Mercury bogging down like it is starving for fuel

[mercsmp]

I have a 1982 Mercury 115 inline 6 cylinder. The motor stops when I go just a little faster than idle. It idles perfectly and runs at a slow speed perfectly. When it stops if feels like it is starving for fuel. When I push the throttle slowly, I can tell at a certain point that it is about to go dead. It doesn't sputter, it just seems like the fuel is shut off. I took the boat out on the day the problem started and went about 5 miles on a lake and fished a few hours with the trolling motor and then when I started the motor to leave the symptoms I described started. Here is what I have done so far. I checked the compression and each cylinder was near 110 psi. I checked for spark on each plug and I had spark. I changed the fuel line from the tank to the motor and I also changed the bulb. I tried new gas in a portable tank. I rebuilt each of the carburetors. I replaced every fuel line. I rebuilt the fuel pump. I replaced the stator based on specs from the shop manual. The new stator tested the same as the old one so I guess the specs in the manual were not the same as the CDI stator that I had and the CDI stator that I replaced it with. I checked the trigger and it is within specs. Does anyone have a suggestion about what may be causing this problem?

 

[j_martin]

Re: Mercury bogging down like it is starving for fuel

You didn't say you checked the linkage. (link n sync procedure)

Throttle pickup too early could do this. (maybe) What happens when you just slam petal to the metal?

 

[mercsmp]

Re: Mercury bogging down like it is starving for fuel

I checked to see that all three carburetors were opening up in sync and they were. I also installed new plugs today and took her to the lake. I had the exact same symptoms of bogging down and stopping. I started her back up and slammed the pedal to the metal as you suggested. It took off like a scalded dog. I went several miles at WOT and it ran great. I stopped and started it and it didn't go dead but still seemed a little sluggish going through the zone between slow and WOT. Is there something that I need to adjust so it doesn't go dead when gradually getting up to speed? What causes this to happen? Thank you very much for your suggestion.

 

[j_martin]

Re: Mercury bogging down like it is starving for fuel

I checked to see that all three carburetors were opening up in sync and they were. I also installed new plugs today and took her to the lake. I had the exact same symptoms of bogging down and stopping. I started her back up and slammed the pedal to the metal as you suggested. It took off like a scalded dog. I went several miles at WOT and it ran great. I stopped and started it and it didn't go dead but still seemed a little sluggish going through the zone between slow and WOT. Is there something that I need to adjust so it doesn't go dead when gradually getting up to speed? What causes this to happen? Thank you very much for your suggestion.

You confirmed what I suspected. You need to get a manual and go through the link and sync procedure. Timing events and carburetion events are unique to each motor type, but from the symptoms you provided, the carburetors are opening at the wrong time, probably too early. The linkage adjustment and timing procedure, followed accurately, will correct that. It will also accurately and safely time your motor, giving you maximum available performance and minimum risk of damage. If you keep fiddling, you will eventually damage the motor and claim it never was any good anyway.

You will need some tools, like a depth gauge to set the check the timing pointer position, and a timing light.

When you slammed the pedal to the metal, you went through the timing sequence so quick it didn't make any difference.

hope it helps
John

 

[ABQGeo]

Re: Mercury bogging down like it is starving for fuel

Boy does this sound familiar! I hope you don't consider this thread stealing but perhaps with the info we have between us, we both might come up with a fix?

I have the exact same problem with my 1990, 150 HP. I did the same as you: rebuilt the carbs, waste of time; rebuilt the fuel pump, waste of time (although the pressure is now higher); replaced fuel hoses, waste of time; replaced both CDI units, waste of time and $! Link & Sync IS PER THE MANUAL. Compression is 105 on all six cylinders. Starts great, idles......er, ah, so, so. I've never been impressed with the way this engine idled. It's just that when you try to accelerate (very, very slowly or rapidly, it doesn't matter which) at about 1200-1500 rpm she dies. Per other threads recommending this, I have disconnected the idle stabilizer module.

Here's the catch with mine. If I make it work hard......that is, if I push the accelerator just up to the point where it wants to die and make the temperature gauge get up to about 1/4 of the way..........bame! All of a sudden I can punch it and away we go. It runs flawlessly at the high end. Shut 'er down, fish for awhile and I'm back to square one.

I'm about to look into the test procedure for checking the reeds. I should have inspected them when I had the carbs off.

The electrical engineer in me immediately had me looking at some sort of electrical problem as you did, since it seemed to be temperature related. I finally broke down and bought a DVA for my voltmeter and the voltages were right on spec. (I should have bought the DVA before I expended the $ for the CDIs)

I've found that I can improve my situation somewhat by plugging a vacuum line that goes from all three carbs to a "Thermal Valve" screwed into the right side head. According to the manual, this valve leans out the air/fuel mix when the head temp gets above about 110 degrees or so. Mine is working correctly, but the motor runs better with a golf tee plugging this line????? This indicates to me that it's definately a "too lean" condition.

