1971 Mercury 115 (1150) Thunderbolt

[falcain]

I recently acquired a 71 Glastron GT-150 with a 71 Mercury 1150 115 HP outboard that is in great shape with low hours. It had been stored inside the previous owners barn more than it was run over the past 5+ years. I have had the boat out twice so far, and it has had the same issue both times.

When I get it out on the water, it will run 3/4 throttle (4,000 RPM) with no issues. As soon as I try to go to WOT, it breaks up and act like it's experiencing fuel starvation. As the day progresses, it will start harder and harder and run worse. By the time I took it out of the water last weekend, it wouldn't even get the boat on plane or break 1,800 RPM.

I have replaced the complete fuel line with primer ball. Had the carbs torn down and checked for sediment, replaced the water pump and impeller, and cleaned out both fuel tanks.

I am at a loss about what is causing my issues. I need to figure out what to check next before I waste another day trailering the boat back to our cottage just to have it run like crap again.

Any help or advice would GREATLY appreciated

Thank you

 

[hoosiercaver1]

Re: 1971 Mercury 115 (1150) Thunderbolt

I have a 70 115 thunderbolt that I'm havin similar problems with. I'm still plugging away tryin to figure it out , and I feel ur pain about hauling the boat 25 miles just to have it not start after u shut it down the first time. I'm workin on checking my timing right now. I want to make sure it is right. I'm gonna dump the gas and start fresh. I'm worried that the issue is one of the ignition components, but I'm not willing to just throw parts at it hopin to fix it. If I find anything out on mine I'll be sure to post it and hope it helps u. There's a pretty good write up on the forums about how to link and sync the throttle timing and carbs. Might be a good place for u to start to verify everything is set right.

 

[falcain]

Re: 1971 Mercury 115 (1150) Thunderbolt

Thanks for the suggestions. I will search for the write up you refereed to. I think I've reached the point where i'm not going to take the boat back to the lake until I find a definitive answer to the problem. it's too much work to pull it the hour to the lake to have it run the same every weekend.

Please keep me updated on your results

 

[aussieflash]

Re: 1971 Mercury 115 (1150) Thunderbolt

Next time she struggles pump the ball and see if that helps,if so its probably time to service fuel pump.Diaphragm kits are still available at around $5.
First check spark on all cylinders and of course compression test.
Check carbs are set correctly especially float heights.(often neglected).
If you can find Mercury service manual for your model...buy it.Best $$ you will ever spend.

 

[falcain]

Re: 1971 Mercury 115 (1150) Thunderbolt

I did try the "pump the ball" trick last time I ran it. I was driving, so I had a buddy pumping the primer ball when it started to break up. It did pick up a little when he squeezed it 3-4 times, but didn't completely straighten it out. All three carbs have been checked and serviced within the last week. I checked the diaphragms on the fuel pumps, but didn't replace them.

Keep the advice and suggestions coming. I really want to get this issue figured out.

Thanks

 

[sludgeguy68]

Re: 1971 Mercury 115 (1150) Thunderbolt

Are fuel lines 3/4" all the way to the pump? I just got a 1400 and noticed they were all 1/4" for some reason - too small and could have starved it at WOT (haven't had it on the water yet)

 

[GT22]

Re: 1971 Mercury 115 (1150) Thunderbolt

Probably a typo but 3/4" is serious overkill. 5/16" should work though 3/8" probably
better.
My thoughts anyway.

 

[Chris1956]

Re: 1971 Mercury 115 (1150) Thunderbolt

Yes, 5/16" fuel line is recommended, but 3/8" is better. Check the antisiphon valve at the fuel tank pickup, if the primer is soft or collapses. Ditto for the fuel tank vent.

4000RPM at WOT means you are overpropped by about 4 Inches of pitch. That doesn't help with power. Pull all the spark plugs after high speed run and compare them. They should all be the same (or nearly) shade of brown or black, and may look oily. If you have any spark plugs that are real clean, you likely have water intrusion.

 

[Bdivine32]

Re: 1971 Mercury 115 (1150) Thunderbolt

Well, I am NO boat mechanic, I have my own probs with my lowerend leaking water.. But, I have been told that the ethynal fuel is too hard on the fuel systems on these engines. They suggested a completed rebuild of both fuel pumps and make sure to replace the anti-shipon valves in them! Also, there is an additive you can run that is supposed to nutrealiz the eythanol and make the eingine run better. Also, the cheapest fix for ethynal fuel is Lead additive! These engines were never designed to run unleaded fuel let-alone unleaded + ethynal. Its not environmently sound, but eliminates the ethynal prob and also lubes the upper cyls like they were designed for ( more-so than oil alone).
Now, like I said, I am no Marine Mechanic, but it makes a big deal in older vehicles that havnt been updated or rebuilt to accept the unleaded/ethynal gas! Ethynal is second only to Propane as being the biggest engine killer! (if the engine isn't specificly designed to run them)
Starlight makes the additive for the fuel to eliminate the ethynal!

