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MF 7 Tecumseh H70 Engine Help Needed


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#1 Utah Smitty OFFLINE  

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Posted August 22, 2011 - 08:02 PM

I bought a couple MF7s a week ago. One has been "upgraded" to a bigger Tecumseh engine (no data plate or info), the other has what looks like a stock H70. Both have electric start.

I worked over the weekend to get the H70 running, but experienced the following:

I have spark, and I primed the cylinder with a little gas. The engine cranks, but it acts like it's firing too soon, as it will make almost two full revolutions, then strain a few moments then crank for two more revs, etc. When I remove the plug, it cranks fine with no hesitation, etc.

I'm not real familiar with the Tecumseh, but I understand how engines work and have rebuilt car engines. I know the Tecumseh has a coil that I assume is triggered by magnets on the flywheel, like most small engines. What could be causing it to not want to turn over completely? (It does turn over, it just slows way down for a moment then turns normally).

Thanks for any help I can get.

Utah Smitty

FWIW, there were two files under M-F tractor manuals --MF8 Operating Manual Part1 & Part 2. I downloaded them, and sent them to work where I have Adobe Pro. I combined the two files into one file, and re-uploaded it. (I also uploaded files for the Power King Tractors and Kubota Engines)

#2 KIRO-1 OFFLINE  

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Posted August 23, 2011 - 06:27 AM

Did you check the points to make sure they are set correctly ?

#3 MH81 ONLINE  

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Posted August 23, 2011 - 06:49 AM

I believe these engines are set to fire somewhere around 17 degrees BTDC, if your battery or starter isn't up to snuff, they will do what you're describing. Especially if the points gap is too big, that will cause it to fire even earlier.

#4 Utah Smitty OFFLINE  

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Posted August 23, 2011 - 01:30 PM

I think the batter is up to snuff. It's a new car battery I use to jump my garden tractors. It started the other MF7 (with 8hp engine) just fine.

I'm not sure where the plug should be gapped, but it was at about .010" so I gapped it to .030" I'll check the points... I hadn't thought about the gap affecting the timing. What should they be gapped at? Also, is there any other way to adjust timing?

Thanks for your help,

U S

#5 Ranchkingron OFFLINE  

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Posted August 23, 2011 - 02:15 PM

I think the batter is up to snuff. It's a new car battery I use to jump my garden tractors. It started the other MF7 (with 8hp engine) just fine.

I'm not sure where the plug should be gapped, but it was at about .010" so I gapped it to .030" I'll check the points... I hadn't thought about the gap affecting the timing. What should they be gapped at? Also, is there any other way to adjust timing?
Thanks for your help,

U S


I have this engine in a Sears Custom 7 and gave me fits till I finally figured it out, Points are gapped at .020 and the plat at .030 the timing is adjustable but is a major pain to do.

Basically you leave the armature plate (points plate) in place and change the points, if it has been moved then just tighten it down in the middle of the adjustment range and that should get you close then if you like you can fine tune it once you know it runs.

In the pic you can see someone has moved the plate so I had to re-time it and its just about dead center in the adjustment slots.

The downside is you have to pull the flywheel to make any adjustments or change points on an H70.

Ron

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#6 Utah Smitty OFFLINE  

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Posted August 23, 2011 - 05:27 PM

Thanks, I'll check into that.

U S

#7 olcowhand ONLINE  

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Posted August 23, 2011 - 05:30 PM

Look at your flywheel key, as it may be sheared. That will do weird things, but starting isn't one of them! The way the key is made, it might shear in the flywheel, but keep the points opening at the correct time, but the magnets on the flywheel won't be in the right position.

#8 Utah Smitty OFFLINE  

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Posted August 23, 2011 - 07:43 PM

Look at your flywheel key, as it may be sheared. That will do weird things, but starting isn't one of them! The way the key is made, it might shear in the flywheel, but keep the points opening at the correct time, but the magnets on the flywheel won't be in the right position.

Thanks... I hadn't thought of that. The engine is acting like it's firing way too soon--trying to drive the piston back down before it reaches TDC.

U S

#9 Utah Smitty OFFLINE  

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Posted August 28, 2011 - 10:20 PM

Look at your flywheel key, as it may be sheared. That will do weird things, but starting isn't one of them! The way the key is made, it might shear in the flywheel, but keep the points opening at the correct time, but the magnets on the flywheel won't be in the right position.


