Why does one plug foul on startup?

[Doctor Shifty]

My No2 plug sometimes fouls on startup. I have to keep close watch on not having too much choke (but it won't start from cold without a fair bit anyway) and I have to rev the motor a bit - probably to 3k or 4k - so i can lessen the choke until it fires on four cylinders and runs smoothly.

If it fouls I have to remove No2 plug, put a clean plug into the hot motor, and then it runs perfectly. It only fouls on first startup in the morning. When the motor has been running, after stopping for lunch for instance, it starts clean every time.

But!! If I start the bike, then stop it within a few minutes (sometimes I forget something and have to go back into the house) it can foul up on the second startup if the motor is not warmed up properly. :-(

I don't think this is related to the vacuum fuel tap. I've never had a problem with the fuel tap leaking and most problems there are about sucking air and running lean. However, if you have an idea in this area I'd appreciate your thoughts.

Things I've thought of checking.
Float bowl level set too high.
Enrichener fitting for that carby too advanced on the push/pull rod.
Jetting problem.
Spark plug lead or coil problem.

Anybody got any other things for me to check before I pull the carbies off the motor to check jets etc?

Kim

 

[argonsagas]

Things I've thought of checking.

Spark plug lead .


Now that you have thought of it, check it.

 

[focus frenzy]

how much oil are you using over, say 1500km?

any blue, blue/grey smoke at cold start up?

if you have the stock spark plug boots, pull them off and check to see if the insert is screwed in snugly.
the resister that is under the insert could be bad (weak spark)

 

[Roadwarrior]

Your petcock may be bad. I had a problem with my petcock allowing fuel to dribble out when the engine was off. It would flood out the carb that the fuel whent to first, and cause the corrisponding cyclinder's plug to foul.

 

[saaz]

Main things to check are is fuel getting into No 2 from the petcock, and are the leads and plug caps in good condition. I chased myself around in circles until I found a similar problem was just the plug caps. If No2 is low on compression or has a leaky valve stem seal this is another possibility.

 

[Wingnut]

Main things to check are is fuel getting into No 2 from the petcock, and are the leads and plug caps in good condition. I chased myself around in circles until I found a similar problem was just the plug caps. If No2 is low on compression or has a leaky valve stem seal this is another possibility.

I agree with saaz. If I remember correctly the vacume line for the petcock plugs into the #2 cylinder. If the diaphram in the petcock has a pin hole it will dribble fuel into the #2 while the bike sits and also while it is running, thus fouling the #2 plug. I'd start there.