JD 175 backfires when cranking

[Scrubcadet10]

Do you know if this engine was rebuilt or torn into ?

 

[rwgreene]

Nothing tells me the engine was rebuilt although it’s a customers so who knows. Another mower shop verified the coil and trigger numbers purchased from Deere since that was the original diagnosis. A true puzzler.....the shop owner can’t figure it out and he’s about as sharp as they come. I’m just a donkey to roll thoughts off of and contribute what I can. I have a good running LX172 with the same engine that I’ve been “borrowing” parts off of to try.

 

[ILENGINE]

I am wondering about either the engine has been opened up and the camshaft isn't properly timed or even more remote that the gear has slipped on the shaft for some reason.

 

[rwgreene]

A local JD dealer suggested that the intake valve seat came out so we took the head off again and rechecked the valve job we had done....no loose seat and both valves looked good. Then we removed the engine and took off the sump, the cam timing marks were aligned but we wondered if possibly the cam shaft became twisted somehow.....kinda far-fetched but more in line with your thinking that the valves are out of time. We didn’t remove the camshaft but simply buttoned it back up because everything looked good. Cam lobes look good based on the observed lift of the rockers and the valve lifter rods are straight. Tried different combinations of new JD parts and good used parts from my running engine...all to no avail so far.

if I remember correctly, the cam gear is keyed onto the camshaft and we checked for the key to be sheared but it was intact. Quite the challenge.

 

[Scrubcadet10]

Do you think you could make a video of it backfiring?

 

[bertsmobile1]

If it is backfiring out the carb then either the timing is too far advanced or the inlet is not closing properly

 

[rwgreene]

Definitely backfiring out the carb....tried another carb and another fuel supply (day tank) with same results. Video is a possibility....shop owner is more tech savvy than me. Supposed to be getting another small engine mechanic visiting today for another opinion...we’ll see what he has to say.
Watched the valves working with the cover off thinking maybe they were “hanging up” intermittently even though we did a valve job....nothing jumped out at us. Thinking of replacing the engine since the owner is getting impatient and then tearing down the old one for a complete “autopsy” at our leisure. It’s us against the Kawasaki gods now!!! Economics went out the door twenty hours ago when the initial coil and trigger replacement didn’t solve the problem.

 

[AVB]

You did check the valve guides when did the valve job? Worn guide can cause a valve to hang partially open at times. Random miss fire can also be by a leaky intake manifold. I had a Briggs last year that randomly backfired (sneezed) through the carburetor because of a bad o-ring gasket, nearly kill the engine at times. Just a thought.

 

[rwgreene]

I didn’t do the valve job but saw the aftermath after re-removing the head. We removed the intake valve entirely, checked the lapping surface and reassembled....nothing of note. The gasket for the intake at the carb is oversized and not brand new but appeared to be sealing. I had to remove the gasket from the original carb (where it was stuck on) and use it on my carb when checking for an overloading gas supply due to a faulty needle valve/carb. Again, the intake gasket is not new but not torn.
Gotta thank all you guys for the info and suggestions.....nice to have folks take time to try to help others.

Just an addition..The engine does not start and the backfire is not an explosion but a muffled pop with occasional small flame burst visible in the carb throat. I’m stuck on a timing issue. Grasping at straws, the Muffler doesn’t seem blocked and the valve reed that vents into the valve cover seems okay.

 

[rwgreene]

Update. We purchased a good used engine (Fc420v-cs00) and installed it to basically get the John Deere mower out the door and back to the customer. With Spring around the corner and the pressure to repair the mower off, the problem with the “old” engine may take a back seat to more pressing matters although I sure would like to know what’s wrong with the engine. Just for information, we switched the muffler, oil filter system and carb between the two engines to make them compatible and for improvement.