1988 Mercury 100hp w/ acceleration problems

[hkeiner]

Re: 1988 Mercury 100hp w/ acceleration problems

The vacuum test and the pressure test should both be done using a T connection and with the motor running out on the water. You need to see if the meter readings are OK when the motor is actually bogging, otherwise you may not see any drop/spike in the readings. If the readings remain within spec while the motor is bogging, then you have eliminated a lot of possible causes related to the fuel system, although not all of them. That is making progress.

Also, be sure to confirm that there are no air bubbles appearing in the clear hose connected between the T-connector and the fuel pump during the above vacuum test.

If you want to check the ignition system, I have found that using a SmarTach meter to monitor secondary voltage in the spark plug wires is a fast and easy way to do this. The meter connects to the spark plug wires using an inductive connection, which can be easily moved from wire to wire while the motor is running and bogging. If the meter shows that the secondary voltage is dropping on one or more spark plug wires during bog, you can then use an ohmmeter and DVA meter (or other methods) to find out what exact component is failing. I bought my meter on ebay for under half or retail. The meter will not tell you which particular ignition component is bad, but it will tell you if the ignition system is failing or not during bog. That is making progress.

 

[CharlieB]

Re: 1988 Mercury 100hp w/ acceleration problems

The 2 + 2 sytem is quite simple, only the top 2 cyl's/carbs have idle circuits, as sooon as the throttle opens the accel pumps gives the lower cyl's a squirt and they fire, ignition sparks all cyls all the time. As engine speed increases above an idle fuel begins to draw from the transition circuit on ALL carbs.

Try the spark plugs.
Check to be sure the accel pump is at factory specs, the slight gap between the ramp/cam and plunger.

Idle mixture on the top carbs must be slightly richer than 'best idle'

 

[MrLundt]

Re: 1988 Mercury 100hp w/ acceleration problems

I had originally thought that the accelerator pump was my problem so I replaced it (I'll double check the gap size). Sounds like the bottom 2 cyl's wouldn't kick in w/o the "squirt" delivered by accel pump. From where does the accel pump draw this fuel that is "squirted". Looks to me like the pump is positioned between a bleeder tube and fuel line that bisects the line from the 2nd and 3rd carb via a "T". I can't tell which way the fuel flows and how it helps. Also, what is the "transition circuit"? Is there anything else that would prevent bottom two cyl's from kicking in?

 

[CharlieB]

Re: 1988 Mercury 100hp w/ acceleration problems

Transistion circuit on a carb is a slot or another hole in the carb throat that is uncovered as the butterfly begins to open, it allows fuel to begin to flow once the throttle is moved OFF IDLE.

You could have a spot of muck still in your carbs, the floats should be dead nuts level with the carb upside down. Float to high or too low and the fuel level is not right, transition is either too fat or too lean,spray carb cleaner thru ALL ports/holes/passages, follow with compressed air, ensure all passages clear/clean.

Fuel pump and lines must be clear/clean and flowing fuel when the pump strokes or the bottom cyls will not come in correctly.

Top carbs do not get accell pump shot, idles must be set rich, best idle plus almost 1/4 turn CCW, rich enough to accellerate without a hesitation or bog.

Spark plugs must be good, a partially fouled plug(s) will cause problems, headaches, trying to figure out what's wrong.

You wouldn't believe how many times all that's wrong is a bad/fouled plug.

 

[MrLundt]

Re: 1988 Mercury 100hp w/ acceleration problems

Can anyone give me a link to a site w/ a good step-by-step description of how to disassemble and clean a carb. Now that I have a better understanding of what #3 and #4 carbs do, I'd like to make sure the "transition circuit" and various other holes and chambers are actually clean. I know the bowl is getting fuel because it runs out when I unscrew the plug on these carbs but I don't know if fuel is getting from the bowl to where it needs to go.

 

[bryanhs75]

Re: 1988 Mercury 100hp w/ acceleration problems

i have 89 mariner 100hp with same problem it wont go past 2000 rpm rebuilt carbs new plugs new stator but still wont run someone told me it was more than likely the switch box but thats $300 i dunno either

 

[MrLundt]

Re: 1988 Mercury 100hp w/ acceleration problems

If not identifying a problem is making progress then I'm making progress. I had the boat out on the lake last night for the pressure and vacuum tests. There was no reading on the gauge when testing the vacuum and there were no air bubbles. My translation: no fuel blockages. The reading on the pressure test was similar to the driveway results. 3 psi at idle and 6 psi at WOT. My translation: fuel pump is fine.

