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Here is a good one you guys to figure out.

#1

StarTech

StarTech

33R877-0002-G1 2014.

Would not idle below 2500; unless, when it did it was backfiring through the carburetor so bad that I actually seen the flywheel to kick back. This had at time kill the engine. I have since repaired the problem and it now idles at 1750 without a problem.

Another shop sold this mower is this condition and even put on a Chinese clone carburetor trying to resolver the problem. He claimed he repaired the problem but it definitely was never fixed. This shop is an insult to us true mechanics and he actually ripoff the customer as I got to bill for my work even the customer had already paid out $800 for the used Craftsman 46" cut mower.


#2

S

slomo

Ignition timing, cam timing way off, mower hit a log chain or a stump. Check the flywheel key and blades.

Side note, don't get into a habbit of idling down a mower engine. Two things, poor battery charging and poor oil lubrication.

slomo


#3

StarTech

StarTech

slomo said:
Ignition timing, cam timing way off, mower hit a log chain or a stump. Check the flywheel key and blades.
slomo
Click to expand...
Nope, nope, nope, and nope. And the steel flywheel key is fine. And the deck is not engaged so that not in the picture.


#4

R

Rivets

Coil installed upside down, or the wrong coil or improper gap?


#5

StarTech

StarTech

Rivets said:
Coil installed upside down, or the wrong coil or improper gap?
Click to expand...
Nope. Although I did need to set the gap twice my normal gap setting due to keep it from rubbing on the flywheel. But that did not affect the problem; otherwords, it didn't improve or make it worst.

A real brain teaser. What some hints from the troubleshooting I did? The problem was there with the engine both unloaded and under loaded.


#6

B

bertsmobile1

Well this is starting to sound like the wrong flywheel was on the engine if you needed to double to armature gap .
The one & only time I have seen that was a no brand Chinese Honda clone .


#7

Scrubcadet10

Scrubcadet10

Faulty spark plug?


#8

StarTech

StarTech

bertsmobile1 said:
Well this is starting to sound like the wrong flywheel was on the engine if you needed to double to armature gap .
The one & only time I have seen that was a no brand Chinese Honda clone .
Click to expand...
Nope, The gap is now set at two repair tag card stock thickness. Now at about the max specd gap. Normally I set them at one stock thickness.

Everything is original from 2014, except the carburetor which was changed by the other shop because of this problem around Christmas 2020 because it was Nikki carburetor. Something I don't understand is why everyone hates these easily fixed carburetors. It was just needing cleaning which is why I initially had the customer return the mower to the other shop for repairs under warranty. (hint of root cause)

Why the shop returned the mower in worst condition than before just tells me the mechanic didn't know what he was doing and was just guessing at the problem.

I can say everyone so far is being mislead down the wrong path. It was not a mechanical or electrical timing issue but it was a serious tuning problem. (Another hint)

It was not a blown head gasket (leak down only 20% which is great for 7 year old Briggs) or a valve clearance problem either. The valves were out of adjustment a little but after adjustment nothing changed.

I actually fixed the problem without using any additional parts just a modification. Now I just got work on the deck vibration problem but I suspect that is just out balance blades.


#9

StarTech

StarTech

Scrubcadet10 said:
Faulty spark plug?
Click to expand...
Nope. New and tested was installed from a working engine. Done to just check it off my diagnostic list.


#10

Fish

Fish

Bad cam?


#11

StarTech

StarTech

Fish said:
Bad cam?
Click to expand...
Not that one either. Good guess though as I replaced two 793880 camshafts last year for bad exhaust lobes but the backfiring was constant and not as random this one was.

As I said (typed) I used no additional replacement parts to make the repairs.

I only pull the rocker cover to do valve adjustment check and for the leak down test.

It sounds as if I had recorded and posted the engine running it would have even confused you guys even more but I have no means of doing it here.


#12

R

Rivets

You say you couldn’t idle below 2500 RPMs and it isn’t a mechanical or ignition problem. This leaves fuel. I’m guessing the idle mixture screw was turned out too far or missing and possibly found idle speed screw in too far. This would produce an extremely rich mixture, plus intermittent flooding, causing back firing and if the flooding occurred at the right time would kill the engine.


#13

StarTech

StarTech

Rivets said:
You say you couldn’t idle below 2500 RPMs and it isn’t a mechanical or ignition problem. This leaves fuel. I’m guessing the idle mixture screw was turned out too far or missing and possibly found idle speed screw in too far. This would produce an extremely rich mixture, plus intermittent flooding, causing back firing and if the flooding occurred at the right time would kill the engine.
Click to expand...
You now on the right path but it was not a rich burn condition but was lean burn issue. Adjusting the idle mixture screw had no effect; hence, the modification. It was pre-ignition cause by burning way too lean; otherwords, the fuel load was igniting from heat and not spark [reverse dieseling effect]. Applying 3/4 choke at all speeds lead to the lean burn issue.


