[tictac80]

I was given a Craftsman garden tractor that someone else tried unsuccessfully to repair. Its engine is 28N707_0666-A1_970402Z0. The PO replaced the camshaft and tappets (I'm not sure why).

I replaced the battery, cleaned carb & gas tank. I also checked all wiring. It now has good spark and clean fuel at the plug and 100 lb of compression.

The problem is: When starting, it only occasionally backfires as if the timing is off. I checked the valve clearances and they seem OK. How can I check the flywheel timing and camshaft timing by doing the least amount of tearing down the engine?

Thanks you in advance for any advice.

 

[rhenning]

Could be as simple as a broken flywheel key. That is what controls the timing on that Briggs. Roger

 

[Bill Kapaun]

Valve timing----
Since that is an OHV engine-
Remove valve cover & spark plug.
Rotate the piston to TDC.
Using a pencil or similar as a gauge, rotate additionally until the piston has traveled down 1/4".
That's the position where you adjust the valves.
Both valves should have their MAXIMUM clearance at that position.
You may as well adjust them.
Rotate the engine to additional positions and make sure NONE of them provide more valve clearance than the "specified" position.
NOTE_ When you BARELY start rotating the engine from that point, the intake valve should open VERY SLIGHTLY and then close. That's the compression release functioning.

IGNITION timing is solely through the flywheel key being intact.

 

[tictac80]

Thank you for the quick replies.

I have to go out and buy a 15/16" impact socket tonight to remove the flywheel. Then I can hopefully check out if the key is sheared.

As for the valve clearances, they are good, but the compression release has me baffled. When the piston is at TDC on the compression stroke (both valves closed) and I rotate the engine clockwise a bit more, there is NO valve slightly opening to show the compression release. BUT... if I go back to TDC, then rotate the engine a bit counter-clockwise, then I do see the slight opening of the intake valve briefly. Could this be caused by the camshaft installed in the wrong position???

 

[Bill Kapaun]

Maybe?????

 

[tictac80]

I got the 15/16" socket and found that rhenning was right about the flywheel key. It was sheared and the flywheel had rotated about 90 degrees off where it should have been. Now I will start the battle of removing the flywheel.

Thanks guys.

 

[tictac80]

Well I got the flywheel off and replaced the key. After re-assembling everything it started right up and ran OK ... for about 5 minutes at a low throttle level. I wanted to let it warm up a bit and then increased the throttle slowly to 3/4. It stalled. When re-starting it had the same timing symptoms as originally (a shotgun-type backfire). I removed the flywheel bolt and the new key had sheared again.

Any ideas as to why the flywheel key keeps shearing?

 

[Whirly]

did u torque the flywheel nut down .. think it is 90 foot pounds.. r u usin an aluminum key.. do not use a steel key in these engines..

these flywheels r designed to spin & shear the key if the crank stops suddenly.. u can over tighten the nut & it will not spin.. this can cause serious damage inside the engine..

I'm thinkin u did not have the nut tight enuf & the backfire sheered the key.. if the key is partially sheered it will cause a backfire

 

[Bill Kapaun]

Fly wheel nut should be torqued to 100 ft-lbs.

 

[tictac80]

I think you're right. I only gave it a shot with my electric impact gun and did not use a torque wrench at all. I will try again.

After reading some other posts, I have seen torque values anywhere from 50 ft-lbs to 100. What is the actual factory recommended torque? ( I don't have the shop manual).

 

[Bill Kapaun]

I think you're right. I only gave it a shot with my electric impact gun and did not use a torque wrench at all. I will try again.

After reading some other posts, I have seen torque values anywhere from 50 ft-lbs to 100. What is the actual factory recommended torque? ( I don't have the shop manual).

I provides a page out of the service manual if you were to look!

 

[Tony McD]

...also make sure the crank and the cone on the flywheel are clean for a snug fit.

 

[tictac80]

Sorry, Bill, I missed that page in your last message. Thank you for all the advice and I will report back after fixing the flywheel key.

 

[38racing]

I think running these engines at part throttle should be avoided. Max oil splash and air cooling only happen at full speed.

 

[Whirly]

Fly wheel nut should be torqued to 100 ft-lbs.

Bill... ur the man with all the info.. I have a ton of ur pdf's already& just added another.. now if I can find the right pdf when I need it... lol

 

[wwxx]

There should be a 1/8" THICK hardened flat washer used with the flywheel bolt, if yours is missing, you will have to get one.

If the flywheel is cracked anywhere, especially in the tapered area, it should be replaced. Assemble clean & dry.

wwxx

 

[tictac80]

My flywheel bolt has the cup that the rotating screen fastens to with 3 screws and it also has a washer that is about 1/16" thick. I removed these two parts and re-inserted the bolt. and it still did not bottom out. I will add another 1/16" washer just to be on the safe side. I may not get back to this project for a day or two, but will report back when I do.
Thanks to everyone for the help.

 

[Whirly]

My flywheel bolt has the cup that the rotating screen fastens to with 3 screws and it also has a washer that is about 1/16" thick. I removed these two parts and re-inserted the bolt. and it still did not bottom out. I will add another 1/16" washer just to be on the safe side. I may not get back to this project for a day or two, but will report back when I do.
Thanks to everyone for the help.

the cup & washer need to b on the bolt .. & a pic will confirm what u have done..

 

[tictac80]

Yes, I realize they have to be on the bolt. I only removed them temporarily to make sure the bolt did not bottom out. I will be putting them back on when I get the key replaced.

 

[suspicionofignorance]

Wow..I have been down this same road last year...sheering keys...till I learned that you must tap the key down in the keyway..[small punch]..don't leave it flat at surface and think that tightening force is the fix...it's not..