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rpm drops at high speed

GAS engine, transmission and generator repair and maintenance discussion forum.
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mhirschhorn
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Vessel Info: 1987 Carver Riviera 28'

rpm drops at high speed

Postby mhirschhorn » January 31st, 2016, 12:45 pm

I have a1987 Riviera 28 with twin 220hp crusaders. One engine is fine at low rpms, but when advancing the throttle, the revs drop and then go up and down. Checked fuel filter. Rebuilt carbs installed 3 years age because of similar problem, was fine until now. Is the carb bad again or is something else causing the problem?

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waybomb
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Vessel Info: 1987 3697 Carver Mariner
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rpm drops at high speed

Postby waybomb » January 31st, 2016, 2:29 pm

You should have a separate spin on filter as well as one mounted at the carb. Make sure use a 5/8 flare-nut wrench for the fuel line nut and a wrench to hold the filter housing still.

Be very careful reinstalling the filter housing - the threads are fine threads and can be stripped easily. Don't forget the gasket for the adapter.
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Fred
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1987 Carver Mariner
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rpm drops at high speed

Postby SplashyLady » February 1st, 2016, 4:11 pm

Engine surge (not holding RPM) is often a sign of fuel starvation. As said above, check the fuel filter to be sure it's properly seated. Then we need to look at fuel supply.

It could be a fuel pump issue, or sometimes this will happen if the gas tank vent is clogged/plugged. Dirt daubers are famous for getting in the vent line and plugging it up - then the fuel pump can't pull gas out of the tank fast enough to feed the engine. You can check this by simply leaving the fuel filler open and doing a test run - if the engine runs OK - you know the problem. Just get some wire and clean the vent line out.

Fuel pumps are pretty dependable, but check for loose fittings on the suction side - the pump will pull air before it pulls gas, and that can lead to fuel starvation. If you can, put a pressure gauge on the outlet side (between the fuel pump and the carb) and see what pressure your fuel pump is putting out. You should have 4-5 PSI if the pump is working correctly.

Hope this helps - good luck!
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