#14

Scrubcadet10

Scrubcadet10

I think i need a new thinking cap...


#15

R

Rivets

Now I’m thinking you punched the main jet couple thousands over.


#16

S

slomo

StarTech said:
You now on the right path but it was not a rich burn condition but was lean burn issue. Adjusting the idle mixture screw had no effect; hence, the modification. It was pre-ignition cause by burning way too lean; otherwords, the fuel load was igniting from heat and not spark [reverse dieseling effect]. Applying 3/4 choke at all speeds lead to the lean burn issue.
Click to expand...
Applying 3/4 choke will lead to a rich run not lean (if carb is operating proper). Where was the fuel igniting at prior to the cylinder causing it to go lean? Fuel/air path is short on these engines. Intake manifold? Plugged air filter pulling excess air from block vent hose?

Also was thinking gov issue.

slomo


#17

S

slomo

Rivets said:
Now I’m thinking you punched the main jet couple thousands over.
Click to expand...
Like the guys that use torch tip cleaners on soft brass jets LOL.

slomo


#18

StarTech

StarTech

I have been doing repairs for 40+yrs and know better than to use those torch tip cleaners except on my cutting torch tips. Matter of fact I haven't used carburetor cleaner since 2013 to clean these carburetors. I actually gave away my last dip can of cleaner back in 2014.

Yes it was pre-ignition. For it to be back firing it had to be at the intake valve head. Couldn't have been the air filter as it and the air intake manifold was removed, plus the vent was discoonected due the manifold being removed. The carb to cylinder was check for both cracks and warpage. I find the restrictor plate was missing, which I replaced.

It took enlarging the jet involved by 0.0381mm [.0015"] on the new Chinese crap carburetor. Could stand to go another 0.0127mm [.0005"] but there is no bit in that size available The other shop threw away a perfectly good Nikki carburetor because they were too lazy to clean it.


#19

StarTech

StarTech

Rivets said:
Now I’m thinking you punched the main jet couple thousands over.
Click to expand...
And no it wasn't the main jet. Remember these engines run off the idle mixture ports when off load and also run off it under light loads.

Btw going from 1.15mm [.04527"] to 1.18mm [.04646"] on the main didn't help anyway under the light deck load so put back the 1.15mm jet.


#20

J

jgiljr

Plugged-up spark arrestor?


#21

O

olie01

StarTech said:
33R877-0002-G1 2014.

Would not idle below 2500; unless, when it did it was backfiring through the carburetor so bad that I actually seen the flywheel to kick back. This had at time kill the engine. I have since repaired the problem and it now idles at 1750 without a problem.

Another shop sold this mower is this condition and even put on a Chinese clone carburetor trying to resolver the problem. He claimed he repaired the problem but it definitely was never fixed. This shop is an insult to us true mechanics and he actually ripoff the customer as I got to bill for my work even the customer had already paid out $800 for the used Craftsman 46" cut mower.
Click to expand...


#22

O

olie01

The internal governor is broken. when it goes out the engine only runs at high speed or what ever the governor was set at.


#23

StarTech

StarTech

olie01 said:
The internal governor is broken. when it goes out the engine only runs at high speed or what ever the governor was set at.
Click to expand...
Have you even bother to read the full thread? It was a pre-ignition problem.


#24

W

woodysand

Probably not your problem but if the gap has to be set at twice the normal to prevent rubbing then the flywheel is a little out of round or the crank is slightly bent


#25

O

OrtisEvans

Interesting! Help me out: what is a carb restrictor for? Also, are many of the replacement carbs that bad? I have had not had the pleasure of needing one. My newest small, 4-cycle engine is more than 20 years old and I have never had to replace a carb on one.


#26

E

enigma-2

Misadjusted idle adjustment, i.e. screwed all the way in and not initially backed out to the preset level (at a minimum).


#27

StarTech

StarTech

OrtisEvans said:
Interesting! Help me out: what is a carb restrictor for? Also, are many of the replacement carbs that bad? I have had not had the pleasure of needing one. My newest small, 4-cycle engine is more than 20 years old and I have never had to replace a carb on one.
Click to expand...
IT a way to limit HP. Even heard of restrictor plate auto racing? Those engines are limited to a certain no matter how the mechanic tune the engines.


#28

StarTech

StarTech

enigma-2 said:
Misadjusted idle adjustment, i.e. screwed all the way in and not initially backed out to the preset level (at a minimum).
Click to expand...
Come on folks please read the thread completely before posting as this was ruled out. All the initial guesses were way off base too. I am just going shake my head.

