71 Mercury Thunderbolt 1150 runs great o/w i/w bogs under pressure

[keithmercy]

So my 1971 1150 Merc Thunderbolt fires on first try and idels and runs throttled up great with muffs in the driveway.

At the lake still fires strong and idles fantastic. Even runs at low speeds 5-10 like it works fine. Once I try to hit it into 3/4 or full throttle she boggs right down on me. sometimes dropping back to a crawl at full throttle others not.

I had it out one day and she ripped my 2500 pound boat with no problem around at 30 witch has been the norm at full speed. The next time in the water this problem existed from the time I put her in.

I replaced plugs that were not good problem still exists. Not much of a boat motor guy. Just though before bringing it to a marine mechanic I would get some potential ideas or things to look at to see if I can fix.

Any ideas ? Thanks


keith

 

[CV16]

Re: 71 Mercury Thunderbolt 1150 runs great o/w i/w bogs under pressure

I'd guess that you need to clean the carbs. Or try running SeaFoam through it.

 

[Huron Angler]

Re: 71 Mercury Thunderbolt 1150 runs great o/w i/w bogs under pressure

I've had very similar isues with my '71 Merc 80hp an I thought it might have been a spun hub based on some research.

However...I checked the hub and there is a scratch on it that remains lined up before and after experiencing the motor bogging down.

This pretty much eliminates the spun hub as the culprit. You can test yours in the same fashion by marking the hub on either side where it meets the prop and making sure it still lines up after running the motor on the water.

Other than that I'm curious what other problems cause these symptoms since I am still not sure if I have an issue of not.

It has planed out my rig just fine the last few times on the water, but I suspect the problem is still lurking somewhere.

 

[keithmercy]

Re: 71 Mercury Thunderbolt 1150 runs great o/w i/w bogs under pressure

what is sea foam?

 

[Huron Angler]

Re: 71 Mercury Thunderbolt 1150 runs great o/w i/w bogs under pressure

what is sea foam?

A fuel additive that is combustible, is is used to clean gummed up carbs and stuff. Notorious for smoking like a banshee when used properly.

Helps get the fuel flowing a bit better in some cases.

 

[yamahoppermx]

Re: 71 Mercury Thunderbolt 1150 runs great o/w i/w bogs under pressure

i had the exact problem with my 1977 ,1150.my problem was my linkage was dis-connected.when i throttled up,the top carb was the only one working.are all three butterflies opening?could be dirty carbs,fuel pump,or the screen could be full of gunk at the bottom of your gas tank.