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RPMS Vary at 3000 RPMS

J

Jfreeman1412

Regular Contributor
Hello Guys
I am running a merc cruiser 228 with a quad jet Rochester carb in a 30' trojan with twin screws. When we are running at about 3100 rpms the engine will occasionally rev up and then back down 200-300 RPMS and it will not take any more r's up to wide open throttle. It gets right up and goes good out of the hole and will run good at 3100 with an occasional increase in rpms, it also Idles down fine. So far we have changed the both fuel filters(the small one in carb) and water seperator, pulled the intake tube in the take to check the anti siphon valve and pick up tube. We ran sea foam through the carb and the gas. We are 400 miles into a trip and have ran lots of gas through the engines. Any ideas? Thanks!
 
Without me looking it up whats recommended wot rpm.

Say it is 4200-4600 cause most were.

So you must first get the boat engines... both of them at wot to turn 4200-4600 rpm if that is rec. rpm.

Understand me so far ?

And yes i too think your prop is coming out of the water for a second.

But i also think your lugging the engines.
 
Wot is 4100 Rpms. What causes prop cavitation? Trimmed down too much? Only one of the two motors are doing it. I seen these motors run a wot (4100) about 10 days ago.
 
It seems like a engine issue to me. Today it is bogging down sparatically some times 400-500 rpms or more. When I open the four barrel gas flaps by hand sometimes it will take off and sometimes it will bog it down.
 
It seems like a engine issue to me. Today it is bogging down sparatically some times 400-500 rpms or more. When I open the four barrel gas flaps by hand sometimes it will take off and sometimes it will bog it down. Other times it will run for 30+ miles with no hicups.
 
will occasionally rev up and then back down 200-300 RPMS
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Just a WAG but look at the fuel pump leak bleed off hose for fuel. The nipple on the pump where the hose attaches may be plugged somewhat preventing more fuel from the worn diaphragm getting to/thru the hose.
 
Well guys looks like we might have figured out. We turned on the crossover valve for the gas tanks and she started to run good again. The only problem now is we are going through gas in the tank that's engine always ran good on a 3:1 pace. Must have some restriction when debris hits the anti siphon valve in the tank.
 
Well It seemed that I was able to fix it by turning on the crossover. That worked for about 200 miles! But then it came back. Until I shut off the 12volt fridge and the blower. As soon as I shut off the blower the rpms instantly went up 200-300. Also my volt guage up to has quit working? I tested the voltage at the alternator and it tested fine. Have you heard of this? Is it the voltage regulator? Thanks for any advice.
 
As soon as I shut off the blower the rpms instantly went up 200-300.
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RPM increase at the gauge only or did the engine actually increase in power? Alternator is how old? Connect a digital volt gauge temporarily at the helm so you can monitor the electrical system changes w/different loads applied. Old corroding wiring, appliances and elec. devices tend to contribute to gremlins appearing randomly. Sounds like a series of issues have developed over time and now coming to life.

When I open the four barrel gas flaps by hand sometimes it will take off and sometimes it will bog it down.
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It is talking to you...fuel/delivery problem? Water in the fuel or just bad fuel w/junk sloshing around in one of the tanks? Dump the contents of the fuel/water separator into a glass jar and see what settles out.
 
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