Have a problem with my old 88 mercury 25 hp. It idles nice,runs fine at low rpm,but when it comes near too full throttle,engine starts to shake and it seems like its heavy loaded.I have tried another carb,other coils ,cables,ignition box,trigger,stator. Have even dissassembled cylinder block and everything seems fine,compression is good.

But when I had the cylinder block away from the driveshaft housing, I could see that the exhaust tube attached too the adapter plate, was teared(rusted) complete apart and no longer attached to the adapter plate.

Could this be the reason to this problem?

Anyone here?

Anyone here?

Still have the same problem,when trottle up engine to max ,it starts to shake and makes a buldring noise,if I throttle slighty back shaking stops,but I
cant get max rpm.

New exhaust tube
New gaskets on adaptorplate,exhaust manifoil and intake manifoil
New seals on crankshaft(lower end)
Checked the carburator functions
Idle timing-set to 2 BTDC
WOT timing set to 25 BTDC( cranking with the starter)

Could a bad coil or wire,or wrong spark plug course this problem?

I`m about to give up soon,so if anyone has some good advice I will be very happy and give the engine one more chance.
(sorry fore my bad english,need better teachers here in Norway I guess)