3.5 hp b&s has no spark

[mycut]

The sort of thing those 1/2 wits on face book do in their videos showing how they make $ 1,000,000 a year flipping mowers using nothing but junk they pick up for free
There should be a rod linking the throttle butterfly to the governor vane and then 1 or 2 springs linking the same rod to the throttle cable via the spot where the spring is currently attached.The little compression spring between the throttle & the manifold should not be there at all , looks like it was salvaged from an empty ball point pen .
Coil is a cheap Chinese knock off of dubious quality
The engine numbers should be stamped into the blower housing directly above the spark plug and is probably filled with the paint he used to tart it up .

I think the "compression spring" is actually the idle screw poking out a lot. The vane/throttle spring is the right length and matches the images of the correct p/n 260875 so that seems ok too. The governor link is also the correct one.
I agree about the magneto. I bought it as B&S and didn't check until it had been fitted. Still it does (for the moment maybe) spark well and I don't associate it with the regular 1 second speed changes, 1 sec up, 1 sec down.
Still there has to be something that's gone wrong since 1977 when it was built! I just have to find it.

 

[Hammermechanicman]

If it is surging or "bouncing" on the govenor you have a carb problem. Start engine and HOLD the throttle butterfly in the idle position. Does engine idle properly? If no you have a carb problem. The throttle linkage is fine.

 

[mycut]

If it is surging or "bouncing" on the govenor you have a carb problem. Start engine and HOLD the throttle butterfly in the idle position. Does engine idle properly? If no you have a carb problem. The throttle linkage is fine.

 

[mycut]

I never associated the speed variations with surging or bouncing as the speed change is so regular, up and down, and the same each time, in seconds, at any chosen speed. Is that what surging and bouncing does?
However, that aside, holding the throttle at idle does not stop the speed variations, but they seem to change in regularity and intensity.
Does a carb problem include the diaphragm?

 

[Hammermechanicman]

Are holding the actual throttle butterfly and not the linkage? If you hold the throttle butterfly from moving you take the govenor out of the troubleshooting process. Holding the butterfly in the closed idle position does it still change speed every second?
When running the engine with it reving up and down watch the throttle linkage is it moving in time with the speed change? I am assuming you have the air cleaner in place while you are running it. You must have the air cleaner bolt in place for the engine to run properly. And from the picture the idle stop screw is way too far in to be the proper idle speed.

 

[mycut]

Are holding the actual throttle butterfly and not the linkage? If you hold the throttle butterfly from moving you take the govenor out of the troubleshooting process. Holding the butterfly in the closed idle position does it still change speed every second?
When running the engine with it reving up and down watch the throttle linkage is it moving in time with the speed change? I am assuming you have the air cleaner in place while you are running it. You must have the air cleaner bolt in place for the engine to run properly. And from the picture the idle stop screw is way too far in to be the proper idle speed.

I held the linkage and I take your point. The air cleaner stops the access to the butterfly so some tests were done with just the cleaner bolt in place. I have to say that I didn't notice any difference with the cleaner on or just the bolt in.
The idle screw, as it is, has 12 threads visible and idle's at about the right speed but varies upwards. It stalls at a lower speed. When only 7 threads show the idle screw will just touch the point against which is adjusted and the rotating part that holds the screw is touching the carb body. It will not idle from that position, hence the 12 thread position.
I'll try what you suggest and get back.
Thanks!

 

[mycut]

I held the linkage and I take your point. The air cleaner stops the access to the butterfly so some tests were done with just the cleaner bolt in place. I have to say that I didn't notice any difference with the cleaner on or just the bolt in.
The idle screw, as it is, has 12 threads visible and idle's at about the right speed but varies upwards. It stalls at a lower speed. When only 7 threads show the idle screw will just touch the point against which is adjusted and the rotating part that holds the screw is touching the carb body. It will not idle from that position, hence the 12 thread position.
I'll try what you suggest and get back.
Thanks!

Holding the butterfly made no difference to the firing problem. I ordered a new diaphragm so maybe Thursday or Friday I'll have more news...and it made no difference! Added duck tape to flap to replace area I'd cut off and now the flap doesn't move when speed is slow but wobbles as before when speed is higher.