87 merc 4.3 V6 Problems running and now won't start

New to forum and new to boating. Intermediate mechanic. I have read several posts here thus far and finding it all very helpful. I have recently purchased an 87 searay with merc 4.3 V6 (IV thunderbolt ignition). Motor ran when I bought it but didn't realize it had a problem until out on the water and at times boat would not get fully up to speed at times (not over 3000 rpm at full throttle) and seemed like it was not firing on all cylinders. I have now since changed the plug as a couple were fouled and #1 plug had no gap. Wires, cap and rotor all look good. Ignition sensor was rusty so I cleaned it up. Motor started but idled a little roughly so checked out the timing and it was off. Checked distributor ( gear looks fine, not worn) Set motor at TDC at compression stroke and reinstalled distributor with rotor and cap pointing to #1 cylinder. Also base timed it at 8* BTDC. Engine started right up and ran smoothly. Went back out on the lake and on the first run it ran OK but still would not get up to top speed (between 4400 and 4800 rpm I think???) After that it just didn't run right at all and would not run over 3000 rpm at full throttle and was sluggish. It would however idle OK and start up with no problems. I brought it home and was going to work on it some more today and now it won't start. Not sure what is wrong. Could the igniton sensor be part of the problem? I know it is a long post but trying to give a little history. Anybody can help me out I sure would appreciate it. Hope I haven't created any other new problems with my limited mechanic abilities. :redface: This looks like a great forum...glad to be a member now. If anyone out there has any ideas that would be GREAT! Thanks.....hopefully posted this correctly

Re: 87 merc 4.3 V6 Problems running and now won't start

1 plug had no gap ? Like the ground electrode was pushed to the center electrode ? I would start out with a compression test in order to see if everything is OK internally . Something could have went trough the engine and did some damage

Re: 87 merc 4.3 V6 Problems running and now won't start

Dollarten may be on to something, but also what did you do the to ign sensor? They are not serviceable in my opinion, probly need a fresh one if it's really ugly. And, I don't think you have to base time a TB IV, although I am not versed in the V6. My 5.7 with Thunderbolt IV is just timed while running at idle. Did you ground the green wire while timing her? If it's not required it may have sent you to the wrong mark.

Re: 87 merc 4.3 V6 Problems running and now won't start

dollarten said:

1 plug had no gap ? Like the ground electrode was pushed to the center electrode ? I would start out with a compression test in order to see if everything is OK internally . Something could have went trough the engine and did some damage

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Thanks for the reply. Yes to your question about the plug, the two electrodes were touching. I will compression test. Good advice.

Re: 87 merc 4.3 V6 Problems running and now won't start

NHGuy said:

Dollarten may be on to something, but also what did you do the to ign sensor? They are not serviceable in my opinion, probly need a fresh one if it's really ugly. And, I don't think you have to base time a TB IV, although I am not versed in the V6. My 5.7 with Thunderbolt IV is just timed while running at idle. Did you ground the green wire while timing her? If it's not required it may have sent you to the wrong mark.

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Thanks for your response. I scraped off the rust and cleaned up the contacts with emory cloth. It did look pretty ugly when I first saw it. From some of the posts I've read on the forum it appears that you do have to base time the TB IV. I looked at it again and it may be a spark issue. I checked the spark from the coil and it was very intermittent. Any other ideas?

Re: 87 merc 4.3 V6 Problems running and now won't start

new cap and new leads and plugs and a multimeter is a must.
if you manage to start it before in the pitch black see if there is any arcing across coil and leads.yours may be 25 years old now but require full load available for sparking.

Re: 87 merc 4.3 V6 Problems running and now won't start

I thought Marine spark plugs, wires, cap, and rotor had special magical powers that prevented them from arcing?


So the boat gets going but loses power or just doesn't make full power?

Check your FUEL system completely before you go ripping into and replacing a bunch of expensive electronics.


Start with the pickup tube in the gas tank. pull it out and make sure it is not clogged. While that is out, check the Anti siphon valve for nasty stuff.

I started losing top end power. Turned out that the pickup tube in the gas tank was clogged.