91 Force 120hp no fuel to top 2 cylinders

Masonator450

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Hi guys. I just picked up a boat last month with a force 120 L-drive. Hadn't run in a year and a half I was told. Had no spark, I fixed that issue now it's timed perfect and has awesome blue spark to all 4 cylinders. After the 3rd try she started right up in the driveway. Did that 3 times and started every time and idled nice. Took it to the lake the next day and couldn't start it to save my life. Finally discovered the top 2 cylinders aren't getting any fuel at all to them, spark plugs come out perfect clean and dry Everytime while the lower 2 cylinders seem to be getting the right amount of fuel. Took top carb off and cleaned every nook and cranny and can spray cleaner through any one of the ports and have it come blasting out the other end - didn't fix it. Swapped lower carb with the upper - didn't fix it. So I sprayed some fuel directly through the carb against the reeds, still none getting to the spark plugs. If I put my hand over the intake for the carb I can feel it sucking as it should be, and the reeds all seem to have a good seal and look in perfect shape. I've spent every free hour I've had for 3 weeks trying to figure this out and am completely stumped. Id be happy to try and suggestions anyone out the can give me. Thanks a ton
 

Jiggz

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" and the reeds all seem to have a good seal and look in perfect shape. "

The word "seem" tells me you did not actually pulled out the reeds. Please pull out both reeds and do a through inspection. A missing leaf or two will actually prevent the motor from getting fuel into the cylinder. Or if you want to do more troubleshooting, do the paper test one cylinder at a time. With a sheet of paper, place it about 3-4 inches away from the plug hole and try cranking over the motor 2-3 revolutions.

You should see fuel spray pattern on the paper. Do this for the rest of the cylinders and make comparison and you should be able to tell which cylinder is not getting enough fuel.
 

Masonator450

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I have done the "paper test" and got fuel comming out of the lower 2 cylinders. Way too much was comming out of the lowest cylinder so I pulled the exhaust manifold off and about a litre of fuel came out of the bottom of the block. I'm guessing it was from me squirting fuel straight into the throat of the carbs and all making its way down to the bottom of the case on my many attempts to start it before finding I'm still not getting Any fuel to either of the top 2 cylinders. After reinstalling the exhaust manifold I tried turning it over with the plugs out and I just got a consistent amount of mist comming from the lower 2 cylinders. As much as I would expect anyways.
Yesterday I poored fuel into the upper carb and both upper cylinders to see just where it would go, with plugs out it came out of all 4 cylinders so I know there's no (or atleast not a good) seal between the upper half and lower half of the engine. But either way I would expect that the whole block would fill with mixed air/gas. This is where I don't see how it's possible for the top 2 cylinders to be dry.

I can see all the reeds clearly with the carbs off and have a horoscope comming in the mail tomorrow so I'll do another inspection and can post a pic of them
 

Jiggz

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With the top carbs off, you could have easily remove the intake adapter and remove the reeds both on #1 and #2 cylinders for inspection. The reason mist came out of all 4 cylinders with plugs off is because you basically poured it in the upper cylinders and the fact that the bottom cylinders are actually getting air-fuel mix to start with.

Leave no doubts and pull out both reeds on #1 and #2 cylinders.
 

jerryjerry05

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No fuel past the carb can also be a stuck float.
The needle can lock in the hole and shut off fuel.
The needle is it a solid one or one with a rubber tip?
The solid one, in the carb seat there is a rubber o-ring that swells because of additives.

You have fuel spitting back from the carb? No? then the reeds should be ok.

With the plugs out and grounded and the throttle advanced all the way.
Use a jumper and turn the motor over.
While it's turning over spray fuel direct into the throat of the carb.
Then it should show a mist coming out of the plug hole.

One trick is tilt the motor up and try to start.
If it starts , shut it down and lower and try to start again.
Squeeze the ball and see if fuel drips out.
 

