engines balking at 3000 RPM

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Gulfisland
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Posts: 2
Joined: Tue Jun 28, 2022 8:10 pm

engines balking at 3000 RPM

Post by Gulfisland »

New member, 1987 10m mid-cabin, 454 Crusaders
Bought boat 4 years ago with around 600 hrs. Boat lived majority of its live in the big lakes, now on the west coast. Been doing work on it since. Fresh water pumps, heat exchangers, hoses and belts. New plugs and wires, distributor caps, carb overhaul, external fuel filters and so on.
Last year at higher RPM experienced some back fire. Decided to change plugs and wires which fixed it. But every time I try to go past 3000 RPM the engines balk and cut out and RPM drop. It feels like the engines are choking on too much fuel. Pulling throttle back it recovers. Lots of grey/brown smoke.
Only happens under load, otherwise engines rev up to 4500 RPM and running very smooth. Overhauling carbs did not bring any improvement.
Coils crapping out under load?
Another puzzling item. When cruising at displacement speed the tachs randomly "rev up" to 3 or 3500 RPM for up to 30 seconds and then drop back without any changes in engine RPM. Voltage regulator?
Could those 2 items be related to some electrical issues?
Nice meeting you all and thanks in advance for any suggestions.
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prowlersfish
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Location: Lower Chesapeake Bay ,Va

Re: engines balking at 3000 RPM

Post by prowlersfish »

Both engines? If it's truly running rich make sure the chokes are open fully. Open an engine hatch to make sure you're getting plenty of air. clean flame arresters. Other things to check are fuel filters including the ones in the carbs. Is the bottom and running gear clean?
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mrrudely
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Posts: 180
Joined: Sat Jun 08, 2019 11:52 am
Location: Marina Del Rey, CA

Re: engines balking at 3000 RPM

Post by mrrudely »

Welcome
I have a 1989 mid cabin
I know the previous owners and knew that it had been neglected for many years

I would check the anti-syphon valves
Odd that they would both be clogged to the same degree at the same time but worth looking at
fuel crossover setting?
I have had many problems but only one engine at a time (chokes, distributors, coils, valves, fuel pumps)
Is the problem truly both engines at the same time to the same degree?
Maybe someone knows what both engines might share electronically
I cannot speak to the grey/brown smoke

where on the west coast?
I am in Marina Del Rey
rr
10 meter mid cabin
mikeandanne
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Posts: 628
Joined: Sun Sep 09, 2012 11:30 am

Re: engines balking at 3000 RPM

Post by mikeandanne »

Couple things I get from your post.

Sounds like the engines are starving for fuel (lean not rich ), that could be the reason for the grey smoke. If it was rich the smoke would be black colored. Lean can also be a reason for higher rpm backfire along with possible engine mechanical reasons. The pops can be the engine looking for fuel back in the intake.

You cannot tell anything by revving an engine at the dock unloaded, it requires hardly any fuel to do that and could possibly damage something, just not a good practice for diagnosis.

Follow along with some of the suggestions so far and see the results, hopefully it is just the anti siphon valves at the tanks.
Gulfisland
Registered user
Posts: 2
Joined: Tue Jun 28, 2022 8:10 pm

Re: engines balking at 3000 RPM

Post by Gulfisland »

Thanks for the replies
Both engines act simultaneously within a few seconds as soon as they reach 3000 RPM.
Chokes are blocked open.
New fuel filters.
Flame arresters are unobstructed
Haven't found the antisyphon valves.
Running gear clean
It does not feel like engines are running lean. I work lots on old carbureted bikes. We can smell unburned fuel.
Did run the boat with open hatch
Location is Ladysmith BC
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mrrudely
Sporadic User
Posts: 180
Joined: Sat Jun 08, 2019 11:52 am
Location: Marina Del Rey, CA

Re: engines balking at 3000 RPM

Post by mrrudely »

the anti syphon valves on my boat are located on the fuel tanks
they look like ordinary fittings so you may not be able to immediately identify
seems like a long shot at this point but I still would encourage you to check it out
not that it would be the problem but I am assuming your crossover is off?

rr
10 meter mid cabin
swampman
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Posts: 36
Joined: Fri Apr 05, 2019 8:54 pm

Re: engines balking at 3000 RPM

Post by swampman »

Assuming these are equipped with the usual Q-Jets.. have you played with , set /adjusted secondary air valve tension since the carb rebuilds ? Do you know how the secondary barrels are reacting when engines bog?
Brooks
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Posts: 14
Joined: Thu Mar 14, 2013 2:45 am
Location: Long Beach, CA

Re: engines balking at 3000 RPM

Post by Brooks »

Just because I chased this same problem - I found I had several problems that all needed to be eliminated

First - BOTH my distributors were not advancing - check that you're getting 30+ degrees advance. You might get lucky with cleaning / oiling them. My springs were shot, so I replaced both with new pertronix, and have never looked back. It just works better.

Secondarily - after things were running great, they would occasionally stop running due to huge amounts of water and junk in the tanks. had a fuel polisher take 2 pounds of crud from each tank out. it was amazing.

I don't want to curse it, but everything is working great on the boat this year.

Best of luck
Brooks
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