1988 100hp Mercury Outboard Fuel problem

[CSkjerven]

My dad and I are trying to get my very cold blooded engine tuned up.

It is a 1988 100hp 2 stroke Mercury with oil injection.

Here are the problems:

It does not want to idle. Does not seem to be getting enough fuel. If I hit the choke it seems to keep running better. It has the most trouble idling when it is cold.

When I finally get it going it will only get up to 7 mph at full throttle. After a while (5 to 10 minutes) it kicks in and gets up to speed (24-28 mph.)

We replaced the fuel pump and filter.

My dad thinks it could be the thermostat or plugged carbs.

Anyone have similar issues?

Thanks

 

[CV16]

Re: 1988 100hp Mercury Outboard Fuel problem

I'll bet on plugged carbs. I'd buy some kits and get to rebuilding them. It can't hurt!!!

 

[CSkjerven]

Re: 1988 100hp Mercury Outboard Fuel problem

All 4 carbs or just the 2 it idles on? It is the type of motor that only runs on 2 cylinders when idling or at low speed. Thanks

 

[sschefer]

Re: 1988 100hp Mercury Outboard Fuel problem

That motor is what's known as a 2+2 motor designed by merc as the big motor that could troll. Anything below about 1700 rpm it's running on cylinders 1&2. 3&4 are given enough fuel to support lubrication but not enough to support combustion. At between 15-1700 RPM a mechanical accelerator pump adds fuel to 3&4 via two check valve style injectors that are located under the electrical plate. This gets the engine over the hump and smoothly transitions carbs 3&4 to a state where they are providing adequate fuel for combustion. There is also a small 1/8" fuel filter located behind the oil tank.

Despite what sounds like a relatively complex system it's actually very reliable. The injectors can become gummed up and the filter plugged but that usually results in a flat spot as the transition occurs. That motor will go way beyond 1700 rpm and 7mph on just the top two cylinders. I suspect you have a problem with the ignition system or possibly a compression problem.

First thing to do is run a compression test. When new, the cylinders will produce 135psi. 115 would be low and ready for a rebuild. Uneven compression in excess of 10% difference between cylinders would also be reason for an overhaul.

If the compression is good then get a spark tester and see what the quality of the spark is. It should be bright blue and jump 1/4" at least. If you have good spark and compression then suspect fuel. If not then go to www.cdielectronics.com and download the electrical system trouble shooting guide for your engine. Follow the testing procedures to a T and you should find you problem and be able to correct it with the least number of parts.

Whatever you do, do not start throwing parts at it just because someone here thinks they know exactly what the problem is. Do the troubleshooting first and save your hard earned bucks for beer.

 

[CSkjerven]

Re: 1988 100hp Mercury Outboard Fuel problem

We tested the compression. All 4 were 120psi.
It ended up being an ignition problem. The top two cylinders were not getting any spark . We were very surprised because we were under the impression that because of the 2+2 it wouldn't have ran at all. It must have been trying to compensate by attempting to idle and run on the bottom two. We cleaned up all the connections and that was all it took. It runs like a dream. Idles nice now and pushes the boat across the lake at 30mph +. Thanks for your help.

 

[CSkjerven]

Re: 1988 100hp Mercury Outboard Fuel problem

Here is an update on my problems. The connections to the plugs on the top cylinders continue to corrode do to moisture. They are copper and turn as green as the statue of liberty. We continue to clean them up, it runs like a top, and they continue to corrode causing the plugs not to fire. We have tried a gel made to stop corrosion but it doesn't help. They corrode just sitting in my yard. Any ideas?

 

[CharlieB]

Re: 1988 100hp Mercury Outboard Fuel problem

Copper? Someone has changed them with cheaper automotive wires or ends, marine parts are stainless to prevent that corrosion.

Order marine plug wires and ends.