white smoke and hesitation when going full throttle

[slomo]

Yeah. White smoke out the oil tube.

Could be the rings didn't seat if they are new. Or you didn't space them out properly with installing on the piston. Could of been a dirty cylinder bore if you honed or bored. That bore needs to be surgically clean prior to installing the piston. Scrub the bore with HOT soapy water, rinse and repeat. Paper towel with a little transmission fluid or Diesel. Look for a dirty towel.

Did you check ring end gaps?

 

[bostonbear]

All good advice. Thanks everyone.

At this point it would have been cheaper for me to buy a new lawnmower... but then I would miss all this fun

After completely eliminating all other possibilities:

1. The carb has been replaced... as well as the gas line and the fuel pump. I suspect regardless of everything else the carb had issues because it runs MUCH better at idle now.

2. Replaced both head gaskets and adjusted the valves. I do need to verify that the heads are not warped. Thanks for the glass tip.

3. Did a leakdown test after replacing the head gaskets and saw about 6% loss. Seems good to me.

After all this the mower runs poorly at full throttle. Hunts and pops out the breather valve. White smoke in the crank case. Runs even worse now under load and often stalls out. Must be blow-by via the piston(s).

Remember... I originally replaced a piston and both connecting rods.

* The piston I bought is a kit with the rings: part #793647
* I did not hone the cylinder walls... but I have the tool now.
* I did not take any great care in staggering the ring gaps previously. In fact, I have re-opened the crankcase and now see that while they were not 100% aligned... the gaps were fairly close (which is, I assume not great).

Plan now is to hone the cylinder walls, replace the rings, verify the heads are not warped... and hope for the best.

Question: The new piston is certainly not exactly the same as the old one. The rings are different as well. The part I am confused by is if I buy JUST the piston rings they have 2 versions. Before 06082400 and after 06082300. My ENGINE code date starts with 05 so to replace the old piston rings on the old piston I'm going with part #499604. Question is... what rings do I use for the NEW piston? Are the rings specific to the ENGINE or the PISTON? The piston is new (793647)... and the rings appear very different to me whe compared to the old rings... particularly the oil ring.

 

[slomo]

Question: The new piston is certainly not exactly the same as the old one. The rings are different as well. The part I am confused by is if I buy JUST the piston rings they have 2 versions. Before 06082400 and after 06082300. My ENGINE code date starts with 05 so to replace the old piston rings on the old piston I'm going with part #499604. Question is... what rings do I use for the NEW piston? Are the rings specific to the ENGINE or the PISTON? The piston is new (793647)... and the rings appear very different to me whe compared to the old rings... particularly the oil ring.

I would say if you have the correct parts diagram, ordered the correct piston and rings per engine date code, should work okay. Obviously using OEM parts and NOT the cheapest OEM parts off ebay or amazon. Last deal was about "trying" to get around Chinese copied parts if possible with OEM box printing LOL. Buy your parts from a real mower shop or an online mower shop. Do this one time and could actually save you money.

 

[Richard Milhous]

"Torque to factory specs"... urrrhh... some years ago I had a torque wrench that didn't seem "quite right". I got a new one.

I had been torquing critical suspension and steering bolts to a third of spec. On highway cars, not lawnmowers. FK ME.

Never trust anyone.

 

[bostonbear]

Here's the update:

1. Question about what piston rings to use if you had a NEW piston on an engine with an OLD engine code. The answer is, the piston rings have BEFORE and AFTER codes. These codes refer to the engine code... but if you got the NEW piston you want the NEW piston rings (regardless of your engine code). The old rings flat out don't fit in a new piston. Note that the new pistons come with rings so this is normally not an issue. I'm not normal. So... the piston ring parts diagram BEFORE and AFTER really refers to the piston... not the engine.

2. The issue I was having was almost certainly blow by. The symptoms were: hesitation when going from idle to full throttle (followed by a big burp of smoke out the valve), white smoke in the crankcase, crank valve "popping" every 5-10 seconds at full throttle, eventual stalls but normally lots of hunting at full throttle (fine at idle).

I do not believe the blow by was caused by the head gasket(s) and I replaced the head gaskets multiple times and saw no evidence of a leak.

Took the engine apart (again) and did not find anything really wrong... but... when I first replaced the broken control arm and piston and put the engine back together:

• I did not take care to stagger the ring gaps.
• I did not hone the cylinders.

This time I did both of those things. During my original repair I also replaced the connecting rod on the "good" cylinder since I though it was a little damaged. When I did that I mistakenly "flipped" that piston 180 degrees. The piston looked to me to be symmetrical. The manual was very clear about the arrow on the piston head facing the fly wheel so I fixed that as well this time around. Note that, indeed, the pistons are NOT 100% symmetrical. The bottom side has a slight difference on one edge. Didn't notice it before. Not that there is also a "2" on one side of the piston. I assume that means it should align with the "2" on the connecting rod.

So... new rings all around (gaps properly staggered), put the old piston back to the correct orientation, honed both cylinders... and it runs. Fairly well. No more smoke in the crank case. No more hesitation when going full throttle. No more hunting or stalling. It's still making a noise I can't quite figure out. Might just be valves clacking... but waaaaay better than before.

 

[bostonbear]

Bummer. I strongly suspect that noise I'm hearing is rod knock. It would make sense since the original repair was due to low oil. I suspect the crankshaft is worn so there is a small amount of play. It runs and it cuts... but how much life does it have? Not sure. Quite a learning experience for me.

 

[Richard Milhous]

If the noise gets worse when the engine is hot and is reduced by using heavy oil additives, it is rod knock. Don't leave heavy oil additives in your crankcase unless you plan to just beat it to death.

 

[bertsmobile1]

Rods are cheap
SHort blocks are not
Your decision.
BEcause I charge for my time & the customer pays for parts rod replacement in your situation would have been a no brainer.
However the home owner never takes any account of their time because the time is not coming out of their pocket but the parts are so the temptation is to replace the minimal amount of parts rather than think about replacing everything that may have been damaged .

 

[bostonbear]

Rods are cheap
SHort blocks are not
Your decision.
BEcause I charge for my time & the customer pays for parts rod replacement in your situation would have been a no brainer.
However the home owner never takes any account of their time because the time is not coming out of their pocket but the parts are so the temptation is to replace the minimal amount of parts rather than think about replacing everything that may have been damaged .

Doesn't really have anything to do with time or $$. Has more to do with I have no experience with small engines and didn't think... "hmmm.... those wear marks on the journal are probably bad news." I'm doing this to learn so I will probably replace the crankshaft in the winter... Once I get it right... next project is a motorcycle.

 

[bertsmobile1]

I got into mower the opposite way round.
Motorcycles are a lot easier