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Guys: I'm stumped. I have a 1985 B28 with Merc 260's. Thunderbolt 4 ignition. I splashed the boat a couple weeks ago and the Port engine started to lag by about 200 RPM's above 2600. Otherwise it started right up, idled smooth and seems to run fine. No miss, just a loss of RPM and you could feel it fall off. Fuel filters were changed in the fall, so I checked the choke, as it was a real cold day when I brought it down. This checked out, so I went out and noticed a voltage drop when it started to lag, from 13.5 to just under 12 at 2700 RPM and up. I was planning on changing batteries, so I did, and also changed out the coils, because the Port coil had oil leaking under the distributor wire. Checked voltage at the new coil and at the battery with the engine running at 1500 RPM, had about 14 or so. I figured maybe a problem with the alternator, so the 4 year old reman was traded for a new reman. Still doing the same thing. I've switched coil wires, no change. The boat had fresh cap, rotor, plugs and distributor sensor about 75 hours ago. The boat ran great when it was put away. I don't want to just keep throwing parts at it. Any ideas? Thanks

Are you absolutely its electrical?? Sounds like you have addressed the electrical side very well.....the voltage drop above 2700 is peculiar....if it is electrical I would start looking at grounds....or corrosion in the wiring harness plugins

Do you have electric fuel pumps or mechanical

I would swap carbs from port to starboard and see if the problem follows the carb to the Stbd engine.....thats simple and does not cost anything bur a few minutes of your time


Harry

I'm pretty sure it's electrical. Maybe it is a ground somewhere. The RPM drop corresponds directly with the voltage drop. I'm in fresh water, so everything is clean shiny in the engine room, so the corrosion isn't obvious. I really don't think it's fuel. Thanks

Sometimes it is the obvious. You said you changed out fuel filters last fall before layup. Do you Layup on the hard? It could be a fuel issue? You splashed 3 weeks ago, your posting it now, maybe now, you pulled some bad fuel and only partialy clogged the filter but at high end its starving.
Had an issue like that at 2 different r.p.m. scenarios, redo the filters, cant hurt, at worst you have 2 spare filters. Do you have a racor or other Pre/filter before engine fuel filter or are you just using the engine fuel filter itself? thats important

Ric: I'll be changing out the inline filters. The spin-ons are new. I'm also going through all of the grounds and other connections one at a time. Thanks

With the Thunderbolt ignitions, there's is a small pick up on the distributor. I think its called an ignition module. Its about half the size of a credit card and looks like a small circuit board with 2 wires coming out of it. Its about $75. I had the same thing happen to my old 28 back in 2001. It drove me nuts trying to determine what it was. If you do a search for troubleshooting Thunderbolt igintions it will probably come up.

Mine was only about 7 or 8 weeks old and it crapped out.

I'll bet you anything that's what it is.

What a site! Who the hell would have thought of that! Walter

Guys: Went through the primary grounds again and I think I got it. The Port ground terminal has three grounds on it and after flattening them out, putting new flat washers and a hex nut on it, it cranked up and held 13.5 to 14 volts from 800-3000RPM's at the dock. Both engines are consistent (I did the same to all four terminals). I'll confirm later this week when I can get it out for a ride. The battries had been set up the same for four seasons, so it didn't jump out at me. Doug, I had thought about the piick-up sensor and switched them port to starboard. I had changed them about 75 hours ago when I did a complete tune-up. Thanks for the replies. It kept me from getting off track. Leigh

walterk wrote:What a site! Who the hell would have thought of that! Walter

Well I can tell you no one on Nantucket did.

I was stuck out there with the Boat Basin's mechanics swtching every thing back and forth to no avail. It wasn't until I called my mechanic back home that he suggested swapping the modules. What really fried my ass was that the part was only a couple of months old and Mercruiser would not warranty it. The part was $75 and it cost $13 to ship it by Nantcuket Air. Unfortunately, it tool the Basin's mechanics 3 days to find the cause and the bill came to $900!

I have seen the same reaction when an el cheapo Auto zone coil was used. The coil would "heat soak" after a while at speed and fall off, then would not idle...just died below 2,000 rpm. And yes, a Twindisc can be slammed into gear at 2 grand in a pinch.

Once a decent high performance MSD Blaster coil was installed all was well (yes it was me that tried the Auto Zone coil....duh) I really got my $12 bucks worth of coil 35 miles offshore!