Topic: Carburetor tuning help please! CB750K2

[Davez134]

I think I know these carbs pretty well, and I thought I had my bike tuned perfectly until I ran into an issue last night. I've read through the carb faq's so many times, I may have them memorized, and searches haven't really helped so far.

First off, here is my setup:
1972 cb750K2, 836cc engine with wiseco pistons, JMR ported head, webcam41, stock airbox with K&N filter. No number exhaust with the remove able baffles installed still.

Carbs are round top, 086A bodies with 40 pilot, 120 mains, needle clip in 4th position, all genuine honda/keihin brass. Idle mixture screw just over 1 1/4 turn out, floats at 26mm.

2 days ago I did complete tune up.

Here is my issue. I was riding home from a local bike night, when after about 12-15 miles of freeway riding, motor fully warm, I'd estimate throttle position to be a little over 1/4, as I'd go to accelerate and open throttle, I can hear what sounds like pre-ignition (death rattle, bb's in a can, whatever term you use.) it would only last a second or too and would go away, at least audibly, as rpm and throttle position increased. After exiting freeway, for the next mile or two to the house, I would notice it again through all gears, but only if I were to rapidly increase throttle position, for example in second gear going to third if I were to gradually go to 6000 then shift, gradually do the same third to fourth, then the sound is not there. I don't feel any noticeable hesitation from the motor, and it for sure does not do this the first 5 minutes or so of riding. Another thing is that I did not notice this two weeks ago when it was 85 degrees outside, and it was well over 100 degrees last night.
All 4 plugs look EXACTLY the same, like this one pic with flash, one without.



I've heard that sound on my supercharged car before, I know to be a lean spot, and am very concerned I may do damage. The bike seemed extra hot when I got home (20 miles)

I don't know where to go from here. Here is my annoying list of questions:
Will main jet change do anything?(don't really think so?)
Would raising needle to last position be worth a try?
Could this just be related to crazy air temps outside? (Thought I thought that would make it run richer?)

Would appreciate any advice. Please!







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[seanbarney41]

too much ignition advance with that hot weather

[Davez134]

too much ignition advance with that hot weather

Thank you for helping me to pull my head out of my a** and think of something different! With no other changes, the plugs seem much lighter color compared to a few weeks ago. Likely due to engine running hot because of timing being too advanced in this heat? I have the timing set spot on the "f" mark with timing light at idle (900) now. How many degrees (about) would you suggest starting with retarding it?

I've gone about 30-40 miles in this heat the past few days. Think any damage was caused? I'm probably going to change the oil again to be safe since im sure it got pretty hot.

[Scott]

I have had a problem with bad fuel one time causing "pinging".

Just a thought, that's all.

[Davez134]

Maybe I just won't ride when it's over 100 degrees 

[Davez134]

You provided everything (+10 pts) but ignition data (-2 pts for you!) 

In addition to considering a timing adjustment, I'd also consider swapping to a cooler plug

Thanks will keep plug in mind, and try retarding timing a couple degrees. And I'm good with +8 total points, that's a Solid B- all day long! 

[seanbarney41]

You provided everything (+10 pts) but ignition data (-2 pts for you!) 

In addition to considering a timing adjustment, I'd also consider swapping to a cooler plug

Thanks will keep plug in mind, and try retarding timing a couple degrees. And I'm good with +8 total points, that's a Solid B- all day long! 

might try some stiffer advance springs before retarding the timing.  I had a good running 750k7 that would ping when it was hot.  Trimmed one coil off the advance springs and it was fine.  I doubt you did any significant damage to those forged Wiseco's (or even lesser pistons for that matter) and I saw no evidence on your plugs.

[Davez134]

I have a rusty advance unit with good springs that I can scavenge and try/modify if needed. Good idea.

[Davez134]

My timing was set pretty damn close with timing light, but the "f" mark was just BARELY advanced at idle, a degree maybe, then full advance came on before 2500 and ended up being a few degrees past full advance marks. I replaced springs on advancer unit with another set I had that seemed "stiffer" and reset timing focusing on full advance rpm and marks this time too. Seems to be much better now. Didn't touch carbs at all, don't think I need to.

My guess is that in cooler weather it was able to compensate, but between 10.25-1 Pistons and 110 degree temps symptoms started showing. Thanks for advice on the timing.

[lrutt]

what CR are you running with that setup?

Are you running regular or premium?

[Davez134]

what CR are you running with that setup?

Are you running regular or premium?

Wiseco advertises the pistons I am running as 10.25:1 compression ratio. I did have head surface trued by JMR and cylinder surface trued by my machine shop, but I got thicker MLS head gasket and thicker base gasket from CycleX to compensate for difference.

I am of course running premium fuel in it. (91 is best we have here though)