Topic: CB750K4 tuning questions

[meangreen]

 I've got my got my build almost finished, but I'm a little stumped by an issue with the idle. The bike starts and runs great, but if I blip the throttle hard (or pull up to a stop while driving), it idles too low and eventually stalls. It sounds to me like it's loading up, but I'm not sure how to address it. If I don't mess with the throttle, it'll idle at 1200-1500 rpm all day long. The idle is a little lopey, which from what I understand is normal with this cam.

 Carb sync, timing, and valve clearance are right on. I'm running #40 slow and #120 main jets, a Web #41 cam, 392 head with stock valves, Dyna-S ignition, Dyna coils, pod air filters, and 4-1 header with a 2" baffled straight pipe. Carb clean, plugs, caps, and wires are all recent. Idle screws are 1.5 turns from seated.

 Anyone care to weigh in? Thanks in advance!

[OldSchool_IsCool]

What condition are the carb boots in?  Maybe spray some Gumout on the boots while idling to see if the reeves pick up, suggesting a leak

[meangreen]

 They're recent. I did have them off and inspected them when I put the engine together.

[PeWe]

Air screws correctly set?
Try a longer trip of 150-200 miles with speeds within speed limits, low throttles, mostly pilot circuit that might need to be cleaned.

[meangreen]

 Yes pretty sure they're set properly. I warmed it up, then adjusted them 1/4 turn at a time for best idle.
 I will try riding it to see if it clears up, although I'd be surprised since I tore down the carbs and soaked them in hot Pine Sol last winter. Couldn't hurt anything, right?
 The other thing I have been considering is a switch to the two-stage Uni pods, from the China ones on there now, to see if it helps the idle quality. I know the extra filtration won't hurt anything.
 

[meangreen]

 I did put about 100 miles on it yesterday. Might have to pull the carbs apart again and see what's what. I expect the idle to be a little rough and lopey, but it shouldn't be unstable and stalling like it is.

[caller]

the engine got to lean fuelmix. larger jets, warm engine will correct it. Perhaps the carb is adjusted to original airfilter and if you got sportfilters they don't get the right resistance to suck enough fuel from the jets. To low level in the carbs will also make the air/fuel mix to lean

[meangreen]

 I've been through the carbs twice now. I've read a lot about which size jets people are using in their 750s with comparable mods, and #40 slow/#120 main seems about right. With the idle air screws 1.5 turns out, I can get lean popping on decel; turning the screws 3/4 turn in gets rid of it, so that tells me my slow jets are pretty close. It runs awesome otherwise and has good power. Going up a size on the mains gives me a crackle/sputter at WOT, so I'm pretty sure I'm close on the mains too.

 I'll have a little extra time next week, so I'll pull the carbs back apart and recheck everything, as well as valve lash, timing, etc. It has to be something I missed.

 

[flybox1]

I warmed it up, then adjusted them 1/4 turn at a time for best idle.

No. 
Is your ear calibrated to hear when the A/F mixture is correct?
Adjust them based on MIXTURE, and unless you have a dyno to read the mixture at idle, use plug insulator color as your mixture gauge.

[JWExperience]

I have a '76 with 4-1,2.5" open muffler k&n in stock air box no cam/engine mods and I run a 42/120 jetting with needle in stock location. I can run 40 with no noticeable difference but I have a nice tan plug with that setup.
It may be worth your while to go to 42 to stop the stalling, sounds lean. When you go up on the mains like you mention, was that to 122 or 125? What color are your plugs because fly box is onto something with that?


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[meangreen]

 Hmm that's interesting about the 42 slow jets. My fuel needles are one notch up. I tried some 125 mains.

 I haven't read the plugs with the bike at idle yet. I'll give it a try this weekend. Thanks.