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8N Carburetor Nightmare

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8ntheo

04-07-2008 17:41:33




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I have a 49 8N that I rebuilt 6 months ago. Ever since the rebuild the engine runs best if I have the idle adjustment screw tightened all the way in and the main adjustment needle about 2 1/2 turns out. Even then it still sputters and runs quite rich. I've had to get new spark plugs since they were fouled. It runs pretty good under load but idles rough and runs rich. I've gone through everything that I can think of, any suggestions?

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counterclockwise

04-07-2008 19:25:47




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 Re: 8N Carburetor Nightmare in reply to 8ntheo, 04-07-2008 17:41:33  
Had the exact same problem with my 8N. I aquired the tractor not running and after rebuilding the engine it ran exactly like yours. After much head scratching and carb work I finally discovered that the throttle plate was upside down. I had been reassembling it just like it was when I took it apart and it was assembled wrong before I got the tractor. Might not be your problem but it's worth checking.

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ZANE

04-07-2008 18:47:59




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 Re: 8N Carburetor Nightmare in reply to 8ntheo, 04-07-2008 17:41:33  
CARBURETOR WON’T IDLE

First do a thorough cleaning of the carburetor and replace the gasket between the two halves. Be sure that the float does not have any gas in it to cause it not to float properly.

The reason the carburetor may not want to idle is that the gasket that goes between the two halves of the carburetor is not sealing good. The castings are not finished too good to begin and lots of people damage the mating surfaces at the outer edge of the carburetor by banging on the side of the carburetor to stop it from flooding when the float sticks or there is foreign material between the needle and seat. (this treatment does work) :O) This gasket must be tight at the middle where the idle orifice gets it’s gasoline.
Remove the float attaching hinge by carefully prying it up until you can get a small screwdriver under it. Don’t bend the two small blind rivets. They will have to be re used to re assemble the float hinge.

Use a new sheet of 80 grit wet or dry sandpaper that is laid on a hard flat surface and lap both the mating surfaces until it is perfectly flat on both mating surfaces.

Re assemble the carburetor and you should be able to adjust the idle.

Adjust the idle by lowering the RPM and then turning the idle mixture screw in or out until the engine runs at the highest RPM. The RPM is what you will be listening for and not the way the engine runs. MAXIMUM RPM!!! At idle.

First let the engine warm up a bit. With the engine at full throtle,turn the main needle in clockwise untill the engine stumbles (not out) Then turn the main needle out until the engine runs smoothly. Reduce the throttle to idle and then quickly advance the throttle and see if the engine falters. If it does then open the high speed needle a fraction more and repeat until the engine will accelerate without stumbling. Reduce the throttle to a low idle speed. Turn the idle needle in and out until the highest engine speed is found. This is where to leave it, regardless of how many turns in or out. HIGHEST SPEED! Not sound of engine. The highest speed will be the best idle. Reduce the RPMs to where you want them after the highest engine speed at idle is reached when adjusting the idle needle.
To adjust the high speed mixture you should start the tractor with the high speed needle open about two full turns and allow the engine to warm up before attempting adustment of the high speed mixture. With the engine at high RPM begin to turn the high speed mixture needle in clockwise until the engine begins to falter. Be ready to use the choke to keep the engine running. Open the needle counter clock wise until the engine does not falter and then idle it down and suddenly accelerate by opening the throttle to full throttle. If the engine falter or hesitates you should open the high speed needle another 1/8 turn and try the sudden acceleration unit the engine does not falter on acceleration.

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8ntheo

04-07-2008 18:41:31




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 Re: 8N Carburetor Nightmare in reply to 8ntheo, 04-07-2008 17:41:33  
Thanks for the info, I'll take a look at the jets and readjust the carb. I'll post back the results



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Dan

04-07-2008 18:26:13




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 Re: 8N Carburetor Nightmare in reply to 8ntheo, 04-07-2008 17:41:33  
"I have a 49 8N that I rebuilt 6 months ago..."

You should know I have proven the Tisco jets you get nowdays are usually not acurate. Problem is - these carbs require them to be. I had the same issue with my 1948 8N I rebuilt a few months back, and could not get the idle right. Turned out to be an idle AND an economizer jet that was too large an opening. I believe the correct sizes were:

Idle - .028"
Econ - .043"
Main - .048"

Or very close to that.

HTH,
Dan

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Gaspump

04-08-2008 07:28:32




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 Re: 8N Carburetor Nightmare in reply to Dan, 04-07-2008 18:26:13  
Right on Dan, the kit jets one purchases today are sized all over the place. You are fortunate to have found jets as close to correct as you did. Just for everyones info, here are the correct TSX-241B sizes. Remember the TSX-241B supercedes all other N tractor carbs. These jets will be the best for all of them, TSX-33, TSX-241, TSX-241A, TSX-241C all inclusive.

Main=.052
Idle=.0465
Econ=.037

And, in order to run well the N Tractor needs jets on or very near these sizes.

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Dell (WA)

04-07-2008 17:54:17




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 Re: 8N Carburetor Nightmare in reply to 8ntheo, 04-07-2008 17:41:33  
Theo..... ....1st, remember ALL carb drawings "LIE".

2nd, the side-pointing idlemix is BASSACKWARDS; out for lean, IN for ENRICH.

3rd, the down-pointing mainjet is TOO-RICH at 2-1/2 turns. You've already proven it is TOO-RICH by complaining about fouled plugs. Try LEANING yer down-pointing mainjet to 2-turns and LEAVITT!!!

4th, try and re-adjust yer side-pointing idlemix for maximum idle rpms, NOT SMOOTHEST idle, MAXIMUM IDLE. Understand??? Then set the behind the carb idle-speed to an incredably SLOOooow 400-rpms. Now re-adjust the idlemix for maximum idle rpms. Do that song and dance at least 3-times.

The last time I adjusted my eazy starting 6-volt 52-8N, my idlemix was 1/8-turn and mainjet was 2-turns. Yours should be similar ..... ....Dell

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JMOR

04-07-2008 17:47:48




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 Re: 8N Carburetor Nightmare in reply to 8ntheo, 04-07-2008 17:41:33  
Idle Adjustment Needle controls air, not gasoline directly. Screw it out for more air, thus leaner idle mixture. Just the reverse of MAIN POWER ADJUST NEEDLE.



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