1999 Mercury 90HP Fuel System Puzzler

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adbegando

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Jun 8, 2002
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I am stumped by a problem I am having with my 99 Mercury 90HP and am hoping that someone on this board may be able to point me in the right direction !!<br /><br />Problem: When running at low speeds (idle - 1000 rpm) the engine runs very rough/stalls. When running at higher speeds (e.g. 3600-4200+ rpm) engine runs well, however, it toggles between higher and lower speeds as if it is getting a variable fuel flow.<br /><br />Signs/Symptoms: While at slow speeds, when engine begins to run rough, fuel bulb is dry. A few squeezes and it seems to stabilze... however, it is difficult to get bulb to "fill hard". When finishing a high speed run (I regularly run 20-30 miles out to where I fish in FL) fuel bulb seems dry as well.<br /><br />Rule Outs: I have ruled out compression (tests fine) and ignition/spark problems. Verified all groundings. Checked timing. Completely cleaned carb. Pressure tested fuel system. Replaced fuel bulb. Replaced primary fuel filter (Racor drain type). Ran with auxillary (portable) fuel tank. I have had two separate mechanics look at the problem... one was totally stumped, the other (a big Tampa Merc shop where I had the engine certified before purchasing) replaced the fuel bulb, adjusted the idle and said it "seems fine".<br /><br />Other Info: 220 Hrs. Engine was purchased used (@ 160 hrs) with boat 1 yr ago. When the engine was certified by aforementioned shop, they thought that a bearing may be going bad (as evidenced by a "squeel" at very high rpm) and replaced the powerhead under the extended warranty (I did the 20 hr break-in process). I leave the boat in FL and use it about once every 4-6weeks for 2-3 days and burn 50-70 gallons of fuel. I usually use Yamalube or Exxon oil.<br /><br />My thoughts:<br /><br />1. Fuel pump, secondary fuel filter or intake port (@ quick connect) bad ?<br /><br />2. Fuel line hole or vent problems ?<br /><br />3. Another bad fuel bulb ?<br /><br />4. I should have bought a Yamaha !<br /><br />Thanks in advance for any assistance.<br /><br />-- GA_FlyGuy
 
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FlyBoyMark

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Apr 14, 2002
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934
Re: 1999 Mercury 90HP Fuel System Puzzler

Is the tank vented or is the vent screw open? Check your pickup in the tank for clogging and check the lines and fittings before the fuel bulb for suction leaks, try a 3 or 6 gal seperate tank and hose. Are you using the rite size fuel line? If you have a fuel filter BEFORE the bulb, check it.This is how to find what is wrong when the fuel bulb is hard to fill. Do you possiblly have bindings on the harness too tight?
 
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adbegando

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Re: 1999 Mercury 90HP Fuel System Puzzler

Thanks for the quick response...<br /><br />>> Is the tank vented or is the vent screw open? <br /><br />Tank is vented. I tried to pull the fuel through with the cap off to no avail... so it doesn't appear to be a vacuum issue.<br /><br />>> Check your pickup in the tank for clogging <br /><br />Is there a way to do this without draining the tank ?? I've never done this before...<br /><br />>>> and check the lines and fittings before the fuel bulb for suction leaks, try a 3 or 6 gal seperate tank and hose. <br /><br />Our Yamaha guy (who maintains my father's Grady) tride the separate fuel tank test and had the same performance issues.<br /><br />>>> Are you using the rite size fuel line? <br /><br />These are the lines that came with the boat... I'll check that Maintenance manual and verify.<br /><br />>>> If you have a fuel filter BEFORE the bulb, check it. <br /><br />I just did and it seems fine... it's a standard RACOR drain out filter.<br /><br />>>> This is how to find what is wrong when the fuel bulb is hard to fill. Do you possiblly have bindings on the harness too tight? <br /><br />I'll check that out.<br /><br />Thanks for the information.
 
