Topic: CB750 KO Weak Idle Cylinder Suspects

[Simpson]

Somehow I have a weak firing piston in my #3 cylinder during idle. .
Symptoms, uneven idle especially when cold, significantly cooler pipe (I can hold on to it), low air pressure coming from tail pipe.

Here's what I've done:
Checked spark - Consistent, strong
Spark plug after cold start idling for few minute- Very dry and brown resembling, seems to fire up all gas provided
Triple checked intake leaks with new clamps- Balancing idle sync seems normal with vacuum pressure gauges
Float Bowls check - Swapped with another side and still get same result
Idle Jet - Swapped out different idle jets, even up to 43-44 with same result
Air screw - Has little impact, fires maybe better when turned in all the way
Compression test - Performed few months ago
cyl #1 - 158
cyl #2 - 150
cyl #3 - 155
cyl #4 - 155

Seems to fire fine off idle circuit...

ARGH!

[Duke McDukiedook]

You sure all the air and fuel passages on # 3 carb are clean? Compressed air/wire in all holes clean?

I had the same problem on my #2 until I had to reclean them and voila! It worked.

[Simpson]

You sure all the air and fuel passages on # 3 carb are clean? Compressed air/wire in all holes clean?

I had the same problem on my #2 until I had to reclean them and voila! It worked.

Seems like that would be the most likely at this point. I took 100 psi to it this winter on all the circuits. Is there a chance that a nasty deposit will not break free? Wonder how I can test that...

[Simpson]

Perhaps it weak all the way around, meaning off the idle jet also...

[Duke McDukiedook]

Run the wet compression test for #$%*s and giggles to see if your pressure significantly changes.

I would reclean the carbs and recheck float levels (with a clear tube hooked up to the drain screw).
 

[Simpson]

Run the wet compression test for #$%*s and giggles to see if your pressure significantly changes.

I would reclean the carbs and recheck float levels (with a clear tube hooked up to the drain screw).
 

Took air box off blew air through all ports, swapped spark plugs and wires...

Starting to think I have low compression.

Have to recheck compression this weekend.

[Simpson]

Riding back from the post office it comes to me. 

 I bet my tappets somehow got wacked when I put the bike back together. It would explain everything.
Now to let the bike cool.