Four barreled carburetor not opening last two?

[JustPhish]

Morning folks, I have two chevy 350 engines on my 29' chris craft. Yesterday was the first day I took the boat out for any length of time this season. First, I had a problem with my starboard engine where I couldn't get it to go past 2600rpm. As soon as I would throttle above that the engine would start to die down and lose rpm's until I brought back the throttle and readjusted it back to 2600rpm. Back at my dock I took off the air filter and tried revving the engine without it in gear. it had no problem going over 2600rpm, however regardless of what speed the engine was at, the butterfly for the last two barrels would not open. I could manually open it myself and immediately hear a difference in the engine, but using the actual throttle would not work. But it's the same on both engines. Shouldn't they be opening up? Could they both be stuck? Do I have to adjust something to get them to open using the throttle? It doesn't look like there is anything there that needs to be tightened or adjusted.

Thanks,
JP

 

[BaileysBoat]

Re: Four barreled carburetor not opening last two?

If you can open them manually they are not stuck. The secondary throttle plates should open at WOT, you can check with the engine off, but the top air valves/butterfly are opened by air flow only. These will come into play at high throttle/low vacuum.

 

[RogersJetboat454]

Re: Four barreled carburetor not opening last two?

Morning folks, I have two chevy 350 engines on my 29' chris craft. Yesterday was the first day I took the boat out for any length of time this season. First, I had a problem with my starboard engine where I couldn't get it to go past 2600rpm. As soon as I would throttle above that the engine would start to die down and lose rpm's until I brought back the throttle and readjusted it back to 2600rpm. Back at my dock I took off the air filter and tried revving the engine without it in gear. it had no problem going over 2600rpm, however regardless of what speed the engine was at, the butterfly for the last two barrels would not open. I could manually open it myself and immediately hear a difference in the engine, but using the actual throttle would not work. But it's the same on both engines. Shouldn't they be opening up? Could they both be stuck? Do I have to adjust something to get them to open using the throttle? It doesn't look like there is anything there that needs to be tightened or adjusted.

Thanks,
JP

What kind of drive do you have (manufacturer, displacement, model, SN)? Do you know what brand of carburetor you have on the engine (any embossing cast into the carb, or tags)? You most likely have a vacuum secondary carburetor which only opens it's secondaries when the engine vacuum is high enough to allow it. There are adjustments for the secondaries that can be made so they can open sooner or later.

 

[JustPhish]

Re: Four barreled carburetor not opening last two?

What kind of drive do you have (manufacturer, displacement, model, SN)? Do you know what brand of carburetor you have on the engine (any embossing cast into the carb, or tags)? You most likely have a vacuum secondary carburetor which only opens it's secondaries when the engine vacuum is high enough to allow it. There are adjustments for the secondaries that can be made so they can open sooner or later.

I'd have to check for the info as I don't know off the top of my head. But yes the secondary are vacuum and the first butterfly stays wide open when the engine is warmed. How can adjustments be made to the secondaries?

I'll have to check the carburetor when I go down next with the throttle all the way forward but I don't seem to remember ever hearing the change in the engine when the second valve opens even when I had the rpm's up to 4500 last year. I'm afraid to let it get any higher than that and even that high I think is a bit much. Just doesn't seem right.

Thanks for the advice.

 

[Bondo]

Re: Four barreled carburetor not opening last two?

How can adjustments be made to the secondaries?

Ayuh,... Rebuild it....