Carb adjustment?

Wed May 14, 2014 5:17 pm

I can't seem to find the post or the thread but not too long ago I came across a post that had the rough adjustment for the carburetor. Would someone be kind enough to point me there? I read in one of the downloads that a tach is required to properly adjust the carb for and high and low idle speeds. Looking for a starting point...how many turns in or out on the carb adjustment screws.

I went trough my governor, adjusted the linkages and took out all of the slop but I still am experiencing surging. I can get the surging to stop when I pull the choke lever out a wee bit so it seems to indicate to me it's a fuel related issue. I tried moving the bumper spring in and out and I've also adjusted the throttle to governor linkage but that doesn't seem to make much of a difference.

Re: Carb adjustment?

Wed May 14, 2014 5:28 pm

air screw out 1 and 1/2 turns from bottom , then fine adjust it from there , you should not need to turn it over a turn either way, most likely a half turn will do

Re: Carb adjustment?

Wed May 14, 2014 7:26 pm

Surging sounds like the main jet is clogged partially. I suggest cleaning it with air and a fine copper wire.

Re: Carb adjustment?

Thu May 15, 2014 6:25 am

Thanks for the replies, I'll try both.

Re: Carb adjustment?

Thu May 15, 2014 8:41 am

Besides checking for dirt and sticky fuel additive gunk in the carburetor, there are only three "adjustments" to fiddle with: the idle air mixture screw (near the top of the carb), the height of the float in the carb and the idle speed set screw. The idle speed set screw simply sets the minimum engine rpm's and is found where the throttle linkage connects to the carb...something around 550 rpm's is fine with no load acting on a warmed engine.

Occasionally, the float in the carburetor gets stuck or the floating needle valve that it operates gets hung-up. With the carb removed from the tractor and with the fuel bowl (lower portion) removed, the float is exposed. Slide the float's hinge pin laterally to release the float from the carb and remove and inspect the float needle valve and its seat...clean and/or replace and return the float assembly. Invert the carb with the fuel bowl removed and measure the distance from the bottom of the float to the bottom of the carb flange...it should measure around 1-5/32 inches. Adjust this distance by gently bending the float's hinge tab.

The only reliable way I know to check where the surface of the fuel is in the carb with it on the machine is to remove the threaded plug in the lower rear portion of the bowl and attach a piece of clear plastic tubing. Open the fuel tank valve and see where the carb's fuel level stabilizes...the website information gives a measure for where this should be. If the carb fuel level is too high, fuel will leak through the float's valve and mess up the mixture.

The idle speed mixture screw adjusts the amount of air that can enter the carb throat. It is adjusted at no-load idle speed. A turn-and-a-half open is standard. I close it some during cold weather to richen the idle mixture and open is some to lean the idle mixture in the summer. My carbs are not very sensitive to this adjustment.

An issue I've had with my carbs is leakage around the orifice plug located under the carb's fuel bowl. This is accessed by removing the threaded plug on the lower rear of the carb bowl. The carb rebuild kits include a cardboard washer for this orifice. I find that the cardboard gets mushy from fuel saturation which leads to leakage around the orifice. I replaced my saturated cardboard washer with a small oring and applied Permatex gasket cement to seal this area with some success. If you find the orifice plug loose, it's due to the washer deteriorating.

Moisture in the fuel is a threat to my tractors every spring. By keeping the fuel tank full, moisture is avoided. I have a lot of moisture inside my distributor caps too in the spring which has proven to really choke my tractors' performance.

Re: Carb adjustment?

Thu May 15, 2014 10:58 am

Ironlegs...thank you for the detailed post, I'll make sure to archive that. I now have a surge-free Cub. I took the easiest route and blew out the main jet, tinkered with the gov-to-carb adjustment and fiddled with the bumper spring and it's purring right along and I have a full range of throttle which I did not have earlier in the day. Took her for a spin around the yard a few times and climbed a hill with no issues at all.

More questions though. I think the fellow who 'fixed' my tractor installed a different throttle lever on the carb that I don't remember seeing before and it precludes me from making all of the adjustment that should be possible to the gov rockshaft. Though it's running fine now, finally, is there a preferred lever?

Also, I static timed the mag and it starts just fine but I was playing with the distributor while the tractor was running trying to find the 'sweet spot' but my ear probably wouldn't know it if it heard it. Leave well enough alone, go with my ear, get a timing light or just go with when the mag trips?

Lastly. I've had two (2) no-start situations that were related to excess oil on the rotating block under the magneto coil itself. Something else to consider as a last ditch effort when all else fails.

Re: Carb adjustment?

Thu May 15, 2014 10:58 am

Ironlegs...thank you for the detailed post, I'll make sure to archive that. I now have a surge-free Cub. I took the easiest route and blew out the main jet, tinkered with the gov-to-carb adjustment and fiddled with the bumper spring and it's purring right along and I have a full range of throttle which I did not have earlier in the day. Took her for a spin around the yard a few times and climbed a hill with no issues at all.

More questions though. I think the fellow who 'fixed' my tractor installed a different throttle lever on the carb that I don't remember seeing before and it precludes me from making all of the adjustment that should be possible to the gov rockshaft. Though it's running fine now, finally, is there a preferred lever?

Also, I static timed the mag and it starts just fine but I was playing with the distributor while the tractor was running trying to find the 'sweet spot' but my ear probably wouldn't know it if it heard it. Leave well enough alone, go with my ear, get a timing light or just go with when the mag trips?

Lastly. I've had two (2) no-start situations that were related to excess oil on the rotating block under the magneto coil itself. Something else to consider as a last ditch effort when all else fails.

Re: Carb adjustment?

Thu May 15, 2014 11:21 am

Posco wrote:Leave well enough alone, go with my ear, get a timing light or just go with when the mag trips?
Magneto, static time. If you want to double check the point where the impulse coupling trips - timing light - cranking engine over - coil wire grounded to engine block.

You are checking the spark at engine rpms below around 250 rpms, engine cranking speed.

Re: Carb adjustment?

Fri May 16, 2014 5:17 am

Static time is all that is needed in most instances. Magnetos are not easy to time dynamically. Some prefer the "ear" method, I find a proper static time is sufficient.

Re: Carb adjustment?

Fri May 16, 2014 10:20 am

Static time it is. Thanks for the help.

Re: Carb adjustment?

Fri May 16, 2014 2:07 pm

I defer suggestions on adjusting the mag on your machine to others with real life experience...both my tractors are coil ignition units.

On the topic of finding the "sweet spot", I have used a timing light and dwell meter but ended up with less than optimum settings although the metrics were right on the specifications. The "ear" method is what I use now and listening to and feeling how the engine is performing while under load. I leave the distributor clamps slightly loose while doing a job with the tractor and try small adjustments until I get the setting optimum. Placing a Sharpie marker witness mark on the mating surfaces of the distributor to show where I started with the timing adjustments helps.

Re: Carb adjustment?

Fri May 16, 2014 5:56 pm

Ironlegs, if the advance weights are working as they should, static time the Distributor then check it with a light to see that the advance is working If it is working you should not need to move it. If you have to move it to get optimum performance the advance is not working, that is what you need to work on. I am referring to a distributor not a mag.
I will leave the mags to Rick as I do not care for them
Boss