'92 Force 70 firing issue

[callit10]

I have read through this forum a bit and I see that it is a powerful resource.

Background: Motor had not been run in 3-4 years.

1st time out: Motor cranks on old gas and runs with good power and continuously better.

2ng time out: runs good until tank is run dry and motor dies. Fresh tank is mixed on the boat a bit oil rich (150% required oil based on 50:1 being correct). Motor cranks, but runs poorly.

3rd time out: fresh tank mixed at 50:1 + 1 can Seafoam. No change in performance. Installed new inline filter between pump and carb.

4th time out: visually verified good fueling in carb throat. No change in performance even when pulling the top, then middle cylinder plug caps off their plugs. The only change noticed is when the third (bottom) cap is pulled, the engine dies.

So new plugs are installed and verified firing (by the above method) while in the driveway.

5th time out: in the water, same performance as prior to new plugs. But here's the rub: we put the boat on the trailer and pull OUT of the water and each of the cylinders WILL FIRE per the above test! What could the boat being in or out of the water have to do with combustion?

Besides putting out to the middle of the lake and dumping the motor then rowing back in and installing a different motor, do you have any advice?

 

[john from md]

Re: '92 Force 70 firing issue

The difference between in and out of the water is the back pressure in the exhaust system caused by the water. If you notice, when you idle your engine on muffs, it will run @ 1200 rpm. When you idle it in the water it drops off to @ 900-1000.

Your engine sat for four years but you don't say if you pulled the carbs to clean the varnish or if you replaced the fuel pump diaphram.

I recommend that you disassemble and clean the carbs, replace the fuel pump diaphram, replace the water pump and check that the fuel recirc passages are clear.

If you haven't bought a Clymer Force Outboard Shop Manual, you need to do so.

John

 

[callit10]

Re: '92 Force 70 firing issue

Thank you John for taking the time to reply.

We have the Clymer manual for this model.

There is one carb on this model and I have disassembled and verified that it is clean and I can see good fueling during operation.

I agree that the fuel and water pumps should be replaced before the boat is put back into service, but we wanted to make sure the core motor was OK before we spent time and money on the consumables. At this time, the fuel pump is supplying the required fuel and the water pump is moving plenty of water out the tt. What we need to figure out is why only one of the three cylinders fires while in the water. If the only difference is back pressure, does this mean the reed valves are able to do a sufficient job out of the water to support combustion, but not in the water?

 

[john from md]

Re: '92 Force 70 firing issue

When you pulled the plugs off one by one, the number one and two cylinders showed no difference in operation. Only when you pulled the number three wire did the engine shut down. This tells me that number 1 and 2 have some sort of problem. The problem could be fuel or spark. Since you only have one carb on this engine, the fuel problem would have to be a vacuum leak, compression leak or two sets of bad reeds. The spark problem can be CDI, Coil, stator or trigger.

Coils rarely go bad but are easy to swap. CDI units can be swapped as well.

Have you checked your compression? Are the cylinders within 10 pounds of each other?

Reeds usually go bad from two reasons. One is over revving the engine from using a prop underpitched for the engine. The other is from backfiring through the carb. This usually occurs from poor starting technique or carbs that are clogged.

Do some additional troubleshooting and try to eliminate potential culprits one by one.

John

 

[callit10]

Re: '92 Force 70 firing issue

I'm confident that the ignition is good because of the out of water firing of all cylinders and the fact that I get a good shock from each of the cylinders when performing the firing test while on the water.

When you talk about reeds going bad, do they develop cracks which could leak fresh charge?

I think we need to compare the condition of each set of reeds. I was not on the boat the first three times on the water, so I can't speak about any backfires. I can imagine a misfire could have occurred and damaged a reed(s).

 

[john from md]

Re: '92 Force 70 firing issue

The reeds on your engine are metal. They are harder to damage than composite reeds but they can be bent. You can pull the adapter plate off and look at them. I believe you are allowed .010 off the seat.

John

 

[callit10]

Re: '92 Force 70 firing issue

This is good information. We will take a look and report back.

 

[callit10]

Re: '92 Force 70 firing issue

We removed only the plate that the carb mounts to and noted that the reeds looked identical on all three cages from upstream. Also verified no holes in any of the pistons and they are visually identical (through the plug hole). Next we will measure compression each cylinder makes and focus our attention on the fuel recirc system.

What are the chances of the crank seal between top and middle cylinder leaking?

 

[john from md]

Re: '92 Force 70 firing issue

Sounds like a good plan to me.

John