75 mercury thunder bolt no power unless idle screws all the way out

[bamzam35]

Ok so here is my problem i just bought this boat with the 1150 on it and iv'e heard nothing but good things about these engines, but the engine boggs all the time and will not pull rpm. I have taken the carbs off cleaned them all out i believe or at least carb cleaner blows through all the venturies. I have also done a link and sync via the you tube vids because i do not have a timing light. I have noticed when we are out on the water and i finally get the boat to plane off the more i screw out the idle mixture screws out the more the engine opens up but still wont open up all the way. I did do a compression check all cylinders where at 120 but number two which was at 105 can some one please help me i just want her running right so wife doesnt kill me for buying this boat. also the prop is a 19 pitch and boat is like a 15 foot ebko trihall please help me out before it becomes an ankore.

 

[Chris1956]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

The prop should be fine on that boat. You need to get a timing light. No way to link and synch w/o one. Carb idle mixture screws will end up being about 2 turns out from seated, but adustment must be done on the water, warm engine, in gear. Your motor is obviously well out of synch.

 

[bamzam35]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

that is what i was thinking because there is a guy on youtube that does it buy pulling plug #1 and then moving distributor at tdc then at 5 degrees and then again at 21 degrees i need to know how to do this in laymens terms and simplified. i want this thing screaming again cause i think it will be a great engine.

 

[Chris1956]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

Write back when you get a timing light and we can assist.

 

[bamzam35]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

OK so I took the dizzy cover off set flywheel at tdc and the arrow on the dizzy was no where near the three dots on the flywheel so lined them up what a big difference runs smooth no hiccups and sounds deeper now I'm getting trimming light tomorrow so how do I do a true LINC and sync

 

[Chris1956]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

OK, Remove spark plugs 2-6, reinstall them into their wires and tape them together and ground to the block. Put timing light on Top spark plus. Crank motor and observe timing. Advance throttle until timing is 4-6* BTDC. Now, not moving throttle, loosen two bolts on distributor body with 5/16" wrench. Slide brass collar until the plastic fingers just touch carb opening lever. Tighten bolts. Advance throttle all the way. Crank motor and observe timing. Loosen locknut and adjust mas spark advance set screw on plate above distributor until timing reads 21*BTDC. Tighten locknut. Now adjust throttle stop set screw until it contacts carbs. Tighten locknut.

Install spark plugs and water test motor. If you hve trouble accelerating (motor bogs or stalls, just off idle), richen idle mixture screws one carb at a time starting from top, by 1/8 turn. Retest after each adjustment. When acceleration is smooth, and motor will idle reasonably well, you are done. Set idle stop set screw to 900RPM or a bit less. adjust throttle cable to push throttle closed, and add a bit of preset.

 

[bamzam35]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

OK so I got the LINC and sync done sounded good but no power when the carbs open it does get better again when you keep running idle air screw out to the point it is even with the castings but still bogs and doesn't open up all the way I'm lost about to take it to a shop or look for another engine this thing is getting bad and I'm lost I can fix about anything but this engine has me throwing tools and killing 6 packs

 

[Chris1956]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

The idle mixture adjustment is a fuel screw. Turning it open (CCW) richens the mixture. There are some pinholes on the left side of carb throat. Did you get those clean? Are floats in good shape? Are they adjusted properly?

 

[bamzam35]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

OK so just was thinking why would the idle screw affect the topend unless the main was plunged please give me some insite

 

[bamzam35]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

OK I'm lost could the ignition cause all these problems going to check timing again and will post resultes

 

[Dave1027]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

If you got an improvement by opening the mixture screws then likely fuel delivery is the issue. Have you rebuilt the fuel pump? Have somebody squeeze the primer bulb while accellerating on the water and cruising. If it helps then you have a fuel pump issue.

Adding some idle mixture can have a very small effect on cruise but that normally indicates not enough fuel delivery from the main circuits of the carbs or maybe just one carb.

 

[Anchor Marine Repair LLC]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

Ok so here is my problem i just bought this boat with the 1150 on it and iv'e heard nothing but good things about these engines, but the engine boggs all the time and will not pull rpm. I have taken the carbs off cleaned them all out i believe or at least carb cleaner blows through all the venturies. I have also done a link and sync via the you tube vids because i do not have a timing light. I have noticed when we are out on the water and i finally get the boat to plane off the more i screw out the idle mixture screws out the more the engine opens up but still wont open up all the way. I did do a compression check all cylinders where at 120 but number two which was at 105 can some one please help me i just want her running right so wife doesnt kill me for buying this boat. also the prop is a 19 pitch and boat is like a 15 foot ebko trihall please help me out before it becomes an ankore.

