Hesitation problem under load 454 Magnum, I need some idea's
 

Hi,

I have 21' Eliminator with a 454 Magnum carb, with CMI headers turning a 28" bravo.

The problem I'm having is when it get to around 3500 RPM it bogs, hesitates, coughs/sputters & pop's once at half throttle 3500 RPM.

I DID change the fuel filer/water seperater (twice)(no change, in previous years that got rid of the problem)

Added Methly hydrate/ gas line antifreeze.

The carb fully rebuilt including new accelerator pump 2 years ago.

I took the plugs out, looked great.

Replaced the NGK plugs with AC Delco MR43T just to make sure they are working 100% (no change)

Checked rotor & cap, looks fine, no corrosion on the contacts.

The same thing happened two years ago, it was a couple of cracked plugs, and that fixed it, but this time same sypotoms, but its not the plugs.

Not sure what else to try??

Appreciate your input in advance

Thanks!

sounds like timing to me!! check it out

Fixx
 

Check all your power connections and grounds,,had a baja with a 365 mag about a week ago doing the same thing..It turned out to be a loose + cable at the starter..

Dont rule out a bad cap,pull off each wire including the coil wire,they get coroaded under the wire if nobody put diaelectric greece on them..

Visually inspeced connections, unplugged and repluged in connections for a clean connection.

The only corroded ground I could find, was on the base of an electronic thunderbolt ignition block. Cleaned it up.

Twisted, unplugged, and repluged in spark plugs cables on the cap and coil.

Still, no luck.

Tried revving it up in neutral, can hit 5000 RPM, no problem.

I also noticed if I run the trim high (I have a high x dimension, so can run the prop half out of the water) it will rev a bit more.

So the hesitation/ bogging is worse with load.

Fixx
 

does it have a carter carb??

If it does pull the top of the carb off,their will be 2 floats,remove them and unscrew the needle and seats..you will see tiny screens,they usually get plugged up.i usually remove them but you can clean them.. and reinstall everything.

I agree with mrfixxall, sounds like she is running lean with plugged up jetting... But you said the carbs were rebuilt recently(??). Other possibility is a weak spark that gets blown out with load...happens frequently with two strokes and turbocharged engines. The engine acts as if it is paralyzed at a particular rpm when it should be building big torque. The culprit there is a weak primary coil.

still no luck
 

replaced the trigger sensor coil under the cap...no luck

replaced the main coil...no luck

carb is the stock merc carb i think a rochester


could it be bad gas? friends use the same station no problems, but they are running normal boats and probaby dont go over 4000 rpm so they may not even notice

water fue filter dumped out to clear container, no water, did thata couple of times as well

will it still pull to max rpm? or does it only sputter at 3500 and then clears up?

Could be a weak fuel pump or clogged filter at the carb inlet

If it's bogging real bad at 3500rpm's, I would make sure the secondaries are opening correctly and getting plenty of fuel. If the secondaries are not getting enough fuel, (clogged jets ect) it will rev with no load, but not under load.


Darell.