What I know (or think I know): Fuel is getting to the carburetor bowl.
What I assume: Lack of power/rpm due to only running on top 2 cylinders
What I believe: Still a fuel problem ? not electric. When primer ball is squeezed, motor powers up and runs at about 90%. When stop squeeze ? return to dog.

What?s next? If fuel is getting into the carburetor bowl, how does it get to the spark plug? Could that be where the fuel problem is? Does anyone have a link to a site w/ good step-by-step instructions on carburetor cleaning? I?ve had them off and apart. The bowl was not gummed up but maybe I overlooked a port/hole/passage that is clogged. Also, with 2+2 system the accelerator pump ?squirts? gas into the bottom 2 cyls. I just replaced the accel pump but is there some other reason that the ?squirt? is not getting to where it needs to go?

 

[hkeiner]

Re: 1988 Mercury 100hp w/ acceleration problems

Does anyone have a link to a site w/ good step-by-step instructions on carburetor cleaning?

When I cleaned my carbs for the first time last spring, I also looked for some on-line videos of how to clean carbs. What I found were very generic and sometimes confusing videos because the carbs in the videos were always different than mine. They had different jet setups, different choke setups, welsh plugs that I didn't have, etc.

In the end, I decided that my Mercury OEM shop manual for my particular motor was probably the best source for instructions on cleaning my particular carb. It had a section that described what carb components needed to be removed for a normal cleaning. A parts diagram of the carb is a must have. The second best source was asking specific questions on this forum, as they arose during my carb cleaning process.

My biggest uncertainty was deciding what to remove from the carb body before soaking it in carb cleaner. For example, it was clear that I needed to remove any plastic parts, the float valve parts, and the idle speed jets before soaking. I was initially unsure, however, if I really needed to remove the fixed fuel and air jets or the throttle plate assembly, as I didn't want to risk damage to them during removal. As I was in the process of carb removal and disassembly I asked these questions on this forum and got answers pretty quickly. The consensus was that it was not necessary to remove the fixed jets or throttle plate for soaking.

Another uncertainty was whether I could get away with using just spray carb cleaner to clean the disassembled carb parts or whether I really needed to use a carb soaking solution (such as a 1 gallon can of Gunk carb cleaner or similar). The consensus was that the carb soaking solution was much more effective in cleaning than just a spray. The Gunk carb soaking solution container also had instructions on how to use it properly to soak the carb parts.

Lastly, the consensus was to be sure and blow out all the passages with air and don't miss any. I studied the shop manual carb diagrams to understand where all the passages were and I also just blew air through all apparent openings in the carb and then confirmed that the air blew out somewhere else on the carb.

My motor ran better after the carb cleaning so I felt I did it the right way. Others on the forum may have additional or better advice than this. Good luck.

 

[hkeiner]

Re: 1988 Mercury 100hp w/ acceleration problems

The reading on the pressure test was similar to the driveway results. 3 psi at idle and 6 psi at WOT. My translation: fuel pump is fine.

When primer ball is squeezed, motor powers up and runs at about 90%. When stop squeeze – return to dog.

The fuel pressure specs for my motor at WOT are higher than 6psi (see attached thumbnail from my shop manual). Have you checked your shop manual to confirm what the pressure specs are for your motor at WOT?

Were your pressure gauge readings taken while the motor was bogging at WOT and again when you were squeezing the bulb to make the motor run better at WOT? If there was an increase in pressure shown on the gauge (which I would suspect would be the case) then the amount of pressure increase needed to get the motor running better at WOT might help in figuring this out. Just a thought...

 

[MrLundt]

Re: 1988 Mercury 100hp w/ acceleration problems

The pressure readings I got were w/in the specs noted by my shop manual (2-3 psi at idle and 4-6 psi at WOT). The readings were taken while my motor was at WOT and running like a dog. On my way home from the lake it occurred to me that I wanted to squeeze the primer ball w/ the gauge on the motor to see what pressure was achieved... but I forgot to do that. If I could increase the pressure of the pump, that may give me better performance on the water, but I think that would just be compensating for the problem that I believe I have beyond the pump. If my pressure readings were belows specs, I would suspect the pump. Since they are at specs, I suspect something beyond the pump.

