Think I found my possible culprit for low WOT at max rpm

[Nextelbuddy]

So I've made a few different posts about propellers and low wide open throttle at max RPMs on various places..


So so I went ahead and started digging in today

I went ahead and started to replace the cap rotor spark plugs and spark plug wires. I'm using Quicksilver Mercury parts for the cap rotor and wires. The plugs and replacing are the Quicksilver AC Delco MR43T

The The plugs that I'm pulling out are NGK 6 series plugs can't remember the exact number.


One of the spark plug wires I went to unclip and it wasn't even clipped in the boot was just resting on the spark plug itself but nowhere near attached so it looks like for starters my engine was running on seven cylinders and not eight. And then another spark plug fell apart on me when I pulled it out the porcelain just split so that looks like another plug that was not performing well.

In In that scenario my engine was pulling 4200 RPMs and getting a max speed of about 35 to 36 mph the quantity GPS and the onboard speedo.

I I wasn't able to finish the plug job tonight because the spark plug adapter I have is too long and thick to fit over the spark plugs with the current manifold configuration I found out the hard way. So it looks like I'm going to take my spark plug adapter home and cut it short and grind it down some to get it to fit better

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Also I went to replace the coil and the coil that the previous owner had put in was some cheap Napa coil so I replaced it with a quick silver brand coil. I'm hoping that helps get a better spark for more power.

I'm curious if anybody else has any experience with an engine running on seven cylinders and yielding the RPM and wide open throttle I've been getting.


I'm really hoping that the soft issue in the engine runs a little bit better

Al also I tried clearing the flame arrestor clean with brake cleaner it was pretty clogged up with dirt and oil I still need to clean it some more because I can went dry and it was still pretty dirty.

​​I mean obviously the engine was down on power if it's only running on seven cylinders because I'm just looking for a little peace of mind before I try to get it in the water lol

 

[Rick Stephens]

All you can do. Start at A, move to B, work your way through things and try it out.

 

[gm280]

Sounds like it was way over do for some maintenance... That usually is the first thing you do looking for top end speed. You haven't yet reached a baseline with broken and damaged parts...

 

[Nextelbuddy]

Thanks guys I appreciate it

 

[Nextelbuddy]

Well just found another culprit.... previous owner had cylinder 2 and 4 swapped.... Cylinder 2 from distributor cap was going to cylinder 4 and #4 wire on the cap was going to cylinder 2.


So the things I found wrong so far causing my max speed of 36 mph @ 4100rpm were:

- Cylinder 6 spark plug boot not connected
- 2 spark plugs with cracked porcelain
- all 8 spark plugs old looks like they've never been changed in years
- cylinders two and four swapped with each other on the distributor cap

Change change the fuel filter and I'm really looking forward to giving it a try on the water tonight or tomorrow.

It it tooka really long time to get the engine to crank over I guess because there was no fuel in the system because I changed the fuel filter but once it primed the engine started right up. I'm assuming that the base timing and the spark plug water should be fine now because it cranks on the first try and it seems to idle pretty smooth it doesn't sound like it's missing.

I'm I'm just extremely surprised that the engine even ran as well as it did with the issues I found.

I I will report back with my experience

 

[Nextelbuddy]

Issue resolved.. new plugs, wires, cap, rotors, proper plug location from distributor cap to cylinders.. all plugs actually plugged in also this time.

I I was able to get the boat up to 4800 RPMs at around 47 mph trimmed all the way down so I'm pretty sure if I trim up I can get it up to 50 mph.

very happy because the boat feels completely different and it gets on plane super fast now.

Thanks for everybody's help

 

[gm280]

Issue resolved.. new plugs, wires, cap, rotors, proper plug location from distributor cap to cylinders.. all plugs actually plugged in also this time.

I I was able to get the boat up to 4800 RPMs at around 47 mph trimmed all the way down so I'm pretty sure if I trim up I can get it up to 50 mph.

very happy because the boat feels completely different and it gets on plane super fast now.

Thanks for everybody's help

Very glad you got it running correctly. I hope you learned a lesson that it takes more then just adding fuel and boating on. Just like any other engine-d equipment, you have to do the maintenance to keep it running correctly. Happy boating...

 

[boater_stevo]

I think this may be what was causing my similar issues on my 3.0 MPI. Went to change the plugs and found a loose boot. The plug was wet and kind of gummy, all the others were dry/tan. Assume your non connected plug was similar?

 

[Nextelbuddy]

I think this may be what was causing my similar issues on my 3.0 MPI. Went to change the plugs and found a loose boot. The plug was wet and kind of gummy, all the others were dry/tan. Assume your non connected plug was similar?

could very well be 1 cylinder down for you as well. double check all your plug connections and make sure each cylinder is connected to the appropriate location on the distributor cap as well.

 

[boater_stevo]

could very well be 1 cylinder down for you as well. double check all your plug connections and make sure each cylinder is connected to the appropriate location on the distributor cap as well.

Got it out on the lake today. I was back up to 4300 rpm at WOT. Huge improvement. Was definitely a bad plug. Though now I’m thinking the injector is filthy and that’s where my last 400 rpm are hiding. I guess these engines like all cylinders firing, who knew?