1988 Mercury 100hp w/ acceleration problems

I have a 1988 Mercury 100 hp outboard. It has the 2+2 system where the top 2 carbs get fuel for idle and slow speeds and the bottom two get fuel after the throttle lever is pushed farther forward (by actuating the accelerator pump...I believe). The boat has not been in the water for about 4 years but gets started yearly because I have delusions of taking it out. I put it in the water this year in anticipation of taking it on vacation w/ me but it is not running right. It starts and idles fine but will not reach top speed/RMP on the water. I can help it a little by squeezing the primer bulb w/ the throttle lever all the way forward. Helps speed/rpm for about 20 seconds then slows again. Barely gets on plane. I replaced fuel pump, filter and plugs 4 yrs ago. Carbs are clean. I've ordered an accelerator pump in the event that it is not sending gas to carb # 3 & 4. That should arrive next wk. Am I on the right track and can I check anything else until then? Thanks in advance for your help!

Re: 1988 Mercury 100hp w/ acceleration problems

Re: 1988 Mercury 100hp w/ acceleration problems

I would consider changing the fuel primer bulb- only costs $10. They do go bad, and when they do, you need to keep priming then to keep the engine running. Otherwise could be junk in the fuel line, especially check at the barb that comes off the tank.

Re: 1988 Mercury 100hp w/ acceleration problems

By process of elimination... It's not the tank, primer ball or external filter and, therefore, the solution is likely not a simple one. Had it on the water today w/ a new tank and hose. Still will not get up to top speed/rpm. Felt promising when I squeezed the primer ball but turned into a dog again after I stopped squeezing. I don't have a guage to check fuel pump pressure so I would likely just rebuild the pump if the accelerator pump does not fix the problem. UPS tracking says the accelerator pump will be here Tuesday. Hopefully that fixes the problem unless anyone else has any ideas. Thanks for your feedback!

Re: 1988 Mercury 100hp w/ acceleration problems

My bet is on the accelerator pump. However, if replacing the accelerator pump does not solve your problem you might consider getting a pressure/vacuum gauge to troubleshoot your fuel system. It is not an expensive tool and it would help narrow down and/or eliminate possible causes.

To troubleshoot, confirm that the VACUUM in the fuel line between the tank and the fuel pump does not exceed a certain spec and that the PRESSURE in the fuel line between the fuel pump and the carbs does not fall below a certain spec. Your shop manual should indicate what these specs are for you motor. If both are within spec, then a different range of possibilities can be considered.

That is what I would do in your situation. I like to avoid the frustration of replacing parts based upon a guess and then finding that it did not solve the problem. The exception is when the part is inexpensive, easy to replace, or can be considered part of routine maintenance.

Re: 1988 Mercury 100hp w/ acceleration problems

Dealing with almost the same problem. I have a 1989 100 hp. Idles fine and sometimes it runs fine from idle to wot. othertimes it act like its not firing on one or two clys, if I choke it, it picks-up for about a few seconds and then back to running flat.

I rebuilt the fuel pump and cleaned the carbs, havent' had a chance to get it back into the water to see if that fixed the problem. Good luck and if you figurer out the problem let me know.

Re: 1988 Mercury 100hp w/ acceleration problems

Just to be sure, check the floats in your carburetors too. The specification will be in your service manual.

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.

Re: 1988 Mercury 100hp w/ acceleration problems

Well I've just been told by a mechanic that I need a stator and $500 to put it on. I'm going to pay him $69 for the advice and try to put it on myself. Any tips (even if the tip is "don't try this on your own")?

How can this be a stator problem?

How can this be a stator problem?

Does this even make sense to anyone? If I have a stator problem that is electronic, why does squeezing the primer ball improve the motor's performance (even if it is temporary)? I think his diagnosis was based on sight. "Well, you have a black stator and it would be red if it has been updated". He did pull the plugs and the top two were clean and the bottom two were black.

I'll do whatever it takes to get this running but I never dreamed it was an electrical problem based on the symptoms I had. I'd love to hear what you all think!

Thanks

Re: 1988 Mercury 100hp w/ acceleration problems

Re: 1988 Mercury 100hp w/ acceleration problems

Guess what the problem is not... the stator. I checked my stator w/ my multimeter and the high range was 7100 and the low range was 111 - both w/in specs. I'm about to put the new accelerator pump on but I'm not confident that will solve it. Seems to be a switch more than a pump. Gotta put it on - it's all I have left to try. Wish I'd bought that fuel pressure gauge on my way home but NO, why would I do that. On my way home I thought my problem was a stator. Anyway... maybe I'll check the fuel pump tomorrow. The liklihood of this boat going w/ me on my trip in 2 days is fading fast.

Re: 1988 Mercury 100hp w/ acceleration problems

Your ohm readings on the stator still don't rule out stator problems. You need to check it with a DVA to be sure. Other possibility could be switch box. My last stator ohm'd out ok but was nonetheless defective and a new stator fixed my ignition problems.

Still baffled!

Still baffled!

Replaced accelerator pump last night. Looked at all carbs - clean, floats good. Internal fuel filter clean. Good gas flow through hose when primer ball squeezed. Put back together. Took to lake this morning. No improvement. I can squeeze the primer ball until my hand cramps up and I have a great ride while I'm doing it. As soon as I stop squeezing the boat starts dogging. I'm still convinced it's a fuel problem but I'm not a mechanic. At any rate, I leave for Canada tomorrow w/ a 14' boat and a 15 h.p Johnson instead of a 20' boat and a 100 h.p. Merc. I'll continue my pursuit of a solution to the Merc when I get back. Thanks for all your posts. I'll let you know how this all turns out.

