Mercury 9.9 missfires at full trottle

[bjorn]

I have a Mercury 9.9 1988 model<br />The serial number is 9495357.<br />I run fine at approx 80%load.<br />With full throttle it starts to misfire.<br />The switchbox is new (it did not run at all before this was replaced)<br />Resistance in stator coils and trigger is checked.<br />Carburettor and fuel system is cleaned.<br />Any idea what the problem can be?

 

[Backlash]

Re: Mercury 9.9 missfires at full trottle

Hey Bjorn,<br />Could be lots of things but check the easy ones first. How are your spark plugs? Bad plugs could cause the symptoms you describe. Check the compression. A faulty head gasket could also be the culprit. Even though you mention the carb is clean, I'd bet you have some varnish or something restricting the flow in that little devil. You need to soak it disassembled and blow all passages out with compressed air to get it really clean. Let us know what you find.<br />Backlash

 

[12Footer]

Re: Mercury 9.9 missfires at full trottle

Bjorn, something killed your other switchbox. You must find it before running it too hard or far,as whatever caused it, will kill the new one. <br />I suspect a bad plug wire,or an arc-over (spark other than at the plug's anode).<br />run the motor on a dark night, and look for any leaking spark. Try replacing the plug wires,just to eliminate them as suspect. They rarely fail open,because they have a real wire core,as opposed to the new auto-type, which have carbon cores, and break easily when bent funny.<br />But still possable, and more likely, arcing to the frame at some spot.They develope a carbon path sometimes, thru the insulation,and it will jump over to the metal frame there,from that point on.<br /><br />Even the plug porcelin can develope a carbon path to the frame, and be useless, so replace them because they are cheap.<br />Good luck

 

[Mr.Bill]

Re: Mercury 9.9 missfires at full trottle

The 9.9 motors some times cutt out at high speeds due to the fuel pump being worn out .They will run fine at low speeds not get enough fuel at top end ,and cut out .If you still have the original pump in the motor I would suspect that is where I would start.

 

[sloopy]

Re: Mercury 9.9 missfires at full trottle

Is the ball firm?

 

[bjorn]

Re: Mercury 9.9 missfires at full trottle

Hey Backlash<br />Spark plugs are new.<br />I will check compression<br />The carbs are disassemled and cleaned with compressed are, but i will do it again after i have checked some of the other items. <br /> <br />Hey 12 footer<br />I have ordered new plug wires. One of them are not original.It is of the type with carbon core.<br /><br />Hey Mr. Bill <br />Is there an easy way to check the fuel pump, or do I have to buy a new one?<br /><br />Thanks a lot for your tips.

 

[mellowyellow]

Re: Mercury 9.9 missfires at full trottle

carb float problem perhaps??? how did you set<br />the float when carbs were rebuilt? try tapping on<br />the bowl and see if that un-sticks float.<br />of course it could be a number of things....<br />air leak in fuel hose etc.<br />good luck,<br />M.Y.