Low RPM's at max throttle '87 Merc 115

[ianmoore]

I've been in the trenches with a '59 Evinrude 50hp, so this Mercury has been a dream. Starts right up and runs pretty well. I got the motor after it had been sitting in a garage for 10 years. Did some simple work on it, changing plugs, spraying some Seafoam in carbs and cylinders, etc. All cylinders are getting spark, and compression is good across the board. I will be rebuilding the carbs once my kits arrive. I am anticipating an improvement with that.
As it stands right now, with the right prop on a Glastron 17' ski boat, I am getting about 28mph at about 4500rpm with the throttle maxed out. I should be able to get another 1000 rpms out of this engine. Any thing i should be looking for beyond simple cleaning and setup?
Thanks
ian

 

[Bob_VT]

It could be an electronic problem and/or it is a carb problem. The inlines are excellent motors however the carb rebuild is not a job for the newbie. There could be a pile of little problems but, yes you are lacking allot of rpms since the inlines like to run at or above 5500 at WOT

 

[Chris1956]

Well, you have survived the Fat-Fifty, the Merc will be a nice surprise to you. It is like a moped, compared to a Ninja motorcycle. Both will get you there, but one is more fun.

I would recommend you clean the carbs. They should be sidebowl carbs, likely it has backdraft jets. Make sure the main and idle tube jets (brass) are clean. There are some small holes in the side of the carb barrel, which need to be clean for the idle circuit to work properly.

I would think you would be running a 17 or 19 Pitch prop on that boat. You do need that extra 1000 RPM, for the motor to thrive.

 

[SkiDad]

You might just need to adjust your throttle so that the carbs are all opening fully. Did someone rig it for you or did you just hook up the old controls as is.

 

[ianmoore]

I "inherited" the boat from my friends dad. It was pretty meticulously cared for, but sat unused for 10 years inside.
I have rebuilt the carbs and was trying to set them up today. I did find something interesting. I am getting solid compression in all 6 cylinders, as well as a healthy spark. I tried pulling the plugs out of each cylinder while running. 1-4 caused the motor to sputter, but when i got to 5, it made no difference in at all in engine performance. I tried fiddling with the carb for that(and 6), but couldn't get it to respond differently. The carb is completely rebuilt, no air leaks, new float, new needle, etc. Everything is tightened to spec.
Any ideas?

The throttle was hooked up when I got it. I will mess around with that as well, though i'm now thinking the issue could be in the cylinder..I have a hard time checking 6 as its buried in the lower cowling..

I tried a bit further, but as you will see, may have been in over my head
I was limping through the link and sync. Never had used either the dial or a timing light. I may have misunderstood the instructions. I followed both the Seloc and the one in red here at the forum. I used a somewhat clamped dial to find the TDC. I then turned the flywheel counterclockwise to the .464 mark, and let the dial indicator go 1/4 turn further(as i further pulled the flywheel). I then pulled the flywheel back to the .464 mark and set the timing pointer to this point. I then put the plug back in the top cylinder and turned on my timing light. With the throttle in neutral i was seeing +9..Which i think is ATDC.. Does this make sense or did i totally miss this?

 

[ianmoore]

Spent some time on the motor today. Went through all of the timing stuff and i think i've got that dialed in. Got on the water and the boat was pushing 5500 rpm and 36-37 mph when i had it tilted just right! I am still having a little bit of an issue with my idle speed. I actually floated around the harbor for a bit trying to tune up the idle mix and then the idle speed. I found it difficult to definitively tune the carbs with the idle mix. I was being pretty conservative with my moves, staying pretty close to the initial settings(1 1/2 turns out from the set). I guess my ears aren't tuned too well, but i was having a hard time discerning a consistent change when i was moving the idle screw
I had it running pretty smooth, then set the idle speed. It ran pretty well for a bit. I put the cover on and it started to slow down to stalling. I started again with the idle mix screw, but didn't have a similar reset starting point for the idle speed.

I also noticed that when i gunned the throttle, i would have a momentary bog down before it would start revving up to speed. I'm sure this is a common and easily diagnosed problem. I'm just not sure what it means yet.