Re: 1988 Mercury 100hp w/ acceleration problems

Re: 1988 Mercury 100hp w/ acceleration problems

Well, I'm ready to resume my quest to find out what is wrong with this motor. I purchased a vacuum/pressure gauge. It came w/ some different fittings and some instructions on how to use it on a car. To test the vacuum on my outboard I disconnected the fuel line at the outlet of my external fuel filter/water separator (between the tank and the motor) and connected it to my vacuum gauge. The meter topped out at 9 inches of vacuum. The gauge came w/ a “T” fitting. I was not sure if I was supposed to use the “T” in-line and leave the hose attached to the tank or to do it the way I did. Based on one of the posts that said I should test to see if the vacuum does not exceed a certain spec I wonder if I should have tested it that way (w/ the “T”), too. I did blow out the vent w/ an air compressor so that potential problem is out of consideration. Anyway, the next test was done by squeezing the primer ball until the carbs filled up w/ gas and then taking the fuel line off the top carb and attaching the pressure gauge to the fuel line there. The readings were a little erratic at first. The reading topped out at 3 psi at idle and went up to 6 at WOT. The pressure readings seem to be w/in specs based on my shop manual unless I did not perform the tests correctly. The only other fuel system adjustment I made was to connect the fuel line from the tank directly to the “T” fitting just past the hose connector on the motor. I have had trouble getting my fuel hose to clip to my motor. Has been very hard to get off and on. Someone had told me that I might have bad O-rings. I bypassed the connectors alltogether and directly connected the fuel hose to the fitting just before my fuel pump. Have I done all the tests I need to do w/ my vacuum/pressure gauge? Do these readings seem to indicate a problem? Was it OK to directly connect the fuel line to the motor or do the connectors provide a safety function. Thanks to all for getting into this mess w/ me again. I’m eager to hear your feedback.

Re: 1988 Mercury 100hp w/ acceleration problems

The following linked Mercury Service Advisory describes how to test for fuel starvation in the fuel line (between the fuel tank and the fuel pump). Testing for proper pressure in the fuel line (between the fuel pump and the carbs) would be done in a similar manner. I hope this helps.

http://www.marinepartsman.com/Mercury-Marine-service-bulletins/Mercury-Mariner/2001/EN_17.PDF

Re: 1988 Mercury 100hp w/ acceleration problems

MrLundt said:

I can help it a little by squeezing the primer bulb w/ the throttle lever all the way forward. Helps speed/rpm for about 20 seconds then slows again. Barely gets on plane. I replaced fuel pump, filter and plugs 4 yrs ago.

Click to expand...

Just so I'm clear, you stated you changed out fuel pump 4 yrs ago. And when you squeeze the bulb your rpms increase. Would this not dictate that a new fuel pump is required? Even with the limited running of the motor during that 4 yrs, would it not be best to rebuild the fuel pump? But I defer to the experts on this...

geb

Re: 1988 Mercury 100hp w/ acceleration problems

Generally, when squeezing the bulb makes a positive improvement in performance then it IS time for a fuel pump rebuild.

When you took your fuel pressure measurements where you out on the water or in the driveway?

On the water requires a substantial amount of FLOW AT rated pressure, on the hose/NO LOAD it is very easy for a fuel pump to pass and still be defective.

Next subject, 4 yr old spark plugs,
Unless you are running very expensive precious metal plugs AND you are NOT fogging as much as I do every year, those plugs may be a problem, at least the bottom two which you stated are blackened,

My 96 115 is a later version of your motor, douoble check piston clearance and try a set of NGK BPR8HS gapped at .032 and see if it makes a difference.Often we tend to overlook the obvious, marginal plugs will limit rpm/performance. @ $2.50 ea it's a cheap enough attempt in troubleshooting

Re: 1988 Mercury 100hp w/ acceleration problems

Thanks, hkeiner for the link. That answers my question about the vacuum test. I do need to check the vacuum readings w/ the “T” fitting. There were no instructions regarding the pressure test but I believe I did that correctly unless I can leave the line attached to the carbs and use the "T" again. After the readings I got, I thought I could rule out a bad pump but because I did the tests in the driveway w/ mufs, it makes sense that it could pass specs in the driveway but may still be defective due to demand needed under load.

What I don’t know is what actually causes the 3rd and 4th cylinder in my 2+2 system to engage. Is it fuel supply? As RPM’s increase does the pump work harder and supply more fuel? Is it electronic? Does # 3 & 4 even get spark at lower RPM’s? Is there any merit to the mechanic’s diagnosis that my stator needs replaced. If the stator would increase my RPM’s, maybe I would get more fuel? I wonder if my discovery that squeezing the primer ball improved performance has actually thrown us all off track.

I’ll probably replace the fuel pump anyway and will check my vacuum w/ the “T” fitting. I'll replace the plugs, too, but I am considering the possibility that it is not a fuel problem. I’m also feeling a little defeated since nothing has panned out yet. I hate to take it to another mechanic that doesn’t know what they’re doing but, at this point, I’m not sure I know what I’m doing either. I have a shop manual but it covers motors from 1976 to 1991 and 45 to 225 H.P. It hasn’t been much help w/ my motor