"By the book", 48/185 should be the right jet set for your engine. It sounds like you are leaking fuel by somewhere, probably a lot, given the way the throttle is acting and the presence of the exhaust soot. I assume you meant a Yost mixture screw. That's another hint that there's too much fuel getting by if it does nothing within a few turns. I'm wondering, how long are you running the bike before you decide that the plugs are telling you the right story and examining the exhaust? Just in the garage or taking out for a decent trip to get it hot? I would make sure that the float and valve are set properly and that the valve is good. Also have to check all the soft parts of the carb; seals and O-rings.

 

Daddyflip offers good advice. Check float level. Higher the float level, the richer the mix, lower level is leaner. Also consider fine tuning with the needle: The higher the needle, the sooner the fuel (mid range circuit) comes in, and the lower the needle, it comes in later. Make sure the O-ring in your new adjusting screw is in good shape and didn't fall out when switching it out.

Assuming you carb is clean, and your intake seals are making a good seal, It sounds like you had the idle circuit dialed in pretty close with the 44 (maybe split the difference and try a 45), and the black soot is probably caused by the mid range circuit. If it is the midrange, then besides the main jet, the needle thickness, needle taper, and needle height come into play. Any shims on the needle? If so, remove them and see if it helps.

The idle circuit goes to about 1/4 throttle (see diagram below) then the needle and needle jet start entering the picture, but there is some overlap with the idle circuit and the mid circuit (needle and needle jet). So maybe lower the needle a notch if its an adjustable needle. Also, the fatter the needle sitting in the needle jet, the less fuel that gets thru, and of course the thinner the needle, then more fuel will get past the needle jet.

I am kind of playing with the same issue as you right now, but am a touch lean. So I plan on checking float level and adjusting to max height spec, then if need be, switching to a thinner needle to let more fuel in. My main is a 185, and my slow is a 46. I had a 48 slow but adjusting it did nothing, so I changed to the 46 and am at two turns out. I am using a NOK M needle with two washers under the needle. Bike runs great but it seems a little lean. No blisters or specks on the plugs, no engine ping, just very little color to the plugs. I might go to a N65c needle or the CVP velocity needle next. If I do, then I need to consider that I will probably have to start over with the tuning. Might even need to switch out the main jet because I will be getting more fuel into the mix with the thinner and shorter needle.

 

That graphic shows why it's important to take a ride, let the bike warm up, and run in that 1/4-3/8 throttle range for a little while to tune the mix and check the plugs. Notice that three of the four fuel metering components are in the meat of their contribution while the fourth (main jet) doesn't matter yet. In the case of the OP, it shows why going to progressively smaller pilot jets is helping matters; it's restricting fuel that's leaking by and overfilling the bowl. That's my read, anyway.

 

Thanks for the response DaddyFlip.

Yes - mixture screw. I had rebuilt the carb in 2016 before getting into the tuning of it so it's pretty tight. I kept notes of everything I did and just checked them. When I dropped to #44, I was able to get some engine stumble at 1-1/2 turns. I left it at 2 tuns and she ran great. When I dropped to the #42 last night I noticed the mixture screw was out to 3 turns. Damn thing is vibrating out! This would explain why it's running rich.

I warmed it up completely in the garage, shut her down and put in new plugs. Then fired her up, rode about 1 mile to the freeway, then cranked on here with high RPM's in 3rd, then in 4th for about 1 mile. I pulled off and killed it before she dropped to idle, then pulled a plug. I'm new to all this carb jetting stuff, but its more white than grey and shows some blistering on the top of the ground electrode.

All still pretty fresh.

 

I posted a reply once but it seems to not have taken...

I warmed her up completely in the garage, shut her down and dropped in new plugs. Then fired her up and hit the road. about 1 mile of normal riding at 35ish, then hit the freeway and opened her up for about 1 mile. Revved high in 3rd and 4th, pulled off and killed her before she dropped to idle, then pulled a plug. I'm new to this carb tuning thing, but it was pretty white and showed blisters on the ground electrode.

I found my notes from when I rebuilt the carb in '16. I put the #44 in and got the engine idle to stumble at 1-1/2 turns. I left it at 2 turns and she ran great. when I dropped in that #42 last night, I noticed the mixture screw was out to 3 turns. I think that Yost is vibrating out. This might explain the rich mixture.

I'm going to pull the carb again and check the float and drop it a bit if needed. If I do drop it, I'll put in the #48 again and see what's what. If I don't need to adjust the float, I'll drop in a #45.

How can I make sure that Yost isn't going to vibrate out?! Oh, and the Yost doesn't currently have any shims. When I rebuilt the carb in '16 I found that the original owner had the mixture screw in without an o-ring or washer!?

Mike

 

Plug looks decent. On the Yost, most folks like to complain that it doesn't have a slotted head, but it doesn't need one. It's a knurled head that you turn with your hand and if there was a slot, and the slot happened to line up so you couldn't see it from the front of the bike, it would do you no good anyway. Your "check" to make sure it hasn't moved is to get it set where you want it and make a reference mark on the knurled edge with a Sharpie. You can then cut a small slot with a file or Dremel there so you can get a quick visual at any time. To keep the screw from moving, first make sure there is only one O-ring in there; use a paper clip with a tiny bend at the end to reach into the hole and carfully probe and pull out any/all O-rings. Then replace with O-ring - washer - spring - screw. You can put more tension on the screw by stretching the spring a little bit.

 

delete

 

Delete

 

delete