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#4737100 - 04/24/18 10:51 AM Tecumshe 7hp spark problems, flywheel magnets etc.
trouts2 Offline


Registered: 01/06/11
Posts: 16
Loc: Eastern Massachusetts
I have an older Tecumseh 7hp, H70, with no spark. The coil tested bad so I replaced the coil with a new coil. The points were pitted a bit so I replaced those also and put on a new condenser. The engine still did not have spark. I removed the coil ground wire which did not help.

I checked everything out a few times and no spark. I tested the coil and it tested ok. I had some other stock so replace the coil, points and condenser with new parts again and no spark. I checked everything a few times, took everything off and on a few times checking for mistakes but no spark. The flywheel magnets seem to have good strength.

I was frustrated so removed the condenser and points and installed a Megafire module. I wired it up and still no spark. Iím checking spark with two different spark testers which I checked on another engine and they work fine.

The Megafire is attached to the block and tested for being grounded with a multimeter. The short wire on the coil goes to ground. The coil long wire runs to the Megafire plus. It was spliced to run to the shut off switch but not attached. It is not touching anything. The Megafire minus terminal wire runs to ground. The wiring has been checked several times. No spark.

At first when I replaced the points assembled everything and pulled the cord to get spark. That was a lot of work so for testing Iím turning the flywheel by hand past the coil. I have the flywheel magnet position marked on the outside to insure Iím causing the magnets to quickly
pass by the coil. I had a coil tester setup I made with an old engine and moving the flywheel quickly by the coil was enough to get a spark so I think doing that now is enough to cause a spark.

Given the failure to get spark I checked the flywheel magnets. There are two magnets. I have installed Megafires in the past with two magnets and it worked so think that part is ok.
One of the magnets is north and one south. I checked online and Ed Stoller page about this stuff. He shows and old Tecumseh H70 flywheel with the magnet polarity as follows. Looking from the center of the flywheel to the magnets on the inside the left magnets is south and the right north. Mine are reversed. The one of the left is north and the right south. I donít that that would make a difference for the points system but may for the Megafire. Iím not sure.

There is some glue residue around the magnets. I donít know if that was from the factory install or someone later put the magnets on. ? The flywheel is a 13-0-44 which is for an H70 so should be ok.

Given that installing points is a fairly simple thing and so is installing a Megafire Iím lost.

I checked for voltage at the coil sparkplug. I moved the flywheel by hand quickly over the coil and only get a few volts out on the AC and DC scales. The voltage may be low or it may be too fast to get recorded by the mulitmeter. Since there are not points to open maybe the field is not collapsing quickly enough to induce a large voltage.

Iíve installed points a number of times and a few Megafires and all went well but not this time. Any input would be appreciated.
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#4737107 - 04/24/18 10:58 AM Re: Tecumshe 7hp spark problems, flywheel magnets etc. [Re: trouts2]
Eddie Offline


Registered: 12/07/03
Posts: 9724
Loc: Florida, Cape Coral
You need to find an old fashion outboard motor place and have then reZap the magnets. The rezap by putting the magnet in a coil and hit then with a high current DC pulse. ED
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#4737128 - 04/24/18 11:14 AM Re: Tecumshe 7hp spark problems, flywheel magnets etc. [Re: trouts2]
trouts2 Offline


Registered: 01/06/11
Posts: 16
Loc: Eastern Massachusetts
Why do you suggest that? Do the magnets being N or S on the right or left make a difference? Would it make a difference for the points system and also the Megafire?
The magnets seem strong or at least as other flywheels I've tested by passing a screwdriver past them.
When I test for spark I can run the flywheel clockwise or counter clockwise and both directions do not produce a spark.


Edited by trouts2 (04/24/18 11:22 AM)
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#4737169 - 04/24/18 12:24 PM Re: Tecumshe 7hp spark problems, flywheel magnets etc. [Re: trouts2]
oldhp Online   content


Registered: 06/28/12
Posts: 1686
Loc: USA
I did just about everything you've done and still no spark. The problem was the flywheel key. I guess I hit something when last used and it 1/2 sheared the key moving the flywheel on the crankshaft which is timed to the points. Took off flywheel, new key, re-installed it, engine fired right up.
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#4737215 - 04/24/18 12:59 PM Re: Tecumshe 7hp spark problems, flywheel magnets etc. [Re: oldhp]
GravelRoad Offline


Registered: 05/30/15
Posts: 170
Loc: Virginia
New points contacts have a coating to prevent rust/corrosion in storage. cleaned off?
After gap was set, did you insure they fully closed/contacted?
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#4737239 - 04/24/18 01:18 PM Re: Tecumshe 7hp spark problems, flywheel magnets etc. [Re: trouts2]
trouts2 Offline


Registered: 01/06/11
Posts: 16
Loc: Eastern Massachusetts
Oldhp. The flywheel key sift will change the timing but there should still be spark. The engine will not fire as the spark does not happen at the right time on the compression stroke.

