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8n runs rough


 
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lorretta
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Joined: 02 May 2012
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PostPosted: Fri May 25, 2012 4:24 pm    Post subject: 8n runs rough Reply to specific post Reply with quote

8n front mount 12 volt.
Replaced wiring with new started switch and new terminal fused block ..starts great
every time and right away...thanks all for previous advice!
new points, timed it and cleaned carb, drained old gas...
but any adjustments on carb do not seem to make a difference.
With troottle high I have adjusted main mixture screw,and that does respond..
I turn in until it starts to stall and then turn out about a 1/4 turn
that's sounds good but when I return throttle to normal run position it just runs rough, and seems to get worse under load...finishing mower.
I also checked valve pressure ..about 100 - 110 in all four.
and they all read very close to each other. spark is good in all four and they are 1243
This is so disappointing ,I had so many projects planed
So my question is how do I tell if I did a bad job timing or if I did a bad job with the carb cleaning /adjustment.
 
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Bruce (VA)
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Joined: 24 Feb 2003
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Location: Old Church VA

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PostPosted: Fri May 25, 2012 4:31 pm    Post subject: Re: 8n runs rough Reply to specific post Reply with quote

Start over w/ the carb adjustments.

Make sure the tractor is at operating temp; that usually takes 10 15 minutes at idle depending on ambient temp.

Both Ford and Marvel/Schebler (assuming you have a M/S carb ) say to set both the side-pointing idlemix and the down-pointing mainjet to 1-turn as a starting point. I set the down-pointing mainjet to 1-1/2 turns and don't fool w/ it until the final step.

Then adjust the side-pointing idlemix for fastest idle; not the smoothest idle. Next, adjust the behind the carb idle-speed set-screw for very slow 400-rpms idle. Do that idlemix adjust for maximum idle at least 3-times. Make sure that you turn the screws slowly, like 1/8 of a turn at a time & wait a second or two for the engine to catch up. Take your time!

Do it like this:

1. Adjust idle mix jet until RPM increases

2. Adjust idle-mix set screw until the engine nearly stops (as slow as you can get it unless you have a tach that tells you 400 rpm)

Repeat steps 1 - 3 three times.

Remember the side-pointing idlemix is out for lean, in for rich.

If you do not have any problems inside the carb, it is easy to get the idle down to 350 - 400 rpms.

Your last step is to go back to the main jet. Remember, in for lean, out for rich. If you end up turning it OUT more than turn for max power (remember, you already had it 1 turns out) then stop right there because you have a dirty carb or a fuel problem.
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lorretta
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PostPosted: Fri May 25, 2012 5:38 pm    Post subject: Re: 8n runs rough Reply to specific post Reply with quote

Thanks again Bruce ..Dark here now.. I will try that tomorrow.
In the past when I have adjusted the idle mixture screw i was looking for an immediate result/effect.. I will slow ....down but in the past it never seemed to matter.. which way or how much i turned that idle mixture screw ..i sensed no change.

The carb is faily new, I bought it from this site last year ..black cast one. i took it apart and cleaned it.
I do wonder about a fuel problem that is why i drained and put in new fuel. but while is was draining it i noticed it did not flow perfectly smooth out of the fuel line at the carb.. It had a consistant small air burp every five/ten seconds or so
so i removed the sediment bowl from the bottom of the gas tank. I cleaned and replaced sreen and gasket.
As i write i wonder if i should go back and clean cab again now
that i have fresh gas in it ...i have not used this tractor since last
fall...except for numerous fustrating test runs this spring.
still love this old tractor. Loretta is a nick name i gave it.. after my wife.
John
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Dell (WA)
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PostPosted: Fri May 25, 2012 8:10 pm    Post subject: Re: 8n runs rough Reply to specific post Reply with quote

John.......you write........"but while is was draining it i noticed it did not flow perfectly smooth out of the fuel line at the carb.. It had a consistant small air burp every five/ten seconds or so"........that burp was caused by CLOGGED gastank vent. Its in the round dome on top of yer gastank where you cannot gitt to it. Mud daubers like to clog the vent. Just loosen yer gastank capple and that will relieve the vacuum. Simple, eh? .......the breathing Dell
 
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lorretta
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PostPosted: Tue May 29, 2012 8:06 am    Post subject: Re: 8n runs rough Reply to specific post Reply with quote

Hi Bruce,
Took the carb off numerous times, checked and cleaned then once back on I went through your reccommended steps each time...
thanks for those clear instructions which make sense but are not producing improvment on my tractor.
still runs rough at idle speed and if i do not slowly bring it down it just stalls out. it still runs very rough under load with finishing mower.
It sounds a lot better at full thottle when sitting still or when i pull the choke out a little. had a nice nieghbor who has a tractor go through this whole process with me: off,clean,back on,adjust
still the same..so after all this i am going to just buy a new carb.
take a chance..my question is: rebiuld or new from this site?
thanks john
 
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Bruce(VA)
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PostPosted: Tue May 29, 2012 8:45 am    Post subject: Re: 8n runs rough Reply to specific post Reply with quote

" rebiuld or new from this site? "

The general rule is never buy a new carb until someone who knows what they are talking about tells you it can not be rebuilt. And that would be unusual.

In order of preference:

1. Gary in MT slk2526 AT blackfoot.net

2. A local rebuild shop.

3. These guys do 'mail order' work; give them a call to get a quote & shipping info.

And, they also do brakes, starters, generators, water pumps, clutches, etc.

Automotive Manufacturers Incorporated
2400 N Lombardy St
Richmond, VA 23220
(804) 321-6861
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