Masonator450

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Thanks for the help everyone, I really appreciate all the experience.

I know the floats aren't sticking in the carbs because I've had both apart half a dozen times last week looking for anything that could be wrong and they were always moving freely, the inlet needle has a rubber tip and moves up and down with no resistance as well.

I do see a single puff of fuel come back out of the carb maybe 50% of the times it backfires but not on every rotation.

Also, wondering how having the leg tipped up can help? I have the boat facing up hill on a steep driveway with the leg up. I figured since it's a 2 stroke it should be able to run on its side or even upside down long as the carb bowls are full
 

Jiggz

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Thanks for the help everyone, I really appreciate all the experience.

"Also, wondering how having the leg tipped up can help? I have the boat facing up hill on a steep driveway with the leg up. I figured since it's a 2 stroke it should be able to run on its side or even upside down long as the carb bowls are full
"

Not quite true. From experience whenever my 125HP is tilted, meaning it is past above the trim, it always have a hard time starting. If you have a steep drive way, trim or tilt the motor so it is horizontally level. If not sure use a carpenters level. Then try again before removing the reeds for inspection.
 

Masonator450

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That makes sense when it's re-angleing the carbs. Mine is an inboard model L-drive, the carbs are on the port side of the engine so the fuel should be perfectly level when looking at it from the stern. When looking at it from either side the engines tipped 18? toward the stern. It still baffles me that I started it 3 times in the driveway just how I have it now then it decided not to start at the lake and now not in the driveway.

I do agree with Jiggz though, better to have no doubt about the reeds so here's some pics
 

Masonator450

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The insides are identically clean and it was a pain trying to get the file size small enough to post so I only have one inside pic, and from the outside they also look pristine, and each Reed has the tiniest bit of suction to the plate theyre mounted on, not sure the actual term.. so I know they seal good

Honestly I was hoping to find 1 or 2 broken and finally have found the issue haha
 

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Jiggz

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Great. At least now you know the reeds are pristine. When you try starting does the engine even try to "cough" like it wanted to start but will not continue on instead causes the starter to disengage? If it does this have you tried inspecting the upper carb bowl to make sure it is full of fuel?

If it is not full of fuel with the top carb bowl off, hold a small cup under the location of the bowl to catch fuel oil mix while have someone crank over the motor. Let the float hang normally. Is fuel pouring out of carb's inlet valve? If not, check the float to make sure it is not stuck in the up position or not set too low not allowing fuel to get in. Try priming with the priming bulb, is fuel pouring out of the inlet valve? If it does when you prime but not when the engine cranks, there is a good possibility the fuel pump is not putting out enough pressure for the fuel to reach the top carb. Or it could be a stuck or improperly set float. Or there could be a restriction between the lower carb and upper carb fuel hose connection.

Your case is a little different due to the fact it is an L drive and motor has a different orientation especially with the carbs. Compounding the problem is the steep driveway. So I would focus on the float especially the top one. You might want to set it a little high but not too high it starts leaking fuel at the vent hole. Also double check both throttles to make sure at your idle setting both are a little open to allow idling. Do not confuse the choke (located at the mouth of the carb) with the throttles which are located at the back or throat of the carbs.
 

jerryjerry05

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I didn't catch the L-Drive untill post #8 SORRRRY
The tilting makes the float shift and IF it's sticking would allow fuel past.
Guess this wouldn't help yours??

Like Jiggz suggested adjust the float.
What kind of needle seat?????
The kind with a solid needle the seat can swell and allow very little to no fuel past.

​One of my motors wouldn't start.
Decided to just go on one I tilted it up.
By accident I hit the key trying to start the other motor.
It started.
I dropped it and it started right up??

It continued for a long time and only started while up????
Then it fixed itself and worked without tilting??
 

