1987 Mercury Classic Fifty - RPM/Power Issue

[DTPHawk]

I just bought a Bass Tracker Tournament V-17 boat with a 1987 Mercury Classic Fifty 45HP engine on it.

My primary use for the boat will be to fish the Missouri river in Montana. It came with a 13 pitch prop (49-73136-13).

I had the engine looked over by a mechanic (timing, carbs, compression, etc.) and everything is running great. Out of the water with muffs I can get 5000+ RPMs but on the river with a load I'm only getting 3800 RPMs max. This is resulting in very low speeds and decrease in hole shot. About 15 mph (according to fish finder) and getting back up river can take quite awhile.

After reading through all of the similar topics in this forum my first guess is to get a lower pitch of the prop to a 9 pitch or a 10 pitch.

Side Note (Not sure if this is normal or related): Full a full load on the engine (15mph @ 3800 RPM) I'm not able to trim the motor up past the brackets on the motor that must hold the engine in the lower trim region.

Does anyone have any experience with this type of issue or have any additional suggestions?

 

[zorak]

Re: 1987 Mercury Classic Fifty - RPM/Power Issue

First off DO NOT rev the engint to 5000 rpm's on the muffs.I have been told its not good on them.2nd. I have a 15' procraft with the same engine on it and i can get 25 mph at 5000 rpm's on it. Verify if you have compression on all 4 cylinders first. Also make sure that you have spark on all 4 cylinders. 3rd make sure the throttle is actually opening all the way. It sounds to me like you might have a spark issue .i know you had it looked over by a mechanic but a few simple tests will confirm that the above items are not a factor.My engine seemed to be running great on the muffs but only had spark on 2 cylinders.

 

[DuckOfDeath]

Re: 1987 Mercury Classic Fifty - RPM/Power Issue

I have the same boat, motor, and prop pitch. Mine's decked out for bass fishing and is real heavy. I have no problem getting up over 5000 RPM's. I'm getting closer to 5600 RPM's since I switched to an SS prop. SS or not, sounds like something isn't right with your motor.

 

[DTPHawk]

Re: 1987 Mercury Classic Fifty - RPM/Power Issue

Thanks for the suggestions. I only ran the motor to 5000 RPMs on the muffs one time to make sure the motor could get there (to rule out a different issue).

I'm going to try and get my hands on a compression tester kit and make sure that is not an issue. Also, what is the easiest way to check the spark on each cylinder?

Is there any way for me to check the throttle or am I better off taking it back to the boat shop to bring all of these issues up?

Since you guys have a similar setup, do you think getting a smaller pitch prop will help at all? Like I said this will be used on mostly fishing down river and trying to fight the current back up to get back home. Not sure if this requires a different prop versus fishing on a calm lake etc.

 

[zorak]

Re: 1987 Mercury Classic Fifty - RPM/Power Issue

I don't know much about props so i hope someone with more knowledge will chime in on that one. you should crank the engine over without gas and use a spark gap testor or use 1 plug with the outside grounded to the block with the other end hooked to 1 coil at a time. Be careful as the merc ignition is quite powerful and will nail you if you aren't careful.As the throttle goes look at the arm at the top of the top carb. with the throttle closed note the position relative to the top of the carb. Open the throttle all the way and see which way it moves.See with the throttle open how much farther you can move the throttle arm on the carb. Not exactly folling the service manual but it is quick to do.

 

[DTPHawk]

Re: 1987 Mercury Classic Fifty - RPM/Power Issue

I contacted the mechanic that looked at the engine and he told me the spark and compression were both good, but it can't hurt to double check.

Tested the compression in each cylinder and the results were 113, 115, 120, and 120. I ordered an inline spark tester that should be here Wednesday so I can confirm that the spark in all cylinders are good as well.

After looking at the top carb I'm still not sure if I know exactly what I'm looking for. Is it the butterfly plate?

Is it possible that the carb needs to be cleaned/rebuilt? Would I be experiencing the symptoms that have been described so far.

 

[zorak]

Re: 1987 Mercury Classic Fifty - RPM/Power Issue

Yes you are trying to see if the butterfly is open. I suggested looking at the top linkage as its easier to see if it's open.Another quick test you can do is start it up and put a timing light on the spark plug leads one at a time. That will show that at least you are getting some fire.I think you might have to get a manual if you are going to troubleshoot this yourself. Personally i try to do my own troubleshooting as i don't want to be at the mercy of a mechanic.The top linkage on the carb isn't actually hooked up to the linnkage.The throttle linkage pushes against it.Turn your steering wheel all the way to 1 side and you might be able to see the linkage move when you open the throttle from your control box.

