how do i correct my timing on my 1971 7.5 hp merc

[tony11905]

i have a 1971 merc 7.5 hp ,when i try to start it it backfires witch leads me to beleave timming is off, can anyone tell me or even send me a link on how to set the timming on this motor to get it running,thanks you

 

[Texasmark]

Re: how do i correct my timing on my 1971 7.5 hp merc

I responded to this elsewhere.

 

[tony11905]

Re: how do i correct my timing on my 1971 7.5 hp merc

where do i go to fix the problem what do i do?

 

[Texasmark]

Re: how do i correct my timing on my 1971 7.5 hp merc

I'm confused. When I find this thread, I can't find the one I responded to yesterday. When I'm on it I can't find this one. I will look around and see if you found the other one and responded to my answer.

Mark

 

[Texasmark]

Re: how do i correct my timing on my 1971 7.5 hp merc

Well I can't find it so in short:

Did you hit an underwater object? If so, pull your flywheel nut off and look straight down at the crankshaft and the flywheel. There is a square slot with a square steel peg in it called a Woodruff Key. If it has ANY damage, it could be your problem.

Otherwise, what did you do to change your timing? If you did nothing to the linkage and all, forget that as the problem.

Next guess would be a broken/bad/defective reed valve. The reed valve is behind the carburetor and allows the fuel mix into the crankcase and blocks any combustion that comes back that way. A defective reed valve could cause your problem.

Mark

 

[Zookeeper1955]

Re: how do i correct my timing on my 1971 7.5 hp merc

Have you disconnected any wires? The trigger wires on some old Mercury's were both brown if you switch them it will throw off timing buy 180 degrees witch will cause it to back fire , try switching the trigger wire connection see if that helps ?

 

[Texasmark]

Re: how do i correct my timing on my 1971 7.5 hp merc

Have you disconnected any wires? The trigger wires on some old Mercury's were both brown if you switch them it will throw off timing buy 180 degrees witch will cause it to back fire , try switching the trigger wire connection see if that helps ?

He hasn't answered the question about how the timing got off in the first place. Hopefully he was messing around, like some people do, and got things messed up. But since these engines are fired from a flywheel mounted magneto, the trigger magnet goes by the trigger coils every rev and one of the plugs fires hot and the other cylinder is dry firing so I'm not sure it matters. If it did, I think Merc would have color coded them like they do others. Just my thoughts.

Mark

 

[Zookeeper1955]

Re: how do i correct my timing on my 1971 7.5 hp merc

They do color code now but a lot of the 70's trigger wires were both brown I have one and was trying to determine witch went were when I found the info. of the back firing on there trouble shooting guide .

Ken T

 

[Texasmark]

Re: how do i correct my timing on my 1971 7.5 hp merc

They do color code now but a lot of the 70's trigger wires were both brown I have one and was trying to determine witch went were when I found the info. of the back firing on there trouble shooting guide .

Ken T

Ok my man, back to basics. He hasn't answered the question as to why he thinks he has a timing problem. What has he been fooling with?

I thought about what I said today and have come to this conclusion. There are 2 magnets on the flywheel, 180 degrees apart. There is one pickup coil that feeds triggers to both cylinders. As I said one is hot when the other is dry firing and 180 degrees later the opposite happens.

If it weren't that way, there would be one magnet on the flywheel and 2 coils requiring 4 wires; one set per coil, per cylinder, not just 2. Just makes sense to me. Have no engine schematic, manual or what have you to back it up. Just some commons sense and knowledge of the process.

Mark