High idle after opening secondary throttles

[nibjo]

Hello,

I have a idle problem with my marine Volvo Penta 4.3 V6 engine (GM 4.3 L). The carb is a Q-jet with number 7046222 and it has been rebuilt according to the previous owner. The idle is just fine at 650-750 rpm until I hit the throttle enough to open the secondary throttle plates. When I do this, the idle will rise to and vary between 1100 ro 1400 rpm. This also occurs if I touch the secondary throttle a little bit by hand when the enigne idles correctly. If I restart the engine the idle is back at normal again.

It seems like the gas will continue to flow through the secondary jets when they are once opened and the intake vacuum is still present. My best guess is a faulty float or an incorrect float height, but I would like to see if anyone with more expertise on the subject has more ideas. If anyone here has the solution to the problem I would be very greatful.

Best regards,
Niklas Bjorkblad
Sweden

[Cliff Ruggles]

Double check the part number, that is not a good number, might be 7045222 instead.

The symptoms indicate that the secondary throttle plates are sticking.  I would carefully loosen the screws and re-index and center the plates so they close each time without sticking.

This is actually a pretty common problem especially if anyone ever removed the plates or loosened them up for any reason......Cliff

[nibjo]

Hello Cliff, thank you for your answer!

I will check the number again, there was some paint and dirt that might have changed the 5 to a 6.

Ok so the small opening that occurs when the plates are not centered would be enough for the eninge to contunute to draw fuel from the jets once it has started to flow? I am asking because I have manually pushed the secondary throttle towards its closed position when the idle was high and it did not change the situation, the throttle seems to close completely by itself. So the only opening that could exist is from the thorttle plates not being centered.

//Niklas

[Cliff Ruggles]

The huge secondary throttle plates seal tight in the bores each time they close.

So it is imperative that they be centered, indexed and verify full closing by holding the baseplate up to a light.  You should see almost no light anyplace around them when they are where they need to be.

Also verify that the linkage is not only bringing them open to 90 degrees, it also must have enough slack to allow them to fully close.  Common problem we see here is that the linkage is actually holding them open slightly due to lack of clearance between the parts.

I'd add here that a 7045222 is an automotive carburetor, not Marine, so for sure check the part number to see exactly what you have.

Most Marine carburetors with the exception of Volvo Penta units will be divorced choke models, the 7045222 has a "hot-air" choke on the side of it, but could have been converted to electric at some point.......Cliff