1987 85 hp mercury force fire from exhaust

[Pan84]

Hello
I finish my engine rebuild and still have same problem. Wont start, only make very loud bang with 1 feet flame from exhaust. Compression is 130 all 3 cylinders, spark in all cylinders. Now there can be only timing wrong. How check the timing? I need timing light, for check timing? Or somebody knows any tricks?
Thanks for any respond

 

[JB]

Re: 1987 85 hp mercury force fire from exhaust

The most common causes for the exhaust backfire are:

Sparking in wrong cylinders. . . wires on the wrong plugs.

Sheared flywheel key allowed flywheel to slip out of time far enough to fire when the exhaust port is open.

My guess is that you have the sparkplug wires mixed up.

 

[Pan84]

Re: 1987 85 hp mercury force fire from exhaust

If space between towershaft and trigger wrong, engine will start? and runs rough on hi rpm? or engine not even start?

 

[Pan84]

Re: 1987 85 hp mercury force fire from exhaust

I try change 2 wires today, engine start but runs only 10 seconds

 

[JB]

Re: 1987 85 hp mercury force fire from exhaust

Well, you have now fixed the backfire trouble.

Next is the fuel supply trouble. Check your fuel pump, tank vent and lines from the tank.

 

[pnwboat]

Re: 1987 85 hp mercury force fire from exhaust

In looking at the picture, the wires for number 1 and 2 cylinders are backwards. Wire from the top coil should go to the upper-most spark plug. Wire from the middle coil should go to the middle spark plug. If these are the two wires you swapped as a test, looks like they were originally correct. Like JB suggested, check your timing.

Not sure what you're referring to as the "space" between the towershaft and trigger. There is some linkage that connects the towershaft and trigger. The length is adjustable. Adjusting the lenghth changes the ignition timing. Is that what you're referring to?

 

[Pan84]

Re: 1987 85 hp mercury force fire from exhaust

I play with screws on carburettor, now it is runs perfect. I has checked up 2 different wiring diagram, but motor start only if connect spark plug wires wrong. I will leave as is. I will go on water on these weekend to check my boat and motor first time. I hope i can come back
Thanks every one

 

[JB]

Re: 1987 85 hp mercury force fire from exhaust

Could be (probable) that the wiring to the coils was switched, but she is running so don't mess with success.