Rochester Quadrajet Carb Issue

T

tjl5709

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Have twin Merc 260 HP inboards. 1983 model year. GM 350 small block with rochester 4 barrels. I replaced the port engine carb with a rebuilt unit as it was leaking gas thru the jets at idle making it rough. The engine runs perfect up until I start to get into the 4 barrels. This is especially an issue when getting on plane. I hit a spot were the engine starts to lose power and the rpms will drop as I move the throttle forward. It appears to me that there is something holding the gas away from the secondaries as they appear to be opening, but no fuel is being fed and I am losing vacuum in the primaries.

I know there are allot of mechanical linkages with these cards, I just don't have a complete grasp of the intricacies of these things.

Anything simple to try? or am I over my head?

Any good manuals to get for these things?
 
You might want to try a "search" on the net for Quadrajet carbs.

I saw a decent article on "Classic Trucks" magazine (internet) that had pics and explanations, etc.....

Sounds like your secondary metering rods are not lifting.....stuck/binding piston or spring is weak/ missing.

This can be accessed while carb is on motor, keep an extinguisher nearby.

Keep in mind that automotive carbs are different from marine carbs, the basic operation is the same.
 
If jets are leaking, you should be able to tighten them down. Never quite heard of leaking jets. Who diagnosed?

A little more info of what is happening with secondaries when throttle is applied...like sound (hollow vacuum sound), any popping.

Don't fiddle with the linkage unless you understand how to adjust them.

One problem with secondaries happens when the secondary spring becomes or is set too loose and the secondaries open too soon or too much. Two things you can try, have someone driving as as the throttle is advanced so the secondary upper plates open, manually close them slowly. Be ready for immediate acceleration is this is the problem(too loose spring). Another thing, not so dangerous is wiring the upper secondary plates shut. You can drive forever with only the primaries and get better overall mileage. You just won't have the extra umph if you need it for 'emergencies'.
 
I believe the initial problem was a high float level or bad float/needle valve. The "leaking gas" was just too much fuel in the bowl and it runs out the primary jets at idle. New problem is probably the secondary metering rod isn't set properly though it could also be the choke circuit. A vacuum brake is used to prevent secondaries from opening when the motor is cold. These units are pretty reliable but takes a little setup. Search the web for quadrajet tuning. You can do this procedure easily.
 
You might want to review the videos regarding Rochester Quadrajet rebuild. It is a sticky at the top of this forum. Alpheus provided good info there.
 
In addition to all the other suggestions above, bogging on acceleration is generally an indication of an accelerator pump malfunction. It is a simple check, move the acceleator linkage as you peer down the throat of the carb, should see a little squirt of fuel. Gets more difficult from there if you don't see the squirt.

James
 
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