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Force 1996 120hp problem

JSWIFT

Member
Ok, I have just finished going through a 1996 Mercury sport jet 120 (Force 120 power head) that had sat for three years. I put it in the water, it started within 6 seconds and i let it idle for about 15 minutes to let everything circulate and the temperature to stabilize. It was running like new. I was using an external fuel tank so I know the fuel was clean. I slowly powered up and it took off strong. I ran it about 5 minutes and I slowly powered back to idle when it died. I couldn't get it started again. I found fuel puddled in the bottom of the attenuator box. The top cylinder was pressurized when I removed the plug. The compression on all cylinders are all around 120 psi. I checked the carb floats and enricher resistance. I now suspect the reed valve assembly. Does this sound like a reed valve problem?
 
Re: Force 120 problem

I need a little help here folks if someone can tell me the symptoms of a Reed Valve failure and if it may be my problem.
 
Re: Force 120 problem

it seems to me that it would be hard to get good compression readings if your reed valves were broken. Also, what is year? Before Mercury (1992?) the reed valves almost never had problems. For example, if you are sitting on an '90 then your reed valve and stator are not equipped with the Mercury engineered self-destruct upgrade.

Fuel > compression > spark = running engine.

check compression on all cylinders.

check for blue spark on all cylinders

if both of those are good then it must be fuel. puddles of fuel inside cowling would make me think either ethanol ate up a fuel line or a carb needs rebuilding. Which reminds me ... don't blow up your boat when checking for spark.
 
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Re: Force 120 problem

The engine is a 1996 as mentioned in the original post. The reason I suspect the reed valve, is because of the large puddle of fuel in the attenuator box. I rebuilt both carbs and it ran great for about 15 minutes. Spark and compression are good. I replaced the fuel lines before I took it out. I've not ever had a reed problem on an engine and understand its uncommon, so I need to get an idea of the symtoms of reed failure before I tear into it.
 
Re: Force 120 problem

I checked the compression again and this time the no 2 cylinder was about 15 psi low compared to the others(120 psi). This of course is a cold compression test on an engine that sat for a while so it might not be accurate( rings gummed up). Anyone have any suggestions? Would a bad reed valve have such a small compression drop?
 
Re: Force 120 problem

Mercury "upgraded" the reed valves from Chrysler's metal design to their own composite. Force engines cold sneeze and it cracks them. That's why pre-mercury engines last forever if you take care of them and post-mercury engines will blow up if you look at them wrong.

I'd go up to the mercury parts dealer and find somebody old enough to remember and ask them how to check reed valves.
 
Re: Force 120 problem

Thanks for reply RRITT. Now that I know the reeds are fragile I'll just go ahead and check them since I already have the carbs off. My service manual has the reed gap and inspection items listed. I've not ever had to get into reeds and a local old-timer mechanic I talked to has had very little experience with reed failure either. I'll post my findings.
 
Re: Force 120 problem

I don't think Mercury ever really understood the Chrysler design. Chrysler was durability and economy through simplicity. Mercury kept trying to make the engine cheaper by using cheaper parts. They never understood that the key to Chrysler's design was simplicity.
 
Re: Force 120 problem

I pulled and checked all four reed sets and they looked very good. Maximuim allowable gap is .010" and all mine were 0" ie tight to the block. The reeds by the way are steel and the block is phenolic and brass. Looks quite stout to me. I did notice a float in one of the carbs was tight in its bracket which could possibly have become stuck. I'm going to recheck my fuel pump diaphram for leaks. Since I put a fuel bulb between the tank and the pump, this could cause flooding which is what I'm experiencing.
 
Re: Force 120 problem

Flooding? I might have misunderstood your problem. I thought you said after about 15 minutes it lost power, slowly dropped to idle, and died.

flooding from primer bulb? that's pretty easy. the needle valve in your carb float bowl isn't working right.
 
Re: Force 120 problem

The motor died as I was throttling back toward idle. I checked the floats when I overhauled the carbs. They were free and the float needles seated when I turned them upside down and blew into the fuel inlet. I checked them after it died and one was tight and could possibly stick. I opened up the float bracket on the carb with a file and they seem fine now. Its almost as if the white polymer floats expanded a bit and put it in a bind. Maybe the ethenol they're putting in gas caused this.
 
Re: Force 120 problem

If the reed valve is bad, than the gas will spray back out the throat.
Follow the Link & Sync in the manual. The S-Cam roller needs to centers between the marks exactly or keep stalling out. If it keeps sneezing, clean the carbs again. Pay attention to the slow straw in the middle on the venturi. Pour toilet cleaner in there if you have to. Rebuild the fuel pump, too. The Merc ones last 5 years.
 
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Re: Force 120 problem

Thanks Timebandit. The S-cam roller was aligned as specified but the fuel pump might very well be the problem. It initially checked out fine but if the fuel pump diaphram developed a leak with it being pressurized by the fuel bulb I installed, it might cause flooding. Waiting on parts now. I think I cleaned the "slow straw" you mentioned when I rebuilt the both carbs.
 
Re: Force 120 problem

New to the forum.
I have a 1998 Nitro w/ tracker (Force) 120
I just posted a reed problem I had in the Merc. forum,I thought my engine was a Merc.
Bob
 
Re: Force 120 problem

Big Iron, Mercury bought out Force in the 90's and put their name on it. If its a 120, its most likely a Force. Mercury does still supply many (not all) parts for the later Force( Mercury) engines. I believe I already reviewed your post when I searched for reed valve problems.
 
Re: Force 120 problem

Thanks for the reply Fernando. The first thing I did before I put the boat in the water was to replace the black stator ( known for problems ) with the new style red stator. I checked the spark and it was strong on all four cylinders before and after I ran it for the first time. I'm getting ready to tear it down to inspect the exhaust gasket/plate when I get my parts in.
 
Re: Force 120 problem

new stator? you don't think the trigger moved do you? cause if you got good compression and you have good spark there's nothing left but fuel.
 
Re: Force 120 problem

I don't think the trigger moved but I'll check it. I marked the location ( with a sharpie) before I removed it and secured the linkage with tape. As I said, it ran great after that for about 15 minutes so it was initially fine but anything could have happened here. I just received my gaskets so the reeds are going back in and I'll recheck the timing. If that checks out, exhaust gasket /plate are next.
 
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