpm drops 400 then picks back up

I have a 1989 200 hp carbed Merc and it has done the same thing, but no constistency and not very often, when it does drop rpms, it is usually around 4000 rpm. You may very well have figured it out, as a transition point for the jets. I thought the transition was around 3k, but that could be it.<br />I have had water in my filter and you can tell the difference in that surge. The only other thing I have thought about was that the water in the filter would get mixed up real good with the gas when in a chop and maybe cause it.

 

[Texasmark]

Re: When cruising rpm drops 400 then picks back up

On adding water absorbers to the fuel. I did that once on a built in tank as I knew the fuel would get stale. <br /><br />In short the alcohol in the stabilizer absorbed the water and the gooey blob made it's way thru the fuel line and at the engine connector blocked the fuel line and completely shut down the engine. <br /><br />Was running fine in the upper rpm's, in the middle of a very rough lake when it happened. Not good I assure you.<br /><br />Had a similar problem with my windshield washer. Used the blue stuff and decided in the summer to add water to dilute it as I didn't need the thermal effect but liked the wash. It too blobbed up and clogged the inlet screen on the spray tank. No more water and no more squirter problems.