1976 merc 150hp prop

[spike6969]

Hello all. I have a 76 merc 150hp inline 6 1500. we had a 4 blade 4x13.5x15. 15 being the pitch from what I was told.I broke the carbon fiber prop and bought a 21 pitch 3 blade and was told that should do fine. however before when we had the 4 blade we were reving 5500 RPM and now with the new prop we are getting 1800. So today we went to a 15pitch 3 blade and we were able to get to 2200RPM the the motor started to bog. we cannot get anything more trimming up down, dont seem to matter.My question is what pitch and what size do I need. merc dont have or stopped selling this part. Thanks all!!!

 

[j_martin]

Re: 1976 merc 150hp prop

Hello all. I have a 76 merc 150hp inline 6 1500. we had a 4 blade 4x13.5x15. 15 being the pitch from what I was told.I broke the carbon fiber prop and bought a 21 pitch 3 blade and was told that should do fine. however before when we had the 4 blade we were reving 5500 RPM and now with the new prop we are getting 1800. So today we went to a 15pitch 3 blade and we were able to get to 2200RPM the the motor started to bog. we cannot get anything more trimming up down, dont seem to matter.My question is what pitch and what size do I need. merc dont have or stopped selling this part. Thanks all!!!

If you were in the power band with the 15 pitch, why in the world would you mount a 21, which would seriously overload the engine.

2200 rpm is another thing, though. Something went wrong with the engine, from broken rings to stator failure. You have to systematically troubleshoot that.

John

 

[spike6969]

Re: 1976 merc 150hp prop

hey man thanks. Well for starters I know nothing about a boat. Not sure if I said it in my other post. We broke the prop that was on the motor.

This is my first time working on one. I went by what I was told by a "boat mechanic" that if we bought this used prop he said that he will credit the money towards a new prop if this one didnt work. He said that he would swap it back out due to he thought that this would work for us. Well however it did not. As far as damaging the motor yeah me may have overloaded this boat with a prop, but it never really hit the power again. Never over revved, only hit 1800RPM

So how would we of damaged the motor? Keep in mind I have never worked on a boat before now.

Also Merc said that the 15 pitch that we have now is the replacement part for the old one. well something is different, it is now the same pitch and we got a little motor back.

I see that you have listed some part on here I would try and find that thing where it is, and what it does. I have rode dirt bikes all my life and have abused 2 stroke motors, I really dont see this couls have damaged the motor.

Again thanks all I would be losy without you all!!!

 

[j_martin]

Re: 1976 merc 150hp prop

Don't believe me, go with your dirt bike experience.

Many of us have grenaded an outboard by overloading it. One difference is that an outboard is running against maximum resistance at all times, and usually at a constant speed. Put your dirt bike on a dino, put it at Max RPM, load it at full throttle, then see how long it runs. That's what an outboard has to put up with, and it is a severely oversquare engine just like the bike.

Overloading an outboard is almost guaranteed to break it in relatively short time. Your mechanic lied to you to try to sell you a prop he needed to unload. If your boat has been in his care without your supervision, I would be suspicious about the sudden loss of performance.

It's impossible to tell what's going on at this point without going through some systematic diagnostics. If you want to learn, this site can guide you through it. If you want to keep copping out with "I don't know and I can't understand......", find a mechanic you can trust or go back to the bikes.

It's your move.

just my o2
John

 

[spike6969]

Re: 1976 merc 150hp prop

Im not trying to cop out. By saying I dont understand. However I have worked on alot of bikes and cars. What I mean by Im new at this was just that. the info that you just gave me I had no Idea that there was that much stress on that motor. So should there be any difference with the four fin prop to the three if they are the same pitch. And as for the guy in the boat shop, He will have hell to pay for making me hurt the motor by intentionally selling me a prop he new was wrong for the motor. I do thank you all, and sorry if I happen to say that the wrong way this morning. That just was not what I wanted to hear.

I do thank you for the info.
Im kinda lost now, on boat motors where do you start?? compression check?
I seen someone said maybe it was you could be broken rings?? I really hate to take the head off unless I have to.

