1977 t-bolt 850 bogs down after half throttle or above

[kevblac]

ok guys, I have been working on this old motor for about a month or so. Hope someone out there can tell me whats going on. Im at a loss. Its a 1977 mercury thunderbolt 850, 85 horse power.

This motor had sat for about 5 years, I purchased a 1986 angler 18' center console with 40 gallon internal tank and placed the engine on it. Boat sat for some time as well. I put everything together and it ran great, no problems. I put almost 100 miles on it. Then I sucked up some bad gas that had been in the boat for god knows how long, and she quit running. I changed the filter, and siphoned out the tank and replaced with fresh fuel. Everything ran great for about 2 hours. Then she started bogging down at WOT. Found out that it would run okay at half throttle. But you feed in more fuel and it bogs like its starving for fuel. If I pump the primer ball it takes off and runs great but you have to keep priming it. Got home an rebuilt the fuel pump. Changed the filter gasket and screen and installed another fuel seperater/filter. still doing the same thing.

At this point im not sure what to do next. Any help would be appreciated.

Thanks

[bktheking]

If you find out the stator is bad I have a good spare for you.

[kevblac]

Thanks for the offer, however I have an extra parts engine, hopefully that stator is good. But if thats the problem and the other is bad I will look you up. Thanks

[Chris1956]

It sounds like you should clean and rebuild carbs as you know that is at least part of your problem. You might test out the highspeed winding of the stator. That motor has 4 ign coils correct?

[merc850]

I had the "pump to go " problem and it was a clogged carb screen
carb-filter.jpg in this type of carb, I took them out and threw them away.
I pay attention to what I put in my gas tank - unlike the POs!

[CharlieB]

When performance increased while squeezing the primer you proved it is a limited fuel supply problem.

Double check the fuel pump, a slightly UN-sealing valve, or even a soft hose between the fuel pump and crankcase, will not allow the pump to operate at full capacity.

Be sure all fuel hoses are in good shape, older hoses do not like the alcohol in todays fuel and should be replaced with modern compounded material.

[kevblac]

yes, it has 4 coils. how do you test the stator?

[kevblac]

Update: I put a test tank on the boat this morning. I removed the fuel line fron the out going side of the filter and ran the boat. No changes, still bogs after half throttle and still runs perfect if I prime the ball while running. that should eliminate everything from the filter to the tank.

[CharlieB]

Re-read post #6

[kevblac]

I will be replacing all the fuel lines in the engine and rebuilding the carbs this weekend or early next week. also, I have checked the fuel pump twice and the check valve. I can blow through the check valve in one direction but cannot in the other. Im thinking the check valves are seating fine unless you think that is not a good test. Let me know what you think and I will update after the carb rebuild. Thanks again for all the help!

[CharlieB]

Does your fuel pump bolt directly to the crankcase with a gasket or does it have a 'hose' to the crankcase?

If it is the old 'hose connected' type, that hose can 'soften' and slightly collapse instead of transferring full vacuum to the fuel pump, reducing fuel pump stroke, volume, and pressure.

[kevblac]

The fuel pump is bolted to the crankcase.

[kevblac]

Ok gentlemen, the boat is running! This past weekend I replaced all the fuel lines from the primer ball to the carbs, then I rebuilt the carbs as well. After a few minutes of "on the the water needle adjusting" the boat shot out of the hole and took off like a bat outta hell! I can't thank you all enough for your time, trouble and advice.

Till next time, safe boating everyone!

Kevin