Merc 80 HP won't go past 2200RPMs

tphops

Recruit
Joined
Apr 8, 2011
Messages
2
Hi, I've got a1976-1980? 80 HP Merc that won't go past 2200RPMs. It's a bit of a Frankenmerc, FYI. Serial number is 4368118, which says it's a '76 85HP. Cowling and other stickers say 80 HP. Sticker on the Carb cover says '80 M80. I bought it cheap knowing it needed work and compression seemed good. The internal wiring harness matches a '80, but the coils I just replaced I got as a'76 because the picture matched mine, it didn't have the grounding tab that connects to the neg terminal like the '78-'80's do. It had the little pigtail on the back. Resistance to the spark plug wires to ground is ~900 now, was no reading (open) before.

Anyway.Replaced the Rectifer and it now charges OK. All new wiring harness and wires to the coil (all were cracked and burnt) and spark plug wires. I replaced the stator because it wasn't getting spark and the old stator ohmed out bad. The new stator ohms out OK on the low speed wires (blue, blue/white). High speed wires ohm out high at 148 ohms. My manual says 56-76 ohms for both '76 and '80. The CDI specs say 125-155. Judging by the manual I'm twice as high, CDI specs say it's OK. I DVA'd the trigger wires, stator wires and coil at cranking and everything looks good. Triggers are ~5V, low speed is ~230V and high speed is ~70V. I DVA'd it out on the water and the coils are all 300-500V, low speed ~440V and high speed was ~350V. CDI says that as you get up to 3000-4000 RPM the high speed circuit should read higher than the low speed and if not you have a bad stator. I didn't notice any dips in voltage as the RPMs increased to 2200. This is also right when the carbs primaries really start to open up. That makes me think it's a fuel problem but I've replaced the fuel lines, rebult the carbs and cleaned them up a couple times since then. Also rebuilt the fuel pump as it had some crud in the screen. I also pumped the bulb till it was firm at those RPMs and that didn't help. Timing is 4BTDC and 27BTDC max per the manual. Compression is 135, 125, 120, 120. Plug #4 doesn't look as burnt as the others and has a little fuel on it.

Any thought? Maybe the switchbox? Thanks.
 

tphops

Recruit
Joined
Apr 8, 2011
Messages
2
Re: Merc 80 HP won't go past 2200RPMs

OK, I'm a knucklehead. I replaced the trigger though it tested OK, mostly because the wires were shot and electrical taped together. Also repaced the switchbox with the cheapest new one I could find. Still nothing past 2200 RPM. Next time out I brought a squirt bottle with mixed gas and squirted it in there and it raced up past 2200 RPM. I originally ruled out fuel because everything fuel related was rebuilt. I thought I saw a bit of water on one of the plugs and was ready to throw in the towel with a cracked block. One last try I tried new gas, old gas, two different tanks then de-carbed the motor....pulled the carbs apart for the 4th or 5th time and poked every hole and blew and flashlighted in there and everything was clean but I noticed the plastic nozzle didn't look right and finally realized it was BACKWARDS.

I'm pretty sure that's what the problem was, I'll find out this weekend and check the plugs again for water. Hopefully I can get some money back on e-bay for the good parts I replaced, or just keep them for spares or another motor some day.

Oh, and I blew ANOTHER rectifier because I forgot to turn the key on when using a remote starter switch while setting the timing.

I'm working on knucklehead of the year award.
 
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