1977 Mercury 115 goes 10mph then 35mph very sporadic

[summerbum]

My Mercury 115 starts right up but has to be started at fast idol. It will only go 10 mph sometimes for 30 minutes but eventually will hit a power band or something and go 35. The compression was checked and fine. The carbs were cleaned and adjusted. The spark plugs all have spark. The fuel filter was clean. Sometimes when I play with the throttle it does go a little faster. I can only get to 3000 RPMs and 10mph. Any help would be much appreciated.

 

[daveswaves]

Re: 1977 Mercury 115 goes 10mph then 35mph very sporadic

My Mercury 115 starts right up but has to be started at fast idol.
That is normal for the inline 6 engine, the fast idle lever must be up and the choke on when the engine is cold. If it starts good then thats half the battle.

It will only go 10 mph sometimes for 30 minutes but eventually will hit a power band or something and go 35.

Are you cavitating, is the engine too high on the transom, what kind of boat are we talking about here.

The compression was checked and fine. The carbs were cleaned and adjusted.

Who adjusted the carbs? An experienced inline 6 merc expert? They do not adjust the way you would think. If a link and sync was not done you are just not going to get anywhere with it. It is critical that the timing and the carbs are in sync with each other, engagement of the throttle advance and proper opening of the butterflies , all three, are important.

The spark plugs all have spark. The fuel filter was clean. Sometimes when I play with the throttle it does go a little faster. I can only get to 3000 RPMs and 10mph.

Need to know what pitch prop you have, obviously the RPMs are too low ant the speed is way off. When you hit the "power band" that you mentioned what was the RPM then, at 35 MPH

Any help would be much appreciated.

We need more info to get you going

 

[summerbum]

Re: 1977 Mercury 115 goes 10mph then 35mph very sporadic

Sorry I wasn't more specific. The boat is a 1977 16ft Baja. It has a 19in prop. When the boat finally did go about 30-35 the RPMs were at 4200. The carbs were taken out and cleaned by a guy who has worked on the same motors for at least ten years. His father is a life long boat enthusiast who worked at a local marina for 20 years and actually advised him on the way to adjust the carbs (whatever that means). I have never seen the boat run at idle in neutral and he adjusted everything until that happened with muffs on today. I did notice that the last three inches of throttle do nothing. The RPMs don't change\, nor the speed or sound of the motor. I just got off the phone with him and he wants to change the throttle cables immediately to see if that helps. Thanks for replying I am new to this forum and appreciate any/all input. I am trying to get as many opinions to try and help my friend troubleshoot.

 

[hkeiner]

Re: 1977 Mercury 115 goes 10mph then 35mph very sporadic

If you are sure that carbs and spark are OK as you state, I would suggest that you carefully do a complete and accurate link and sync to confirm that everything is set correctly and that the linkages are not restricted in movement. Incorrect settings or a restricted movement of something could cause the symptoms your describe.

Also, the idle should be set with the motor running in the water in gear, not running on muffs. If you set the idle while on muffs, the motor will not idle correctly on the water in gear. The link and sync procedures include how to set the idle. Just my thoughts...

 

[Chris1956]

Re: 1977 Mercury 115 goes 10mph then 35mph very sporadic

Summer, I have a rig very similar to yours.(16' Avenger/ '77 Merc 1500, 21 Pitch prop) There are some inconsistancies with the description you gave, Your boat should be on plane at 3000RPM. Speed should be 25+MPH.

30-35MPH at 4200 RPM is about right. If the last three inches of throttle do nothing, you have reached the max power (with that prop)the engine can handle.
OK Specific recommendations: Check spark plugs to see if they look new. If they do, you may have water ingestion. If the plugs are full of gas, rebuild the fuel pump. Make sure there is no arcing form the plug wires to the block. Replace them if necessary.

There is a link and sync posting in the FAQ. Check idle spark timing and max spark advance.

BTW - your mechanic may be enthusistic, but he is an amateur. New throttle cables will not fix anything, if the carbs are opened all the way with the old ones. That is very easy to check.

 

[daveswaves]

Re: 1977 Mercury 115 goes 10mph then 35mph very sporadic

OK, thanks for posting the info, on the 16 foot baja with a 19P you should easily be able to reach 5200 rpm. The 35 MPH at 4200 tells me your prop hub is not spun and your motor set up is reasonable (height on the transom). As I mentioned in my first response and others have mentioned as well it really does sound like it needs a link and sync procedure done. With all due respect to the person that tuned your carbs, if he tuned your idle on muffs he is not familiar with this particular engine. You do not need a new throttle cable. The existing linkage on the engine needs to be brought into alignment (link and sync) While you are in there you need to bring the max advance down to 21 degrees, not the 23 stated on the sticker or the manual. Changes in fuel since 1977 mandate this. You will not notice any loss in performance.
There is a very easy to follow link and sync procedure in the FAQ section of this forum, by Clams Cannino. It is generally accepted in the merc inline 6 community as the gold standard. Follow it carefully and you will likely solve your problem.

 

[daveswaves]

Re: 1977 Mercury 115 goes 10mph then 35mph very sporadic

Gee Chris, you typed 4 minutes faster than me!

 

[Chris1956]

Re: 1977 Mercury 115 goes 10mph then 35mph very sporadic

Dave, Your post had to come all the way from the Great White North! That is why I posted first!