Carb issues

[Covelli33]

1996 kat 600.

Working on a friends kat which he just acquired. Carbs needed cleaned out upon purchase. After cleaning them out, the bike runs great when wide open in just about any part of the power band. Bike has a pipe on it, and no airbox. Seller mentioned bike having a jet kit done. What I can't figure out is why it cruises so choppy in the midrange. It almost feels like the diapraghms in the carbs are fluttering. Any opinions? I'm stumped.

Thanks

[Kreylyn]

1996 kat 600.

Working on a friends kat which he just acquired. Carbs needed cleaned out upon purchase. After cleaning them out, the bike runs great when wide open in just about any part of the power band. Bike has a pipe on it, and no airbox. Seller mentioned bike having a jet kit done. What I can't figure out is why it cruises so choppy in the midrange. It almost feels like the diapraghms in the carbs are fluttering. Any opinions? I'm stumped.

Thanks

No airbox... Major issue 1.

Check the needle jets for out of round... possible issue #2

Were the carbs fully disassembled and dipped as part of the cleaning process? That includes drilling out and removal of the A/F screws, resetting them to 2.5 turns out, and a new sync once re-installed on the bike. Possible issue #3

Krey

[92xjunker]

I guessing here but, what size jets were installed to try to over come the lack of an airbox......way too large?

[maniac1886]

The diaphragms are likely fluttering. CV carbs use the back pressure in the Airbox to lift themselves. No Airbox, way less vacuum, inconsistent diaphragms. That's why hiflo K&N and similar as well as pod filters don't work well. Also, with pod filters, air currents throw off the intake and the carbs will go in and out of sync. I've seen people do all kinds of cooky stuff to try to fix that. Ultimately, an air box and oem paper filter works best.

Go Figure.

[Covelli33]

No airbox... Major issue 1.

Check the needle jets for out of round... possible issue #2

Were the carbs fully disassembled and dipped as part of the cleaning process? That includes drilling out and removal of the A/F screws, resetting them to 2.5 turns out, and a new sync once re-installed on the bike. Possible issue #3

Krey

Needle jets looked great. Everything was cleaned properly, and the fuel screws are at 3 turns out. Feels better than 2.5. Carbs were synced by me, and are all really close. I'm guessing the filters and lack of air box are the culprit here.

I guessing here but, what size jets were installed to try to over come the lack of an airbox......way too large?

I believe they were either 120's or 122's iirc.

The diaphragms are likely fluttering. CV carbs use the back pressure in the Airbox to lift themselves. No Airbox, way less vacuum, inconsistent diaphragms. That's why hiflo K&N and similar as well as pod filters don't work well. Also, with pod filters, air currents throw off the intake and the carbs will go in and out of sync. I've seen people do all kinds of cooky stuff to try to fix that. Ultimately, an air box and oem paper filter works best.

Go Figure.

To me it feels like they are fluttering. His bike doesn't have pods but it looks like knock off K/N filters. I wonder if stiffer diapraghm springs would help. Either way, i'm sure the stock setup is probably best.

I have sourced a factory air box for 35$. Should I expect to need to modify the jetting on his bike if I switch out his filters and go to the factory box/filter?

The bike feels really good when cranked open. Doesn't feel too lean, or too rich. It just gets a tiny bit choppy when you cruise in the midrange. Open in up low or high rpm and it takes off and cleans up. Darn CV carbs!

[GaryS-NJ]

Does it idle ok? Does it rev up through midrange without load but just stumble midrange under load? Did you check for air leaks including the intake O rings?

[maniac1886]

I don't remember off hand what the suggested main jets sizes are. It's in this forum somewhere. I specifically remember arsenic explaning it. It's likely the jets that are in it now are too big in an attempt to make up for the lack of Airbox. I would say use. The suggested jet size for slip on exhaust with an Airbox when you get it, and go from there.

GaryNj is right also. The orings behind the intake boots in the head do have a tendency to flatten out and cause a vacuum leak once it warms up. If after you adjust the jets and put in the Airbox, you still have clunky idle issues I would do a vacuum leak test, starting at the intake boots

[Covelli33]

Updates

You guys are awesome.

So the idle is okay, it was hanging up a little not wanting so drop back to idle so initially I thought vacuum leak, but no leaks. The o rings are also in good shape. I messed with the fuel screws and the idle cleared up and it's fine now. It was too lean at idle and getting into the main. I think a size up on the pilot would be a good idea though. i cruised on it at about 5-7k rpm and when I pulled the choke open gradually it got worse. That tells me you guys are right and the mains are probably a size too rich, and that the needle may need to move one spot. I'm going to start with 118 mains and the needle on the reccomeneded spot as per the kit, with a size up on the pilot to stay at 2.5 turns. I think that's going to be pretty close to where this bike needs to be when the air box arrives.

My bike has 122's with an open header and it runs great so i'm thinking 118 main or 120 main on his bike should be very close. I need to finish repacking my can. It's way too loud right now on the highway.

Best,
Nick