The idle mixture of the carburetor had effect except to kill the engine when fully screw in. Even the idle speed screw only sped up or slowed down the engine. There was a restriction in the idle circuit called an idle jet that was way under sized by Chinese OEM as they were just selling a will fit carburetor.

Anyway the customer came by yesterday and was very happy with the way the engine is now running when compared to a week ago. Just got though fixing a factory defect on the deck and got to a set blades for a 46" deck since I am currently out of them. Probably will buy a couple dozen.


#29

S

sallyphilip

StarTech said:
33R877-0002-G1 2014.

Would not idle below 2500; unless, when it did it was backfiring through the carburetor so bad that I actually seen the flywheel to kick back. This had at time kill the engine. I have since repaired the problem and it now idles at 1750 without a problem.

Another shop sold this mower is this condition and even put on a Chinese clone carburetor trying to resolver the problem. He claimed he repaired the problem but it definitely was never fixed. This shop is an insult to us true mechanics and he actually ripoff the customer as I got to bill for my work even the customer had already paid out $800 for the used Craftsman 46" cut mower.
Click to expand...
out of time


#30

L

LarryO

This guy is really an expert.


#31

R

Rivets

See what you started Star, a never solved thread.


#32

T

TT398

Well, you stumped a lot of folks. Stop the tease. Reveal the secret fix. :)


#33

S

slomo

Man that Slick 50 really works. Won't idle below 2500rpm.... Extra 16hp, having to run a NASCAR restrictor plate.



Slick50ClassicET_720x.jpg


#34

StarTech

StarTech

TT398 said:
Well, you stumped a lot of folks. Stop the tease. Reveal the secret fix. :)
Click to expand...
Well it was revealed to those know had do proper carburetor modifications. It down to a factory induced restriction in the metering of the fuel causing the engine to run extremely lean; therefore, the fuel load was igniting premature (pre-ignition). If I post the the complete details there are DIYers out there that will trying it on every carburetor screwing many of them up using these micro drill bits For them it would be better just to install the correct OEM carburetor. I do get tired of hearing that Nikki carburetor is one of the hardest carburetor to repair when it is one of the easiest.

slomo said:
Man that Slick 50 really works. Won't idle below 2500rpm.... Extra 16hp, having to run a NASCAR restrictor plate.
Click to expand...
Now you are only trying to make fun of the problem.

Maybe I should never posted anything about this for the real mechanics to learn from such a problem. Yes it hard not to get distracted and keep going the same old worn path. Having an open mind when approaching is key to troubleshooting.

Rivets said:
See what you started Star, a never solved thread.
Click to expand...
Oh a suggested solution was finally got very close to heart of the problem. Post #12. For mechanics that know what they doing they know what I did when I resized a under sized jet.

The problem here I see is no one fully read the thread. Without reading everything it like troubleshooting with only half a mind working. You may not ever see that red herring as it is on another path.

It like the Husqvarna YTH2348 steering tie rod replacement I did last week. If I just keep trying to get a wrench on the backside of the sector gear I would not have realize they Husqvarna redesigned the area. What was once a 30 minute job is now a 1-1/2 hour job as the deck has to come off and the steering completely disassemble just because put a plastic cap bushing over the nut. So glad I brought the mower into the shop as it took a large impact to disassemble the sector gear assembly just to get that cap off. As always you get a face full of dirt when working under a mower.


#35

C

CaptFerd

Really confusing thread about what the initial problem was compared to the actual repair. So by what you are saying is that the sole problem is the missing restrictor plate and that the the nikki carb was good if they had cleaned and repaired it properly. China knockoffs need to be drilled and redesigned to work on this engine even before and after the missing part is replaced. I think that it would have been a better thread to discuss this issue than to make it a guessing game on what was actually wrong. Would love to hear more about the StarTech Improved Chinese Nikki knockoff. This would have to be a better choice for a paying customer than the original part coming from a top quality 40 year experience lawn mower mechanic.


#36

StarTech

StarTech

The whole point of the thread was to make it into a training exercise. It was to get fellow techs to learn to think outside the box; otherwords, don't get so focus that you miss the real problem. We must be able to troubleshoot without just throwing parts and hope we hit on the solution by guessing.

This guessing just what many shops in my area are doing ending up costing the customer more as they charge for unnecessary parts. And forget if they need to troubleshoot an electrical problem.

The mower in this thread was sold by a local repair shop. It was in a mess from the get go. My customer did make the mistake of buying the mower and should have return it when the very first lie was told about it. He was told it had a Kohler engine and it had a Briggs when I raised the hood. It was acting up so bad I had the customer return it for the other shop to straight it out. The shop put on this junk clone carburetor and replaced the deck. Neither of which solved the problems I had the customer return the mower for.

