My 1977 1250 QL

[justanengineer]

Just thought I would post an update on my 1250. After a small break which was spent between the house and a few other projects, I dug back into this last week. Initially, I decided to pull the flywheel cover and locate the timing & tdc marks. I was able to find one through the timing hole, but not the other. Unfortunately, I decided after 5 mins effort that removing the cover with the engine in the tractor is a royal PITA, so sought out another option. I then removed the tin to access the head, removed the head, and squeezed my hand between the flywheel cover and the block at that point. Removing the head to access the flywheel allowed me to 1. locate the two marks on the flywheel, and 2. decarbonize the engine. A quick minute with some 120 grit paper on the flywheel, and a few more with a soft wire brush on the head, piston, and deck surface, and the first two tasks were done. While decarbonizing, I also thought it prudent to pull the side cover and adjust the valves while I could easily/visually discern TDC.

Things I discovered:

1. My previous "guesstimate" based off the seat of my pants and experimentation regarding timing was off a few degrees, but close. The mark I located through the hole was the TDC mark, and thus setting timing to it resulted in the backfiring and lack of power I experienced earlier, followed by my experimenting. I fine tuned the timing based off of the timing mark that I located by sanding the flywheel.

2. ~400 hours carbonized this engine a good bit, and I was surprised by how much came off once I started scraping bc initially I didnt think it was too terrible. Reinstalling the head, the head gasket looked strangely good with only a slight discoloration in one small area of the fire ring. Bc it appeared almost brand new, being a cheap SOB, and bc these are air cooled, I simply reused the old gasket and torqued to factory spec.

3. What I suspect is the biggest change - Despite the valve lash on the intake valve being within spec, the exhaust had ZERO lash (or at least .003" shim stock wouldnt fit, which was the thinnest I had), and I suspect most of the exhaust valve sealing was done via the carbon buildup (aka, the exhaust valve wasnt closing fully). While cranking for startup, the tractor now fires significantly harder/more violently during the first few revs before the engine really "catches" and I suspect the compression has risen significantly bc of this.

The result - EASY starting, ZERO backfires, and a TON of power. I was able to give this the acid test - having her mow the really thick, weedy, knee deep backyard acre, and it passed superbly.

Thanks for all of the help thus far fellas, especially to Matt for creating the various downloadable how-tos. I have learned a lot in a short time and hopefully will continue to do so.

[Matt G.]

I would strongly suggest a new head gasket. They permanently deform a bit when the head is torqued initially, so it's probably not going to seal right the second time around. The fact that the head is aluminum and the block is iron makes it worse.

Not to mention there is a lot of crap to remove and put back on when changing the head gasket on a QL, so it's best to only have to do it once

[Donovan M.]

Well after you have done everything mechanically done on a QL there one of the best tractors to use for mowing. I put all new ISO mounts on my 1250 and i love it, don't vibrate as bad as it did. But make sure your drive shaft is good and the fan is in good shape for optimum cooling. 1250s are powerful on my account. I love mine. if i could own another one i would. and good luck.