Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Marine 454 no throttle response between 3,000- 4,000

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7228Sedan:
Not sure where to look on this one...

New build Mark IV 454 Mercruiser. Motor runs great up to 3,000-3,100. Instant throttle response nice idle. If I advance throttle to try for 3200, 3,400 I get travel with no response followed by the bog. If I continue to push the throttle it will bounce between 3,000 and 4,000 until there is enough throttle to support the 4,000 revs. Once there it will hold 4,000. As it's only got 4-5 hours on it, I didn't want to rev it any higher then that so I don't know where it goes from there..

I have the following set up on the carb:

Primary jets: 70
Primary rods: 40J
Secondary Rods: CP

I tried tightening the secondary air valve spring and it improved slightly. Prior to adjusting the spring, It would bog really bad and never get to where it would hold 4,000.

The spring is really tight beyond the recommended setting. I'm a good turn and a half past initial contact.

I feel that there is something else up...

Piston spring possibly?

Any and all thoughts are appreciated.

Cliff Ruggles:
What else has been done to the carburetor and why does it have "J" metering rods in it.  Those are incorrect for a Marine unit.

The first thing I would do would be to order our Marine rebuild kit to get the correct high flow N/S assembly in the carburetor and the correct primary metering rods. 

I suspect it's sucking the bowl dry on hard runs, but at least installing the correct parts would be a good place to start looking.  You will also get a new secondary cam/spring which may also be part of the problem with stumbling.

A weak spring can also cause high RPM running issues as you have to wind it too tight to prevent stumble/hesitation/bog, then it doesn't want to allow the air flaps to open fully at high demand.......Cliff

7228Sedan:
Thanks Cliff!

The Qjet on the original motor had the following:

Primary rods: 41
69 Jets
"DP" Secondary Rods

Should I swap them back into this unit?

The old carb air horn was warped, throttle bushings leaked etc, so I went with an eBay reman special to try to save time and aggravation... my mistake. It did have all new parts however. The air valve spring doesn't feel weak when compared to the original carb.

I'm wondering if the high end of the primary circuit isn't hold up its end of the bargain as the secondaries come on?

If I have to lift the air horn off, I'll double check the float level.
I didn't nail it off the line, it was a gradual increase from 3,000 up. There seemed to be a flat spot up to the point where it would bog as well.

7228Sedan:
Adding that while it does Idle great, it loads up if the choke linkage is attached when cold. Idle mixture screws are also fairly ineffective... I've been running it with no choke and it's been fine. Not sure how all of that plays in the big picture.

Cliff Ruggles:
Don't tell me you bought one of those I-5 or similar Marine "remanufactured" or replacement units........JUNK!......

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