One other characteristic that I have and was wondering if yours is the same: The fast idle lever is nearly impossible to set! It way overshoots and then way undershoots. No way can you set it on say 1500 rpm and forget it???

This is very reminiscent to me of a loose carburetor that I had on my pickup one time. I've checked all the mounting bolts, and the bolts for the reed plate.

Could one of the experts on here tell us if the reeds could be suspect?

Unfortunately for me, living in the desert (Albuquerque) makes it very difficult to get to water for testing. Under no-load on the earmuffs in the driveway, I can give the throttle linkage a pretty good rip and it takes right off......although there is sometimes a slight hesitation before it "catches"?

I'll be anxiously following this post for sure.

 

[BoobieBouncer]

Re: Mercury bogging down like it is starving for fuel

After my Link and Sync I had to re adjust the idle mixture screws and the idle speed screws.

Have both of you tried turning your idle mixture screws out to richen up your fuel mix so you don't bog down with the hole shot?

 

[ABQGeo]

Re: Mercury bogging down like it is starving for fuel

Thanks Boobie. During the carb rebuild, I noted the exact setting for the idle needle valves. All were exactly at 1 1/2 turns off of the seat just where the manual tells you to put them when finished. So that's what I did and put the plastic limiter caps back on. While adjusting the in-gear/idle timing for best idle performance, a smoother idle was obtained by opening all the needles to about 1 3/4 turns off of the seat (still within the limits of the limiter caps, barely). I feel this is a bit odd since all my lakes are at 4,000' plus (and have been that way for all 19 years that I've owned this boat )

Where are your idle screws sitting mercsmp?

John, your comments that the linkage could be starting the opening of the carbs too soon is interesting and certainly would be easy to check. The manual says the center of the roller on the carb linkage should align with the mark on the cam that drives it (at idle). It is curious that the ONLY place where timing linkage and carb linkage really comes into agreement is at wide open throttle? I've read on this forum that idle timing varies anywhere from about 4 to 10 degrees ATDC depending upon the engine/poster. Mine seems happiest at about 10 degrees? High altitude probably has something to do with that? That's a pretty broad range and if those carbs open a bit too soon?.......BTW. I've never touched the flywheel cover so my timing mark should not have moved. I checked it anyway with a dial indicator and it was spot on that 0.462" mark. I don't believe in random "diddling" either Getting the spark advance just a tad ahead of the throttle plates moving just might be the ticket?


mercsmp. Since you've replaced the stator, for sure you need to check that the timing pointer is EXACTLY where it's supposed to be. Also, where does your carb linkage start picking up?

 

[thelm]

Re: Mercury bogging down like it is starving for fuel

mercsmp

My 82 90 HP inline 6 was doing the same thing. I went through the link and sync procedure and fixed the problem. The synchronizing J martin is talking about is synching the carbs to the ignition timing, not the carbs to each other. The procedure is not difficult. Having said that, you might try just opening the idle mix screws a quarter turn and test run. If you see any improvement try another quarter turn. The inline 6's are notorious for hesitating or stalling on accelleration if the idle mix is too lean.

Check out this post.

http://forums.iboats.com/showthread.php?t=322071

 

[j_martin]

Re: Mercury bogging down like it is starving for fuel

Thanks Boobie. During the carb rebuild, I noted the exact setting for the idle needle valves. All were exactly at 1 1/2 turns off of the seat just where the manual tells you to put them when finished. So that's what I did and put the plastic limiter caps back on. While adjusting the in-gear/idle timing for best idle performance, a smoother idle was obtained by opening all the needles to about 1 3/4 turns off of the seat (still within the limits of the limiter caps, barely). I feel this is a bit odd since all my lakes are at 4,000' plus (and have been that way for all 19 years that I've owned this boat )

Where are your idle screws sitting mercsmp?

John, your comments that the linkage could be starting the opening of the carbs too soon is interesting and certainly would be easy to check. The manual says the center of the roller on the carb linkage should align with the mark on the cam that drives it (at idle). It is curious that the ONLY place where timing linkage and carb linkage really comes into agreement is at wide open throttle? I've read on this forum that idle timing varies anywhere from about 4 to 10 degrees ATDC depending upon the engine/poster. Mine seems happiest at about 10 degrees? High altitude probably has something to do with that? That's a pretty broad range and if those carbs open a bit too soon?.......BTW. I've never touched the flywheel cover so my timing mark should not have moved. I checked it anyway with a dial indicator and it was spot on that 0.462" mark. I don't believe in random "diddling" either Getting the spark advance just a tad ahead of the throttle plates moving just might be the ticket?


mercsmp. Since you've replaced the stator, for sure you need to check that the timing pointer is EXACTLY where it's supposed to be. Also, where does your carb linkage start picking up?

