Topic: stumped!!

[TomL]

73 350f.  All four carbs cleaned numerous times.  Needle on middle position.  New plugs.  Timing set etc.  All four carbs set up identically, yet 2 plugs have carbon, 1 is just right and one is super lean.  I even trimmed the plug cable to make sure I was getting good spark.  Confused.

[mattcb350f]

Is it firing on all four? idling can and will make plugs look like crap despite the fact that they are indeed firing.

Variable compression between cylinders will also produce different plug readings on worn engines too.

...make sure you've got z valves adjusted too

 Matt.

[TomL]

I haven't adjusted the valves.  My guess is that the carbon is from idling.  But why not all 4?  I have adjusted timing, cleaned and rebuilt carbs.  Only 9500 miles on the bike.  Is it going to be bad for the engine to run it as is?  I don't want to blow it up because of lean conditions on 1 cylinder.

[Gordon]

I haven't adjusted the valves. 

Do the full tune-up first and then see if you're still having problems.

[flybox1]

I haven't adjusted the valves. 

Do the full tune-up first and then see if you're still having problems.

+1
..then do your carb sync.

[mattcb350f]

I haven't adjusted the valves.  My guess is that the carbon is from idling.  But why not all 4?  I have adjusted timing, cleaned and rebuilt carbs.  Only 9500 miles on the bike.  Is it going to be bad for the engine to run it as is?  I don't want to blow it up because of lean conditions on 1 cylinder.

Running lean is bad because it creates excessively high exhaust temperatures which overheats the piston/valves etc...but this usually happens at wide open throttle settings where EGT's are the highest to begin with.

Running it lean at idle or midrange shouldn't (Disclaimer: but may) do any damage but the issue must be fixed regardless.

How long has the engine ran before pulling the plugs to check them? It usually takes a little while of running to get a good read on them. As well, if the engine was at idle a long time before pulling them, then the plug condition that you see is an indication of the fuel/air mix at idle only.

In this case, the super clean one is probably not getting any fuel at idle meaning the idle circuit in the carb is blocked somehow.

OR,

It was a clean plug that has never fired because of an ignition issue and it never burnt the fuel that was passing through the cylinder enough to blacken it.

So, If the valves havent been done then check the tolerances and adjust them acordingly. Too tight of valve clearance will cause the most problems at idle where you seem to be having trouble.

 Matt.

[tlbranth]

I'm not familiar with your particular model - mine is a CB750. But on mine, 1&2 share a fuel feed as do 3&4. 1&4 share a coil as do 2&3. So sometimes you can weed out a problem by noting which cylinders share what. You can also switch 1&4 or 2&3 plug wires to see if the symptoms move. On my K0, if I mess with the tank or anything near it I have to re-seat the throttle cables veeeeeeery carefully. Probably not a problem for you though.