30 years as an Automotive Machinist, Mechanic, and Auto/Heavy Truck Parts Sales.
My 1.5 cents worth!
Hope it might help?
Bryan

 

[falcain]

Re: 1971 Mercury 115 (1150) Thunderbolt

Bryan,
Thanks for the suggestions. I have been using the Starlite fuel additive every time I fill my tanks. I can't say what the previous owner was or wasn't using, so there is a chance the fuel lines have already been attacked by the Ethenol gas. I have considered replacing all of the fuel lines between the tank and the carbs just as a precaution. I already replaced the line between the tank and the engine including the primer ball and ends. It made no difference in resolving the issue. It actually ran worse after the line replacement. I have two tanks and the engine has the same issue on either tank. The pickups have been pulled out of both tanks and checked for damage and blockage. Both looked good. I also considered replacing the diaphragms and seals in both fuel pumps. I'm willing to try anything at this point. I have to track this issue down.

What would be the best way to check for air leaks in the fuel system? Could I spray starting fluid at certain areas to check for leaks? If so, where and how.

Again, any help or advice would ge greatly appreciated. Either that or I need a name and number for a good classic outboard mechanic in the NW Ohio/SE Michigan area.

 

[Chris1956]

Re: 1971 Mercury 115 (1150) Thunderbolt

Bryan, I think you have been told a bunch of crap. Ethynol is a solvent that cleans tanks and lines, the first time it is used. That can cause issues. However, once you are past that, it works fine as a motor fuel. Also, 2 cycle motors are fine with unleaded gas. Lead was never needed for the two cycle motors, as they have no valves or valve seats. Also, there are one-way valves in fuel pumps, but not Anti-siphon valves. A-S valves are a different animal.

If you want to make the fuel additive companies rich, it is your business. I consider all additives to be snake oil, and unnecessary. I use a fuel stabilizer over the winter, but only because it is real cheap. I doub't it makes any difference at all...

 

[mr 88]

Re: 1971 Mercury 115 (1150) Thunderbolt

Not to nit pick Chris but he said @3/4 throttle he is running 4 g.. Link N Sync would be my first step after verifying fuel delivery.I put see through blue fuel lines on mine and that shows the fuel flow,I had a new bad primer bulb and it started hard. Pulled the wrap and watched the gas being sucked backwards,new bulb,no problem.

Yes, 5/16" fuel line is recommended, but 3/8" is better. Check the antisiphon valve at the fuel tank pickup, if the primer is soft or collapses. Ditto for the fuel tank vent.

4000RPM at WOT means you are overpropped by about 4 Inches of pitch. That doesn't help with power. Pull all the spark plugs after high speed run and compare them. They should all be the same (or nearly) shade of brown or black, and may look oily. If you have any spark plugs that are real clean, you likely have water intrusion.

 

[Chris1956]

Re: 1971 Mercury 115 (1150) Thunderbolt

Yes, Mr. 88, I misread that. I doub't it is a link and synch problem, however, it is worth checking, as nothing else comes to mind. Maybe a bad anti-siphon valve, maybe water infiltration.

 

[Bdivine32]

Re: 1971 Mercury 115 (1150) Thunderbolt

Bryan, I think you have been told a bunch of crap. Ethynol is a solvent that cleans tanks and lines, the first time it is used. That can cause issues. However, once you are past that, it works fine as a motor fuel. Also, 2 cycle motors are fine with unleaded gas. Lead was never needed for the two cycle motors, as they have no valves or valve seats. Also, there are one-way valves in fuel pumps, but not Anti-siphon valves. A-S valves are a different animal.

If you want to make the fuel additive companies rich, it is your business. I consider all additives to be snake oil, and unnecessary. I use a fuel stabilizer over the winter, but only because it is real cheap. I doub't it makes any difference at all...

NOT to start an argument, but you are wrong about ethynol. Ethynol is not a solvent, it is another form of alcohol. Alcohol dries out rubber! What do you think "Heat or Iso Heat is" is? It is used to dry the water out of your gas! Any engine, 2000 and Later, Was not designed to run gas with at least 10% alcohol in it! Thats why they came out with ISOHEAT when cars started going to Fuel Injection. Alcohol is a solvent to a point but gas is more of a solvent than alcohol! Race engines that run on Alcohol use braided steel fuel lines for that reason (as well as others). There is more, but like I said. I dont wanna start an argument. But Ethynol will dry everything out and crak hoses, seals, and gaskets and eat the old type of floats.
ANYWAYS... sorry to go off point.
And LEAD additive with help! They didnt use the same hard steel they use now days to accomadate unleaded fuel. Not, 2 cycles dont matter that much, but it will extend the life of the cylinders. As well as eliminate the ethynol. That was my point. I know how 2 cycles work.