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trevorcday

Petty Officer 1st Class
Joined
Dec 2, 2001
Messages
216
Re: 1999 Mercury 90HP Fuel System Puzzler

I would look for an air leak on the suction side of the fuel line, anywhere between the fuel pickup in the tank and the fuel pump. We had one boat that didi something like this and the cause turned out to be a bad fitting where the fuel line exited the tank. I realize that you said you tried a portable tank so that should eliminate the plumbing in the boat but maybe there is a problem under the hood of the motor.<br /><br />Good luck.
 
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FlyBoyMark

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Apr 14, 2002
Messages
934
Re: 1999 Mercury 90HP Fuel System Puzzler

If you have access to the tank, MOST fuel pickups are removable. If not, try blowing into the fuel line BETWEEN the fuel bulb and tank and listen for bubbling in the tank. Use a bottle of soapy spray water while blowing to check for leaks too. Use a low pressure air chuck.<br /> Were you present when this MECHANIC tried another tank? As a last resort..when was the last time you replaced your fuel pump diaphrams? Screw on the pump snug? Some engines have a filter on the engine, didja' check it if ya have one? Check the hose and fittings between it to the cowl fitting as MRMERC says.(cracked hoses, loose fittings or broken fittings.
 
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dpski

Cadet
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Sep 2, 2002
Messages
12
Re: 1999 Mercury 90HP Fuel System Puzzler

GA_flyfisher,<br /><br />Did you ever get resolution to this issue? I'm looking at a 1999 90hp 2stroke classic that appears to have the same problem. The dealer tried to tell me that at low rpm the motor cuts out 2 cyclinders to save fuel. Sounded like a load of BS to me for a carb 2 stroke, specially on a 3cyclinder engine. So I've been searching the web forums and found this thread. Any help would be appreciated.<br /><br />Thanks,<br /><br />DP
 
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orca

Chief Petty Officer
Joined
Jul 12, 2002
Messages
422
Re: 1999 Mercury 90HP Fuel System Puzzler

G'day. The 90 Hp runs on 3 Cylinders at all times, it is only the 4 Syl motors that cut out on 2 cylinders at idle.<br />When you say that the motor runs rough at idle is this in the water or on the muffs, these motors always run rough at idle on the muffs but will smooth out on the water.<br />these motors can only be tuned on the water. you cannot tune the motor on the muffs, in a test tank or on the dyno. If the motor has had a new powerhead the carby setup is critical.
 
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bryanredfish

Petty Officer 2nd Class
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Apr 22, 2002
Messages
182
Re: 1999 Mercury 90HP Fuel System Puzzler

dpski and Whaley:<br /><br />A little off-topic, but my mechanic also told me that the 3 cylinder Merc. 90 (2002 - Saltwater) ran on 2 cylinders for fuel economy purposes! Now I'm confused. Are there some certified mechanics who can confirm this? I read a number of complaints about the seemingly rough idle of these 3 cyl. engines, and this might explain quite a bit.
 
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ob

Admiral
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Aug 16, 2002
Messages
6,992
Re: 1999 Mercury 90HP Fuel System Puzzler

I'm with whaley on this one.motors that sit for 4-6 weeks or longer are proned to fuel deposit issues within fixed orifices and carb porting.Get some kits and rebuild or clean and service existing carb components.Sounds like you've checked everything else within fuel system
 
clanton

clanton

Rear Admiral
Joined
Jul 9, 2001
Messages
4,876
Re: 1999 Mercury 90HP Fuel System Puzzler

The Merc manual Im looking at dated Dec 1997 shows idle screws in each carb for the 3 cylinder and idle screws only in the top 2 carbuators for the 4 cylinders.
 
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orca

Chief Petty Officer
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Jul 12, 2002
Messages
422
Re: 1999 Mercury 90HP Fuel System Puzzler

G'day. I am a certified mechanic with merc.!! The 3 cylinder 90HP runs on 3 cylinders at all times
 
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