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Can't get over 3600 rpms!

J

jerry and sons

Member
I have twin 1991 454's that ran fine two weeks ago, but now one will not go over 3200 rpms and the other over 3600 rpms. I have changed all fuel filters and seperators. with out any change in top end. I ran approximately 70 gallons out of the tank and replaced with 93 octane, but no change. Should I be looking towards fuel lines, Plugs, wires, distributor? Carbs were rebuilt in the begining of the season. Any help or suggestions would be appreciated.
Thanks!
 
How old are the fuel lines, are they ethonal proof? I would lean towards fuel issues as both moters are acting up.
 
Hook both of them up to a portable fuel tank and go for a ride. This isolates the engines from the ship's fuel supply hardware. Hook up right at the engine.
 
what RPM was achieved when it "...ran fine two weeks ago"? in an area with 'hot water' in the marina? any other work done in the recent past?
 
jerry and sons said:
I did reach 4400 rpms on both motors, no work has been done recently. Water temp is no more than 65 ish degrees!
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Do you have traditional distributors with mechanical advance? If so I'd check the advance/spring mechanism on the engine producing the lower RPM's. Perhaps it's not advancing and not allowing it to reach WOT, and in the process the additional drag is holding the other engine back.
 
Fuel is most likly the problem, there straved. Do they share a fuel pump. Check fuel pressuses on both engines and compare, must check at high rpm where problems exsist, if same and normal. Is there filters at carbs, check them. Also test spark strenth, not likely though! These are two most common causes let us know.
 
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If this was my issue, and since it has developed within a two week period, and given that both engines are apparently being affected similarly......., I'd look at this systematically and methodically using a PoE (process of elimination) giving each potential of a 1 thru 10 likelihood.

What's the likelihood of ______ in a two week period:
  • a change to the fuel quality for both engines
  • a change to fuel filtration for both engines
  • a change to engine tune.... again, for both engines
  • a change to Ignition timing involving a change to TA (total advance)....... and again, for both engines
  • a change to marine growth on the hull
  • a change to marine growth on the props
I'd think that the first items on the list would render a very low likelihood.... but the last two items may be higher.

Just a thought!
 
Top speed is 17 to 18 knots, I should top out at 24. I won't be back to the boat until Monday afternoon, I will start the hunt for the problem. Thank you for the input and please keep the ideas coming.
 
We recently had the exact same problem on a 2008 8.1 Gas Crusader. We could only top out at 3400 when we were getting 4400 at WOT previously. Of course engine is newer Crusader model than yours but symptom very similar. We changed everything fluid wise, and filter wise, then sending units too ... still no relief. Then the basic light went on in our heads ... yep still simple fuel, spark power logic - so we changed all the spark plugs AND BINGO, problem solve! Turns out plugs had not been changed in almost 2 seasons and gaps had been burned too wide ... My suggestion having owned Crusaders for almost 15 years. Use the KISS principle first. Keep it simple. Good luck, Allen
 
Rpms got worse. I was able to spin up to 2800 on port and 2200 on starboard. I have replaced all plugs, wires, cap, rotor, fuel line and filters. I am now able to get both motors to stay at a top rpm of 3600, but no more. I think that maybe the fuel has lost octane? Waht do you think?
 
Hook up a portable fuel tank as mentioned and you eliminate the possibility of bad fuel tank pick ups, anti-siphon valves, clogged tank vents, bad fuel, etc etc. Also, if you have fuel tank selector valves, make sure port engine valve is set to port tank, starb engine valve is set to starb tank and crossover valve is shut off.
 
I tend to agree with the bottom needing cleaning. 6 or 7 months ago I had my bottom painted. Afterwards, I could jump up on plane quickly and could do so for a few months. Now I find I need a bit more power to get it up on plane. I do have a bottom cleaning service monthly, last month they replaced all my zincs so I will wait until they clean this month before I complain. My boat sits 5 or 6 days a week, and here in the lagoon, growth is rapid.

Your problem could be engine related, I am only mentioning how a clean bottom helps.
 
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