Masonator450

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I got a chance to play with it some more yesterday, parked it on the road to make sure everything was level and the floats were where they should be. It acted just like it did on the angle.
I took both float bowls off and both were full so I know they're getting supplied properly by the pump.
When I hold the choke on and turn it over fuel will come dripping out of the lower carb but barely a drop out of the upper one. I held my hand over each carb and they both pull air through and don't blow back at all.

Did another compression test today just to continue my double checking everything. I have, from top cylinder to bottom: 130, 130, 120, 170.
I assume the bottom is high from excess fuel flooding the cylinder.

I also tried taking the inlet needle right out of the top carb and it did sputter and cough more than in any other test so everything seems to point to the carb still yet I switched them and sprayed throttle body cleaner right through every needle and port hole I could find on the things and saw it come out another spot.

When I spray a gas filled squirt bottle in the top carb with throttle wide open as it's turning over I still take the plugs out to find them bone dry.
 

Jiggz

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Wow! I can only think of two things that will not allow fuel sprayed directly into a wide open throttle carb into the cylinder. The reeds not opening wide enough. And the second, the carb's throttle not actually opening. The first is a long shot but it's possible. The second is also a long shot but the main reason the throttle will not open on the top carb is because the tie bar mid point adjustment is loose or not connected at all with the top carb's throttle shaft.

The throttle cam directly pushes against the lower carb's throttle shaft. This is probably the reason the lower carb always get fuel into the lower cylinders. However, the top carb throttle shaft is controlled via the tie bar. The tie bar is connected to the bottom and top carbs at its ends (with "E" rings) while the middle is an adjustable slotted bar tightened with a screw.

I highly recommend checking the top carb's throttle actually opens instead of just relying on the position of the cam. Use a flashlight and look into the throat of the carbs. Do this while directly turning the throttle shaft NOT VIA THE TIE BAR OR THE THROTTLE CAM. Does the throttle butterfly opens? If you can post a pic I'll sure appreciate it.

Lastly, and this is probably the last thing I can think of. The fuel maybe entering the carb, reeds and into the crankshaft. But will not go into the cylinders due to blown gaskets on the transfer ports or faulty fuel recovery/recirc system. If the port covers gasket are blown then you should see fuel running down on it. However, if there is no fuel leaking from the port covers, then it could be the fuel recovery/recirc fittings check valve might be stuck open.
 
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Masonator450

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I don't think it's the reeds cause the carb is still sucking a good amount of air through it. I know it isn't the throttle position on the carb because I sat beside it and held the butterfly valve open by hand while my wife turned the key.

That last paragraph though ^^ that sounds super promising! This is where I'm totally new though. Where would I see the "port covers gasket" ??

And how would I check if the "fuel recovery/recirc fittings check valve might be stuck open" ??
 

jerryjerry05

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For the reeds to stop the flow there would be 2 things that would need to happen.
​The reeds would need to be glued in place or no compression???
The main nozzle supplies fuel at high speed and when the primer or choke is activated.
Maybe plugged?
Does it run at all? Use starting fluid and spray around the motor as it runs and under the electronics as that's where the "port covers" are.
The recirc system is on the stbd.part of the block.
Just in front of the port covers.
The recirc check valve shouldn't have anything to do with the carb not feeding??

boatsdotnet has parts diagrams for your motor. Mercury section then open the Force tab in the lower half.
 

Masonator450

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Jul 26, 2016
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Had to take a break from my impossible project.
Anyways I got a set of carbs off a guy and despite my every effort to clean mine, with these ones I now have Fuel, Spark, and compression.
Still can't get the thing to fire.
I've tried putting a squirt of fuel in each cylinder and don't get anything more than the odd sputter. So now I'm back to thinking it has to be a spark issue but I just took all 4 plugs out, 1 at a time, hook the threaded part to a jumper cable and the other end to the block, and all 4 had bright blue spark. Any more ideas?
 

jerryjerry05

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Possible the flywheel key has sheared?
​Check for TDC?(top dead center)
Top posts in this forum cover the timing/ TDC .
 
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