Iím not trying to start the engine. The carb, gas tank and muffler are off the machine. Iím looking for a single spark.

Gravelroad
Yes, they were cleaned and very lightly sanded then cleaned after. I did that a few times. The close was full so good contact when off the peak of the dwell.

I tried the points and went through the setup 3-4 times then gave up. I then switched to the Megafire and still have not been able to get spark. The flywheel seems fine but I suspect it may somehow be in the mix.

The only thing I can think to do is setup the Megafire and coil off the machine to a workbench setup. I have not done that yet. Something I did try was running a magnet over the coil to simulate the flywheel magnets. I donít have a strong magnet around so I used the back side of a speaker. I could get a reading off the spark plug end but not much. About as much as with the flywheel.


Edited by trouts2 (04/24/18 01:18 PM)
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#4737330 - 04/24/18 02:34 PM Re: Tecumshe 7hp spark problems, flywheel magnets etc. [Re: trouts2]
old1 Online   content


Registered: 01/04/11
Posts: 1024
Loc: Columbus Nebraska
At this point I think if you can find one, I would try another flywheel. I don't know anything about how to trouble shoot the mega fire set up, but if the points setup is correct, it should have good spark, ALTHOUGH, sometimes I have to spin the flywheel faster than you can without pulling the rope. What point gap do you have? Have you ever seen this engine run, or did it come to you not running? I have seen those glued on magnets come off. Maybe someone glued them back on in the wrong place. That might make spark with the electronic set up, but it sure won't work with points. 50 years of repairing OPE, and I never found one I couldn't make good spark.
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#4737355 - 04/24/18 02:57 PM Re: Tecumshe 7hp spark problems, flywheel magnets etc. [Re: old1]
trouts2 Offline


Registered: 01/06/11
Posts: 16
Loc: Eastern Massachusetts
The gap was set from the Tecumseh factor book which I think was .20. The machine is new to me and has not run for many years. It was a garage fine in excellent shape but has sat for many years.

Iíve reglued magnets in the past and replaced many points without problems but not this time. The magnets are in the right place. Where they are on the inside diameter should not matter just to get spark as long as they are at the right height. Iím not trying to get it running Iím only trying for spark. They are in the usual place.

The only odd thing is as I mentioned before in reference to Ed Stollerís page. He shows an H70 flywheel with the polarity of the magnets being opposite mine. I donít think that would matter for points but may for the Megafire. Iím not sure.

In the past the only checks Iíve done for flywheels is tap the magnets to see if they are securely on and pass a screwdrive by them to see if there is a strong pull on the screwdriver. The magnets have a strong pull. I may take the Megafire out and put the points back in. I have some extra new sets so will probably replace the coil, condenser and points with new parts again.
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#4737442 - 04/24/18 04:16 PM Re: Tecumshe 7hp spark problems, flywheel magnets etc. [Re: trouts2]
JimPghPA Offline


Registered: 08/22/09
Posts: 3878
Loc: Pittsburgh,PA U.S.A.
The gap between the coil and magnets might be too large.
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#4737476 - 04/24/18 05:04 PM Re: Tecumshe 7hp spark problems, flywheel magnets etc. [Re: trouts2]
trouts2 Offline


Registered: 01/06/11
Posts: 16
Loc: Eastern Massachusetts
JimPhgPA
A friend mentioned that and a possibility. The flywheel has marking to indicate it is
what is supposed to be there 13-0-44. The magnets do not seem damaged and I think correct
thickness. Everything is under the flywheel so not easy to take an exact measurement but
I don't know what the gap should be anyway. I think that part is ok.

I've been out that last few hours mucking with it. I took out the Megafire and put points
back in and went through the setup. When they close they are not consistently making my meter
go to zero ohms. The new points are not Tecumseh OEM and I've always had a hard time setting
them. The issue is the non OEM points do not go firmly against the crank to get a solid
close even thought they appear closed. Gravelroad's suggestion made me check again.
I checked my piles and found Tecumseh OEM points and put those in. I think the spring
metal is better as pushing the points arm into the crank. But getting a consistant close
off the dwell is difficult. I played around with the points housing that sets the timing.
Moving that around and adjusting the gap I can get a good close with the points under .20.
I don't think it will matter if the gap is under as long as the open happens at the peak
of the dwell and they close properly when off it. That is now happening.

So I think one issue is taken care of. Maybe it will get spark now but I doubt it as the
Megafire bypassed this issue and would not work. I'll try tomorrow to see if I get spark
and if so leave the points in.