 

[Chris1956]

The carbs are a bit tricky to set. First, use brand new 50::1 fuel mix. That will help a lot. Next adjust the carbs from initial setting to run smoothest, by ear. Now the tricky part...when you go to accelerate, it is likely the carbs idle mixture will be too lean and she will stumble or die. If so, open the top carb 1/8 turn and retest. Repeat with mid carb and then bottom carb, testing acceleration after each adjustment. Repeat top-mid-bottom carb sequence until you have reasonable idle and smooth acceleration. It may take a couple of attempts....

 

[ianmoore]

Thanks Chris! I have a lot of boating to do today(taking our scout troop to a nearby island) so will report back. I did a bit of reading on the parallel carbs of the inline and was going to try the descending carb adjustment. Pretty excited to have the motor opening up the way it is!

 

[ianmoore]

The triumph, the tragedy...
Complete failure on the water yesterday. I took off, with the initial revving up with a slight bog when accelerating. Motor died as i accelerated, and upon dying, would start up, but die as i put it into gear. I took the cover off, tried adjusting the idle speed screw. This didn't help at all. I then reset the idle mixture screws on the carbs(1 1/2 turns from seat). I was able to start motor and rev it up pretty easily in neutral, but it would die as soon as i put it into gear. I messed with this for about an hour, trying different settings on the idle speed, resetting carbs, etc. I tried adjusting the carb adjusting screw. When i initially got the motor, the roller was off the cam when in neutral. I adjusted it to barely sit on the cam in neutral(as told in the Seloc manual). This seemed to properly situate the carb where they were completely closed while in neutral and open to sideways when the throttle was pegged. This all seems right, but when the motor woudln't start i tried resetting this to the way it was initially.
I finally was able to put the motor in gear, and with the throttle 3/4 of the way up was able to get about 1200 rpm and 5 mph. If i pushed the throttle the rest of the way up, the motor would bog down and die. It seemed that it was getting too much gas
I got back to the dock..

Today i opened the motor back up. Tried checking for TDC. Dead on the pointer. I reset the carbs again. I checked the timing and in neutral it was dead on at 0. With my remote, i have to press the neutral throttle switch, which i did to throttle max, in order to start it, and i think i was at around 5 BDC(possibly 3) at that position.

There is a step in the Seloc manual that says to "crank the engine while adjusting the idle speed adjusting screw until mark on the throttle cam aligns with the arrow on the powerhead bracket". This was already aligned while in neutral, though i don't know why you need to crank the powerhead to achieve this.
​I rechecked all cylinders for spark, and tried running it with the muff on. I am able to start it, but it runs roughly, with a low RPM, typically dying unless i rev up the RPMS.

I have new plugs in. Gas is non-ethanol. Mixed 50-1 with good outboard oil. It is new gas.
Here are things i'm thinking of doing:
-I will replace the fuel filter(water separator). That is pretty old and probably needs to be replaced.
-I will rebuild(new seals) the fuel pump.
-I am going to get a fitting for my portable tank to see if the internal tank has some residue or something that is gumming up the carbs..

I guess my lack of experience is murking up a logical next step. Is this possibly something with my timing? I don't have a test tank, and following what seems to be the main 'lync and sync' thread here, I get confused at this step

"Timing Event One - throttle pickup.
As the throttle is advanced, the spark timing advances toward TDC and then to before TDC. As the timing is just passing TDC the 1st (minor)throttle pickup should hit and start to open the carbs. Use the timing light. Adjust the tang or trigger screw to make this a reality. All of them are speced different, but if the 1st pickup hits at 3-4 degrees BTDC advance - you win. Use the timing light with a friend cranking it to complete this setting."

If I'm at TDC at neutral, what is the first throttle pickup? what is the trigger screw? It seems that the carbs are being progressively opened from here on out? Is there a way to change my open throttle settings to the 34 degress while maintaining my intitial neutral, which seems to be right? I don't seem anywhere near the 34..I'm at around 5 or so, which seems to explain my boggy response when i rev it up(too much gas not enough spark//
Am i on the right track? Or am I completely in the dark here?