You have a bad powerhead. The answer to everyones question is right in the second to last sentence. You have 120 compression, which is low for any of the 75's. And you have a cylinder that is 105. That cylinder is dead. It won't pull it's weight. Therfore, you'll have a motor that bogs out of the hole, and won't reach max RPMs. The extra fuel you're adding just perks up the motor because the engineered amount of fuel can't be delivered to the combustion chamber

 

[bamzam35]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

OK checked compression again at marina guy told me they were all at 125 this time and the early mercs only had like 140 new and thery were lower comp engines till 77 then they jumped to 160 due to a high rise piston dome is this true or smoke

 

[Anchor Marine Repair LLC]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

So you checked compression, or 'marina guy told me they were all at 125 this time'

Feel free to use punctuation

In your OP, you said #2 was 105. If the reading was different at a later date, there could have been an accumulation of gas/oil mix to thro the reading off.

Compression readings are only as good as the technician taking them. It's information, but you have to have a large knowledge base to decide what they mean. Anyway you cut it, a cylinder with 105 vs 125 is a bad cylinder.

The true test of an engines condition is leakdown test.

V-4 Leakdown Test - YouTube

http://www.youtube.com/watch?v=kuAqfZUfoXQ&feature=c4-overview&list=UUJ7rjVI2seAL5Ei9SUkey6A

If I were you, I'd spend $80 and get it properly diagnosed by reputable repair shop.

 

[Chris1956]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

Boy, I disagree with Anchor marine Repair. 120 PSi si fine for that motor. Also, if your compression on all cylinders is within 15%, your are good to go. 15% of 125 is 25PSI, making the 105 PSI cylinder within range. You have other issues.

Idle mixture will affect idle quality and acceleration. After you pass 1200 RPMs or so, it has no effect at all.

 

[Anchor Marine Repair LLC]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

Boy, I disagree with Anchor marine Repair. 120 PSi si fine for that motor. Also, if your compression on all cylinders is within 15%, your are good to go. 15% of 125 is 25PSI, making the 105 PSI cylinder within range. You have other issues.

Idle mixture will affect idle quality and acceleration. After you pass 1200 RPMs or so, it has no effect at all.

So Chris, if a 3 cylinder motor, say a Mercury 75 HP 3.500 bore has 120 psi on all three cylinders, is that acceptable? By your definition of "15%", how will that motor run? They're all even right?

 

[Chris1956]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

Actually it is a 1975 merc 1150 inline six. Also the accuracy of a lot of compression gauges varies widely. The reliability of the compression gauges is usually better, so if you get the same reading on all cylinders (within 15%) I feel the motor does not have any significant damage in the cylinders. Also it is a very old motor. If it is close in compression, I recommend it be run...

 

[Bob_VT]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

Ok so here is my problem i just bought this boat with the 1150 on it and iv'e heard nothing but good things about these engines, but the engine boggs all the time and will not pull rpm. I have taken the carbs off cleaned them all out i believe or at least carb cleaner blows through all the venturies. I have also done a link and sync via the you tube vids because i do not have a timing light. I have noticed when we are out on the water and i finally get the boat to plane off the more i screw out the idle mixture screws out the more the engine opens up but still wont open up all the way. I did do a compression check all cylinders where at 120 but number two which was at 105 can some one please help me i just want her running right so wife doesnt kill me for buying this boat. also the prop is a 19 pitch and boat is like a 15 foot ebko trihall please help me out before it becomes an ankore.

I am going to jump in here

The inline 6 motors are fun by very picky. The carbs should be completely rebuilt. There are MANY passages not accessible without complete dis-assembly. All of the carb kits are available. They are NOT easy to remove since most of the front of the motor has to come off and is NOT for a novice undertaking. I have rebuilt a number of the inline 4's and 6's and the carbs of the 6 are not fun to take off.

You have a distributor. A VERY expensive hard to get part. The cap and rotor and many parts are hard to find. Treat them like gold but inspect each piece. It is also not uncommon for the bearings to wear on the distributor shaft and replacing them is a near pro level job.

Yes, your motor will run with your compression ratings however, unless carbs and ignition are not correct .......you will have running problems. Be extremely careful with the ignition since if it is not done properly, you can blow a hole in a piston.

Great motors but no longer a great motor for a novice due to the age of it. Look at the parts available on ebay and the distributor normally gets more money then an entire lower unit or engine block!

 

[bamzam35]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

Well hate to say it but engine is off of boat being replaced by a 85 HP Johnson no one in Nebraska will toutch these things they say buy another engine not worth fixing so putting it up for sale as parts just bums me out one month of trying things tried ordering carb kits and always get sent the wrong one put new water pump in to but for nothing what a waste

 

[jagans]

Re: 75 mercury thunder bolt no power unless idle screws all the way out

Trying to get those old motors to run on today's fuel is an exercise in frustration. Did you change all your hoses to alcohol resistant hoses? Did you use the right check valves to rebuild your fuel pump? It goes on and on and on, and the old Mercs were like Thoroughbred horses. They run like raped apes when they are happy, but they are always pissed.