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8n carb mystery

5156 Views 11 Replies 8 Participants Last post by  JohnW
i just got this 8n and the previous owner couldnt figure this carb problem either. he rebuilt the carb including the carb dip cleaning. it will only run with the main jet removed.....or the choke pulled. i looked at it and thought it was a simple plugged passage. i removed the bowl and found a bit of soft rusty mush in the bottom (there is the problem..."im so smart") cleaned it and put it back on. still not fixed... took off the carb and took it apart and cleaned all the passages in the top half and recleaned the bottom half (i must have missed something "hard to believe since im so smart...") put it back together and put it on.... still the same... checked fuel flow and it was good. looked in the manual found the screen at the carb.checked it and cleaned it (that has to be it it looked a little plugged) put it back in....still not right ("maybe im not so smart")

the needle looks corroded so today im going for a complete kit and try again.

so here i am...humbled....asking for ideas ( when you dont know...ask)

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elair, the choke pulled means less air going in, which effectively enriches the air mixture. This means either too much air (vacuum leak)...

Or since these carbs adjustment screws are actually fuel screws...opening them allows more fuel in...removing the main screw means you are supplying more fuel.

So its very possible from your description there is not enough fuel available during running conditions thus you must modify the supply of air/fuel to get it to run.

So...I'd look at the float level, needle and seat, fuel flow to the float bowl, etc.

If its only getting a little fuel in there, choking it or removing the screw but no choke levels out the air/fuel ratio.

Another possibility is a major vacuum leak. Like a warped or loose intake. And one last thought, how is the compression?

Keep us posted, this sounds like one of those weird and rare problems we can file away the answer for whenever the next time it pops up!
i havent checked the compression but i should and probably after reassembling the whole tractor from a complete strip down and paint, and full carb kit it wont fire at all....not even on starting fluid. it has spark on the 2 plugs i checked, but it looks weak. i took off the distributer cap and found the rotor has been rubbing on the cap. the shaft is tight (no play in it) and one peice of the cap is broken off. im going to try cap and rotor and come back to this problem. possibly related.....
A crack in the carb someplace????? :dunno:
Im hav ing the same issue with my 8n. In my case Im working with a fresh engine, and have checked for intake leaks, and fuel flow. After 4 or so rebuilds, its time to dump the carb... Im waiting till spring to get a new carb on her. cap, rotor, points, condenser, and plugs and she purrs like a kitten. complete carb kit install was not the answer. thanks for all your advice!
Glad it was an easy and affordable fix.

Been said many times, the majority of carb/fuel problems are electrical.
I agree carbs take alot of the blame
that they dob't deserve.
i had a problem that drove me nuts for a while, i rebuilt my carb (saved all the original carb parts) i cleaned the carb, put all new filters on, cleaned the tank.

she just would not start good, after she warmed up she was ok, she got to the point she would even start! she would just flood out. so u put in a new float, and made sure it was set correct. i found nothing wrong with the original float. replaced it also any way.
still wouldn't run!

what i did was the design of the original needle and seat was slightly different then the one from the rebuilt kit.
so i put the origanal needle and seat in. she she starts right up!!! never had a problem since! i'm still not sure why? but that was my issue apprently
i read about one with the same problem and he had a warped casting causing a air leak. im glad mine wasnt that bad but next time i may consider a new carb over a deluxe kit. new carb $100 and done or deluxe kit for $47 plus tax (so over 50) and risk damaging a jet or something else....not to mention the carb dip cleaner is expensive. i do like the feeling of fixing it myself. anyone have any opinion on this? is it worth the repairing the old or is the new one better? thanks for all your input and help!
I had to replace one once upon a time because of broken castings, it 3 new ones to get one that actually would run anywhere near like normal. The first 2 leaked like a screen door and would not idle after I got them to stop running my gas out on the ground. IF there is a next time I will find someone with an old carb they wish to get ride of and rebuild it myself. Just my $0.02.
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