8N carb, main adjustment screw.

dahermit

Member
In regard to tuning an 8N carb:

1. Start with one turn open.
2. Under full load, turn screw in ntil the engine begins to lose power.

What does it mean "Under full load..."?
If I have the throttle all the way open and the P.T.O. engaged is that a "full load", or do they mean just have the throttle open all the way?

Note: I have a Woods RM85 mower attached to the P.T.O. and could run that to get a "full load" if that is necessary.
 
(quoted from post at 12:19:20 09/06/20) In regard to tuning an 8N carb:

1. Start with one turn open.
2. Under full load, turn screw in ntil the engine begins to lose power.

What does it mean "Under full load..."?
If I have the throttle all the way open and the P.T.O. engaged is that a "full load", or do they mean just have the throttle open all the way?

Note: I have a Woods RM85 mower attached to the P.T.O. and could run that to get a "full load" if that is necessary.


Throttle open all the way with the tractor sitting stationary is "No Load"

Full Load is what it sounds like, the tractor being used close to the max of what it can do.

I see no way of adjusting the main jet while under load. You adjust it stationary with no load.

If the tractor then falters when under load, open the main jet a bit.

I would NOT turn screw in until the engine begins to lose power when under load. If anything, I'd open it a hair more.
 
There seems to be so much contradictory, ambiguous, errors, incomplete posts on how to adjust the Marvel-Schebler carburetor.
 
The full load adjustments referred to in the manuals were likely done
with the tractor setting stationary, hooked to a dynamometer.
How else could you adjust it at full load?
I do it this way: Start at about 1 turn open off the seat.
Does it rev when the throttle is yanked open or does it falter?
If it falters, it may need to be open a bit more. Say 1 and 1/2.
If it revs up good then, then I go work it and see how it goes.
If you need it turned out more than 2 full turns, you need to re-
clean/fix the carb because that's not how they're supposed to work.

Some of the conflicting information you're hearing may be due to
the different models/styles of MS carbs.
The ones for the larger tractors were made entirely differently and
were adjusted differently.
 
(quoted from post at 19:35:51 09/06/20) The full load adjustments referred to in the manuals were likely done
with the tractor setting stationary, hooked to a dynamometer.
How else could you adjust it at full load?
I do it this way: Start at about 1 turn open off the seat.
Does it rev when the throttle is yanked open or does it falter?
If it falters, it may need to be open a bit more. Say 1 and 1/2.
If it revs up good then, then I go work it and see how it goes.
If you need it turned out more than 2 full turns, you need to re-
clean/fix the carb because that's not how they're supposed to work.

Some of the conflicting information you're hearing may be due to
the different models/styles of MS carbs.
The ones for the larger tractors were made entirely differently and
were adjusted differently.
Differently" is correct. Farmall SMTA manual says that for heavy loads & full power, adjust the needle out 5 full turns, leaving the size of seat as the fuel metering control, where needle is effectively out of picture. Then for light loads, the needle can be screwed inward to reduce fuel flow for greater economy.
 
One further comment......use the often quoted 1 1/4 to 1 1/2 turns out as a starting point & then adjust for best performance, depending on how you use the tractor. If puffing black smoke, too rich. If sluggish & maybe even running hot, too lean. Just play until you find the sweet spot.
 
(quoted from post at 01:25:22 09/07/20) One further comment......use the often quoted 1 1/4 to 1 1/2 turns out as a starting point & then adjust for best performance, depending on how you use the tractor. If puffing black smoke, too rich. If sluggish & maybe even running hot, too lean. Just play until you find the sweet spot.


JMOR is spot on.
 

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Back
Top