MF135 main jet adjustment

[Sid Dunn]

Posted earlier about dry black sooty plugs that foul after running for about 45 minutes or so on my 135 Cont. Z134, and after checking everything (Fire seems good because it runs great with clean plugs), I began to think about a fuel related problem causing it. Saw somewhere that the float level being high or low would cause rich or lean mixture and either could cause sooty plugs. It"s a Marvel-Schebler tsx882 carb. Found out the float was not at specs. Calls for it to be 1/4" when measured from gasket surface to nearest part of float when held upside down and resting against the needle jet. It was more than that before adjustment. Probably 3/8". Put back in tractor. It runs good and seem to be getting less fouling. Anybody ever had this? Now the main question: Carb prelimenary adjustments say about 1 turn out for idle screw, (the small one) and 1 1/4 to 1 1/2 turns open for main jet,(the big one). The thing is it makes no difference at all where the Main jet is set as far as how the tractor runs. Only after screwing it all the way out and taking the jet away from carb does it start to sputter. I"m pretty sure I cleaned all tiny holes throuout carb when apart. Anybody with one of these engines I"d be glad to hear what their motor does when adjusting main jet. Any input at all about this and fouling plugs. Thanks, Sid

 

[Jerry/MT]

The dry sooty fouling you described is from too rich a mixture.the MS carbs are very sensitive to correct float setting and you are not the first person to have that problem.
As far as the main jet goes, never go below 1 turn on it. I would try it at 1-1/8 turns and set the idle air(Yyes that screw sets the idle air, not the idle fuel. Out is leaner.)to about 3/4 turn out. with the engine warmed up, slam accelerate the engine to max rpm. It should accellerate without hesitation or coughing if the mixture is correct and the float level is correct.
You say "The thing is it makes no difference at all where the Main jet is set as far as how the tractor runs." How does it run? Does it idle smoothly and run under load well? What"s the problem withthe way it"s running after you reset the float?

I suspect you have a dirty carb. How did you clean it"

You are focuing on the fuel but have you checked the spark? Pull the center wire from the distributor cap, hold it near a good ground and try a start. You should see a FAT, BLUISH-WHITE SPARK, the color of lightning. If its blue, reddish, or yellowish,or spindly, your points need adjusting or replacement or you don"t have the proper current through the primary ignition. circuit.

 

[Sid Dunn]

It runs great until the plugs end up fouling. Before I started on the carb, I had already changed Plug wires, Cheked distributer cap, set points to .022". Cleaned carb.comletely, everything out, checked all passages in it and did have to reset float height. After float was reset, it runs even better Great from slow idle tohigh rpm with no sputter or staggering. I'm just trying to clear up the fouling plugs. Again , black dry fouling. Put good solid copper plug wires. I'm probably open 1 turn on idle air as you say. The 2 and 3 cylinder are the main ones that get fouled. I've got good compression. #1 cylinder- 131psi, #2-136, #3-136, and #4-136 so don't really suspect rings. Of course I'd like to find problem in the carb or weak spark before any internal problems. Was wondering why main jet had no affect on running engine until it's being pulled out of carb. Thanks for your input. You say I need to lean it out? The float previously was low in carb, which would shut fuel off to bowl at a lower level. So after my adjust to float, I probably have Higher fuel level in bowl, Sid

 

[Jerry/MT]

You"re correct about the float setting. I mis-spoke.

The main jet only comes into play whe you get above a certain throttle setting. Their is a an idle fuel flow port just above the closed throttle plate and a transition port I believe that supllies fuel at part ttrottle off idle. The main jet comes in after that. So if your comparing runnning at idle, or just off idle,. the power jet won"t have any effect.


Taking the main jet needle out vents the fuel bowl to the atmosphere. This carb is a pressure balanced desing that vents the fuel bowl to the carb inlet so that a restricted air cleaner will not affect the mixture ratio, It just affects the max power.
The normal pressure in the fuel bowl is below atmospheric pressure slightly and it"s the pressure that drives the fuel out of the main nozzle and the other fuel ports. When you vent the bowl to the atmophere, the pressure ratio across the metering orifices increases and you get enrichment of the mixture beside opening the main jet fully.
Rich mixtures would cause all the plugs to foul but you"re only fouling #2 and#3. Is your firing order correct(1-3-4-2 and the distributor turns CCW)? Do you have the same plugs in #2 an #3 that you have in #1 and #4? I wonder if you might have a manifold gasket leak on #1 and # 4 that"s leaning the mixture? Get some starting fluid and CAREFULLY(THE STUFF IS FLAMBABLE!)spray that around the intake manifold near #1 and #4 with the engine running. If the engine speeds up you have a manifold leak. It could be the gasket or cracks or rust holes in the intake manifold itself.

 

[Sid Dunn]

I've got Autolite 386 plugs throughout. Some age on them. Firing order correct and counterclockwise dist. I was at first kind of doubtful of fuel problem causing only 2 and 3 cylinder fouling, unless theres something about the delivery to the combustion chambers inthat engine. I'll check the intake manifold next. thanks for help

 

[Anonymous-0]

SID: I have a TO-35 with similar problems, but I'll run 20-50 Hours before I have to clean/change the plugs. Try a set of Autolite 388's, they are a hotter plug-last longer, if you can get them. Good luck!

 

[Jerry/MT]

Do your self a favor and put some Autolite 3116 plugs in that tractor. They are non-resistor plugs and these Continental engines seem to really like them. The 386"s are resistor plugs and really don"t belong on these old gas tractors, in my opinion.

 

[Jerry/MT]

Rich,
Try Autolite 3116 plugs. These Continentals seem to like them and they are a non-resistor plug unlike the 386"s.

 

[Sid Dunn]

Thanks guys. I got the parts store to get Autolite 388 in the other day but never got around to pick them up. Autolites charts show the 3116 as a powertip Non-resistor that is one notch hotter than 386 and the 388 one notch hotter than 3116. Sounds like a plan.Tried the starter fluid around intake gasket, didn't get any variations. So maybe it's ok. The only other thing that I did was run the idle air screw in to 3/4 open from a full turn or so open. Should it be as far closed as possible or open as much "as possible" to help alleviate plug soot. It probably starts to run rough below 3/4 turn open and I can open up to about 2 turns before it runs rough. All said, I guess I'm pretty happy with it. Runs GREAT. Probably can go a few hours to plug cleaning Thanks again

 

[Jerry/MT]

Screwing it out leans the mixture so I leave it out as far as ar as you can where it still idles well and see if it helps reduce the fouling.

I used 386"s and had a hard time getting them to run correctly and a guy on the Ferguson board(phil(VA)) told me to put the 3116"s in and I never had plugs make as dramatic a channge in performance before. That"s why I recommended them. I think ther emy bea QC problem with the 386"s because I had two fouling.