3.5 hp b&s has no spark

[mycut]

I bought a 1978 B&S model 92988 Mountfield mower which was hard to start, missed almost regular beats at all speeds and wouldn't run at low speeds. So I looked into it. Now it won't start at all and has no spark, but it did fire just once with a cloud of smoke.
The 2 wires to the condenser had been cut out, so the magneto was an electronic replacement. It has 9504P marked on it but I can't find that online.
Its 2 wires have a resistance of 0.9 ohms and 4.5 kohms.
The kill wire is not shorted to ground and the meter shows over 1000V (its limit) when connected to the plug cap with the plug out. I've tried 2 mower plugs and a car plug with its gap reduced to 5 thou but no spark.
The flywheel has 3 magnets that will support a fair size screwdriver so that seems OK.
The parts manual shows a 398593 and another magneto 209812 with a trigger unit for this motor. I assume that 398593 is electronic as it uses 398593 for the part number of both the combined 209812 and trigger, and that combination is electronic.
If 398593 is the correct magneto, will any of its replacements be OK?
Unfortunately I just got rid of a 1979 model 92982 Mountfield mower as the roller drive shaft broke, otherwise I could have converted it back to points.
Any help would be appreciated.

 

[Scrubcadet10]

398593 is superseded to 591420, which is a new magentron coil requiring no points or condenser and has the trigger module built in.

 

[mycut]

398593 is superseded to 591420, which is a new magentron coil requiring no points or condenser and has the trigger module built in.

Thanks. I fitted the replacement magnetron and had to cut the governor flap a bit so it would pass over it.
The spark's now very good (visible blue in daylight) but I'll have to learn the correct starting procedure as I tend to flood it every time.
Unfortunately the regular missed beat's still there at all speeds and temperatures so that'll need some work.

 

[mycut]

Thanks. I fitted the replacement magnetron and had to cut the governor flap a bit so it would pass over it.
The spark's now very good (visible blue in daylight) but I'll have to learn the correct starting procedure as I tend to flood it every time.
Unfortunately the regular missed beat's still there at all speeds and temperatures so that'll need some work.

Update.
I learnt to start it and it's easy. Since I had to cut a fair bit off the governor flap to miss the magneto I wonder if that's contributing to the variable firing problem.
The speed varies up and down continuously from just above quite fast to very fast, up and down regularly every few seconds and the flap moves with the speed change. Now I've reduced the area of the flap the airflow will be affecting it differently and its balance with the governor spring will be affected. Is that my potential problem area?
I imagine that these old engines have needed magneto replacements so cutting the flap probably's been done before.
Has this caused a similar problem and what's the solution? All answers welcome!

 

[slomo]

Sounds like a dirty carb. You need heat, reason why I recommend cleaning the carb in the wife's dishwasher. Course you remove all the plastic and rubber parts. Pull the jets and rod out with a nylon bristle off a nylon brush. Spray out with carb cleaner both ways. Then hit it with compressed air both ways. Might take 3-5 cleanings to run proper.

From what you are saying, you are trimming the air vane that the flywheel blows on to close/open the throttle? Post a picture of your carb and all linkages.

slomo

 

[mycut]

I did the cleaning of the pulsajet carb before I changed the magneto and everything was nice and clean, as is the petrol.
I'll take the motor cover off and take a few pictures.
Cheers!

 

[mycut]

Right, here are the photos.
I hope they help identify the problem.
You can see a diagonal piece was cut from the flap, about 3/4" x 3/4".
Should the governor spring have a loose connection to the governor link? You can see what I mean on the 3rd photo.
The "vibration" of the flap is only a few mm and it happens about every second no matter what speed is selected.

 

[slomo]

Post up the ENGINE numbers off the cowl. Where's Rivets at???

Clean the cylinder cooling fins all the way around the cylinder. See a bunch of dirt and jazz in/on them. Get them clean down to fresh metal.

Looks like the idle speed it set up pretty high. Screw is turned in pretty far. Looks like maybe the flapper vane spring is incorrect. Need to see the parts manual to verify. Also the connections look bent around a bit. Someone's been playing around in there.

De-carbonize the cylinder and check valve clearance or lack of.

slomo

 

[bertsmobile1]

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 .

 

[mycut]

Post up the ENGINE numbers off the cowl. Where's Rivets at???

Clean the cylinder cooling fins all the way around the cylinder. See a bunch of dirt and jazz in/on them. Get them clean down to fresh metal.

Looks like the idle speed it set up pretty high. Screw is turned in pretty far. Looks like maybe the flapper vane spring is incorrect. Need to see the parts manual to verify. Also the connections look bent around a bit. Someone's been playing around in there.

De-carbonize the cylinder and check valve clearance or lack of.

slomo

92988 1766-01 77022802 Rivets??
The spring measures 2" which matches p/n 260875 in the manual but it doesn't seem right to be loose on the vane end of the throttle link. It looks like screwing in the idle screw back could correct that but then the idle's going to be wrong!
I did the idle adjustment and when the throttle's closed there are still 7 screw threads protruding and the spring's still loose.
Back to the drawing board.