Carburetor issue?

91 Dancer

91 Dancer

New Member
Jul 14, 2007
76
Athens, Al.
Boat Info
270 Sundancer
Engines
454 Mercruiser/Bravo 1
Just wanted to confirm my suspicion. '91 7.4 Bravo. Runs fine at idle and up through about 2800 rpm. At that point it starts to cough and sputter, even backfiring if I keep increasing the throttle. All this is only when in gear. In neutral it revs fine. I suspect it is time for a carburetor rebuild. Does that sound right?

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Sounds like the secondaries are not opening. Rebuild or replace with an Edelbrock 1410.
 
check the inlet screen at the carb. sometimes this is a quick easy fix since the screen rarely gets cleaned as a routine maintenance issue.
 
Will do. Thanks. If worse comes to worse, is that Edelbrock match?

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Before you get too far into slinging parts, look at fuel delivery as mentioned, mechanical linkage at the carb is sound, plugs/cap/rotor/wires in good shape, etc. Could be as simple as a stuck flapper. The basics first, then go for the fun stuff.

The Holley is a good carb of course - but it's hard to beat a properly working QJet in a marine application.
 
Oh, don't worry. Once I get a good list of things to check, the new carburetor will be way down said list... I can't afford to start slinging parts. That is why I feel so privileged to be part of this forum!

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Sounds like the accelerator pump plunger may be the issue.


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When was the last time the fuel filter was replaced, some have one in the carb. A rebuild is cheap and easy. Sounds like the plunger is stuck.
 
What Carb is on the engine. Carter or Rochestor?
 
It's The Rochester. Fuel filter was replaced last season, and is first thing on my list to try. I will also check the inlet screen this week.

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Yes it is. Looking the the carb on the right side, when you advance the throttle you can see the top of the plunger move down. Rochester's are very easy to rebuild.

Another issue could be the power piston. That is located right in the middle at the front of the carb. Sometimes soaking it with carb cleaner and working the primary butterfly's will loosen this up. It would be wise to disconnect the fuel line while doing this.


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Check out boats.net, they have good parts drawings.


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One of the things I would check is the primary metering rods to make sure there not stuck down. You can do that by lookind down the bowl vent (witch is the large vertical tube under the back fire arrester) and taking a small screw driver or something like that and make sure they push down and move back up. Ussually if you put the screw drive down there and it just feels solid, there struck down. You should be able to feel a light spring movement before it hits hard. IF they are struck done, you will need to get a kit and take it apart.

Something else to do is.... put the boat in the water. Get the engine cover and the back fire arrester off. Take the throttle cable off the carb and prop out of the way. Make sure you have it warmed up. Get it reven up, maybe bring up to about 3500 really quick and close the choke all the way, open throttle all the way and run the choke to keep rpms up or down in a safe limit. Do this for about 5 seconds and do it a few time's. Don't over rev the engine but must keep the R's up to keep from stalling. This will pull any "loose" crude right throught the jets if there is something in them. I bet I have fixs a 100 car's and a few boats by doing this. Keep you face off to the side of the carb, you don't need a back fire coming up at you in the face. Hopefully I explaind this right.

Must make sure the full delievery is good

Have not see to many secondery cause an issue over the years, but it chould be. If Top back flaps don't open up. still should run because the secondary rods don't lift and mixer is still good, The older 70's cars, a leaver was hooked to the choke so the top flaps chould not open up till the choke was off. I have seen secondary throttle plates seize, but thats just like having a two barrel if that happen.
 
Thanks bigblock. All good suggestions. I'll be checking all these things as soon as I can and will report back.

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unless i missed it you really have not determined if your problem is fuel supply related or ignition related....both can produce the symptoms you described.....you will need to determine this before you can identify the part that needs to be repaired or replaced...from the symptoms you posted it could be as simple as fuel starvation at higher rpm's from a partially clogged fuel filter, obstruction (or kink) in a fuel supply line, or a partially clogged anti-siphon vlave on the fuel tank or maybe a choke that is not pulling off all the way once the engine is warm....it could also be from a failing ignition coil that cannot put out sufficient voltage at higher rpm's to fire the spark plugs, or it could be one or more plugs and/or wires breaking down.....it you have not performed a good basic tune-up of the engine including ignition parts (spark plugs, plug wires, distributor cap, rotor button, ignition coil, coil wire), fuel filters (water/fuel seperator and in-line filter screen at carb fitting), and flame arrestor cleaning recently i would suggest you start there....many times problems like this can be and are corrected with a good tune-up.....if you still have the issue then you can move into more in-depth diagnosis such as issues with the carb or issues with ignition components such the ignition sensor in the distributor and the ignition control module....

good luck...

cliff
 
Thanks for the input Cliff. All those are now on the checklist.

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if the fuel has ethanol in it and is at least several months old and not treated with a stabilizer you could have some separation happening which can cause an engine to run rough....also might want to insure the fuel tank vent hose is not blocked by a spider web or dirt dauber's nest......

to help diagnose the problem from a fuel supply perspective you can run the engine directly from a portable fuel supply (with fresh gas and a different rubber fuel supply line to the fuel pump) such as a 5 gallon gas can instead of the boat's fuel tank....if the problem goes away while running from the alternate fuel supply then you know you have a problem with either the original fuel supply line, anti-siphon valve, fuel line filter in the tank, or the fuel tank vent hose.....if the problem does not get better while running from the gas can you still could have issues with the fuel pump, water/fuel seperator, or carb that would require a little deeper diagnosing.....

cliff
 
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