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Discussion Starter #1
Hello,
I have a ‘98 gt275 that i Just finished rebuilding the engine on due to a blown rod. I think the rod blew due to overheating because of the cooling fan being clogged with cotton wood tree fuzz. My routine has been to check the oil before mowing so the next time I mowed I noticed the air intake block with the cotton debris so the mower may have run over a hal hour with little or no air flow without me knowing it. I blew out all the blockage but next mow it blew. (Also bent the flywheel key, which of course I replaced)
Always started right up and ran great .. well cared for machine.
Anyway, after mounting the rebuilt motor, I turned it over with no gas hooked up so I can get the oil circulating first. Sounds fine turning over.
Hooked up the gas and tried starting it, tried to start but backfires through the exhaust and carb. The new ngk plug appears wet & sooty. Tried another plug, same problem. Spark looks fine.
I did spray carb cleaner into the carburetor when it was on the bench for cleaning but did not take it apart as I never had fuel issues with it. Maybe some junk came loose and causing a blockage?
To me, it seems like a fuel issue but am not familiar with these carbs.

any ideas?

thanks for any kind help.
michael
 

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Backfiring through the carb sounds more like a timing or valve lash issue. Any chance you accidentally flipped the magneto or didn't get the valve lash set correct?

I'm assuming that you had the timing marks on the crank and cam correctly orientated.
 

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Discussion Starter #3
Lapped the valves and did the adjustment, 0.006, per the manual. Rechecked the valve adjustment after I turned the engine over.
the magneto orientation looks correct and is dictated by the input and output wires routing.
I was very careful setting the timing marks on the cam & crankshaft. Even rechecked it before torquing the crankcase cover.
I agree with you, it does exhibit a timing type issue but I’m pretty confident it went together ok.
 

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Not to beat a dead horse or question your abilities, but I suggest rereading the valve lash procedure for your engine, and if you have any concerns about the process repeat it. I believe your engine has a compression release and the valve lash might not be set at TDC.

With that out of the way, maybe try a little gas directly in the carb and give it another try.
 

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Starting fluid is a little easier to control, some people don't like it and say it wears out your engine. I think a little probably won't hurt. It will indicate if gas is the problem or not.
 

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Discussion Starter #7
Ok, I’ll check I’m at tdc on the valve adjustment.
I did try starting fluid in the cylinder
 

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Discussion Starter #8
Re-checked valve adjustment per manual, .006”
still backfires through exhaust and carb.
sounds like timing but it’s a fixed setting Between crankshaft and cam. Same with mag.

any ideas or am I into pulling the engine and remove the crankcase cover ?
 

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Does it run at all, of just backfire?
It really sounds like the timing is off, possibly way off. I don't know if there are timing marks that could work with a timing light to see where the timing really is? That seems easier to check than pulling the crankcase cover and verifying that the timing marks align with TDC (or whatever the manual says) for the correct cylinder. Perhaps you aligned it to the wrong cylinder? That would explain they symptoms you are seeing.
 

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Do you have access to a compression gauge? I would test the compression before pulling the engine.

Also, you stated that you threw a rod, so new rod, piston, crank? Anything that could change the timing or be different from the original? Such as cam, crank, flywheel?

To your earlier post, the cam timing is pretty foolproof unless something is different from the OEM.

Here is what I would do at this point:
  1. Compression test (make sure the throttle plate is forced wide open and choke is open) - due to the compression release, I think you are only expecting 70 psi.
  2. Take the nut off the flywheel and make sure the keyways are aligned.
  3. Timing test - if you have a timing light use that - I know I got rid of mine years ago. If not mark the flywheel at TDC to a reference point and see if the spark is close. You should be able to rotate the flywheel by hand quickly enough to test it with the plug out.
  4. If you replaced the cam, crank and/or flywheel, see if the alignment marks are the same.
Hopefully something there will be off. Maybe someone else has better suggestions.

Best
 

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pull the bolt/nut on the flywheel and double check that the key didnt slip out under the flywheel on installation... i have had that happen more than once....

also, try a different spark plug. i had a GT275 come thru the shop that would not run, backfire, etc, turned out the spark plug was at fault. showed good spark on the plug, showed good spark on a spark tester, but would not run the machine.
 
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Discussion Starter #12
When the rod blew, it tore a chunk out of the bottom of the cylinder and bent the cam so I bought a block, piston, rod & cam from a guy. The crankshaft, head and valves are the originals. The piston is much different than the original, longer skirt and thicker rings but were original to the block I bought.
the code on my engine was fs15, don’t know the code of the purchased block.
I tried a new plug also.
inspecting the flywheel key has merit. I’ll give it a try.
 

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I skipped ahead again, so if this was brought up, sorry. I have a 265 which I think is the same engine. Mine was running fine one day, next it was just backfiring through the carb. Called my JD buddy and asked if they could skip time like a car. He said, No, its your electronic module. Then he said don't get a JD one they are expensive. I asked if a Nova II would work. He said yes. So, for 12 bucks I put a Nova II on it, fired right up and ran great for another 10 years and that module went. That was last summer. This is the 265 with over 3000 hours on it. Did you put a new module on the new engine?
 

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Discussion Starter #15
DING DING DING !!! Yes Folks, we have a winner!
Replies concerning the timing/flywheel key nailed it. The fly wheel key is broken but I have absolutely no idea why. Got it from a mower repair shop, had the correct part number on the bag.
How can it break ?! Anyone ever experience this?
I’ll try to find one tomorrow (it was the last one the mower shop had) and report back.

thanks everyone.
 

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The fly wheel key is broken but I have absolutely no idea why.
I have heard that if you don't torque the flywheel bolt/nut correctly, i.e. if it is loose it will shear the key. The key is only for initial alignment. The torque of the bolt and the pitch of the flywheel/crankshaft are what keep them in alignment.
 

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I have heard that if you don't torque the flywheel bolt/nut correctly, i.e. if it is loose it will shear the key.
Congrats - I think Frogmore likely nailed the why.
 

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Discussion Starter #18
Ahhh, Frogmore, you da man! I just tightened the bolt, not paying any attention to a particular torque setting. Didn’t realize that was so critical.
I will definitely check the required torque when I RE-install the flywheel.
thanks very much !
 

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as a heads up, per Deere, the torque spec for the flywheel bolt is 127 ft lbs, i always make sure to wipe down both the bore and the crank with a degreaser/solvent like brake cleaner to ensure theres a completely dry fit, with no grease or oil that could allow the flywheel to slip on the taper.
 

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Discussion Starter #20
as a heads up, per Deere, the torque spec for the flywheel bolt is 127 ft lbs, i always make sure to wipe down both the bore and the crank with a degreaser/solvent like brake cleaner to ensure theres a completely dry fit, with no grease or oil that could allow the flywheel to slip on the taper.
Great information. Thanks very much. I’ll do that.
 
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