By the way, I had mentioned in a prev post that my manual was a Clymer Mercury outboard shop manual for motors from 45 to 225 HP built from 1972 to 1989. My specific motor is not mentioned very often in that manual. I just ordered a manual specific to my motor from Mercury today. That should help me w/ my carb cleaning and w/ issues that are specific to my 2+2 system. Thanks for your posts!

 

[hkeiner]

Re: 1988 Mercury 100hp w/ acceleration problems

It seems you have definitively eliminated the fuel pump and everything upstream of the fuel pump as possible culprits. That is true progress. Since squeezing the bulb does make the motor run better, the extra pressure is apparently making the fuel flow faster through the carbs. As you indicated, the solution is not to up the pressure created by the fuel pump (it probably can't be done anyway) but to find out what is restricting the fuel low under normal pressure. The most likely culprits are incorrect float valve adjustment, dirty high speed jets, or partially obstructed fuel passageway somewhere inside the carb body.

I have read of a carb troubleshooting technique where one shines a timing light into a carb throat to view the spray of fuel while the motor is bogging and then visually compare the spray pattern between the carbs. You have to do this when it is relatively dim (dark) outside and you have to move the timing light connection to the proper spark plug wire for the carb/cylinder being tested so that the comparison is meaningful. I never did this myself but I have never had your type of perplexing problem before either. Sorry but I can't advise more than that as I don't know enough about the unique design (and additional carb failure points) of the 2+2 system. Hopefully someone else on this excellent forum has experience with the 2+2 system and can provide better advice. Good luck.

 

[MrLundt]

Re: 1988 Mercury 100hp w/ acceleration problems

I just confirmed w/ mercury that the max pressure output expected from my fuel pump is 4-5 psi. My generic shop manual said 4-6 psi and my reading out on the lake was 6 at WOT, but the way the boat runs, it acts like it needs more fuel so I wanted to make sure that the tests that I did to rule out fuel pump problems were to specs and they are.

Sunday I removed all carbs (1 at a time) and dipped in carb cleaner. That was a very rewarding experience. You may think they?re clean but after the dip, you know their clean. Blew out all holes, etc. I couldn?t tell if there was any pre-cleaning blockage by looking at it but after the cleaning, I know there is no blockage. Haven?t had a chance to take it to the lake to see what it does under a load. Sounded good in the driveway but still no way to tell if bottom 2 carbs are contributing. Maybe I?ll get it out Tuesday.

I?ve suspected a fuel supply problem all along and have worked my way up the line ruling out problems as I went along. I know the fuel is getting to the carbs, and, after this cleaning, I am pretty certain that fuel will be getting to the cylinders. Is there anything beyond the cyls that involves the fuel system? What to the reeds do? I?m hoping the boat will run well after the dip but I?m also looking ahead to what my next point of interest will be if it does not work.

Thanks

 

[MrLundt]

Victory at sea!

Victory at sea!

I guess after several failed attempts to resolve my problem it would be expected that I might be a little pessimistic about my latest attempt but I shouldn?t have been. A $20 bucket of Chem-dip was all I needed. I soaked all 4 carbs in chem-dip on Sunday. There was no way to tell if I had made any progress w/ mufs in the driveway but running it on the lake today provided all the answers I needed. I was pleasantly surprised when my boat climbed up on plane w/ no hesitation from the motor. I don?t have a tachometer on it but it sounded and performed like it should. I guess my carbs were not as clean as I thought they were when I first looked at them in August. The moral of this story is: when someone that doesn?t know what their doing tells you their carbs are clean, better ask them what that means to them. To me it meant there was no gunk or varnish in the bowl. I did not take the top cover off to look at the jets or other holes. Something must have been clogged because after the dip and the air compressor, the motor works like it should! Thanks to all who posted. It was the suggestion to get the pressure/vacuum gauge and instructions on how to use it that helped me rule out a lot of the problems and lead me to focus again on the carbs. I couldn?t have done it without you. THANK YOU!!!!

 

[hkeiner]

Re: 1988 Mercury 100hp w/ acceleration problems

I am glad to hear that you got it working OK. Good thing that you didn't buy the $500 stator that the mechanic recommended.

Regards

 

[jonhstebbins!3]

Re: 1988 Mercury 100hp w/ acceleration problems

I had the same issue with my 150hp Merc black max. I just had the stator changed and now its picking up speed. I also had to change out the seal to the oil tank in the boat that sends to the the reserve tank. I was not getting a good vacuum which caused loss of speed.

How did the stator help?