1979 mercury 900 small running issue

[terry kozmo]

Thanks Chris1956, I will try what you stated here, so run one gallon of gas 50:1 with the seafoam at idle until it quits running?, and then let sit overnight, then run on fresh gas a wot until its smoke cleans up. and then change out the plugs?, or should I change the plugs the next morning before the wot test..... . Is this the plan? thanks again.

 

[terry kozmo]

and I did rebuild the carbs, but I can't remember if the floats were an issue, new floats were installed then and maybe your right here. maybe I did not set them correctly. I do not see gas coming out the vent holes when ball is pumped..... and the ball appears to stay hard.....

 

[terry kozmo]

also Chris, maybe you can answer this, Plug one and three looked the worst, do these cylinders run on carb one? this may help me narrow down witch carb to take off and adjust

 

[Chris1956]

Your Seafoam procedure is correct.

Carbs feed 1&2, 3&4 and 5&6, however, plug one and three will run a bit leaner than plug 2 and 4.

Water infiltration will normally occur on plug 6, and possibly plug 5, if lowr crank seal is leaking. If water jacket cover is leaking, any spark plug could be affected. Look for a spark plug that looks brand new. That one could be getting water.

 

[terry kozmo]

Did the seaform cleanout, worked good, , seemed to make it better, I have a strange problem, The motor will start and run, and runs good at first, after about 5 min of run time at full throttle on the lake it will start to really run with no power, no take off at all, I take it out of gear, and I get full power fine, put in gear and boughs down again. I checked the lower unit grease, no water in it, and looked clean, so I changed it a anyhow, and again it ran fine on the trailer, good throttle, and put in gear and ran fine on land,,,, since I changed the lower unit grease, I have not had time to put back in lake, But feel it will still act up. I was told maybe reed valves acting up? any help here? I am confused, can I do some kind of test on the reed values? and why does it run great until warm and loose power, but will have power out of gear i get full throttle fine with good response, It only boughs down on the water after about 5min or so....? man, these motors are touchy..... any suggestions would be great. checked plugs after this, and they all appear fine, burning clean, and getting fuel.

 

[Chris1956]

If you rev up the motor in neutral, you are not testing anything. Even a moor running on 1/2 it's cylinders will rev up in neutral.

Bad Reed valves will affect the idle, engine warm or cold. I doubt if they are the problem.

I would inspect your spark plug wires for arcing to ground, especially if they are originals, when the motor warms up. Also check for engine overheat.

 

[terry kozmo]

plug wires new last year, but had to make them, so maybe one bad.?, motor compression was checked and good, Do not think its overheating, but do not have a gauge, so will have to figure that out, It idles good, and will run strong on lake for about 5 min, then just looses power........Could I have some bad gas? I did put fresh gas in it, and it has a new alum tank..... weird what its doing...............also I have a good blue spark at all cylinders

 

[terry kozmo]

The # 1 plug fouls out, the motor will run great for about ten min on the water, and then looses power, I pull the # one plug and it is carboned up , clean the plug, put back and it will run fine until warmed up again?. weird problem. The plug wires appear fine (new last year), compression good, carbs appear working fine, no known fuel leaks, fuel ball will pump to hard, and It will start fine. checked oil mix and it appears fine, Just trying to figure out what's causing this plug to foul out. It will get like an alum dust on it?, and not real wet when I pulled it out,.. change it with another plug, it will run good again for a bit? I am soooo confused...... runs really good until that plug fouls out again........ had this little 16' boat up to 45 mph and runs strong until this issue happens.....Could this be a trigger problem?, or the reed values?

 

[Chris1956]

Terry. if the #1 plug is aluminum fouling, that is not good. That is a symptom of overheat, pre-detonation or lean running. I would rebuild the waterpump.

I would also clean the top carb, and clean the #1 plug real well and check max spark advance. If it is 21*BTDC, you might retard it to 19* BTDC to save your motor..

