PTO Spark Plug keeps fouling on new rebuild

[sookati]

Hey Everyone,

I have a 96 GTI dual carb. I just installed and broke in a new SBT motor. While I had the motor out, I cleaned the carbs and re-set the low speed screws to factory (1 1/4 turn out i think?). I replaced the spark plugs after the break-in process.

I am on my second tank of gas after break in, and I noticed that after a 3 minute no wake zone, it wants to cut out if I gun it (normally from idle its fine when not idling in no wake for a long time). I look it up, and it looks like it's loading up, so I turn the low speed screws 1/4 turn in. It seems to be bogging less with the fix, but after about an hour of running I start losing top rpm. As I'm running it back to the marina, it's getting slower, and slower, and slower.

I think that maybe it's a charging issue, but I threw my multimeter on it, and it's charging normally/the battery is fine. Then I decide to pull the plugs and this is what I'm looking at:



The PTO plug is totally fouled, but the mag plug looks fine. Do you think this is a carb issue, or maybe I need to snip 1/4" off the plug cable? Full disclosure, I left the gap on the new plugs on factory settings, it was probably .25 or so.

What could this be? Do I need to turn the low speed screw on the PTO carb in a little more?

 

[sookati]

bump .

 

[Sportster-2001-951C-Stock]

Looks like fuel soot from being too rich (assuming your ignition is good) Anything that keeps the plug electrodes from reaching that happy temperature that allows them to turn brown (evidence of self-cleaning) Check your carb mixture settings and ignition system (coil and plug wire) if it's an ignition miss causing this.

Generally a slight ignition miss will show up more under a load than idling around b/c the plug voltage will go up as the larger volume under compression requires the voltage to rise higher before the plug gap can be jumped. Higher voltages can leak off somewhere else where insulation is weak, or voltage may not be high enough to jump the plug gap (if plug is shorted by carbon from fuel soot the coil voltage cannot rise enough to jump the gap).

You're barking up all the right trees.

 

[Sportster-2001-951C-Stock]

Also, if you didn't cehck the pop-off and/or replace the metering needle or seat this may be the cause of running too rich.

I always replace the metering needle when rebuilding a diaphragm carb, the rubber tip on the needle wears out after a few years of bouncing off the seat. Sometimes the seat itself might have a burr on it and cause the needle to stick, causing inconsistent pop as well, some tuners use a q-tip and lapping compound to clean the seat up and smooth it before installing it.

 

[sookati]

I'll pull the carbs again and take another look. Do I need to go through both carbs again, or should I just be focusing on the PTO carb? I didn't know if each carb only controls one cylinder, or if they mix together.

 

[sookati]

Also, could a choke valve on that carb that's 10% closed cause this? When I put the carbs back together, it seemed like they would both fully close and stay open correctly, but when I adjusted the low speed screws last time, I think the PTO carb's choke valve was slighty angled in when the choke was off.

 

[sookati]

Ahhh, I think I might have found the problem. The nut that keeps the choke linkage tight on the PTO carb came loose, causing the butterfly valve to act up. Thank god for easy fixes sometimes.

 

[Sportster-2001-951C-Stock]

Good find, a partially closed choke will definitely cause the cylinder to run rich.

 

[roundtopjeff]

just a hunch but it might be a bad coil on the PTO side. I had a similar problem on a 03 RXDI...turned out to be bad coils. Are the coils interchangeable...if so try switching them.