Now we're getting some real data. If it likes to be at max late idle timing, and at altitude, I'd say you're considerably lean on idle mix.

Try, while on the water, adjusting the idle mix screws richer 1/4 turn. It should slow down the engine. Reset the idle timing to correct idle rpm.

one or 2 passes of this should do it.

hope it helps
John

 

[ABQGeo]

Re: Mercury bogging down like it is starving for fuel

Thanks John. I understand clearly what your saying. That's good info.

Darn, I'm just going to have to schedule myself another fishing trip real soon so I can check this out.

mercsmp. That link thelm provided looks like it might be the ticket for you?

George

 

[j_martin]

Re: Mercury bogging down like it is starving for fuel

I know the feeling. Just ripped the motor off'n the bass bote so I could fix the steering. Gotta hang it back on tomorrow, cause it looks to me like Thursday morning is gonna be another lunker day.

My son and I put together a 35 lb stringer of bass on Friday in 3 hours. What a blast. Top was 5 lbs. This is in MN, where the state record is under 9 lbs.

John

 

[mercsmp]

Re: Mercury bogging down like it is starving for fuel

I adjusted each idle screw 2 1/4 turns out from lightly seated. These adjustments allowed the motor to go to WOT while gradually giving it throttle as it should. I still want to do the link and sync procedure. This is my first time working on a boat motor so I am learning as I go along. How does the dial indicator work? Do you screw it in or just hold it in place? How do you keep it steady to get an accurate reading?

 

[ABQGeo]

Re: Mercury bogging down like it is starving for fuel

That's great news mercsmp and exactly what John was referring to. The relationship between the idle timing and where the linkage starts opening the carbs is critical. I haven't had a chance to get mine back out to the lake, but I'm now convinced (with your success) that my problem is identical. Wow, 2 1/4 turns seems a long way out. Did you say you had rebuilt or at least looked at the fuel bowls for the carbs? Sounds like you might have some varnish/gunk built up on the carb passages? Mine was clean as a whistle even after 19 years and not running the fuel out at the end of each season either?

What begs the question is, "Why all of a sudden?" I'm suspicious of our alcohol based fuels. I can no longer find a supplier in Albuquerque without alcohol. The next thing to go will be the hoses and tank pick up tube!

As to the dial indicator to check the timing mark: I picked one up at Harbor Freight for about $10 or so.... it was on sale. I then busted the insides out of an old sparkplug (that was a real chore!) and then drilled and tapped for a set screw in the side of the spark plug to secure the shaft of the dial indicator. It's sort of Mickey Mouse and really doesn't work that well, although it got me the verification I needed (I've never been into the top end and since I bought it new, it shouldn't have moved) I need to make a better holding fixture and braze it to the top of the old plug. Harbor Freight has the same dial indicator with a magnetic base........ but where to attach it? Aluminum heads

I believe another worth while process is to try de-carboning. I did this to mine and it greatly reduced the smoking and improved the idle. I did this process to my 15 hp kicker and WOW! I thought I had gotten a new engine.

http://forums.iboats.com/showthread.php?t=158076&highlight=seafoam

In the meantime, another problem has reared its head! My tach quit working. I sure hope it's not the 40 amp regulator! The old style is apparently no longer available and going to the new one is $$$$$$

Definition of a boat: "A hole in the water that you continuously pour money into!"

 

[mercsmp]

Re: Mercury bogging down like it is starving for fuel

I did rebuild the carbs. The top two were fairly clean but the bottom one had a little more gunk in it. Could have caused the float or float needle to stick. I also changed all of the hoses and inspected the tank pickup tube.

I saw the exact dial indicator you were talking about at Harbor Freight. That's a clever idea about using the spark plug to secure the dial indicator. I may give that a try.

My tach has been acting up for some time now. It registers way too high for the actual RPM's. I don't know if it is the tach itself or some of the electronics. Like I said, I am just learning as I go along about working on a boat motor. So far, it hasn't been so bad with people like you that are willing to help me out.

I will also try the de-carboning. Thanks for the suggestion.

 

[j_martin]

Re: Mercury bogging down like it is starving for fuel

Here's how I use a dial indicator on a Merc V6

Instructions:
Install adapter.
Install dial indicator with ty-wraps
rotate engine clockwise so piston is below tdc
open jaw on dial indicator into the cylinder.
rotate engine slowly clockwise past TDC
zero the dial indicator
rotate engine prox 1/2 turn CW
open dial indicator past .462
rotate engine slowly CW till .462
set the timing pointer.

Notice: Never rotate the engine more than a few degrees backward. It's hell on water pumps.

hope it helps
John