QuestionHi,
I'm having problems getting an older Allis-Chalmers mid size tractor to idle. It has an Onan (around) 20 hp (Mdl CCKA MS 20266) with a Marvel-Schleber downdraft carb. Carb numbers are VD 75 and on the top: 3871S or 6871S, the first number isn't well stamped.
I set the float by eye, made the tops of the floats sort of parallel to the top of the carb. It was lower and hunted/ surged at idle. So I raised it ( to the above setting), but can't get a nice idle.
I've sprayed carb cleaner through all the passages but it seems like the idle circuit isn't working. I can screw the fuel idle screw in (to closed) with little result. The screw is in good shape, not grooved or broken.
I sanded the mating surface of the top of the carb flat. It was a little warped from the three screws being over-tightened. The bowl seems to be flat, I put a straightedge over it.
I did make a mating gasket for it, and it seems to line up well with the top and bottom passages.
If you have any tips I'd sure appreciate the help. Thanks!
Answer Hello Greg:
Spray WD40 or Liquid Wrench(use the Red Tube supplied with the Spray) Around the Mounting Gaskets and Check for Air Leaks. If the Gasket is Leaking the Engine will Smooth Out when the Spray Locates the Vaccum Leak. Leave the Air Filter Hooked Up for this Test. Otherwise the Intake will Draw the Spray in and Give a False Reading. Has the Carbutrator been Soaked Overnight in Cleaning Solution(carburator cleaner)? Then Blow All the Passages Out with Compressed Air. All the Spray Carburator Cleaner does is Loosen Some of the Varnish Inside the Carburator and Helps Stop it Up. Soaking will Loosen All the Varnish Up and Allow the Passages to Be Cleaned when the Compressed Air is Blown through them. On the Updraft Carburators, if the Float is Set Level with the Bowl Mounting Surface and it Surges, Raise the Float 1/16" and Try Again. If the Engine Still Surges After Raising the Float 1/8', then there is Still Trash in the Carburator or the Valves on the Engine are Leaking by and Not Drawing enough Vaccum at Idle Speeds. This is Seldom the Case. Hope this Helps. I am here to help if you Require more Assistance. Let me know what Happens, Please. I enjoy knowing the Outcome. Thanks.
Good Luck
Respectfully
John
