'71 Merc 80hp idles, planes boat but no top end

I used the motor last summer and it starts up great, idles fine and will plane my 16' Starcraft out and top speed has been around 29-32mph.

I didn't run the motor too much since I knew it had issues.

After testing compression last night I might be willing to invest some cash to get it running 100%.

The results are as follows...

Cylinder#4-----3-----2----1
Test 1: 105--120--124--125
Test 2 110--122--125--125
(Oiled added to cylinders)
Test 3 124--130--131--130

I was surprised and glad to see that the compression #s were so tightly banded. I fully expected to see horrible results that would make me inclined to start pricing used outboards.

Obviously it would be nicer to have them in the 150 range, but at least they are not all over the board.

I hired a mobile marine mechanic that looked at it and told me that rather than rebuild the carbs, he thinks the wiring harness needs to be replaced first.

The wiring is terrible on the motor, to the point where it's surprising that it runs at all.

I plan on either re-doing all the connections or ordering a new harness altogether.

Some folks have expressed doubts that the electrical issues would cause a motor to lose top end RPMs.

I have a tach on the way so I can get more info on what's happening once I'm finished with a transom rebuild.

The mechanic was pretty cheap($40/hr) so I'm not 100% that he is the best guy to get advice from in this case.

Any thoughts?

Re: '71 Merc 80hp idles, planes boat but no top end

i have had 3 mercs from 71-74 and 65hp-115hp on all the old motors replace all the wiring harness including the plug for the motor. iave found and little cheaper web site for the harnesshttp://www.outboardparts.com/ I would replace it if you can afford it for peace of mind

Re: '71 Merc 80hp idles, planes boat but no top end

Maybe 32MPH with an 80HP on a 16' boat isn't that bad? How heavy is the boat?

Your compression variance is significant; 105 to 125 would indicate a problem with #1.

If I were you; I would look at the tach #'s....you might be surprised to find 5500 or so....

If that's the case; then a de-carb, carb cleaning and eventually re-wiring night be all that you need right now; unless you feel like pulling it apart and doing a full re-build.

Chuck

Re: '71 Merc 80hp idles, planes boat but no top end

douglas1978 said:

i have had 3 mercs from 71-74 and 65hp-115hp on all the old motors replace all the wiring harness including the plug for the motor. iave found and little cheaper web site for the harnesshttp://www.outboardparts.com/ I would replace it if you can afford it for peace of mind

Click to expand...

Thanks for the link, that is a bit cheaper than all the places I've checked.

Re: '71 Merc 80hp idles, planes boat but no top end

SeaKaye12 said:

Maybe 32MPH with an 80HP on a 16' boat isn't that bad? How heavy is the boat?

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I've seen videos of Mike's boat at WOT. He's definitely not revving all the way. And it's a light tin hull. He should see closer to 40 I would think.

Re: '71 Merc 80hp idles, planes boat but no top end

Maybe 32MPH with an 80HP on a 16' boat isn't that bad? How heavy is the boat?

Your compression variance is significant; 105 to 125 would indicate a problem with #1.

Click to expand...

It's an aluminum boat and only weighs around 700lbs dry. I should be closer to 40mph according to a few guys that have identical boats and similar motors. I'm getting a top speed that is about the same as a fellow iboater that had a 50hp.

As far as compression I am hoping that the last test is a good reading.

Re: '71 Merc 80hp idles, planes boat but no top end

Here's a video of the motor from idle thru wide open. You can tell by the sound that the RPMs are not probably not hitting too much higher than 3000-3500 ballpark.

Thanks for the replies, I will be getting more info as soon as I can. I have a tach that I will be using once I can splash the boat again.

Re: '71 Merc 80hp idles, planes boat but no top end

I've got the identical motor pushing a 16ft heavy fiberglass hull (1100 Ib). GPS speed is between 28 and 32 mph depending on conditions and load.

From your video it's really hard to tell if the motor is turning at the right RPM or not. Only a tach will give you the real numbers. What prop are you running ? Don't forget, that branded 80hp is really only about 70hp at the prop compared with newer motors.

How does the boat react when you put the hammer down from stopped? How did you measure speed?

Re: '71 Merc 80hp idles, planes boat but no top end

Moody Blue said:

I've got the identical motor pushing a 16ft heavy fiberglass hull (1100 Ib). GPS speed is between 28 and 32 mph depending on conditions and load.

From your video it's really hard to tell if the motor is turning at the right RPM or not. Only a tach will give you the real numbers. What prop are you running ? Don't forget, that branded 80hp is really only about 70hp at the prop compared with newer motors.

How does the boat react when you put the hammer down from stopped? How did you measure speed?

Click to expand...

I agree that getting a tach rigged will be a huge help. The prop that came with the motor when I bought it is a 2-blade aluminum, with a few nicks. I'm sure a 3-blade prop will be better for my use and I plan on buying a new one this spring.

The prop could be in better shape but I don't think it accounts for the loss.

I've heard from folks running 70hp J/E 3 cyl motors on similar boats and they are getting better performance than what I've seen.

It accelerates pretty well if I drop the throttle, but always seems to be missing another 5-10 mph or 1-2k rpm.

The only speed measurement I've gotten has been from GPS, and the normal top speed has been about 29mph. I think I was going downriver when I hit 32mph.

It's not that I need to go much faster than that, but I'd like to be able to get a skier up, and make sure the motor is running as efficiently as possible.

