2000 Yamaha 100hp Four stroke running on three cylinders...I think

[608WB]

I recently installed a second hand 2000 100hp Yamaha 4-stroke on my runabout and ever since the install it seems to be dropping a cylinder and running on three instead of four. When I purchased the motor I went over it with a fine tooth comb and took it for a rigorous test drive, during which it did not have this problem. Before I installed the motor I had it serviced by a dealer who indicated that everything was great on the motor and it should be a runner for many years to come.

Now that I have it installed, when it comes under load while trying to plane, it appears to drop a cylinder and the motor bogs a little bit, doesn't cut out but changes pitch and loses a little bit of power. It sounds/feels like it picks the cylinder back up intermittently while planing and once on plane it will generally pick it back up and run fine unless I hold it wide open, at which point it seems to drop the cylinder again. This problem gets worse when the boat is loaded down and never does it at idle.

The motor isn't cutting out completely by any means, it changes pitch and loses a little power. If I push the throttle quickly it will bog a little, but it seems most times when I apply the throttle steadily it doesn't do it as often.

It does not appear to be a fuel problem as I have gone over the whole system replaced filters and feeder tubes.

Does anyone have any suggestions, to me this sounds like a spark plug may be pooched or on the way out...any other thoughts would be greatly appreciated.

-Go Blue Jays!

 

[mommicked]

Re: 2000 Yamaha 100hp Four stroke running on three cylinders...I think

My 03 F90 would seem to run fine at wot w 1 cyl dead, but bucked and shook at lower rpm's/idle. Mechanic said a float needle had come loose and was starving the carb of gas but i'm not sure if thats exactly what he said. Check for a wet plug, mine was the bottom carb and I still have runnin on 3 symptoms at startup occasionally, for about a minute, when it's not been cranked in a while. Have the carbs ever been rebuilt?

 

[608WB]

Re: 2000 Yamaha 100hp Four stroke running on three cylinders...I think

I haven't been able to find a wet plug, I pulled all of them out and had a look, but they are look the same...pretty black and I can't quite remember what a spark plug is supposed to look like when it is running well. There is no bucking or shaking when it drops off, it just seems to lose a little bit of power and change pitches, it doesn't surge and bog, if it loses power it will stay at the same rpm until I hit the throttle or it picks it back up, where it will stay at a level rpm until I hit the throttle or it bogs again. Basically if I leave the throttle the same and it bogs and the picks up, it will always bog and pick up the same rpm increment so long as the throttle isn't touched.

 

[99yam40]

Re: 2000 Yamaha 100hp Four stroke running on three cylinders...I think

You need to see if it is dropping spark to a cylinder, an inductive timing light will show that fairly well.

might check fuel pumps to see if leaking and dumping extra fuel into crankcase.
Too much fuel is just like not enough, neither will let cylinder fire properly

 

[multimech]

Re: 2000 Yamaha 100hp Four stroke running on three cylinders...I think

You need to recheck the health of the engine. LEAKDOWN and static spark test. If that checks out, if it is carburated, then do a synch on the carbs with a manometer. Check the fuel pump pressure. Check the spark on all cylinders under load with a test light on each cylinder. Look at each cylinder with the test light where the problem occurs. Get back to basics or you could be chasing your tail.

 

[siny]

Re: 2000 Yamaha 100hp Four stroke running on three cylinders...I think

I recently installed a second hand 2000 100hp Yamaha 4-stroke on my runabout and ever since the install it seems to be dropping a cylinder and running on three instead of four. When I purchased the motor I went over it with a fine tooth comb and took it for a rigorous test drive, during which it did not have this problem. Before I installed the motor I had it serviced by a dealer who indicated that everything was great on the motor and it should be a runner for many years to come.

Now that I have it installed, when it comes under load while trying to plane, it appears to drop a cylinder and the motor bogs a little bit, doesn't cut out but changes pitch and loses a little bit of power. It sounds/feels like it picks the cylinder back up intermittently while planing and once on plane it will generally pick it back up and run fine unless I hold it wide open, at which point it seems to drop the cylinder again. This problem gets worse when the boat is loaded down and never does it at idle.

The motor isn't cutting out completely by any means, it changes pitch and loses a little power. If I push the throttle quickly it will bog a little, but it seems most times when I apply the throttle steadily it doesn't do it as often.

It does not appear to be a fuel problem as I have gone over the whole system replaced filters and feeder tubes.

Does anyone have any suggestions, to me this sounds like a spark plug may be pooched or on the way out...any other thoughts would be greatly appreciated.

-Go Blue Jays!

I have the exact same problem with my 2003 F100, have You got Yours repaired?? and wat was the problem?

 

[Silvertip]

Re: 2000 Yamaha 100hp Four stroke running on three cylinders...I think

If the spark plugs are blackish in color you have a fuel "mixture" or oil consumption problem. Plugs on a four stroke should have a light tan/brownish tint. Plugs are not a budget buster -- replace them and see if the problem goes away -- at least temporarily.