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Heart Transplant

3513 Views 14 Replies 9 Participants Last post by  VlvtnRbt
Well, except for some trim colours and stickers the machine is finally all back together.

This is a 1975 ST12 that had the ignition on the Tecumseh motor finally go the way of the Dodo. Became uneconomical to fix as I had already tried 2 modules with no success. Other than that this mower is damned near indestructable.:i_praying and that 48" deck makes short work of the lawn.

After looking through recyclers and at some of the new junk offered out there finally decided to bolt in a new engine. Could not overlook the cost of one of the knockoff 13HP PowerFist motors so gave it a try. Only problem was the output shaft was slightly smaller, meaning I had to grab two taper-lock pulleys as well, (more on that in a bit).

Surprise! The mounting holes lined up when I was setting up the crank location:bannana: . Same with the original pulley guard/guide for the drive and accesory belts.

As this motor has an integral fuel tank I pulled the stock battery/tank mount and fabbed a .200" U-Shaped plate to fit between the frames. Only bit of fun there was aligning the drive belt clutch pulley and, as I flipped the plate over to give more room between the top of the battery and hood, it took a couple of tries to get the stack height correct for the pedal.

Even though the schematic was a joke it was not that complex to remove the integral control and install the components to the dash. That includes the breaker. Still looking for a 0-5 or 0-10A ammeter as the stock one barely jiggles with the 3A charging system output.

The other pain came from the fact this was an angle-cylinder engine, compared to the straight vertical stock plant. Due to this I used part of the stock battery plate removed earlier to move the front hood mount forward 2". This cleared the motor, but created its own set or problems. Now the tapered hood would not fit and I needed an exhaust deflector to keep the grille and glass from getting cooked.

The deflector ended up coming from a B/S motor and I had to fab a 2" extension bent out of an old industrial shelf.

Final analysis? This motor fit, but required a bit of shoehorning to get it to integrate and line up with the stock drives. I probably made it more difficult by trying to eliminate some of the original complaints of access to certain areas, such as the back of the dashboard and still have a few tweaks to do to the throttle and choke. The former lever range does not match that of the new carb very well and the choke cable does such a severe angle change at the carb there is more friction than I like. On that note, to get the choke cable system I had to shell out close to $55 for the Honda kit. Could not figure any other way to hook up a remote and this motor absolutely requires choke to start. I think a few R/C aircraft linkages will have to get sacrificed to rig it so it will be more reliable and easier to operate.

p.s. Ignore the Ridiculous pic at the end. It's in response to a Father's Day card my son sent me. At least it gave me an excuse to dig out my old Helicopter Flight Gear:sidelaugh .

Oh yeah, that bit about the pulleys? First try had me nearly run it into the neighbors brick wall as I could not get the drive to disengage. The problem ended up being the fact they powder coat the belt shieves along with the outside surfaces! Got nice and sticky and required a bit of work with a hand file and sandpaper to remove. Belts slide nicely now.


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Sorry, could not get all the pics in with the first post. Will have to resize and add them later.
Sorry, could not get all the pics in with the first post.
Post the pictures in a reply and I will add them to the first post..
Still listing them as too large and I'm way overdue to hit the sack. I'll be back in a while.

Still listing them as too large and I'm way overdue to hit the sack. I'll be back in a while.

If using WindowsXP, CLICK HERE
Sorry, got busy and was out of town.

Let's try the pics again. What you can't see in these are the exhaust deflector and the choke cable. Must have added them after these were taken.

Still have to add the stickers.


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Now the mower has a few passes around the yard I've found a couple of issues I'll have to deal with. Nothing major and not unexpected when doing a graft such as this.

Anyone work with one of these Honda clones in regards to turning the exhaust? Find with it coming out the front it's baking the paint on the grille and lower hood segment. Other than fabbing a plumbing adapter and threading in a disposable muffler I would like to turn it 90 degrees CW, (the only other option available unless you want the outlet to face the carb or tank!:hide: ).

Also, a week ago went to start it and found it backfiring and running rich. Checked the choke mech and carb for a stuck float. Source was low oil, even though it was still showing level in the middle of the dipstick range. Topped it up and will be changing it out soon anyway, as the motor should be setting in with approx 12-15hrs on the meter now.

At the end of the season I will be working to refine the throttle and choke linkages as they cause more friction than I'm comfortable with.
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Found one more issue that I wonder is endemic of this breed motor.

The wife started it up a few weeks ago and found it ran like a pile of p00p, backfiring and chucking black smoke. Thought it might be mixture or a spark system breaking down, but could not see anything I could attribute to this behavior.

As this has the oil level cutoff I took a look. Oil was down to 1/2 the hash mark, not unexpected with a new motor and approx 15 hours on the hobbs. However, decided to top it up again and problem disappeared.

Think that sensor is way off the mark.
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nice job! what size is the motor and how much did it cost?
Cougar429:This is off topic,but do you by chance have a 429 powered cougar?

It's not that far off. much lower on oil and there would be motor damage. better safe than sorry.

My new generator with a briggs did exactly that. Same 1/2 mark on the stick.
are those sensors reading oil level or oil pressure?
Great job! Gotta say you look special in the pic! Pretty funny!
I'll have to check to see if my Powermax (same honda clone, different name) 16hp has this problem. I installed it new, it ran great for about 15-20 hrs, then starting running rich with black smoke all over the inside of the hood,sputtering, and generally running like crap. Now it quit running and has no spark. I bet low oil is the problem.
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On another post, I installed a 13HP Harbor freight Honda clone on a '67-68 SS12 I had to move the mounting bolts 9/16" to left for pulleys to line up removed battery/fuel tank tray installed new transverse tray similar to yours I moved the grill mount 2-1/2" forward and 1/2 inch to the left I trimmed the grill to clear carburator, and the left frame horn 2-3/4" to clear electrical box.
I have yet to extend the dash to make up the gap, and I have to weld a "C" on the upper hood strap around the new fuel fill location. The 2 biggest headaches were the 3 groove pulley I ground the engine side with a 1" grinding point in a drill press to clear th 1/16" step the crank has I used a brass 1" male x 3/4" pex adapter From Lowes, to create a bushing for the outer end of the pulley. (I recommend find an indulgent machine shop) I used 2 , 3/4" galvanized pipe stantions (spelling?) to create two matching exhaust flanges wihich I welded to a 8"x1" pipe nipple to extend the exhaust out from under the hood, I reused the the original manifold and I used some copper sheet to make more gaskets. I did have to trim a little metal from the muffler heatsheild
I used some 2"x1/4" angle to create a new mount for the belt idler which I mounted on the plate behind the motor. Again at Lowes, I got a very nice sintered bronze bushing and steel sleeve to reuse the original idler arm(although I tried to mount it in as close to the original location to use the original belt size, and the keep geometry the same and had to grind the belt guard on the idler arm to clear the starter motor) I got the motor for $299.00 and probably used another $50.00 dollars worth of additional materials (bought a lot a more but that's what I used, my mistake hopefully not yours)
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