4.3L Mercruiser stalls and backfires at apprx. 2,200 rpm

[TurtleTamer]

That carb is gross. Looks like it had water in it instead of fuel. Is it yours or an example?

I am unfamiliar with your ignition system. I'm used to EST with the module under the distributor cap. I had one go bad that wouldn't allow base timing mode. I later found out it wasn't even GM brand. The engine still ran but again, no base mode so I couldn't set base timing, and I didn't get as far as checking advance/total timing since I had a spare on-hand (GM brand) and it had to be changed.

 

[adm7789]

It is my carb prior to tearing it down and letting it sit in carb cleaner overnight. I cleaned and rebuilt the carb after this photo was taken. AD, thanks for letting me know what this is as I had no idea. I only knew that it did not come with my rebuild kit and it appears to be damaged because it doesn't move at all and is fairly corroded. I bought the boat and was told it needed head gaskets, it turned out to be a cracked intake manifold. The previous owner let the boat sit with old fuel(most likely with ethanol) and probably water as well. What is the role of this piston and could it cause the engine to run lean. My thought was why didn't this come with the rebuilt kit if it's function contributes to the overall carb performance.

As far as the timing, the ICM is the original (Denso I believe), I confirmed this with the part number and Mercruiser's OEM site.

Overall, this appears to be a lean condition and I'm not sure if this is because the timing is off or if that 2GC power piston is causing it.

What's the purpose of this piston? I will look into it now that I know what it is.
Thanks everyone

 

[alldodge]

The power piston is used when throttling up. When this happens the vacuum is reduced and the power piston starts supplying more fuel. Once the motor catches up, vacuum increases and the piston closes. If the piston is not working then it will have a hard time getting up, and also at high rpm (lower vacuum levels) when more fuel is required (lean condition)

They sell new ones, just google rochester 2GC power piston

 

[adm7789]

Thanks AllDodge. How do I know if this will fit my Mercarb carburetor part No.

3310-807764A1?

I'm finding some discussion online that states the Rochester 2GC power piston will not work on a Mercarb 2bbl.

 

[alldodge]

Sorry but don't have an answer for that question. Suggest call one of the places that sell the piston and ask them

 

[hollis900]

It is my carb prior to tearing it down and letting it sit in carb cleaner overnight. I cleaned and rebuilt the carb after this photo was taken. AD, thanks for letting me know what this is as I had no idea. I only knew that it did not come with my rebuild kit and it appears to be damaged because it doesn't move at all and is fairly corroded. I bought the boat and was told it needed head gaskets, it turned out to be a cracked intake manifold. The previous owner let the boat sit with old fuel(most likely with ethanol) and probably water as well. What is the role of this piston and could it cause the engine to run lean. My thought was why didn't this come with the rebuilt kit if it's function contributes to the overall carb performance.

As far as the timing, the ICM is the original (Denso I believe), I confirmed this with the part number and Mercruiser's OEM site.

Overall, this appears to be a lean condition and I'm not sure if this is because the timing is off or if that 2GC power piston is causing it.

What's the purpose of this piston? I will look into it now that I know what it is.
Thanks everyone

Hello,
I know it has been 7 years but I have the same engine and had a cracked intake and after replacing it am having exact same issues and have done everything you listed as well but didn't see that you ever found a solution or did not post it. I am hoping that you figured it out and can please enlighten me. Thanks in advance for any help