Sparkplug diagnosis

[LaborOfLove]

Hi folks - longtime lurker, first time posting. Site has been a major help getting my 2002 2900 SCR back into shape! I’ve done a bunch of work to get the engines going well. Still struggling with my starboard engine, however nothing major(yet...). Can’t seem to get it above 3600 rpm or so. It will go up to 4000 however only if I’m running the port side wide open at 4800. Engines are v6 4.3l thunderbolt v. At 3600 on both engines I’m running about 24 mph on plane. Basically at 3500 or so it starts to lack power, no backfire or strange noises or smells, just seems to run out of juice. I’m going through the troubleshooting options...changed water separator(which had a bunch of junk in it), new props with the factory spec(and same as port side), new exhaust manifold and risers, new temp sensor, new dist cap sensor and wires. Still need to do a compression test, which will come soon(I know I know)Just changed the plugs too. In looking at the old ones I can’t really tell what category they fall into. Rich/lean/oil etc. Would greatly appreciate all thoughts on the old plugs and what they look like could be the issue. Thanks in advance for your thoughts on the plugs.

[LaborOfLove]

Closer pictures

[LaborOfLove]

The other 3 closer. Thx again.

[jrsick]

Your plugs look good to me.
What size props are you running? The starboard engine has a larger prop on it than the port?

[LaborOfLove]

Thx. I believe they are 19 pitch. Same ones on both engines replaced at the same time.

[mmwjr]

Have you checked the timing? Note the Thunderbolt V needs a temporary jumper installed to set the base timing.

[LaborOfLove]

Ah good. Forgot to mention that. Yes. Checked timing. It was way behind initially. Was like 0-2 btdc and supposed to be 8. I corrected but didn’t make much difference at high rpm. Seemed to idle a bit smoother and start up a smidge quicker though. All in all it isles and runs great up to 3500 rpm. Never backfires or stalls out at any rpm. Since then I changed the dist cap so I’ll probably recheck the timing once again.

[mmwjr]

Originally Posted by LaborOfLove

Ah good. Forgot to mention that. Yes. Checked timing. It was way behind initially. Was like 0-2 btdc and supposed to be I 8. I corrected but didn’t make much difference at high rpm. Seemed to idle a bit smoother and start up a smidge quicker though. All in all it isles and runs great up to 3500 rpm. Never backfires or stalls out at any rpm. Since then I changed the dist cap so I’ll probably recheck the timing once again.

Did you install the jumper wire when setting base timing? If not it will not be correct nor will the advance curve.

[LaborOfLove]

Yes, i did install the jumper wire. At any rate I’ll be down there tomorrow and do it again just to be sure. If not timing, could it be that the carb is Gunked up? Or would I be having other issues if that were the case? I had it rebuilt before last season, but the mechanic was suspect unfortunately. Not 100% sure he did everything correctly.

[jrsick]

Your problem could definitely be carb-related. Rebuilt carbs seem to have a 50-50 success rate. In my case it was a 33-67 success rate. Third carb was charm.
I recommend you download manual #25 in the documents section of this forum. See the 8 point carb checklist on page 5B-9. It will help you understand if carb is getting enough fuel and is set properly. If your carb fails any checks, then you have good basis for going back to mechanic.
Also see Testing Thunderbolt V Ignition checklist on page 1C-10, and the troubleshooting steps for "Engine Runs Poorly at High RPM" on page 1C-15. This will provide more detailed info than we can give on the forum, and gives good info for moving forward if you still have problems.

[mmwjr]

If the base timing is good the next thing to check is the timing advance to ensure you're getting proper advance at higher rpm, this can be found in the mercruiser manual. Another item is verify fuel pressure as a week pump will starve the engine at higher rpms, note you have electric fuel pump on this vintage engine.

[LaborOfLove]

Thank you. I will. However in downloading manual #25 it appears it’s only a partial copy. It only goes to page 3A-109. Is there another way to obtain the full version?

[jrsick]

I'm not sure about the V6 manual. I know the V8 manual, #24 is complete and has same checklists that you can use.

[LaborOfLove]

Okay found the #25 online somewhere. I previously compete the thunderbolt ignition troubleshooting, where I learned the sensor in the dist was faulty. Replaced. Other than that we seem to be good. Just re did timing. Made sure purple white was grounded. Things were pretty good. Base timing was about -12 vs the -10 that it’s supposed to be. Also reset the idle while I was in there. Idling at 550-600 in base mode per spec. Then i checked the advance curve. Not sure if I’m reading that chart correctly. My markers are on the wheel and go from -16 to +6. At least that’s what I assume and not the other way around since spec is -10. When throttling up to 3000 rpm with the jumper removed it appears to increase to about -25. About 15 degrees further to the right of my initial timing in base mode. Does that sound accurate? Also engine sounds good where it’s set and runs well. Just for kicks I moved the dist far the other way and it stalled out.

[mmwjr]

Sounds about right. To get a better check but a timing tape from Summit Racing and install it on the balancer, note get the right one for the diameter of your balancer.

[LaborOfLove]

Cool. Thanks for your help and thoughts. Also cleaned out the carb fuel filter. Had a couple tiny pieces in it but looked pretty good. Insane thunderstorm just rolled in so won’t test her out tonight. Hopefully can test this weekend to see if timing made any difference.

[LaborOfLove]

In doing some more research it looks like the knock sensor may be kicking it. Anyone know where that sensor is located on the 4.3 so I can inspect?

[mmwjr]

The knock sensor is on the starboard side mid-engine down low by the oil pan gasket. Do you hear any knocking or pinging?

[LaborOfLove]

I’m no expert, for sure, but I don’t hear anything that sounds unusual to me. Ugh on the starboard side. Hard to get back there! Do they go bad? Engine was sitting for 5 years before last year. Thus far lots of parts have needed replacing.

[LaborOfLove]

I should also ask, I’m struggling to find a part number. Any recommendations on where I can find the part number?