Topic: Cyl 2 firing intermittently - good spark no sign of vacuum leak 1978 CB750 K8

[Tdubsy]

Hey Everyone,

Noticed a bit of an issue today.

I was using a colortune system to adjust my mixture and I noticed that cylinder 2 was firing intermittently below 2000 RPM. You are able to see the spark but there is only ignition every few cycles.
There wasn't any obvious signs of a vacuum leak as I sprayed both engine and intake boots with WD-40 and the RPM's didn't even flinch.


I did notice a popping whenever it would ignite and I could see the carb boot expand when this happens (Had to set the pilot screw a touch rich to prevent this)

I have removed the carbs and cleaned them a few times also done some re jetting.

Is there anything I should triple check?
Cyl #4 Acting Normal at idle

Cyl #2 Igniting intermittently

List of things I have done so far
- Rebuilt Carbs with new floats, gaskets,seals
- New Carb boots
- Installed dyna electronic ignition
- New ignition coils
- New spark plugs and wiring
- static and dynamic timed ignition
- adjusted valve spacing to OEM specs
- Synced carbs
- 115 Main jet (up from stock 105)
- 40 Slow jet (up from stock 30)
- Checked cylinder pressure (within 5 psi of stock value)
- Valve clearance within spec

Thanks


Thomas

[TimV5]

I've seen a couple of posts recently about NGK spark plugs going bad very quickly.. (possible fakes?) might be worth swapping the plugs around just to check.

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[Tdubsy]

It fires intermittently using the colortune plug only on cylinder 2. Same test plug works fine on the other 3 cylinders

[seanbarney41]

Check float level?

[Tdubsy]

Level was checked during original carb rebuild but I can try that again