Well, I pulled the flywheel Saturday. The key is fine--the taper inside the flywheel and the end of the crankshaft are clean and no damage. I pulled the points, filed them smooth (they were actually pretty smooth, but were burned black--even the baseplate looked like it had been hot...) Anyway, I reinstalled the points, set the gap to 0.020" and put her back together.... still no luck... The starter and jumpers get very hot very quickly, which tells me I have a large load on the starter and a lot of current draw. Also, it does the 1.9 turns and pause if the plug wire is attached or not, and if I try to start with a key, or just touch the positive to the starter motor.

I'm not very experienced with small engines, but the principle is the same as with multi-cylinder engines... I'm wondering if there is a bent connecting rod, or a bend valve, etc...

Any other thoughts on this???

Thanks

Utah Smitty

#10 Ranchkingron OFFLINE  

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Posted August 29, 2011 - 11:11 PM

Hmmm ok I would pull the head and see if the valves are even opening, decarbon it and clean the cooling fins/shrouds while it was off anyway so that would be a normal thing in my garage but I would do this last if I were you because it could be something else.

I'm guessing it has a starter/generator so take the belt off and see if it is running free with power, also check all the connections for good contact (a dirty ground will cause heated wires) I don't think the high load is the problem but a symptom of something else, if all is as it should be then it goes back to the engine.

You have compression obviously so that's a good thing, does it have nice blue spark cranking it with the plug out ? If yes then then I would do a compression test and/or pull the head clean it and adjust the valves.

I really don't think its a bent rod or valves, a rod you would more than likely hear and valves would present other problems i.e. a stuck/bent exhaust valve would give a no compression symptom etc.

When you pulled the flywheel did you use a puller or a prybar under the flwheel ? I've seen people do that and broke one of the two magnets off and the it wont fire right, but that doesn't explain the heavy starting load.

Just a thought a friend had one that had sat for some time and the cylinder had rusted (cast iron sleeve) from about the middle to TDC and acted the same as yours is but if I remember right the H70 is all aluminum (mine is) so that may not be the case here.

I'm sure one of these fine gents can get to the bottom of this better than I can, all I can do is give you guesses.

Ron

Edited by Ranchkingron, August 29, 2011 - 11:17 PM.


#11 MH81 ONLINE  

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Posted August 29, 2011 - 11:36 PM

I think I would pull the plug and see how it turns over. If it turns over smoothly, check for spark while the plug is out. If no spark, you may have shorting points. Pulling the flywheel again and doing a continuity check is the best way to test.

If the engine is still struggling to turn over, see if you can tell if it's a worn out starter or in the engine. A whooped starter will have a high load and very little torque.

#12 ncb OFFLINE  

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Posted August 30, 2011 - 12:07 AM

one more thing to check is if it has an internal compression release . my kohler k321 was not turning over every third or fourth start.ended up finding the internal comp release spring was missing altogether.was a $5 dollar part and was installed with only removing pan from bottom. no other dissasembly was required.gave me a chance to inspect the rest of the guts as well. worth a shot.... rex knightly

#13 Utah Smitty OFFLINE  

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Posted August 31, 2011 - 06:16 PM

Thanks to all for their replies... The engine has an automotive-type starter, not the starter/generator like the Cub Cadets, etc. The starter turns the engine freely without the spark plug, and there is spark at the plug. I guess I need to pull the head and see if the valves are seating properly. Most of my experience is with Kohler K-series, but I understand that the Tecumseh's can have a problem with valves or valve seats sinking into the block...

I have a Neway seat cutter and a crank type valve cutter, plus a lapping "drill". I'll check this out and try again.

U S

#14 MH81 ONLINE  

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Posted August 31, 2011 - 10:07 PM

Pull the valve cover first and see what your gap is. I think that engine is 10 to 13 thou. Check me with the manual tho. You also should be able to see the valve stems moving while turning over by hand. If the compression release is working the one valve will never quite close.

EDIT: I'm not sure that engine had a compression release. Some of the smaller ones didn't. :anyone:

#15 DH1 ONLINE  

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Posted August 31, 2011 - 10:15 PM

Did you check the timing?
As you can see by Ranchkingron's pictures that coil points assembly is adjustable, can be rotated to change the timing.
The position of that unit and the point gap both affect the timing.
If it is firing to early it will cause a kick back.




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