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3.5 hp b&s has no spark

#1

M

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.


#2

Scrubcadet10

Scrubcadet10

398593 is superseded to 591420, which is a new magentron coil requiring no points or condenser and has the trigger module built in.


#3

M

mycut

Scrubcadet10 said:
398593 is superseded to 591420, which is a new magentron coil requiring no points or condenser and has the trigger module built in.
Click to expand...
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.


#4

M

mycut

mycut said:
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.
Click to expand...
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!


#5

S

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


#6

M

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!


#7

M

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.
20210611_164246 (2).jpg20210611_164055 (1).jpg20210611_160309 (1).jpg


#8

S

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


#9

B

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 .


#10

M

mycut

slomo said:
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
Click to expand...
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.


#11

M

mycut

bertsmobile1 said:
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 .
Click to expand...
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.


#12

Hammermechanicman

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.


#13

M

mycut

Hammermechanicman said:
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.
Click to expand...


#14

M

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?


#15

Hammermechanicman

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.


#16

M

mycut

Hammermechanicman said:
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.
Click to expand...
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!


#17

M

mycut

mycut said:
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!
Click to expand...
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.


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