Cannot hit WOT on mercrusier 4.3

[Magnum220]

This was my original post, which became slightly distorted as the tread progressed.

I have a 1989 Thundrcraft Magnum 220 with a 4.3 Mercruiser Alpha 1 drive.
It is a heavy boat at 4000 LBs
When I purchased the boat it had a 14 D 19 P 3 blade Piranha (replaceable blade)prop.
The boat never achieved WOT with that prop. and with a load would not even plane.
I changed to the spare prop that came with the vessel which was a 14 D 14 P.
It now rides and handles much better and cruises at a speed of about 25 MPH. at about 3200 RPM
When I research all that I could most recommendations come back at a 19 P. prop
Is a 14 P prop seriously under propped for that boat motor & O/D?
The motor is solid, starts good and runs good except when approaching WOT.
When hitting about 3500 RPM it seems to lag and almost seems to stall.
I have never pushed it above that point.


Just wanting some "reasonable" feedback from those in the know.

I have now had the following done by a Mercruiser mechanic and still cannot hit WOT, used all OEM parts
I emptied the boat of all non essentials for weight.
Still using a low pitch prop!

• New plugs / wires / distributor / full tune up all OEM parts
• Had the carb taken out and cleaned rebuilt with a carb kit ( Holley 4 barrel)
• Changed primary and secondary inline fuel filters.
• Did notice almost a black jelly substance on the primary bowl filter which i changed.
• Thought the fuel line might have been deteriorated causing the sludge so I put on a new fuel line.
• Changed the ignition coil
• Set the timing which had been slightly advanced
• Compression checks within spec. I'm told.
• Hot wired from the battery to the coil to ensure full voltage.
• The boat starts well/ idles well/ under low power runs well.
• Trying to accelerate the motor bogs down and will not hit WOT
• Even took a buddy to spray gas down the carburetor while accelerating in an attempt to isolate a carb problem under acceleration. No better.

I will pull the boat out and ensure the out drive has the proper ratio for that motor just in case.
I just bought the boat in May it did run OK at trial (believing a tune up was required)

Before all the above was complete I could get up on plane and hit 32-3500 RPM when trimmed.
It now does not go above 2700 RPM and almost seems to perform even more poorly than it did.
When accelerating the motor just bogs and will not accelerate.
Really perplexed as to what other conditions may contribute to this.
Any additional assistance brain storming would be appreciated as we are running out of potential causes.

Thanks, One sure can learn a lot from the intelligence on this forum

 

[Scott Danforth]

stop throwing parts at it and start diagnosing the issue

list of causes for low WOT is in the stickies

check the compression yourself
if all the cylinders are within 10% and all above 125psi with a known good gauge, you should be ok

most likely its carb related and most likely who ever went thru the carb did not clean each passage. most likely its crud in both the power valve circuit and the accelerator circuit.

 

[Magnum220]

The carb was re-built and put through a sonic cleaner. Checked carb while under power getting lots of fuel. BTW I have read all the posts and am trouble shooting according to those recommendations.
So "throwing" parts at it such as tune up, I would have thought this is the right place to start.

 

[Bt Doctur]

with a 14 x 17 you should have no problem with WOT unless your running a 1.50 gear

 

[Magnum220]

Great! Thanks!

 

[Scott Danforth]

The carb was re-built and put through a sonic cleaner. Checked carb while under power getting lots of fuel. BTW I have read all the posts and am trouble shooting according to those recommendations.
So "throwing" parts at it such as tune up, I would have thought this is the right place to start.

unless each passage was chased with a wire to visually confirm that the passage is clean, do not assume that an ultra-sonic cleaner does anything but vibrate the finish off the bowls.

I have had carbs so packed with crud it took pin-drills to clean the passages.

everything you describe points to either bad fuel or crud in the carb.

BTW, its almost never the coil

 

[Cuda1719]

You may want to check the exhaust this is how they will act when the exhaust is restricted. One of the shudders may have broke and is restricting flow

 

[Magnum220]

Yes, good points as well and I will check those issues as well.
Going to also do an advanced timing check as it may be the ignition amplifier. ( Just another thought)

 

[Magnum220]

Here's my logic behind the ignition amplifier. (hope I'm on the right track)
On the 19 pitch prop it would only max at 2500 RPM (consistently at WOT)
When i changed to the lower pitch prop revs would only get to 3200 RPM (consistently at WOT)
When checking the timing it had been advanced so it was adjusted back to 8 degrees (from 12 degrees)
Once timing was adjusted it now only gets 2700 RPM (consistently at WOT)
I am speculating that the previous owner may have advanced timing to get higher WOT perhaps knowing the module was bad.
I'm hoping that by checking timing while accelerating the timing may not advance correctly or be inconsistent.
I will check all other suggestions as these modules are very pricey..

 

[Bondo]

Here's my logic behind the ignition amplifier.

Ayuh,..... Welcome Aboard,...... yer scenario is highly unlikely,.....

T-Bolt ignition either works,........ or it don't,......

 

[havoc_squad]

Here's my logic behind the ignition amplifier. (hope I'm on the right track)
On the 19 pitch prop it would only max at 2500 RPM (consistently at WOT)
When i changed to the lower pitch prop revs would only get to 3200 RPM (consistently at WOT)
When checking the timing it had been advanced so it was adjusted back to 8 degrees (from 12 degrees)
Once timing was adjusted it now only gets 2700 RPM (consistently at WOT)
I am speculating that the previous owner may have advanced timing to get higher WOT perhaps knowing the module was bad.
I'm hoping that by checking timing while accelerating the timing may not advance correctly or be inconsistent.
I will check all other suggestions as these modules are very pricey..

Max timing advance can easily be verified on the water using timing light and compared with what is shown in mercruiser OEM service manual to be the correct range.

I also seriously doubt you have timing issue, hitting a wall on throttle midway is most likely lack of fuel delivery by carb causing a lean condition or possibly dead cylinder(s) either from misfire or compression issue.

If there was an intake, exhaust, or valve issue suspected, those can be ruled out using a vacuum gauge operating on intake manifold vacuum.