89Mach1

Hey guys, a little background first... The motor in my boat is a 96 454/330hp carbed. Last week had the first run of the year and had some water in the fuel, maybe 10 gallons of fuel at most, so I treated it and added another 10 gallons of 93. Ran fine. During the week I had bought a K&N Flame Arrestor from a guy, $25, and installed on the boat. Went out today, put $100 in fuel in it, ran it up to WOT (4700RPM) for a bit, back down to 3500 rpm cruise for a bit, then back up to WOT, but this time I felt and noticed the RPM was bouncing back and forth between 4200-4400, almost like it was hitting a rev limiter. I ran it to where I was going, stayed the day and then ran it back and same thing was happening. Motor runs perfect all the way up to 4400 then bounces back and forth between 42-4400. GPS speed is correct for the RPM it's running also. Any thoughts on whats going on? Bad fuel maybe? K&N Flame Arrestor maybe? Combo of both? Something else? I'm going to change back to stock arrestor tomorrow, and put some more fuel treatment in as well. Anything else I should look for or think about? Thanks in advance!

Jay

tinman565

Could you actually HEAR the rpm difference, or did you just see it on the tach ?

89Mach1

I could hear it and feel it, felt like a small brake check in a car. Speed was correct for the rpm I was running, and because I'm not reaching full rpm I'm about 10mph slow... As in, I run about 48-50mph at 4000rpm, and 58-60mph at 47-4800 rpm. So when this was happening I was running about 50mph. The tach is correct according to my gps speed is what I'm getting at.

tinman565

Ok..lets start from the begining. When was the last time you did a tune up ? Cap/rotor, wires, plugs, water seperator, fuel filter, coil ? Have you looked in the cap lately ? You could try running some dry gas in the tank (isopropyl alcohol). If everything above is good, or been done recently, you may end up pumping the tank out and filling it with fresh gas.

Oh...and going back to the stock arrestor is a good idea too. If it runs better...sell the K&N to some other sucker...lol.

danlb25

have you dumped out the water separating fuel filters ? sometimes ya gotta dump them several times before all the water is gone

89Mach1

Plugs, wires, cap, rotor mid last year, fuel/water separator end of last season while winterizing, then took off last week and dumped out to see it there was water in fuel because it wasn't staying running. Had this happen before so I knew the signs... Treated fuel last week and it ran fine after. Tank was just about empty this morning when I put $100 in it, I was pretty nervous last week on my way back and thought I might run out... ha!

tinman565

Sir...in my opinion (and no offense to the above guy)...for roughly $13...I wouldnt dump out a filter and re-use it. I'd put a new one on.

And...running outta fuel would REALLY suck...lol. Good luck.

89Mach1

I do plan on changing back to the stock filter tomorrow. Honestly, this is about the only thing different from the last run with it, so I'm kinda leaning towards the K&N being the problem, but who knows... Couple years ago when I had some water in the fuel and treated it and added fresh I lost about 100rpm on top end for a couple days running it. I think I just had to run the treated fuel through it and then it ran fine after, so I'm not so sure it's a fuel problem, but I've been wrong before so.... I'll treat it again tomorrow and change the filter back to stock and see. Any other thoughts? I've never done the coil, could it be going bad maybe? I thought when they go they just go?

tinman565

Do you know when the coil was replaced last ? I personally like to change things...that way I know when they were done. Just like you knew exactly when the last tune up was done. If you dont know about the coil...you can get an MSD Blaster High vibration coil from Summit racing. These are the ones we have always run sir.

http://www.summitracing.com/parts/MSD-8222/

89Mach1

Honestly I don't know exactly when it was done last. I've owned the boat for 2 years now, and the guy I bought it from was a mechanic and extremely picky about things so I'm sure it wasn't long before I bought it that it was done. I can call him and find out tomorrow probably. Do they "start" to go bad or when they go they just go?