Rochester Quadrajet-problem

[diesel1960]

Have my project boat out of the water, try to run engine GM350 (1984) with Rochester 4MV Quadrajet carburetor.
Did accelerate engine to 5000 RPM ,but notice the large air valve plate (secondary power system) never opens.
When had boat in the water , under the load couldn?t reach more than 3200 RPM ,but never open flame arrestor to check choke plates.
Now troubleshooting the engine ,and this carburetor is new to me.
If anybody is familiar with Quadrajet ,please advice. Thank you.

 

[alldodge]

Re: Rochester Quadrajet-problem

This carb uses a vacuum release which when the throttle is opened more then half or so, the vacuum is reduced enough to release the hold on the secondaries. If the throttle opens more the secondaries top butterfly will start opening from air flow and gravity. Need to remove the flame arrestor and have a second person to take a look. With only 3200 rpm there could be other issues then the carb.

 

[diesel1960]

Re: Rochester Quadrajet-problem

This carb uses a vacuum release which when the throttle is opened more then half or so, the vacuum is reduced enough to release the hold on the secondaries. If the throttle opens more the secondaries top butterfly will start opening from air flow and gravity. Need to remove the flame arrestor and have a second person to take a look. With only 3200 rpm there could be other issues then the carb.

But shall it open at 5000RPM with no load ?

 

[alldodge]

Re: Rochester Quadrajet-problem

No, because no load does not use much fuel, you need to have the engine under load to get the vacuum to be reduced enough to allow the secondaries to open. You can test this with the engine off. Hold the vacuum release in with your finger and move the throttle full forward, the secondaries will not open. Release the vacuum and do it again and the secondaries will open.

 

[diesel1960]

Re: Rochester Quadrajet-problem

OK thanks,it work .

 

[NHGuy]

Re: Rochester Quadrajet-problem

Actually the secondary air doors are what you see on top. They are spring loaded and only open when the secondary throttle plates down at the bottom are open with the engine running. When the boat is under around 3/4 or 7/8 throttle the secondary throttle plates begin to open and engine manifold vacuum pulls open the air doors. Once you get to the point where the secondaries start to open they continue opening with added throttle til you are at wide open. You can open the secondary air doors and primary choke plate for a look, they are spring loaded and it won't hurt anything. Have someone move the control to full as you watch. You will also see two shots of gas from the accelerator pump in the primary side as the throttle is opened.
You really can't get them to open without a load on the motor or it would over rev.
The reason you didn't see the secondaries open is because they don't come into play til there is much more load on the motor than just freely revving it up in neutral.
It's bad to rev it unloaded, don't do that any more. 1500 or 1800 is enough vroom to clear things if it's rough.

 

[diesel1960]

Re: Rochester Quadrajet-problem

Thank you NHGuy.