 

[Bdivine32]

Re: 1971 Mercury 115 (1150) Thunderbolt

Bryan,
Thanks for the suggestions. I have been using the Starlite fuel additive every time I fill my tanks. I can't say what the previous owner was or wasn't using, so there is a chance the fuel lines have already been attacked by the Ethenol gas. I have considered replacing all of the fuel lines between the tank and the carbs just as a precaution. I already replaced the line between the tank and the engine including the primer ball and ends. It made no difference in resolving the issue. It actually ran worse after the line replacement. I have two tanks and the engine has the same issue on either tank. The pickups have been pulled out of both tanks and checked for damage and blockage. Both looked good. I also considered replacing the diaphragms and seals in both fuel pumps. I'm willing to try anything at this point. I have to track this issue down.

What would be the best way to check for air leaks in the fuel system? Could I spray starting fluid at certain areas to check for leaks? If so, where and how.

An air leak big enough to be causing your prob would be noticeable! it would have to be so big that your carbs fuel bowls would be more than half empty.

I agree with whomever said about the prop. Has the boat never gone over 4000 rpm since you have had it?
Have you checked the timming? The control module could be the prob. That happens in cars often. I dont know if they made/make a control module with a governer or the distibutor governed?
If it wont start after you run it for a while would make me think its the control mod. Again, I am NO BOAT mech, but that used to be a VERY COMMON Problem in cars! The control mods get hot then quit until they cool down then they will start and run again just fine. I have a 73 1150 and I have an extra control mod.. you might try running the boat at full throttle and spray some fuel mix in the carbs and see what happens. Or you should have a choke lever on your control box, start lifting the fast idle (choke) lever as you go. That will advance the timming. See if that makes it worse or better. are the plug wires all good? are you useing the old style plugs or the new?
When the carbs were gone through, did whomever rebuilt them spray cleaner in each jet to make sure they are not plugged? Did they replace all the needles and seats? (if the seats can be changed. dont remember. If it sat for that many years without being started every once in a while, seals and gaskets could be cracked. spray starting fluid around the base of each carb while running. If it alters the idle or WOT, bad mounting gaskets.
nyways, I am getting a new manual for the 1150 today and also going to the shop to have my lower end pressure tested and I will see what they say.
My tips are based on my knowledge of Automotive engines and small engines. Thats why so maney differnt questions.
But the more I think about it, I think I remember reading somthing about a governor of some kind on these engines, but I havnt looked at my manuals for a while.
Anyways, thats my 3 cents this time. lol But What size is your prop? I am sure someone on here can help you with the correct one. But reguardless of your prop, you should still be able to get over 5500 rpm!
Can you run your engine in a tank at home?
ok. enough for now. Hope I am helpping, if not let me know... But like I said I have a 73 1150 and mine runs like a rapped ape so, I can help ya with pics or whatever.
Bryan

 

[Chris1956]

Re: 1971 Mercury 115 (1150) Thunderbolt

Bryan, I recommend you only give advice on subjects you are sure about. The OP has an issue that he needs help on. The advice you have offered is partially true, filled with guesses and not applicable to your Merc nor his. As you say "I am NO Boat Mech"

 

[Bdivine32]

Re: 1971 Mercury 115 (1150) Thunderbolt

Bryan, I recommend you only give advice on subjects you are sure about. The OP has an issue that he needs help on. The advice you have offered is partially true, filled with guesses and not applicable to your Merc nor his. As you say "I am NO Boat Mech"

Thanx for the heads up Chris! I will make sure I no longer so that statement. But considering, We both share the same powerhead, My IDEAS are par for the issue. But since I can't just say with exact certainty what the problem might be, I wont offer any more ideas.
Sorry to have wasted your time Chris, and I will leave it too you and others to help and will just enjoy the fact that mine runs great, and when I get my lower end back tomarrow, I will will be wishing Falcain all the best and hope you figure out your problem soon.

And to You Chris, sorry to have offended you and waisted your time reading something that was never inteded for your consideration. But please be sure to let me know what the problem was when someone with more engine experince or happenes to have had the same problem and solved it, comes along. It will be something worth knowing incase I ever expierence the same issue.