That just the tip of the iceberg. After I got the engine problem resolve, the mower had three flat tires, dry rotted. Mower deck shakes badly. Bent blades, bad idler pulley, both spindle pulleys are bent, and the blade brakes are working. When you have brought a used rider for $800 and then having to spend another $300 on it just to get it to an usable state is not fun or right.

Anyway I get the mower out this coming week after the deck parts get here. Didn't have the blade brakes in stock.


#37

Fish

Fish

I think that the thread is a good indicator that we need to go outside and enjoy the warmer weather.


#38

StarTech

StarTech

Fish said:
I think that the thread is a good indicator that we need to go outside and enjoy the warmer weather.
Click to expand...
Yes, It is Spring cleaning time and a good time to go fishing. I'm off to nursery to check out the new perennials for my yard. And stop stop by a place to see if I can get a couple daffodil bulbs of one I don't have.


#39

Fish

Fish

I am needing to buy a new garden tiller, which is proving to be very difficult.


#40

StarTech

StarTech

Yelp a lot of the new ones are just junk that why I keep my TB Horse in good repair.


#41

M

mystic240

Vacuum leak?
I had a neighbor a couple of years ago that put high alcohol gas in her mower. It would run but needed to be choked to run.


#42

Scrubcadet10

Scrubcadet10



#43

Fish

Fish

StarTech said:
Yelp a lot of the new ones are just junk that why I keep my TB Horse in good repair.
Click to expand...
Everyone is sold out


#44

VRR.DYNDNS>BIZ

VRR.DYNDNS>BIZ

not enough data. 1) governed idle, 2) decompression system, 3) governor links/throttle links/governor spring or setting, 4) static governor adjustment, 5) idle stop screw, 6) carb adjustment or replace original carb, 7) combination of any of the above.


#45

StarTech

StarTech

VRR.DYNDNS>BIZ said:
not enough data. 1) governed idle, 2) decompression system, 3) governor links/throttle links/governor spring or setting, 4) static governor adjustment, 5) idle stop screw, 6) carb adjustment or replace original carb, 7) combination of any of the above.
Click to expand...
  1. No governed idle. Please refer to the engine IPL.
  2. Yes, It was working correctly. Of I check it and adjusted the valves since it is the 793880 camshaft. I also did a leak down test while I had the rocker cover off. Rate 20%.
  3. All were as intended.
  4. It was fine and I never touched it before or during the final repair solution.
  5. That why it would even idle at 2500 rpm. Dropping the idle stop below that would cause the engine finally die after heavily backfiring, coughing, and surging.
  6. Original carburetor was replaced and mixture adjustment only killed the when fully seated. The new was even worst than original which just needed cleaning but the other shop replace the carburetor because they were too lazy to clean the Nikki.
The main problem was caused by the lousy tech that so called repaired it in the first place. It should have never left their shop in the condition that I saw when it came into my shop.


#46

S

slomo

Got to be the spark plug for sure. :p

slomo


#47

Scrubcadet10

Scrubcadet10

slomo said:
Got to be the spark plug for sure. :p

slomo
Click to expand...
Nah , i bet it is the drive belt.


#48

B

Brian - Modern Mower

Fish said:
Bad cam?
Click to expand...

Thats my guess - wiped exhaust lobe.


#49

B

Brian - Modern Mower

bad ground at the coil.


#50

Scrubcadet10

Scrubcadet10

Bad reading comprehension


#51

D

DELTASCIENCES

StarTech said:
33R877-0002-G1 2014.

Would not idle below 2500; unless, when it did it was backfiring through the carburetor so bad that I actually seen the flywheel to kick back. This had at time kill the engine. I have since repaired the problem and it now idles at 1750 without a problem.

Another shop sold this mower is this condition and even put on a Chinese clone carburetor trying to resolver the problem. He claimed he repaired the problem but it definitely was never fixed. This shop is an insult to us true mechanics and he actually ripoff the customer as I got to bill for my work even the customer had already paid out $800 for the used Craftsman 46" cut mower.
Click to expand...
carburator problems are usually the result of pooooor gas management. using that 5 gal can that you have had foe 10 years. the bottom of can is full of crap. mechanic cleans carb customer puts more crap in from dirty can. you cant fix stupid


#52

StarTech

StarTech

DELTASCIENCES said:
carburator problems are usually the result of pooooor gas management. using that 5 gal can that you have had foe 10 years. the bottom of can is full of crap. mechanic cleans carb customer puts more crap in from dirty can. you cant fix stupid
Click to expand...
Well the carburetor problem was not caused by bad fuel as the fuel was purchased the same day using a clean can. It was a flawed carburetor that I had to modify to get to work correctly.

Btw way I definitely know not use a dirty can or old fuel. Forty plus years of experience here. So please don't assume things... So what that my fuel cans are 20 years old; as long they are clean and seal tight they are still good. Plus they are probably better made than the current fuel cans on the market in places like Tramlaw.


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