Johnson TN-26 / TN-27 backfiring after carb rebuild

brockj

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Jul 9, 2008
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I recently picked up both a TN-26 and TN-27, and after initial cleanup, tore apart the carbs and cleaned them real well. Now after reassembling everything both engines are doing the exact same thing. I can get them started, but they are backfiring now and as such will not stay running longer than about 30 seconds. I went through the ignition systems (cleaned and adjusted points) and they both look good. I am fairly certain it was something I did with the carb rebuild as I was able to get the one running for short periods of time (10-15 seconds) before it would act like it ran out of gas, but it never backfired. I am not a complete novice with these carbs as I have rebuilt a couple HD-25's which are similar. Any thoughts or suggestions??
 

Chinewalker

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Re: Johnson TN-26 / TN-27 backfiring after carb rebuild

Are you sure it's a backfire and not a lean sneeze? The lean sneeze on those pops out the hole in the crankcase near the tip of the low speed needle valve...
 

wilde1j

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Re: Johnson TN-26 / TN-27 backfiring after carb rebuild

Did you have the flywheels off? Key might be sheared, especially if the flywheel nut wasn't properly torqued. Backfiring is almost always due to timing being whacked, which a sheared key would cause. BTW, you only get a few tries to get it right before the flywheel and crank become junk.
 

brockj

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Re: Johnson TN-26 / TN-27 backfiring after carb rebuild

I did have the flywheel off, and the key is not sheared. It could possibly be a lean sneeze, I know you can see something shooting out of the front (the same hole that squirts when you prime), I also notice it around the carb as well.
 

brockj

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Re: Johnson TN-26 / TN-27 backfiring after carb rebuild

So I just did some quick searches on sneezing/lean sneeze and it is very possible that is the problem. One post I found referenced enrichening the low idle, any other suggestions or places to look?
 

kfa4303

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Re: Johnson TN-26 / TN-27 backfiring after carb rebuild

Hi brockj. Adjusting the lo speed needle should fix the lean sneeze. Backing the needle out makes a richer mix and lowers the rpms at idle, while turning it in leans the mix and raises the rpms at idle. You'll want to lightly seat the needle, then back it out about 1 1/2 turns as a rough starting point, then tweak it from there. Here's a great link for your files that explains dialing carbs of all sorts including yours. Give it a quick read and see if it helps.


http://forums.iboats.com/showthread.php?t=167352
 

brockj

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Jul 9, 2008
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Re: Johnson TN-26 / TN-27 backfiring after carb rebuild

Got to spend a little time with each one last night. I was slightly off as it was my TD-20 and TN-26 that were sneezing, the TN-27 still needs the recoil rebuilt...

Anyway, I got the TN-26 running pretty darn good last night. I was going based off the label on the front that (still quite legibly) reads 3/4 turn out for each. Moved it closer to 1.5 turns out and sneezing stopped! Motor runs great, hope to throw it on the duck boat this coming weekend for the early season goose hunt and see how she handles running full bore across the lake!

Unfortunately the TD is still having the same issue and turned it out a ways. Any other thoughts on where to look? I did replace the gasket that carb to the intake, do you think that could cause the sneezing.

Thanks again!
 

Chinewalker

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8,902
Re: Johnson TN-26 / TN-27 backfiring after carb rebuild

I have seen some of those TD/TN motors that get wear at the center main bearing. The center main on those acts as a rotary valve of sorts for the low speed circuit as it draws fuel and air through that small hole. If the bronze bearing wears it loses its seal and can draw more air and less fuel - resulting in lean condition and inability to adjust it away. Bottom line, it's possible your motor is simply showing its age...
 

brockj

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Jul 9, 2008
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Re: Johnson TN-26 / TN-27 backfiring after carb rebuild

Well that motor definitely has a bit more wear on it than the TN-26. I will play with it a bit more when I get a chance, heck, that's half the fun with these old guys! Thanks again for the help!!
 
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