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Old 07-16-2019, 09:24 PM   #1
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Default Sparkplug diagnosis

Hi folks - longtime lurker, first time posting. Site has been a major help getting my 2002 2900 SCR back into shape! Iíve done a bunch of work to get the engines going well. Still struggling with my starboard engine, however nothing major(yet...). Canít seem to get it above 3600 rpm or so. It will go up to 4000 however only if Iím running the port side wide open at 4800. Engines are v6 4.3l thunderbolt v. At 3600 on both engines Iím 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. Iím 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 canít 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.
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Old 07-16-2019, 09:28 PM   #2
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Closer pictures
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Old 07-16-2019, 09:29 PM   #3
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The other 3 closer. Thx again.
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Old 07-16-2019, 10:47 PM   #4
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Your plugs look good to me.
What size props are you running? The starboard engine has a larger prop on it than the port?
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Old 07-16-2019, 10:57 PM   #5
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Thx. I believe they are 19 pitch. Same ones on both engines replaced at the same time.
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Old 07-17-2019, 02:00 AM   #6
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Have you checked the timing? Note the Thunderbolt V needs a temporary jumper installed to set the base timing.
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Old 07-17-2019, 02:12 AM   #7
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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.
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Old 07-17-2019, 02:24 AM   #8
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Quote:
Originally Posted by LaborOfLove View Post
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 didnít 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 Iíll 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.
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Old 07-17-2019, 02:47 AM   #9
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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.
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Old 07-17-2019, 10:51 AM   #10
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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.
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