Thanks for any advice. Sorry about the novel!
Best
ian



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[Chris1956]

Well, your post is a bit rambling. That motor has two spark timing points. The first is when the carbs are closed, but about to open. The second is max spark advance. The following is an outline of how to set the timing. It presumes the carb throttle plates all close at the same time and the trigger linkage length is correct.

So remove spark plugs 2-6, stick them into their wires and ground their bases. Remove prop. Put a timing light on spark plug #1. Advance throttle until carbs are about to open. Jumper solenoid. ignition on and read timing. Adjust the middle screw on the throttle arm to read 4-6* ATDC.

Advance throttle to maximum. Jumper solenoid to crank motor and read timing. It should read 21* BTDC. Adjust the top screw on the throttle arm to make it so. The very bottom screw on the throttle arm adjusts the idle speed. Of course the throttle cable must be adjusted to push the throttle closed, and to an extent allow the idle speed to be turned up.

 

[ianmoore]

Agreed about the rambling. Trying to put out all necessary details and probably too much info.
I will try to recheck the timing using this procedure.
couple of quick questions:
1. on my motor I have 2 screws not on the throttle arm itself. They are the 'primary pickup screw' and the 'spark advance screw'. I am assuming I adjust the 'primary pickup' for the first adjustment and the 'spark advance' for the throttle.
2. Are you just taking the prop off for safety? Would that in any way affect the load?

Thanks Chris. Sorry to be such a noob, but gathering the knowledge.

 

[ianmoore]

Do you have a handy way to ground the plugs? I came up with a wire thing using the spark plug jackets.

 

[Chris1956]

Ian, Removal of the prop is for safety, when cranking the motor in gear. I usually tape the spark plug bases together and use a clip lead to ground their bases.

Your throttle arm is a bit differentdesign than I am used to. Primary pickup screw should synch the carbs to open at 4-6* ATDC. Spark Advance screw should set max advance at 21*BTDC. Hope that helps/

 

[ianmoore]

Chris,
Thanks for the detailed explanation. It all makes sense to me now.
I checked the timing and things seem to be in line. I am getting 4-5 BTDC with my first instance, not ATDC. 2nd is 21BTDC. All the other instructions I have read have said the 1st instance should be BTDC. Is this not correct?
Thanks
Ian

 

[Chris1956]

Ian, I do not have the primary pickup timing specs for that motor. The earlier Distributor-ignition IL 6 motors were indeed 4-6* BTDC, however, when Merc switched to the ADI ignition, like your, they changed it to a few degrees ATDC on the V6 models, as well as some others. The online reference for the newer models shows slightly different timing specs, based upon serial number. You can look your motor up. Try pages A10 - A12, and section 6 in the following service manual.

http://www.boatinfo.no/lib/mercury/manuals/mercury_1965-1989_40-115.html#/504.

 

[ianmoore]

OK. interesting.
Timed the engine today. I actually went out and got the Mercury manual. Much clearer. I timed the first instance at 5ATDC and kept max at 21ATDC. Motor was having a really hard time starting. I went ahead for grins and tried it back at 4 BTDC and it turns over immediately. this is where it was set when i got the boat.
I do have my remote throttle linkage pretty screwed down, which seems to be necessary to keep the idle happy, which in my case seems to be a minimum of 800-850 on the muffs. I am trying to give myself some room for the water difference. It makes the throttle basically not engage until you start to crank it.

 

[Chris1956]

That Idle RPM is OK. Actually, 1000 RPM on the flusher is kinda normal, so if you can get a lower RPM, and she still runs in the water, that is better.

 

[ianmoore]

Wanted to update and say that I have the motor running really well. the problem with the "bogging down" was total user error. When i rebuilt the carbs, i put some threadseal on the screws, and a bit of that was clogging the jets. I am now able to throttle right up to 5500 rpm, which is getting me around 35-37 mph on the Glastron.
I'm still curious as to why the timing seems to sit better at the 5deg BTD. Assuming the engine is stock, which i think it is, is there any way for that to happen?

 

[SkiDad]

that's great news - just curious, what pitch prop are you running ?

 

[ianmoore]

Its a 13" 19P.