wd833

have a 2000 35 fountain single step with twin carbed 500hp in the spring boat ran 5300 to 5350 rpm at 84 to 86mph on spedo now motors will only turn 5000rpm at 80 to 82 mph boat is kept on hoist so bottom is clean but the cap and rotor and plugs have not been changed in two seasons could this be the problem. or could the valve springs be worn. the boat has 125hrs and never a problem. boat runs great other than loss of top end rpm and mph. thanks in advance wd833

jpclear

Temp. and humidity could be the culprit. I loose 300 rpm going from cool and dry to hot and humid. --- Jer

Dennis Moore

The plugs, cap and rotor are probably not the problem. Check the small fuel filters inside the Holley carbs (just behind the fuel line inlet flare nuts) and replace the Water/Fuel separator. You didn't overfill the oil pan with oil did you?
Sincerely
Dennis Moore

wd833

hi dennis thanks for the responce. i did not over fill oil and the fuel water seperators were changed last week. i did not check the fuel filters in carbs . i will do that later today. i just thought that if the fuel filters were cloged the motors would break up or missfire under load. any other ideas?

Dennis Moore

Pull out the plugs and check the compression.

If the valve springs were weak the engine would miss at top speed.

Check the float levels and clean out the air bleeds in the top of the venturis with spray carb cleaner.

Run some fuel cleaner through with a tank of fresh gas and clean the inside of the engine with a spray can of Mercruiser engine cleaner. The kind you spray into the carburetor while the engine is running (follow the directions on the can).

Install new RV15YC4 Champion spark plugs gapped at .045 (after the spray in engine tuner).

If anything you will have performed a good tuneup!
Good Luck
Dennis Moore

formulaF302

I lost 300 rpm also from the cold spring weather to the hot fall weather....dropped from 4800 to 4500 rpm with 502's ....talked to 2 other guys locally who had the same problem...I think its weather or hull drag related.....the motors are running sweet except for the rpm loss...just my opinion...running on Lake St Clair....

wd833

thanks for your responce formulaf302 but my problem is not weather related my loss came this fall when the weather conditions and water conditions were perfect

daven

Change all points and plugs or pull plugs see if they all read the same, also check timing, see if any extra water is in bilge (sometimes overlooked) If you have a compression guage see if you can check and look for week cyclinders , if cyclinders are more than 10% off you could indeed have a week or broken spring..

Dave

Gary Anderson

How do your props look? Just a few dings in a Spinelli I used to run would take off that much speed.
Gary