1987 interstate, running rich on idle

[micfitz]

I need help! 34,000 miles. I’ve removed and replaced the carbs 4 times now but the problem remains. Plugs foul up completely black with soot by the time the engine has warmed up. Exhaust smells very rich and acrid. Floats set to 7.5mm. Pilot jets set to 1 3/4 turns. Main jets unbranded #102, idle jets Kleinin #40. Carbs balanced. Choke cables and pistons ok. Starts easily without choke. Good compression all cylinders. Smooth and powerful at all speeds on fresh plugs but turns rough after a few minutes on idle. It is very, very rich on pilot/idle jets but seems ok cruising on main jets at 50-60mph.

[CrystalPistol]

I need help! 34,000 miles. … etc … Floats set to 7.5mm. Pilot jets set to 1 3/4 turns. Main jets unbranded #102, idle jets Kleinin #40. … etc … Starts easily without choke. … etc …

Because it is rich.

Does the bike have only 34,xxx miles? If so, should still have OEM hard parts, though maybe new rubber parts. Looks like should have #35 slow jets & #108 main jets. Looks like pilot screw initial opening should be 2-1/4 turn out. Looks like float is all that is correct or close.
These are for 1984 & '1955 ...much like your 1985 Interstate, a 49 state model likely.

Above specs page for your bike.
And while posting pics, here's one on float adjustment.

Below is for 1986 & 1987 bikes.

[micfitz]

Thanks for posting those fuel system specs and I can confirm the mileage 34,000. The original plugs were in a box in the trunk and judging by their condition the bike had been running beautifully up to 2009 when the previous owner must have given up on it and parked the bike in a barn for 9 years until I bought it in 2018. I had to remove the tank to clean it. But the carbs were in pristine condition. The jets that were fitted were OEM kleinin main/slow 100/40. I lost one of the main jets during overhaul and initially replaced all jets with kit jets 102/35. That setup ran rich at idle. I then put the original 40 slow jets back in. Still running super rich at idle on all 4 cylinders. Changing the main jets shouldn’t have any effect on slow mixture right? And I’ve already tried #35 slow jets with no noticeable change. I tried setting the floats at 9mm and the bike Would not idle below 1500 rpm and still ran rich so I returned the floats to the standard setting of 7.5mm. Today I’m going to try non-standard pilot screw initial setting. It should be 1 3/4 turns. I’m going to try 1 1/4 turns to see if it has any effect but first I have to clean the soot off those plugs yet again. I think I need to look for something that overrides jets and settings at idle and slow running but not at main jet speeds. Like ignition timing? Or valve timing? Or some vacuum circuit? Maybe one of those big valves on the bottom of the carburetor assembly or the reeds or an electronic controller?

[Maz]

Possibly a silly question, but is the air filter clean?
Maz

[micfitz]

The original air filter is still good but I put a new one in there anyway. And the specs posted yesterday by CrystalPistol, thanks, I’ve noticed that your posted specs call for idle speed 1,000 rpm +/- 100 on all models 1984-1987. My Haynes manual says 800 rpm +/- 100 rpm. I have been setting it to 900 but I’m going to try 1,000 rpm tomorrow In conjunction with half a turn clockwise (leaner) as a starting point on the Pilot screw settings then I’ll attempt to synch the carbs before I have a go at fine tuning the Pilot screws followed by a final synch. If it’s still running rich after that I’m going to take the carbs off for the 5th time, getting good at this, and have a close look at the slow air cutoff valve and vacuum line between the intake and the electronic controller. Either of those could cause a rich idle/slow running if faulty right?