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Author Topic:   Mercury 90 2-cycle won't turm full RPM

BilgeRat posted 10-09-2010 02:02 PM ET (US)
I started this thread on the "Performance" forum thinking all I needed was a new propeller. I have a 1998 Mercury 90 2-cycle on a 1998 Montauk 17. With the OEM 13 X 21 Aluminum propeller, it turns about 4800 RPM and 41 MPH at WOT. The compression checked good, I had the carbs cleaned and rebuilt, I de-carbonized the engine, new spark plugs were installed, the gas tank was completely drained and re-filled with fresh 87 Octane non-ethanol gas, and a brand-new Laser II 13-1/4 X 20 stainless steel propeller was installed. RPM's increased by about 100 RPM, and top speed decreased to about 38 MPH. I don't have a lot of faith in the accruacy of the tachometer- it sometimes gives obviously erroneous indications, like 3000 RPM at idle- but the speed is measured with GPS. I have looked at the throttle linkage to see if the carbs are actually being fully opened, and there appears to be only a very small amount of "play" at WOT. Could there be something electrical going on that is causing this? Any ideas?
jimh posted 10-09-2010 04:08 PM ET (US)
The only way to check the accuracy of the tachometer is to compare its reading to the reading from another tachometer whose accuracy is verified.
Boat speed measured by a GPS is not precisely accurate. The typical marine GPS receiver has no way to measure speed, but deduces speed by comparing two position of the boat and the time interval between those positions. Speed measurements made that way are generally reasonably accurate, particularly if the GPS receiver has been able to use a precision-fix augmentation.
One method to test the maximum engine speed of an engine is to operate the engine with a test wheel, a special propeller-like fitting which creates a load on the engine but does not generate much propulsion thrust. Another method is to run the engine on a dynamometer, and measure the propeller shaft horsepower.
When an engine cannot turn a particular propeller to the maximum rated RPM for the engine, the most reasonable assumption is that the propeller represents too much load for the engine.
Newtauk1 posted 10-10-2010 01:11 AM ET (US)
Bad stator?
10000 Lakes posted 10-10-2010 01:24 AM ET (US)
The main problem is finding out if there really is one. The owner is relying on unrelibal information in the first place. Until he hs reliable equipment, he is just chasing ghosts.
weekendwarrior posted 10-10-2010 08:48 AM ET (US)
There must be a ton of that boat around with the merc 90 on it, ask around and compare your prop to what they are using. From reading here it seems low 40's mph is average with 90HP so you're not far off. GPS gives very accurate GROUND speed, it does not account for the speed of the water current. So when checking speed with gps, be sure ro do it in both directions then average the results for your real speed.
Tohsgib posted 10-10-2010 10:51 AM ET (US)
38knots would be the correct speed, double check GPS settings.
10000 Lakes posted 10-10-2010 12:22 PM ET (US)
Actually, GPS units become more accurate the faster they are moving through space so as to speak.
The margin of error is greater the slower it is moving.

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Author	Topic: Mercury 90 2-cycle won't turm full RPM
BilgeRat	posted 10-09-2010 02:02 PM ET (US) I started this thread on the "Performance" forum thinking all I needed was a new propeller. I have a 1998 Mercury 90 2-cycle on a 1998 Montauk 17. With the OEM 13 X 21 Aluminum propeller, it turns about 4800 RPM and 41 MPH at WOT. The compression checked good, I had the carbs cleaned and rebuilt, I de-carbonized the engine, new spark plugs were installed, the gas tank was completely drained and re-filled with fresh 87 Octane non-ethanol gas, and a brand-new Laser II 13-1/4 X 20 stainless steel propeller was installed. RPM's increased by about 100 RPM, and top speed decreased to about 38 MPH. I don't have a lot of faith in the accruacy of the tachometer- it sometimes gives obviously erroneous indications, like 3000 RPM at idle- but the speed is measured with GPS. I have looked at the throttle linkage to see if the carbs are actually being fully opened, and there appears to be only a very small amount of "play" at WOT. Could there be something electrical going on that is causing this? Any ideas?
jimh	posted 10-09-2010 04:08 PM ET (US) The only way to check the accuracy of the tachometer is to compare its reading to the reading from another tachometer whose accuracy is verified. Boat speed measured by a GPS is not precisely accurate. The typical marine GPS receiver has no way to measure speed, but deduces speed by comparing two position of the boat and the time interval between those positions. Speed measurements made that way are generally reasonably accurate, particularly if the GPS receiver has been able to use a precision-fix augmentation. One method to test the maximum engine speed of an engine is to operate the engine with a test wheel, a special propeller-like fitting which creates a load on the engine but does not generate much propulsion thrust. Another method is to run the engine on a dynamometer, and measure the propeller shaft horsepower. When an engine cannot turn a particular propeller to the maximum rated RPM for the engine, the most reasonable assumption is that the propeller represents too much load for the engine.
Newtauk1	posted 10-10-2010 01:11 AM ET (US) Bad stator?
10000 Lakes	posted 10-10-2010 01:24 AM ET (US) The main problem is finding out if there really is one. The owner is relying on unrelibal information in the first place. Until he hs reliable equipment, he is just chasing ghosts.
weekendwarrior	posted 10-10-2010 08:48 AM ET (US) There must be a ton of that boat around with the merc 90 on it, ask around and compare your prop to what they are using. From reading here it seems low 40's mph is average with 90HP so you're not far off. GPS gives very accurate GROUND speed, it does not account for the speed of the water current. So when checking speed with gps, be sure ro do it in both directions then average the results for your real speed.
Tohsgib	posted 10-10-2010 10:51 AM ET (US) 38knots would be the correct speed, double check GPS settings.
10000 Lakes	posted 10-10-2010 12:22 PM ET (US) Actually, GPS units become more accurate the faster they are moving through space so as to speak. The margin of error is greater the slower it is moving.