1979 mercury 900 small running issue

terry kozmo

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Jul 12, 2016
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I have this 1979 merc 900 that I have done a lot of work too. It starts a little hard, most of the time I have to use starting fluid to start, once its started, it will run good with fast throttle up, it gets up and goes.... and usually starts right back up....After warm, and maybe after I just putt around a little at low throttle on the lake, it looses its punch, I throttle it up and it seems laggish, and wants too almost stall. I tried something when this happens, I choke it just before I throttle it up, and it takes off and is then fine???? think it may be a carb adjustment? Bad gas? It has had the carbs rebuilt, I changed the plugs with new ,and it still does it, Its had new plugs and wires, new alum tank, new coil, and rebuilt the fuel pump, If I just run around the lake for awhile and not stop, or go slow, it appears fine, Its usually when I go slow for a bit or stop, and then it acts up, but when throttling up, I tap the choke for a second, and it takes off? Any suggestions here?
 

jimmbo

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May 24, 2004
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Try richening the idle mix screws on the carbs 1/8 of a turn steps counter clockwise. If that doesn't help, have the float setting rechecked. BTW, a 79 90 hp mercury is just called a 90. 1978 was the last year for the extra zero
 

Chris1956

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When starting it for the day, cold start lever all the way up, pump primer bulb very hard, hold choke button down and crank her until she fires. Let her run at high idle speed, and use choke (if necessary) to keep her running until she is warmed up.
 

terry kozmo

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thanks Guys, I will do both these things., "cold start lever" I assume this is the throttle, all the way up? Thanks, I'm thinking the carb adjustments may be a bit off, The motor does state 900 right on it, its a 1979 glasstron (original). but maybe a 1978 motor, I know its all original, I am the second owner, and I new know the first owner, thanks again for all the cool help here,
 

Chris1956

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Terry, MerControls have a small lever next to the larger throttle lever. That is the cold start lever. Normally a motor of your vintage would have the MerControl.

However if you do not, maybe your controls have a throttle-only button. If you have that, advance the throttle an inch or so, for cold start. Be prepared to pull it back, if she runs over 1500RPM.
 

terry kozmo

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Jul 12, 2016
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yes, i know what lever your talking about, this one does not have that small lever, I will try what you suggest when I start it this weekend, I will also try a carb adjustment, I think that's more likely the problem, I have not figured out the proper adjustment yet. like I said, when it starts to bogg down when throttled up, I just push that choke button and it takes right off? weird....
 

jimmbo

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thanks Guys, I will do both these things., "cold start lever" I assume this is the throttle, all the way up? Thanks, I'm thinking the carb adjustments may be a bit off, The motor does state 900 right on it, its a 1979 glasstron (original). but maybe a 1978 motor, I know its all original, I am the second owner, and I new know the first owner, thanks again for all the cool help here,

This is a 78 merc 900
merc78b.jpg
 

opalized1

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Apr 5, 2017
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MAKE SURE NO LEAKS IN FUEL LINES, COULD SUCK AIR AND CAUSE THIS, BUT THEN RUN FINE ONCE STABALIZED IN RPM
 

Chris1956

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Terry, it is not weird for a bit of choke to help your acceleration. If you adjust the carbs on your motor for best idle, they are almost always too lean for acceleration. They need to be richer for acceleration to occur. So the adjustment is a compromise between best acceleration and acceptable idle.

Not all Merc Carbs have this "feature", but your inline motor and lots of others do.
 

terry kozmo

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yes, the 1978 900 looks just like the pic here. I want to say thanks for this site and all the great help here, I am leaning so much about these motors, and have done a lot to this motor. Jimmbo, you are my hero, I adjusted the idle mix screws 1/8 turn counter clockwise on all three carbs and it took care of the throttle hesitation problem. it runs great. I had it pulling tubbers all weekend, and with great take off and started up fine all day, a simple fix. I do smell gas a lot, but have been told these motors will smell fuel when in use, (if thats correct), but I see no leaks, none at the carbs or in the lines that I can see. I plan on checking up by the new alum tank and its fittings this weekend, Thanks for all the help, I had this little 16' boat going 42mph this weekend, and that's with others in the boat with me, It runs great..... thanks again.
 

