bizarre coincidence needing help - two twins running lean on left side


City Limit Moto:Parts-Service-Apparel (716)8038606
SO - I've been working on two bikes here that I've gone completely through and both are exhibiting similar symptoms that I can't seem to figure out the root cause of.

Bike 1: 1975 CB360
- ultrasonic cleaned and rebuilt carbs - floats set to 20mm
- rebuilt petcock
- new points and condenser
- valve clearances set to spec
- camchain tensioned to spec
- new NGK plugs
- disassembled (unstuck as it was seized) spark advance mechanism and lubed/reassembled.
- carbs synced on vacuum gauges and checked headpipe temps with infrared thermometer (within 4* Left to Right)
- fresh tank ethanol free 91 octane

Bike 2: 1972 CL450 scrambler
- ultrasonic cleaned and rebuild carbs, including new brass floats
- rebuilt petcock
- set valves to spec
-camchain tensioned to spec
- new NGK plugs
- throttle cables synced up and idle set screws synced using head temps/infrared thermometer


Both bikes fire right up and idle nicely, even rev quite well parked in nuetral.

Both bikes when I go to ride them fall flat on their face over half throttle and spit/sputter with lack of power.

Pulled plugs on both bikes, BOTH have proper brown color on the right side plug, but both quite lean on the left plug :eek:

I sprayed all over the carb boots and carbs themselves while running, no vacuum leaks present.

I was working on the 360 first, and I originally thought maybe it was an ignition issue so I threw a charlies place electronic ignition in it and it has not changed the symptoms.


Paging CrazyPJ?

I'm just pretty blown away its basically the same symptom on both bikes and can't seem to figure it out. wondering if anyone has gone through this before and I'm maybe missing something simple?
I've experienced where they don't exhibit a noticeable air leak through the boots at idle, but then start to show leakage at higher rpm's.
how were you able to determine that? revving to an RPM and attempting to hold it steady enough while spraying at the boots with some carb clean to see if it jumps enough that you know it wasn't your throttle control raising it?
It was my CB750, which has 4-into-2 carbs. #1 was normal, even a bit rich at idle, then went lean above 3k. Did plug chops and you could notice in the way it ran. Since 1 and 2 run off the same carb, it was pretty obvious.

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irk miller said:
It was my CB750, which has 4-into-2 carbs. #1 was normal, even a bit rich at idle, then went lean above 3k. Did plug chops and you could notice in the way it ran. Since 1 and 2 run off the same carb, it was pretty obvious.

Please enlighten me on what a plug chop is
cb360j said:
Please enlighten me on what a plug chop is
Went for a ride, ran it up to 4K and cut it of. Coasted to a stop, pulled the plug, and inspected.

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OK, on the 360 last night I swapped coils and the problem didn't follow. So I pulled off the airboxes to see whats going on with the carbs while running as I had a hunch maybe the choke plate was maybe faulty or something like that. Low and behold I realize the right side throttle slide wasn't moving under throttle while running.....pull the cap and somehow when I was putting these back together the diaphragm pulled out of the groove in one spot. re-seated that and fired it back up and the slide is now moving under throttle while running. Then the starter button quite working - opened that up and the guts for the switch exploded. started swapping another switch on which I'll wrap up today to test ride but I'm pretty confident the issue on this bike is resolved. I will get into the CL450 probably tonight or tomorrow once I've ridden the 360.
In my experience it's almost always something stupid that I did. Like putting the slide in backwards or not checking the diaphragm lol
So the points in the 450 are in fact stamped “Japan” - but I took them apart and did the common-motor mods reccomended in the video. Filed and set gaps to .014 , and noticed the advancer springs were so loose it would wobble a bunch so I snipped the springs down a touch and re-hooked the ends to get that back in order.

Went for a spin, same’s a little boggy and sputtery Under load until it gets WOT and then it screams pretty good. Came back pulled plugs, same deal - fouled right side plug.

Swapped left to right coils threw fresh plugs in went back out. Came back, still fouled right side plug.

Checked both carbs again for proper fuel level and mixture screws are set at 1.5 turns out.

Sprayed carb cleaner all over the boots and connections while running, no sign of vacuum leak.


Any further ideas fellas?
May have found the culprit. The brass air port at the bottom of the inlet on the right carb had some shmootz
In it. I pulled both carbs again and I can blow carb cleaner through the left one but not the right one. Guessing that’s causing the rich condition.....they’re now both in the ultrasonic cleaner with a fresh batch of solution.
I am finding electronic ignition pickup kits with the stock mechanical advancer causes bad pre-advance timing issues due to slack tolerances in camshaft diameter on the magnetic rotor piece. Points or fully electronic should fix it if all else fails. Manifests as a cough at idle and up to 4K on one side gets worse as it heats up.
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