Force 120HP exhaust (fire) coming from fuel pump pulse port

I am working on a 120HP Force OB about 1999. Would not idle, so R/R carbs, cleaned and retested. Result was high rpm's & popping and high surges coming from the fuel pump, enough to make the fuel bowl on the top carb spit out of the air vent on carb. I have never seen this sort of problem before. The plastic fitting on the block at the fuel pump pulse port got extremely hot and actually melted. I have replaced the pulse port fitting with metal one & inlet needle on the top carb & still get the same problem. Does anyone know what would cause the pulse port on the strbd side of the block to spit fire???

Re: Force 120HP exhaust (fire) coming from fuel pump pulse port

Do a compression check. If compression is OK. Check the timing. If that's OK check the reed valves.

Re: Force 120HP exhaust (fire) coming from fuel pump pulse port

i did check compression yesterday and all were 150psi. I intend to check timing today. I will post results. Thanks.

Re: Force 120HP exhaust (fire) coming from fuel pump pulse port

Just for clarification...is the pulse port for the fuel pump that you are referring to attached to the by-pass cover on the....I think on number 3 cylinder? About 3/4 of the way down from the top of the block?

You may also have a damaged fuel pump diaphragm too. You probably need to check it out, especially if the plastic piece was melted.

Re: Force 120HP exhaust (fire) coming from fuel pump pulse port

Hello and thanks for the replies. I have since checked the reeds, the compression, cleaned the carbs, replaced reed gaskets. I have found that the problem is basically that the lower carb is running lean and it is the leaness that is causing the backfire,, I just have never seen one actually fire into the crankcase chamber. I have switched the reeds from top to bottom and vise versa with no change. The last thing I tried this afternoon, was to switch the top carb with the lower one (can be done with some modifications). This made the backfire on the lower carb to stop. I can't find what was making the bottom carb run lean, but it is working better with the bottom caeb in the top position. My conclusion is that the bottom carb is getting extra air somehow, it dosen't effect the top as much because the fuel pump pulse port is in the cyl#3 position. I am going to leave the carbs the way they are and see how it runs. In the meantime, I will be looking for a replacement carb (used), a new one is NLA. Thanks again for the replies, I will check back for any info you can lend, like where I can get a good carb, or , any suggestions for where the extra air may be getting in the carb vaenturi. By the way, this motor is a 1999 120HP Force by Mercury.

Re: Force 120HP exhaust (fire) coming from fuel pump pulse port

Check E-Bay or sometimes Craigslist for a new carb. I'd be cautious about running it if you suspect it's running lean. Lean mixture is a recipe for a melted piston, especially if you run it at WOT for any length of time.

Carb running lean is either the float is set too low, or one of the internal fuel passages in the carb has some debris that is blocking it. You may have to use a small piece of wire to clean out all of the small internal passages. You may also have to remove the "welch" plugs to access some of the passages. Most rebuild kits include replacement plugs. There's not much to the carbs so it should be an easy fix unless it's cracked.

Also check all of your vacuum hoses that are attached to the carb adapter plate.

Check the very small hoses for the fuel re-circulation system too. There are two one-way check valves that two of the small hoses run to in the block located just below the each of the carbs. Verify that the two one-way check valves are working.

Make sure the four bolts that hold each of the 4 by-pass covers on the side of the block in place are all tight and no leaks.

Re: Force 120HP exhaust (fire) coming from fuel pump pulse port

Double check the timing on that cyl with a timing light, while the light is ON look for another flash at 180 degrees out, see if the timing mark lines also show on the opposite side of the motor from the correct firing position.

I have NOT seen this problem on late model ignitions, but the earlier Force ignition modules sometimes DID develop a 'Double-fire' fault in which the second spark occurring 180 degrees opposite of correct did product these symptoms, an intake backfire, leaning that cyl and soon causing piston damage.

It is a simple test and can save a piston if identified early.