 

[terry kozmo]

hey Chris thanks. The waterpump is working good, I replaced the Impeller a season ago, and it is peeing good. and all three carbs were rebuilt last year also. Maybe an adjustment problem on the top carb, I did put new plugs in also, and it will run great for about 5 min or so, then it decides to foul out.... Maybe that one carb is running too lean, I hope its an adjustment issue, This all started with the use of octane boost in the gas, mistake I now know., It will start fine, and run good until warm. and it does not appear to overheat..... I may find a place to hook up a gauge on motor to check running temp. I also will check that timing, I know now its at the 21*BTDC now. I will try the 19*, compression was checked on #1. and its also within specs....

 

[Chris1956]

The only adjustment on the carbs is for idle mixture. Idle mixture only affects engine speed up until 1200 RPM or so. Since you have a high RPM issue, the adjustment is not a factor. I would make sure the main jet is clear, as is the main nozzle to the venturi.

Remember, the #1 cylinder is highest and farthest away from the water pump. It will overheat first. Maybe you can check the head water jacket near #1 for overheat.

 

[terry kozmo]

Thanks Chris, I will have to see if I can check that water jacket near #1, not really sure how, but will look at it, is there a water jacket plug someplace I can remove up there to see if water present near #1?. Maybe the water jacket is plugged someplace near # one cylinder, not cooling down.. I don't know. I know it runs great on the water, and all of the sudden after about 5 min, it boggs down. Makes since that it may be overheating, I stop and change out the plug, and it runs good again? I will also look at the carb adjustments. Maybe I will remove that carb and check it out good. Do you still recommend to adjust the timing to 19*?. or should I just leave it,

 

[Chris1956]

Gee, I would back the timing down to 19*BTDC, because it won't hurt the top speed much, but might save the motor from pre-detonation.

Some models have a plug near the top of the head water jacket. Remember, that motor should run dead cold at idle and low speed.

 

[terry kozmo]

No Title

Chris1956, I thought of something that may be a factor here, I just recently changed to a synthetic oil, and am now only using merc synthetic oil, could this have any bearing her e? Here is a pic of #1 plug what do you think ?

 

[Chris1956]

The synthetic oil should not be an issue. You are mixing at 50::1, right? If the deposits on the spark plug are hard, it is a bad sign. If only carbon, then it looks OK. I suspect hard deposits.

You might pull top transfer cover and make sure gasket is not leaking and inspect the cylinder walls and piston sides. She may be all scratched up.

 

[terry kozmo]

yeah Chris 50:1, it appears hard deposits, I may be done for the season, I this weekend will look into all that you suggested, I will first check temp around#1 when running, and check the water jacket area, hope find a plug near top to remove to see if water flow good, and cooling properly. I will also set that timing at 19* and if all fails, well, rebuild time??? darn, and it appeared to run soooo well before this.....

 

[terry kozmo]

Thanks for this site and all assistance we get, its really helpful. I never did figure out whats up with my motor, but feel its a season ending thing, It will run, but that #1 cylinder is just bogging down, Like stated here, may be getting to hot, I later this fall will be tearing into this motor, and seeing whats up on the inside. water jacket plugged? scratched cylinder wall, improper carb adjustment. don't know, but again thanks for this site and the advise, until spring fellow boaters..... we will figure this thing out.......

 

[terry kozmo]

ok Guys, it happened, I had the boat stop (at full speed), and pulled the # 1 plug and it has alum dust covering the end of it. (I tried to attach a photo and it would not upload). The motor (1978 merc 900), will still turn over, and all other cylinders look great with good compression. Just looking for advise, could this be rings? reed values? piston? where should I start? I looked in the cylinder and piston looks good.

 

[terry kozmo]

I guess question is, where would this alum dust come from? al the cylinder walls alum? I suppose the top cylinder gets the hottest, so it may have been overheating? I know now its a rebuild, but just want advise to start.

 

[Chris1956]

Usually the aluminum burns off the eyebrow on the piston top. The cylinders are cast iron as are the rings. You should see aluminum from the piston skirt stuck on the cylinder walls, along with scratches. You need to check it, but it is likely you will need to bore out the affected cylinders (check #3 also), and install new oversize pistons and rings.

Of course, the most important thing is to discover the cause for the lean condition or overheat condition or timing issue and fix it. If not found and fixed, the issue will repeat itself.