terry kozmo

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Ok, need more help. The boat (after a full week of use, all was good), I then, the last day of the week, I now have a running issue, I lost power... it will start up fine when warm (hard starting cold, need starting fluid most of the time), and it will run at a very low speed (even out of gear, so no lower end issues here)... It has no power or take off. I know this could be Many things, but wanted any advise where to start my diagnosis. I have new plugs in it, a new alum tank a few years back, new plug wires last year, rebuilt carbs (I could have an adjustment problem with carbs, I am getting a bad fuel smell at the motor, and wet unused gas in the bottom tray and some on the water. When power tilt is up, gas comes out of carbs, is this normal ?) noted fuel on the water when motor is off, around motor....could this be my issue here, carbs ?, but it ran great all week with nothing changed to cause this. .....I did make sure oil/gas mixture was correct and I use only high octane fuel. I replaced the coil last year (seems like the same issue as then, lost power). Could the coil be bad again? do they go out a lot? It has been running awesome and now this. I have got a lot of good advise from this site, just wondering where to start with this issue? compression check maybe? 1978 900 merc.... Thanks for any help here.. where to start ...........
 

jimmbo

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Well the high octane gas is just a waste of money. You say that you can smell gas? With the cowl off and the engine down, squeeze the primer bulb. Does it get firm? Is gas squirting from the vent orifices on the top fuel bowls? If so there might be some dirt holding the inlet needle open and flooding the carb(s).
My inlines would suffer from floats that would jam in the fuel bowl and couldn't rise enough to close the needle.
 

terry kozmo

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Jul 12, 2016
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yes I smell gas even with the cowl on and can see it under the carbs and on the water top when stopped. I checked the fuel lines, and have been unable to see any leaks. I even replaced the fuel ball fitting at the motor. ...... the ball gets firm, but question, does it stay firm? is it supposed too? I am not sure on that , I will have to check, it seems i can pump it to firm, but not sure it stays firm very long....... I will see if the gas is coming from the vents, and check all fuel lines again, Would this cause my loss of power?.........I will have to find those vents and look, I know there appears to be a fuel leak, but not sure where or if its natural that this motor smells of fuel.... . I defiantly smell fuel when in use. And Jimmbo, you say I don't need the high octane fuel for this boat? really, will it run fine on the low octane fuel? that's good news, but want to be sure, Didn't this boat run originally on Regular gas,,,,,
 

Chris1956

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You need to take a compression test. If one or two cylinders are damaged, no other fix will work. Inspect the spark plugs for color as well. Real clean is bad, real dirty is also bad.

Secondly, Jimmbo's test for fuel leaks at the carbs is important to run. Fuel in the water can also mean your fuel pump needs to be repaired.
 

terry kozmo

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thanks Chris1956, I did rebuild the fuel pump last year, but I will check to make sure not leaking, and yes agreed needs a compression test... , I will check those plugs this weekend, and do a compression test, thanks again, i'll keep this updated as to what I find,,,,,,
 

racerone

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Dec 28, 2013
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There is a wee hose on the starboard side of the block.--Goes from bottom to the top bearing area.---If old it could be leaking gas and oil.-Best to replace it.
 

terry kozmo

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Jul 12, 2016
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Thanks racerone, I think I know what hose you are referring to, do I have to remove the flywheel to replace that? and can you tell me what size hose to get? is it 5/16 fuel line, maybe about 24" long, and clamped at each end? (I am currently away from the boat, want to take up what I may need this weekend)... thanks
 

terry kozmo

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Update, checked compression 4 are at 140, one at 133, one at 138, all appear fine, One plug (and not the lowest compression cylinder) bad carbon buildup, I did use a very high octane fuel last run (I know, maybe a mistake), I was given advise on running a separate tank of gas with a little more oil than normal (maybe switch to synthetic oil now), and run Sea foam thru it (maybe a gallon of gas of so. And then change out the plugs. What does others think here. The motor will run, appears carboned up. Any other thoughts here....
 

Chris1956

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Your compression is fine. High octane fuel is unnecessary, but will not harm anything. No need to exceed the 50::1 gas/oil ratio. Seafoam is OK to run, but remember to mix 1 can Seafoam into 1 Gal of 50::1 fuel mix. Idle motor on seafoam mixture and let her sit overnight. Now run the motor at near WOT to "blow" the junk out of her.

BTW - I doubt that is your issue. I still think your carbs have bad or incorrect float settings or defective inlet needles/seats. You could also have some water infiltration.
 
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