Loss of RPM’s on both engines at wot

[Merkinbrumsky]

Had the boat out yesterday and put it up on plane for the first time in a while. Could only reach 3000 rpm and 16 mph at wot. Applied tabs to get up on top but missing about 10-12mph.Port and stbd engines running at same rpm. Normal would be 4300ish rpm. Bottom is clean. Props haven’t been touched. Engines are 305 merc’s on a 28’ carver voyager. Marina did plugs, wires, caps and timing end of last summer and boat was running tip top.Thinking it might be a fuel issue as I filled and stabilized last fall and have only burned half a tank this year so far. Seems odd that it would happen to both engines at the same time when running off separate tanks. Marina just did tranny service and water pumps, a month ago, oil pressure and engine temps are normal. Engines start, idle and run smooth. Just missing the top end. Any suggestions. I have ordered fuel filters and will replace next time at the boat.

[g36]

Did you run it after the marina did plugs ,timing etc last year and verify it was "tip top"? Or was it put away after the work for the season?. Can you tell if your trim tabs are actually affecting the boat when your adjusting them?

[Merkinbrumsky]

QR_BBPOST Did you run it after the marina did plugs ,timing etc last year and verify it was "tip top"? Or was it put away after the work for the season?. Can you tell if your trim tabs are actually affecting the boat when your adjusting them?

Yes, I’ve run it a bunch since they tuned it. It was running good up until sometime this summer. I always open it up in the spring to make sure everything is good and it ran great. Last time I had it up on plane was probably late July and it was running normal then. Since then I’ve been just cruising at trawler speeds. Tabs are working for sure. Marina is suggesting the hull is fouled by zebra mussels but I put the mask on and checked yesterday. Hull is clear except for a small bit of algae along the waterline near the transom. Nothing significant. They are going to run some diagnostics on the engines but the season will likely be in the books by then.

[km1125]

You might want to take a look at the flame suppressors on the carburetors to see if perhaps they're getting clogged up with something.

[bud37]

One thing to remember is that one engine can take the other one down ( hold it back from accelerating ) as the props / rpm will be roughly the same on both sides.

Do you have a sync gage on the dash and if so how does it look as you throttle up, does it favor one side more than the other ?

[Merkinbrumsky]

QR_BBPOST One thing to remember is that one engine can take the other one down ( hold it back from accelerating ) as the props / rpm will be roughly the same on both sides.

Do you have a sync gage on the dash and if so how does it look as you throttle up, does it favor one side more than the other ?

I do have a sync. Tough to tell as the the throttle positions have been off slightly as long as I have owned the boat. The stbd side always ends up further ahead to achieve the same rpms but when both are pushed to the limit the port side won’t travel all the way fwd ( about 90% of the way) but was achieving wot rpm’s. I assumed it was a cable adjustment issue and that job was on the list.

[bud37]

Ok then...do yourself a favor and with the engines OFF, push both throttles to their wot limit and go and look down the carb throats and see if the throttle plates are both open the same and whether they are both full open.

At least this will cross something off the list. Check and clean flame arresters while you are there.

[Cooler]

Flame arrestors are easy to clean, but a lot of people just ignore the fact they need maintenance. Not only do dirty arrestors choke off air to the carbs, they can be a real fire danger. I would bet it is fuel related. Did you top off with ethanol gas last year? Good luck! er

[Merkinbrumsky]

QR_BBPOST Flame arrestors are easy to clean, but a lot of people just ignore the fact they need maintenance. Not only do dirty arrestors choke off air to the carbs, they can be a real fire danger. I would bet it is fuel related. Did you top off with ethanol gas last year? Good luck! er

It would be non-ethanol from the marina fuel dock. I will check and clean the arrestors as well next trip as well as change the fuel filters
and confirm both throttle plates are open all the way. Engines are getting up there in hours approaching 1300.

[Viper]

When you replace the fuel filters, dump the fuel into a clean container and note the condition of the fuel. Look for excessive sediment, water, note the colour, etc.

Can you post engine serial numbers?