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.

will only pull max rpm in neutral, or trimmed way up, but with any load, just bogs and backfires

fuel filter on inlet has been removed and inspected, was clean. still...no luck

mine is stock which i think is a rochester, would it a similar design?

carb is stock and full rebuild 2 years ago with new accelerator pump.

stock, i thick rochester, can the jets be checked without taking the carb off?

Make sure the choke isnt stuck closed. I'd make sure secondary butterflies on the carbs are opening.

Not exactly sure on the marine rochestors, but on automotive ones, if the choke is closed, the secondaries will not open.

Might consider pulling the valve covers if it is not a fuel problem. Let the motor idle (do not rev it or oil will go everywhere). Then check to see all your rockers are moving the same while idling. If one or more don't, you ate a cam lobe. This happened to me a few years ago and the engine behavior was just as you described. You won't necessarily find play in the pushrods since a wiped off lobe can be down to the base circle.

+1 on check the timing advance rate. Use a fuel pressure gauge to check pressure while under a load. Doesn't sound like anything mechanical to me but what do I know? I'm always learning. My guess is the timing isn't advancing quick enough under a load or it's not getting enough fuel. Drain your fuel filters into a clear glass container and see what settles. One crappy load of fuel will take forever to work it's way thru. Just my 2cts.

How do I check for the timing advance rate?

Carbs are off now, and dropping them by the carb place for a quick check (it was rebuilt 2 years ago)

I have two tanks, feeding into one Y (no valve) drained the filters about 3 times now, looks perfect, nothing in it, nothing floating, nothing at the bottom.


Have freinds that use the same gas station, and they have no problems (so doubt it would be bad gas)

This is a reply from Scream and Fly:

Sounds like a fuel restriction or air leak into the fuel system or a bad fuel pump. Put a vacxuum gauge and clear hose on the fuel line where it ataches to the motor and a pressure gauge at the carb. Rub the boay under load and observe the gauges. Vacuum should read 3 - 5 inches, any less and air bubbles in the clear hose indicate an air leak such as a broken pickup, cut or loose hose. High vacuum will indicate a restriction such as a clogged pickup, clogged anti-siphon valve, collapsed hose or clogged fuel tank vent. Fuel pressure should be 6 - 9 psi. Normal vacuum and low pressure indicates a bad fuel pump. High pressure indicates a clogged filter in the carb..

Dropped by the carb place that rebuilt it to check the carb, and they told me "the secondarys can't get plugged because the fuel first has to go throught the primarys before the secondarys and the primarys are much smaller"

That makes no sence to me, (or does it??)

I plan to open it up and take a peak. (after I buy the torx to open it up)

Do I need a new top gasket for a Rochester Quadra jet? (was rebuilt 2 years ago, with very few running hours on it)

fixed! very strange a little spring that keeps the secondary flap closed had a littlw worn notch in it.

and that was stopping the secondary carb to open.

lubed the spring up, and reduced the spring tension

works great now. now that i know that caused the problem, i'll dremal the spring notch or get replace the spring.

i think the carb rebuilder who rebuilt it 2 years ago should have noticed the wear and replaced it (probaly a $2.00 part, and i spent 3 weekends and about $300 in parts....frustrating

but its working now

thank you to everyone that helped out with idea and suggestions greatly appreciated

would be nice to try another carb, sometimes a rebuild does not fix them all time..