IH carb - Main Jet Size

[Underwood]

Cub S/N 6697. I am going to go through the IH carb. While ordering parts I see on the TM site there are 2 different main jets available, 0.030 orifice and 0.034. Not sure what size is in it now but the plugs are always pretty sooted up. How do I determine which main jet I should order? Thanks in advance for your help.

[Bob McCarty]

Can you read the number on the flat part of the carb top opposite the gas inlet?

[Underwood]

Sorry for the delay, had to strip old paint off the carb. In the center of the flat is "1259". The outside edge of the flat has a bunch of old scars and dents. I can see what looks like 36457 ,then some dents, then 791. (Maybe R91 ? Maybe 7R1 ?) Sorry, that area of the top is pretty beat up.

[Larry in WNY]

I put the 0 034" jet in all 3 of my Cubs. No problems using the larger jet.
Larry

[Bob McCarty]

364579 R91 originally came with a .032 main jet. The other thing to check is what spark plug you have and if you might need a hotter one. You probably don't need to replace the main jet. If clogged you can run a thin copper wire (like phone wire) through the orifice and then blow it out with carb cleaner or compressed air.

[Underwood]

Thank you Larry and Bob. The soot covered plugs I just took out were D21's. I replaced them with Autolite 3116's but don't have any run time on those yet. My thought was that the main jet orifice may be enlarged from 73 years of being cleaned out, maybe not using COPPER wire, thus resulting in an oversized hole and a too rich mixture. For the $8 or $9 cost I may go ahead and put in a 0.034 jet as suggested. At least then I will know for sure what I have. Thanks again.