Choke Pulloff Not Letting Air Valves Open, Suspect Too Much Vacuum

[r1elkins]

Hi Cliff,

I have a 22' Cobalt boat with a Mercruiser 454 Magnum. You rebuilt my Quadrajet and yesterday was the first day I got to test drive it. When I floored it, there was a significant hesitation around 3000 RPMs. I suspected the air valves were not opening up and was choking the engine. I removed the flame arrestor and watched the air valve as my buddy drove the boat. Sure enough, they weren't opening at all. In fact, it was hard for me to open the valves with my fingers when the engine was idling. There was no biding in the linkage. I decided to remove the vacuum line from the pulloff and sure enough, that removed the excessive force needed to open the air valves. I plugged the vacuum line and had my buddy floor the throttle again, this time the air valves opened around 3000 RPMs and the motor did not hesitate. I spent the rest of the time on the lake with the vacuum line disconnected and plugged. I tried doing some research on this subject and a few posts mentioned drilling out the metering hole to create less of a vacuum so the air valves will open up. Any recommendation? Any reason not to just leave it as-is with the vacuum line disconnected? Does that disable my choke during cold start-ups? Any advice would be appreciated.

Thanks,

Jason

[Cliff Ruggles]

Vacuum goes to zero when you go to full throttle.  The pull-off is needed to open the choke and it actually helps prevent hesitation going quickly to full throttle.

I would loosen the tension slightly on the secondary air door and hook the pull-off back up.....Cliff

[r1elkins]

Vacuum goes to zero when you go to full throttle.  The pull-off is needed to open the choke and it actually helps prevent hesitation going quickly to full throttle.

I would loosen the tension slightly on the secondary air door and hook the pull-off back up.....Cliff

Cliff, the pull-off is still connected to the air door via the linkage. The only thing I did was remove the vacuum line from the pull-off. This is what's happening:

- When the vacuum line is connected to pull-off, secondary doors do not open.
- Unplug the vacuum line from the pull-off, secondary doors open.

If you still want me to loosen the tension from the spring, I can give that a try. Can you tell me how to do that? It wasn't obvious to me when I was looking at it. Also, I'll hook up a vacuum gauge and see if the line has negative pressure when at WOT. If there is a vacuum, any ideas on how to rectify that?

Thanks,

Jason

[Cliff Ruggles]

The pull-off opens when vacuum drops off when you go to full throttle.  It just takes about 1.5-2 seconds to reach full open point.

The spring on the air door will control the opening rate, the pull-off is just there to prevent the flaps from whipping open too quickly and causing a lean condition (stumble).

Adjusting the spring is easy, there is a small allen screw holding it.  Look at the linkage on the end of the shaft, you will see a slot for a small tipped screwdriver, that winds up the spring, the allen screw comes in from underneath it to allow it to turn.  We set them by the factory setting, usually around 3/4 to 7/8 turns.  To bring the secondaries in faster loosen it up slightly (CCW).....Cliff

[r1elkins]

Cliff - just wanted to give you an update. I loosened the spring tension on the air valve and now they open up at full throttle. I confirmed this on the lake yesterday. I also confirmed the vacuum goes from 15 inches of Hg at idle to 0 at WOT. There's zero hesitation now and the engine runs smooth and pulls hard. 

However, my engine won't go over 4000 RPMs. I went from a 23P to a 19P prop and the engine's max RPM is the same as before. I suspect the fuel pump might be the issue as I've checked everything else (timing, anti siphon valve, fuel/water filter).

[Cliff Ruggles]

It could be fuel delivery related.  A boat doesn't accelerate really fast like a vehicle so less tendencies for fuel delivery issues with a mechanical pump, plus in most cases the fuel tank is ahead of the engine not 14' or so behind it!

We use the high flow Marine N/S assemblies in all Marine rebuilds so plenty for what you are doing......Cliff