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

[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!......

[7228Sedan]

It was a few years ago... I don't remember. But I will be sending you this unit in the off season for a professional clean up  .

I just need to get it good enough for the fall Striper season!

I'm going to change out the primary rods to the 41 that were on the old carb, and check the float. I might pull the air valve spring off of the old carb as well and try it on the new carb for shoots & giggles... The old carb when it was good never had this issue. Who knows what the knuckle head used in the "re manufacturing".

Is there a rule of thumb measurement for the secondary throttle blades to open based on primary angle?

[Cliff Ruggles]

I don't work on anything "remanufactured" or the Marine replacement units sold on Ebay.........lessons in humility.

I will work on any original Marine carburetor, and get it running FLAWLESSLY for what you are doing.  I've done thousands of them at this point and have learned to stay away from anything but the original Marine units........Cliff

[7228Sedan]

Fair enough I guess... Can you point me in the right direction regarding my question on the secondary throttle blades? I still have the original carb, I'll send you that one for perfection :-)

[Cliff Ruggles]

There were two different arms used to apply the secondaries.  Marine units will use the shorter one so they will come in sooner.  There really isn't much adjustment in the linkage as far as when they start.  Never measured it in degrees, but the baseplates that use the taller arm bring it in considerably later, which is beneficial for some applications, such as towing......Cliff

[7228Sedan]

Thanks Cliff, I appreciate the expertise... I pulled off the carb and found that the base plate had a different linkage as you mention above, as well as replaced primary throttle plates.
Whomever "re manufactured" this carb was a real piece of work... the primary blades were too big and wouldn't allow them to close all the way. It's no wonder that I couldn't get the idle circuit to adjust. The primaries were open way too far even with the idle adjustment stop backed all the way out. I took the base plate off of my old carb and assembled a Frankenstein unit using parts form the original as well as the newer one.  Looks promising.. I'll keep you all posted.

[Cliff Ruggles]

I see this often enough that I refuse to work on anything commercial built/remanufactured these days....lessons in humility!.......Cliff

[7228Sedan]

I made the changes above and took the boat out with a helper. I had someone view the secondaries as I was putting on power. I was able to dial them in pretty nice. Still getting an occasional pop through the carb from 3800 RPM up to 4000. Assuming I need to look into the hanger or secondary rods? Once I gave it full throttle she pulled hard up to 4400 RPM, perfect for the prop I'm running.

All pretty good however when I put the flame arrestor back on the bog returned. I am assuming that the flame arrestor isn't flowing enough air?

When testing a carb without an air cleaner on a car, only to find a bog after putting the air cleaner back on, what could be the issue there?

By setting up the Air valve spring with the flame arrestor off, did I not calibrate it correctly?

Thanks all!

[Sonny]

When testing a carb without an air cleaner on a car, only to find a bog after putting the air cleaner back on, what could be the issue there?

Would that not shift the calibration LEANER?

[7228Sedan]

Would that not shift the calibration LEANER?

That is the most obvious...  I have a tendency to glance over the most obvious from time to time. My concerns were related to the difference in the airflow in form, not necessarily the impact the added airflow has to the mixture.

So if leaning the mixture out by removing the flame arrestor helped (possibly a bit too lean based on the popping I was experiencing) I should assume that the mixture is too rich with the arrestor on... fair enough

I will be trying a larger flame arrestor which should lean things out some. Before I make any other changes, I'll test that.

[7228Sedan]

Able to put this one to bed... the larger flame arrestor solved the problem.

Thank you all for the input.