A possibility could that the crank arm the points rest against is worn. I've never seen
that but I guess it could happen. It would make sense given the difficulty in setting the
points.


Edited by trouts2 (04/24/18 05:07 PM)
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#4737485 - 04/24/18 05:12 PM Re: Tecumshe 7hp spark problems, flywheel magnets etc. [Re: trouts2]
Lou_Boyle Offline


Registered: 11/17/17
Posts: 7
Loc: Flyoverland, KY
I once encountered an engine with no spark (don't recall the brand except that it wasn't Briggs) which opened and closed the points with a keyed cam ring that slipped on the crankshaft right behind the flywheel. The ring was designed to only fit properly in one direction, but that didn't stop the last person who worked on the engine from installing the ring upside down. Consequently, the points were opening and closing out of sync with everything else.

As a troubleshooting step, take the flywheel off and turn the crank in the direction of running rotation until the points just open. Then put the flywheel back on and see where the magnets are. It's been a couple of decades since i've made that check, that but I believe the trailing magnet should be aligned with the coil armature at that point.

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#4737537 - 04/24/18 06:21 PM Re: Tecumshe 7hp spark problems, flywheel magnets etc. [Re: Lou_Boyle]
GravelRoad Offline


Registered: 05/30/15
Posts: 170
Loc: Virginia
oldhp is correct about a sheared flywheel key on a point/condenser setup.

The points use a fixed cam on either the crankshaft or camshaft, timed to compression stroke near TDC. Flywheel timing is independent of fixed cam for points.

Newer electronic ignition coils use the location of the magnets to time spark. So a sheared key will still allow spark, just at the wrong time.

Remove spark plug and move piston to TDC on compression stroke. The magnets should be just past coil and your points should just start to open. Give or take a few degrees.
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#4737553 - 04/24/18 06:31 PM Re: Tecumshe 7hp spark problems, flywheel magnets etc. [Re: GravelRoad]
GravelRoad Offline


Registered: 05/30/15
Posts: 170
Loc: Virginia
Also use a starter rope or rewind assy to test. If I remember correctly, it needs around 500rpms +/- to build current in the primary windings before it is dumped to the secondary windings when the points open.
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#4737685 - 04/24/18 08:01 PM Re: Tecumshe 7hp spark problems, flywheel magnets etc. [Re: trouts2]
JimPghPA Offline


Registered: 08/22/09
Posts: 3878
Loc: Pittsburgh,PA U.S.A.
I had a 1976 Olds with the electronic ignition that has a pickup coil mounted in metal that has 8 metal points to conduct the magnetic pulse that then generates the signal that causes the ignition to fire. That setup has a vacuum advance that rotates the coil to change the timing. The weak point is the single strand of wire to and from the pickup coil. The repeated flexing of that wire every time the vacuum advance changes position eventually causes the wire to break. I knew these were common to break after about 60k to 80K miles. When it broke I was in the parking lot of a Boy-Scout camp 60 miles south of home. I found the broken wire on the pickup coil. When I got home (60 miles away) I bought a replacement pickup coil (that is housed in the metal with the 8 points). I traveled back to the car (60 miles away) and replaced the pickup coil in the distributator and the car would not start. I then took note of where the rotor in the distributor was pointing and removed the distributor and took it home (60 miles away). I connected the distributor to a charged car battery and spun the drive axle of the distributor by hand and there was no spark. I then though about what could be wrong. The new pickup coil ohmed out to have a complete circuit so I thought that maybe the 8 metal points that the magnetic path must go through is now too large so I took the metal from the old pickup coil and put the new coil on it. I put the distributor back togehter and again turned it by hand and got spark. I traveled back to the car (60 miles away) and installed the distributor and it fired right up.

If you messed with the coil (installed a new coil) the gap for the magnetic part of the circuit might now be too large to enable it to work. It happened to me with my 1976 Olds, and it could be happening to you with your 7.5 HP Tecumshe.


Edited by JimPghPA (04/24/18 08:07 PM)
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#4737740 - 04/24/18 08:37 PM Re: Tecumshe 7hp spark problems, flywheel magnets etc. [Re: GravelRoad]
trouts2 Offline


Registered: 01/06/11
Posts: 16
Loc: Eastern Massachusetts
I made a test station for coils and could get a spark by turning the flywheel by hand. That was a while ago
and I can't remember if all the coils I was testing were solid state setups. I'm not sure if the rotating
speed by hand is 500rpm or not but I think the magnet passes the coil with enough velocity to trigger
a spark. I done the before on engines versus the test fixture but can't remember if those were SS versus
magneto's.
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