03 F90 dropping RPM

[ncfishhawk]

I have a carb'd F90 that has started dropping 200 RPM intermittently while in the 3500 to 4200 RPM range. It'll drop then come back for few seconds then drop again then it may run ok for a minute or 2 then do it again. I had just replaced spark plugs and timing belt. Does this sound like it would be a dirty carb/fuel delivery problem or something electrical? I have a 10 micron filter and run non ethanol gas. Perhaps something not done correctly as part of the belt or plugs? Thanks in advance.

[ncfishhawk]

I have a carb'd F90 that has started dropping 200 RPM intermittently while in the 3500 to 4200 RPM range. It'll drop then come back for few seconds then drop again then it may run ok for a minute or 2 then do it again. I had just replaced spark plugs and timing belt. Does this sound like it would be a dirty carb/fuel delivery problem or something electrical? I have a 10 micron filter and run non ethanol gas. Perhaps something not done correctly as part of the belt or plugs? Thanks in advance.

edit...i just pulled plugs out of curiosity...top and bottom plugs are very black a sooty and the middle 2 are nice and tan.

[99yam40]

My thoughts are.
spark is dropping on a cylinder intermittently
or fuel to air ratio in a cylinder is changing not letting the fuel to burn properly.

if that motor has pulse port operated lift pumps, then making sure that pump is not dumping extra fuel into a crankcase would be a good idea

timing light hooked to individual plug wires might show if spark on one of them is going away when the problem shows up

[ncfishhawk]

My thoughts are.
spark is dropping on a cylinder intermittently
or fuel to air ratio in a cylinder is changing not letting the fuel to burn properly.

if that motor has pulse port operated lift pumps, then making sure that pump is not dumping extra fuel into a crankcase would be a good idea

timing light hooked to individual plug wires might show if spark on one of them is going away when the problem shows up

thanks for your reply. what are lift pumps? It's a carb'd 4 stroke and does have a fuel pump that i would guess works from vacuum maybe? I will check the oil to make sure it doesn't smell of gas or has increased in quantity. there are 2 plugs getting sooty and they work from different coils from what i can tell. It ran very good after belt replacement and new plugs on the hose in the yard. now idle is rough and i assume that was due to plugs. It's only been in the water 1 time since the belt and plugs and that was for a very brief 3 mile run.

[panasonic]

I have had bad plugs right from the store....swap the plugs around after cleaning the soot from them and see if they are any different in another cylinder.
Also check the plug caps are good condition. Seeing your trouble started after changing the plugs..it could be the new plugs. You could even try putting the old plugs back in.

[ncfishhawk]

I have had bad plugs right from the store....swap the plugs around after cleaning the soot from them and see if they are any different in another cylinder.
Also check the plug caps are good condition. Seeing your trouble started after changing the plugs..it could be the new plugs. You could even try putting the old plugs back in.

I just swapped plugs between cylinders, put clean plug from 2nd up to 1st and vice versa. clean plug was black with just 5 minutes of running on the hose. i have rechecked all the timing marks to be positive something wasn't missed but with the flywheel at TDC the pointers on the cam gears are in alignment. I also looked under the flywheel and can see the hole on that flange on crank is over the little casting on the block just like the directions say it should be. I put timing light on all wires an all have consistent spark. Also swapped coil for those 2 cylinders from a friends same model engine and no change.

could it be carb floats stuck maybe? seems odd it would be 2 at the same time but it is after all me, so strange things happen that way