View Full Version : 28zx max rpm problem

05-21-2012, 08:54 AM
I have a 1998 28zx with twin 350 mags mpi 300hrsp. 2 weeks ago for tickfaw 200 boat ran fine 4 people full of fuel wot 4900 70 mph. Took the boat out last weekend 2 people 3/4 fuel 4400 trying to blow props out 63 mph. Only change was adding 25 gallons of fuel to the existing 50 gallons. Replaced fuel filters and cleaned flame arrestors which were very dirty. Took boat out today Put 28 gallons of fresh fuel non ethenol and still the same problem cannot get the rpms over 4500 and onlt seeing 62-63 mph any suggestions.

Both motors same rpms.

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05-23-2012, 11:50 AM

No one has any input. I am thinking weak fuel and I have about 70 gallons in the boat. I am gonna add some octane booster to the tank and run it empty.

05-23-2012, 01:53 PM
considering it's both engines at the same exact time I'd think fuel as well. did you change the fuel filters yet?

05-23-2012, 02:06 PM
Wasn't sure from your first post whether the suspected bad batch had ethanol. Also wasn't sure whether that was the first ethanol mix that went in the boat. I have heard that most tanks that live with non-ethanol fuel get a buildup of sludgy varnish inside, and that ethanol breaks that stuff loose but then puts it through to the motors. Such that when a boat first starts getting ethanol, it may clog the fuel filters a couple of times until that varnishy stuff is gone. Just a thought, but I'm curious if perhaps the new filters you just put in are already clogging.

05-23-2012, 03:32 PM
I don't know how I missed the fuel filters part, doh!

05-24-2012, 09:47 PM
no still runs the same with new filters both at 4500 max.

05-26-2012, 10:38 AM
Did the batteries go dead? could the computer have reset the rev limiter? almost sounds like "limp mode" on a car with computer issues

06-02-2012, 11:01 AM
Confusing and interesting situation.
At first, I couldn't get past the octane theory...but, if you have a Thunderbolt V ignition system (and I think those motors did), the computer will read a knock code and decrease ignition timing automatically until the knock stops. Effectively reducing HP and RPM.
This could indeed be your situation seeing how it affected both engine's RPMs.
And the only thing that would tip you off is the decrease in performance.

06-02-2012, 02:07 PM
I would think Gene is on to something. Have you read any error codes? You will need a code reader or a scan tool.

06-03-2012, 10:07 PM
Well fellas, I added octane booster and topped off the tank with 93 octane fuel and ran it down in Florida this weekend for the Billy Bowlegs festival and with 9 adults and full of gear and fuel I hit 4800 and 67 mph. By george I think I've kick it's azz in the rear. It had to be weak fuel. It is running great now. Thanks for all the input and help from everyone.

06-04-2012, 07:30 AM
I think some areas are already pumping E15 fuel, you WILL see a 10% power loss and 20-25% increase in fuel consumption.... the pump may say E10 but don't believe it...