Well I will wait until someone gets back to me with some ideas as to trouble shooting this motor, thanks all.

 

[j_martin]

Re: 1976 merc 150hp prop

Best bet is to start with a compression check. Plugs out, ignition crippled by kill switch, or leave ignition off and use a remote starter to operate the starting solenoid. It should be decent compression (100-130) and within 10% cylinder to cylinder.

If compression's good, you're good enough inside so it should run good.

Questions at this time.
How does it start?
How does it idle?
When it balks, have you tried to squeeze the primer bulb?
How's your gas, fresh, no water, proper oil if not VRO?
We'll help ya out.

John

 

[spike6969]

Re: 1976 merc 150hp prop

ok this is where Im at, I have a compression tester I will have to find.
As far as the motor runs great up until the dead spot @2200RPM.
Starts Great, you touch the key and the motor starts.

I have tried squeezing the bulb as the motor was running and did not do anything.

The gas tank was empty, the manual said that this is a 50:1 mix, and it is dead on. Like I said everything leads me to think the motor is fine, I maybe mistaken

you said something about a stater?? what is this and can I test it? and how?

Thanks all again, I'm off to find my compression tester! Happy boating!

 

[Chris1956]

Re: 1976 merc 150hp prop

Spike, A 1976 inline six will have battery-driven CDI Ignition. The stator only serves to charge the battery.

If the compression test is succesful, we can look for other causes. Id there d dead spot at 2200 RPM, or does the motor refuse to rev higher?

 

[spike6969]

Re: 1976 merc 150hp prop

I still have yet to find the compression tester, Thanks for the info on the stator, It does OK.. We never have had a dead battery yet. but we charge it everyday before we go out to. For the motor with this 3 fin 15pitch prop it will not rev any higher. The guy we got the boat from gave me another prop, he said that the boat did around 60 with this prop on there. see we bolted it on and my father is taking it out this weekend.

I will post back with results. Thanks all for the help.

 

[j_martin]

Re: 1976 merc 150hp prop

I still have yet to find the compression tester, Thanks for the info on the stator, It does OK.. We never have had a dead battery yet. but we charge it everyday before we go out to. For the motor with this 3 fin 15pitch prop it will not rev any higher. The guy we got the boat from gave me another prop, he said that the boat did around 60 with this prop on there. see we bolted it on and my father is taking it out this weekend.

I will post back with results. Thanks all for the help.

If it won't rev with a low pitch prop, it sure won't do any better with a steeper one.

Engine needs fixing, then mess with the props. If it turns out to be a fuel problem and you continue to push it, you could burn it down big time, so get on with the diagnostics before you wreck a good machine.

hope it helps
John

 

[spike6969]

Re: 1976 merc 150hp prop

hello all, I'm still out of state. I will be back on Tuesday and will try and find the compression tester. When I left my dad said that he was taking it out. He said that a guy that he works with had a prop that came off of it. This prop he said on the baot would do like around 65MPH

Well my dad took the boat out, He said that the same thing happened he was a little faster, after playing with the Trim, he said that the boat opened up and ran great. He said that if he had to guess that he was doing about 60-65MPH.
So he dont feel that the motor was hurt.

I told him that he might be hurting the motor more!!

He said that he had tried playing with it again and he could not getting it to fast again!!,

So is this something to do with the trim? Did he hurt the motor more? or changing props, does that mean we have to change any carb settings??

I will look for the compression tester when I get home, as soon as I get home.

Any other ideas I can have him try while I'm gone??

 

[spike6969]

Re: 1976 merc 150hp prop

Hello Im back, I still cannot find my compression tester. however we put a new set of spark plugs in and the boat runs alot better. The motor will open up 4 out of 5 times. we are looking for a distributor cap, and rotor button. and help on that ?? we have the spark plug wires on order.

 

[j_martin]

Re: 1976 merc 150hp prop

Yer onto it. I guess what we forgot to ask is if it is missing when it balks.

