Topic: Slow jet size, air screws mostly in? CB750 K1

[Dunk]

I got my CB750 K1 back together after mostly sitting a year or two. Rebuild carbs while engine was out to fix oil leaks, bench synced with 1/8" drill bit. Rode it a bit today, put fresh gas in, rode some more, did a carb sync with vacuum gauges. Still has slight clutch rattle, not perfect smooth as I recall from before. Air screws were 1/2-5/8 turn out when I disassembled carbs. Seems best now around 5/8 turn out.

I'm wondering if I should try a larger pilot jet, I think carbs at at 40/125 now (stock I think was 40/120). Maybe I should going up to 42 on the pilot jet? This spring I observed the huge difference in idle and transition slot AFR on a wideband O2 gauge on a small block Ford and Summit carb (Holley 4010/Autolite 4100) that required idle feed restrictor changes to idle nicely when chanign frmo 10% ethanol to pure gas with MTBE. All I can get locally is 10% ethanol. Was very rich changing to non-ethanol gas, so much that I had to go two sizes smaller on idle feed restrictors and down a size or two on main jets to get it running nice and an acceptable AFR. If a relatively big engine and carb had such a drastic change with only 10% ethanol I'm thinking that a small engine like a CB750 might be impacted more noticeably.

Other bits are K1 originally had 120 jets but I found increased performance years ago changing to 125 jets. Stock engine aside from a lightly ported head.

Thoughts? Anyone go a size up on pilot jets to account for ethanol fuel?

[69cb750]

Four guesses
The idle jets have tiny holes, visually check you can see through them.
If original coils replace them.
Plugs are clean or replace them.
Run engine with points cover off at night, lots of sparking relace condensers.

[HondaMan]

I use leaner jets in these bikes upon rebuild, lest they constantly foul sparkplugs with modern fuels. Our Regular gas now burns even slower than 1970s Premium did.
Try 69CB750's suggestions first: if it still needs bigger mainjets, check the float bowl levels for being too low. Also make sure the bowl-vent pipes on the 2 inner carbs have at least 18" of 3.5mm hose on each one when riding, or the float bowls will run low at speeds above around 50 MPH.

[Dunk]

-Idle jets are clear and clean. I could see through them before cleaning carbs, they are at least as clear after some time in the ultrasonic.
-Coils are newer 5 ohm Dyna. Wires are ~12 year old copper core.
-Plus are older NGK Iridium. Had some light buildup I cleaned off. Measured all around 4.4-4.6 ohms if I recall.
-Dyna S ignition

Being as it ran great the past 12 years got me thinking this morning my first time commuting to work on it since reassembly. When I rebuilt the carbs I used new aftermarket needle and seats. The needles that came out were pretty soft on the springs. Two felt about what I remember being typical and two felt extremely light. The aftermarket needles have very stiff springs. Maybe I should try raising float level 1-2 mm, in case these stiffer needle springs are closing at a lower height due to stiffer springs?

Good call on the bowl vent hoses. I just checked and they are sitting on my bench. I'll reinstall and test. The stumble at tip in seemed worse on my commute vs riding around town, but a bunch of time at 35-60 MPH. Didn't notice any issues on teh highway at 70+ MPH, but by then the slow jet or slight changes in fuel level shouldn't have much effect.

[HondaMan]

Yeah, modern float valves have much stiffer springs than the OEM versions were. This results in slightly lower float bowl levels when running at [engine] speeds high enough to be well into the mainjet range (4500+ RPM). This is why you'll often see me recommending a little bit deeper-than-OEM float bowl depth in the 750 in particular. The smaller engines don't suffer quite as much from this issue. In the 750 roundtops I usually set the floats to 25mm instead of 26mm when using the stiffer new float valves.