 

[DuckOfDeath]

Re: 1987 Mercury Classic Fifty - RPM/Power Issue

I contacted the mechanic that looked at the engine and he told me the spark and compression were both good, but it can't hurt to double check.

Tested the compression in each cylinder and the results were 113, 115, 120, and 120. I ordered an inline spark tester that should be here Wednesday so I can confirm that the spark in all cylinders are good as well.

After looking at the top carb I'm still not sure if I know exactly what I'm looking for. Is it the butterfly plate?

Is it possible that the carb needs to be cleaned/rebuilt? Would I be experiencing the symptoms that have been described so far.

Good news! Compression isn't the issue. Let us know how the spark test works out. If it has good spark on all cylinders, you'll move onto troubleshooting fuel delivery. You have to be very carefull running these motors at all when a faulty carb is starving one or more cylinder. If it's not getting fuel, it's also not getting any lubrication. You can burn them up quickly, causing real damage.
Dropping down to a smaller prop pitch won't solve anything. 13P should be fine for your settup when the motor is running right.

 

[DTPHawk]

Re: 1987 Mercury Classic Fifty - RPM/Power Issue

Good news! Compression isn't the issue. Let us know how the spark test works out. If it has good spark on all cylinders, you'll move onto troubleshooting fuel delivery. You have to be very carefull running these motors at all when a faulty carb is starving one or more cylinder. If it's not getting fuel, it's also not getting any lubrication. You can burn them up quickly, causing real damage.
Dropping down to a smaller prop pitch won't solve anything. 13P should be fine for your settup when the motor is running right.

I just got the spark tester today so I'm hoping to get out and test everything tonight or tomorrow.

I called the mechanic that looked at the engine and he said that they did try a 9 pitch prop and were able to get considerably more RPMs on the engine (5000+) vs the 3800 RPM with a 13 pitch. Not sure if this tells us something or anything at all.

Is rebuilding these carbs to clean them out something that can be done by myself without a manual (haven't has any luck finding one online for less than ~$80)? Are there any guides or videos online for this specific application?

I've also read in a few places that when you get the carbs cleaned it is also recommended that the fuel lines and fuel pump/gaskets get replaced as well. Would you recommend the same?

 

[DuckOfDeath]

Re: 1987 Mercury Classic Fifty - RPM/Power Issue

I just got the spark tester today so I'm hoping to get out and test everything tonight or tomorrow.

I called the mechanic that looked at the engine and he said that they did try a 9 pitch prop and were able to get considerably more RPMs on the engine (5000+) vs the 3800 RPM with a 13 pitch. Not sure if this tells us something or anything at all.

Is rebuilding these carbs to clean them out something that can be done by myself without a manual (haven't has any luck finding one online for less than ~$80)? Are there any guides or videos online for this specific application?

I've also read in a few places that when you get the carbs cleaned it is also recommended that the fuel lines and fuel pump/gaskets get replaced as well. Would you recommend the same?

Get a manual. Consider it a requirement that pays for itself the first repair you do. You just need a carb rebuild kit and replacement jet for each carb. Some of the kits will come with the jets. I don't recall them being that expensive.

Could the sprayed-in foam in your hull be water logged? I don't know how that might effect RPM's. I do know that a 13P works well on my very similar boat, and your's likely came with a 13P because it was ideal for it at one time too. I have a 12P as a backup and it runs well over the recommendation of around 5500RPM on my rig if I let it. I can't imagine running a 9P on it.

 

[DTPHawk]

Re: 1987 Mercury Classic Fifty - RPM/Power Issue

Get a manual. Consider it a requirement that pays for itself the first repair you do. You just need a carb rebuild kit and replacement jet for each carb. Some of the kits will come with the jets. I don't recall them being that expensive.

Could the sprayed-in foam in your hull be water logged? I don't know how that might effect RPM's. I do know that a 13P works well on my very similar boat, and your's likely came with a 13P because it was ideal for it at one time too. I have a 12P as a backup and it runs well over the recommendation of around 5500RPM on my rig if I let it. I can't imagine running a 9P on it.

You know, that's a good question. I did some floor board repair when I first got the boat in front of the steering console. I remember the foam there looked perfectly dry. Is there anyway to check the foam near the rear of the boat (maybe under the gas tank area)?

After talking to the mechanic earlier he said in this area (Great Falls, MT elevation ~3500ft) they normally drop the pitch on new boats by 2 pitches because the motors are normally tuned at the factory at a lower elevation.

Either way I'm going to go ahead and get my hands on a service manual. Hopefully save myself a few dollars in the long run.