Right when you are going on plane is about the most stressfull point for the engine, especially cylinder pressures. Weak ignition fails first at high pressure. Once yer over that point, pressures go down, and maybe the missing cylinders pick up.

john

 

[spike6969]

Re: 1976 merc 150hp prop

Im not sure if it is a spark or timing issue. it seems like the distributor isnt turning to me.. I have watched it tho so it is.. the wires did not help. I really wish I could find that compression tester. I think that you all were correct, the motor is old and when we put that other prop on there we may have hurt the motor. however with a new set of plugs it will open up. when we cannot get the boat to move or plane out, it sounds like a timing issue because you can hear it backfire through the carbs. I will be looking for another coil/ rotor button and distributor cap..Also i will continue to look for the compression tester.

 

[spike6969]

Re: 1976 merc 150hp prop

hello all. Ihave now done the compression test. cyl 1had 126 cyl 2 had 130 ,cyl 3 had 132 cyl 4 had 124, cyl 5 122 and cyl 6 had 112 and im not sure that the tester was sealed because that cyl is hard to get too. however when we checked the plugs were still in the motor. We will warm up the motor again today and compression check the motor again.

 

[spike6969]

Re: 1976 merc 150hp prop

ok, back and compression testing done, all spark plugs out of motor.. cyl 1-5 is 120 give or take 1 lb. cyl # 6 is 102.. every cyl seems to be getting good spark through tester. and yes.. Seems to be missing/ back firing through carb like a timing issue when it misses.. please help. is it the compression? also motor does pee but not like I have seen other motors on the water.. unless we open it up dont seem to be peein like it should... thanks again all!!

 

[sschefer]

Re: 1976 merc 150hp prop

O.K. First, the rule is that you drop an inch of pitch when you go from three to four blades. You should be running no more than a 17 with that engine if it ran well with your four blade.

Next, your compression is not good. You have 5 that are within reasonable wear specs and one that is 18 lbs low. There's a reason for that and it's not gonna be a simple fix like adding stop leak. (that's supposed to make you laugh because I know you're about to cry).

 

[spike6969]

Re: 1976 merc 150hp prop

yeah I figured that wasnt that great, from what I have read they say anywhere from 100 to 150 is ok? I understand that this one cylinder is hurt, that will make this motor to the point where it will not plain out huh?? could be just didnt seem like that one cylinder would make that much of a difference. any other things to look at or should i stop right there?? This head looks to be a pain in the stuff to take off and re-ring...

 

[sschefer]

Re: 1976 merc 150hp prop

yeah I figured that wasnt that great, from what I have read they say anywhere from 100 to 150 is ok? I understand that this one cylinder is hurt, that will make this motor to the point where it will not plain out huh?? could be just didnt seem like that one cylinder would make that much of a difference. any other things to look at or should i stop right there?? This head looks to be a pain in the stuff to take off and re-ring...

You'd be suprised how easy it is. If all is good and a quick glaze break with a hone is all it needs, you could have it done in a week. I can put a 4 cyl back together in day. It takes longer to tear them down. The trick is not to guess, get the block checked by a pro and have the machine work done by a good shop. The biggest delays come when you have to wait for outside work and parts but that's also a good time to clean and paint. Here's the best advice you'll ever get on a rebuild. Never, Never, Never, order any parts until your machine shop says the block, pistons, rods and crank are good. Failing any of that could make a power head replacement a more economical decision.

 

[daveswaves]

Re: 1976 merc 150hp prop

yeah I figured that wasnt that great, from what I have read they say anywhere from 100 to 150 is ok? I understand that this one cylinder is hurt, that will make this motor to the point where it will not plain out huh?? could be just didnt seem like that one cylinder would make that much of a difference. any other things to look at or should i stop right there?? This head looks to be a pain in the stuff to take off and re-ring...

Spike,you have lots of good feedback here that should help you out. The one question no one has asked you is how big and what style boat is this on? I am guessing that since it had a 15p 4 blade composite prop on it the boat is quite large....but then the whole 60 mph bit does not make sense with the 150 and a 15P. Whats the boat??