Losing a cylinder at low RPMs

[Glen007]

I noticed last week that my 2001 was dropping the front left cylinder at low RPMs. At idle I don't think it's firing at all. When revving the bike in neutral I can hear the bike spit and pop ever so slightly in the 3k to 4k range and think the cylinder finally picks up consistently around 4k. Initially I thought it was just the plugs as they hadn't been changed in over 10k miles, but I put in new plugs and its the same thing. Any suggestions would be greatly appreciated. Thanks in advance.

 

[one2dmax]

Carb sync or possible CDI box going out.

 

[jfeagins]

Check your charging voltage during idle w/ bike warmed up. If it's not at least 13.25 volts, or so, AFTER THE FAN KICKS ON, your CDI box will usually begin screwing up.

Mine only did this after the fan was running and taking some of the charging voltage. I fixed my charging system and never have had to replace the CDI (knock on wood).

Of course, like Sean has mentioned, it can also be a carb issue.

 

[Glen007]

I love this site - damn that was fast. Hopefully I'll be talking to you soon Sean about some Progressive 440's if I can get this issue taken care of at a reasonable price. Unfortunately I don't have the tools or time for a carb sync and I'm unaware of anyway of testing the CDI box (other than eliminating everything else). Thanks for the suggestions.

 

[Miles Long]

Possibly the air/fuel mixture circuit for that cylinder is inoperative - I had the same problem with my bike. Do the "Shotgun" cleaning technique. Follow up with a carb sync.
Cheers!

 

[Glen007]

Thanks for the suggestion J. The cylinder doesn't appear to be picking up off the start even when it's cold and the fan is not running (pipe stays cold until I rev it into the 3k to 4k range.

 

[m-cman]

Might try trimming the end(s) of the plug wires on the coil(s) side slightly. Could be a poor connection. :confused2:

 

[[email protected]]

First guess would be plugged pilot jet.
Second would be plug wire issue.
Third would be over my head, LOL.

 

[jfeagins]

Yep, if it's doing it while cold, and you have plenty of voltage while @ idle, I'd switch my best guess to plugged pilot jet(s) in the carbs too.

 

[dwarf717]

I had a similar problem, and finally fiqured it may perhaps be my shiftlight. I unplugged it from the tach, and the problem has never resurfaced. The rpm sensor is fed by that cyl.

 

[dannymax]

First guess would be plugged pilot jet.
Second would be plug wire issue.
Third would be over my head, LOL.

Actually, the third possibility could be something blocking the main jet at idle.
As 'off the wall' as that sounds I had it happen recently where the main bleed pipe was loose in the jet block and had fallen down blocking the main....revving would create enough vacuum to pull fuel up and it would run fine.