Is 1100lbs your total weight or the dry hull weight?

Thanks for the info MB.

Re: '71 Merc 80hp idles, planes boat but no top end

hey huron have you ever adjusted the lean -rich mixture screw on the carbs and what porp are you running 19p? from your other viedo it sounds like you have the fast idle turned up in the driveway or does it idle like that all the time?

Re: '71 Merc 80hp idles, planes boat but no top end

Two blade prop?

Re: '71 Merc 80hp idles, planes boat but no top end

Have you tested spark with a tester on all cylinders? Does your motor have a distributor? If distributor, I figure it's hitting on all cylinders most likely unless a plug wire has an internal break that's opening the circuit as it's bent went you give it throttle. I'd then check my throttle butterflies to see that they're all opening and in sync with the others. If so, maybe check the timing stop/timing to make sure it's getting enough advance. I'd also pull the plugs and make sure there's no aluminum on the tips. I was experiencing detonation on my 65hp merc '73 model and had to retard the timing.

After looking at your video, I agree you're not getting nearly enough rpm's. If the prop pitch isn't too much, I'd think your carbs aren't opening up all the way and check for totally open throttle plates with the throttle all the way forward (I'm sure you've probably done that already, but....). Then check and see that your spark advance is working while a partner works the throttle. It's got to be fuel or timing I would think, unless the #1 cylinder is so bad it's not giving power,which I doubt. Have you tried partially blocking a carb throat during full power to see if a carb is lean or not working?

Re: '71 Merc 80hp idles, planes boat but no top end

douglas1978 said:

hey huron have you ever adjusted the lean -rich mixture screw on the carbs and what porp are you running 19p? from your other viedo it sounds like you have the fast idle turned up in the driveway or does it idle like that all the time?

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I have not adjusted the lean-rich settings. I don't have the specs on the prop, it's pretty scratched up.

That video of it running on muffs in the driveway was the day I bought it, and the owner was operating the motor. He did have the fast idle lever up I believe, to let it warm up.

I usually let it idle just a little higher using the lever before putting it into gear, otherwise it idles normally.

Re: '71 Merc 80hp idles, planes boat but no top end

Pull the plug on #1 and check the "eyebrow" of piston. The compression #s you posted for #1 is waaay low and oil in cylinder is a false test as oil seals imperfections in cylinder walls and piston. The correct way is to check after engine is at temp,cylinder is "wetted" by fuel charge,piston/rings have expansion and throttles are at WOT. Your first 2 compression test were correct and #1 fails the 10% difference.This imbalance and the other cylinders be low also is why its "no fast". That engine new had about 145 psi per cylinder due to high dome pistons and 120psi is minium on that design. The newer low domes have about 125 psi new and 110 psi is about minium on them. Sorry......

Re: '71 Merc 80hp idles, planes boat but no top end

Mi duckdown said:

Two blade prop?

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Yeah I'm guessing that this Merc was bolted on a speedboat of some kind back in the day. Certainly no good reason for me to run a two-blade prop on my Starcraft.

jbjennings said:

Have you tested spark with a tester on all cylinders? Does your motor have a distributor? If distributor, I figure it's hitting on all cylinders most likely unless a plug wire has an internal break that's opening the circuit as it's bent went you give it throttle. I'd then check my throttle butterflies to see that they're all opening and in sync with the others. If so, maybe check the timing stop/timing to make sure it's getting enough advance. I'd also pull the plugs and make sure there's no aluminum on the tips. I was experiencing detonation on my 65hp merc '73 model and had to retard the timing.

Click to expand...

I have not done anything other than test compression and examine the spak plugs. The plugs were clean, no fouling at all. Not sure if it has a distributor.

Thanks for the suggestions, I need to get a real Merc manual and get reading.

Re: '71 Merc 80hp idles, planes boat but no top end

Faztbullet said:

Pull the plug on #1 and check the "eyebrow" of piston. The compression #s you posted for #1 is waaay low and oil in cylinder is a false test as oil seals imperfections in cylinder walls and piston. The correct way is to check after engine is at temp,cylinder is "wetted" by fuel charge,piston/rings have expansion and throttles are at WOT. Your first 2 compression test were correct and #1 fails the 10% difference.This imbalance and the other cylinders be low also is why its "no fast". That engine new had about 145 psi per cylinder due to high dome pistons and 120psi is minium on that design. The newer low domes have about 125 psi new and 110 psi is about minium on them. Sorry......

Click to expand...

OK so that third test was no good...and this motor may not be worth investing in unless I plan on a total rebuild(which I'm not).

Thanks for the info, maybe I will run it until it dies and then part it out. I bought the boat because I liked the boat, the Merc was free with it...so I'm not overly attached to it.

Re: '71 Merc 80hp idles, planes boat but no top end

OK I've been reading other threads, and it seems like other folks are quite often squirting oil into the cyclinders prior to a compression test.

Some of the threads where I've seen this are folks who know quite a bit about working on outboards.

Is it guaranteed that my test was bad from the oil being in there?

Not to kick a dead horse, but if there is any chance the compression is ok I will feel much better about selling this motor. Otherwise I'm gonna sell it as a parts motor pretty much.

Re: '71 Merc 80hp idles, planes boat but no top end

Do a good 'De-carb' then run double check the link and synch, run a couple gallons of fuel thru it then retest compression.