Carbon fouling at 1/3 throttle?

[bogan]

Have just put a custom exhaust and pods on my bike, it seems (from plug chops etc) to be tuned good for half throttle and above, also fine for idle. Noticed the problem when it started stumbling at 100kmhr cruise after a few mins, front cylinder plug shows carbon fouling. It's a V-Twin and only a problem on the front cylinder, rear is fine, carb set pretty much the same as the rear, although I have lowered the floats 1.5mm extra on the front thinking it might help (it didn't help significantly).

I think the next step is to drop the needle a clip on the front, though it's very uncommon to have needles set at different clips. Anyone have any other ideas as to what it might be that I could have overlooked? I'd much rather sort the problem, than tune around it.

I've checked the choke, and ended up taking off the cables (left the plungers obviously) as I don't use it anyway. Verified that the float valves are working and the set height is in range. Looked for wear on jets etc. Inspected the cv rubbers and no visible leaks. Have balanced the carbs too.

[MSTRS:Me]

Front cylinder is usually supposed to run hotter than the rear (or rear should run cooler LOL). Not so important with watercooled engines but I think it still happens. See - the front cylinder blocks some of the airflow to the rear. So the rear should run a tad richer to achieve slightly cooler ignition.

[bogan]

Front cylinder is usually supposed to run hotter than the rear (or rear should run cooler LOL). Not so important with watercooled engines but I think it still happens. See - the front cylinder blocks some of the airflow to the rear. So the rear should run a tad richer to achieve slightly cooler ignition.

The RC31 is backwards in that respect (I guess there is a restriction in the coolant supply to the front), the front is supposed to run a bit richer, the main is generally a few sizes bigger but the rest are the same, which I've done. So if anything the requirement of a richer front should cause the rear to foul plugs when jetted the same.

[racefactory]

Is it lean or rich on the cruise? Float heights have been the only thing which affected steady partial throttle for me... surely something to do with it? Tried with different heights?

[bogan]

Is it lean or rich on the cruise? Float heights have been the only thing which affected steady partial throttle for me... surely something to do with it? Tried with different heights?

yeh rich on cruise, dropped the float height 1.5mm, which eliminated bogging when opening throttle from low rpm, but didn't really affect cruise. Dropping a notch on the needle now so will see if that fixes it.

[scumdog]

Front cylinder is usually supposed to run hotter than the rear (or rear should run cooler LOL). Not so important with watercooled engines but I think it still happens. See - the front cylinder blocks some of the airflow to the rear. So the rear should run a tad richer to achieve slightly cooler ignition.

I wonder how the fuel in my Harley knows to make the rear cylinder richer ??

[bogan]

I wonder how the fuel in my Harley knows to make the rear cylinder richer ??

don't be silly, we are talking about tuned bikes, not harleys, they still use the bit of cloth covered in petrol as a carb right?

[bogan]

When I took the diaphragms off just now, I noticed a very small bit of fiber (probably from cleaning cloth) that was stuck under the seal, may have been causing slightly less vacuum to be seen by the diaphragm. I can't figure out whether this would cause a lean or rich condition though; lean because the lower slide means it isn't operating as high on the needle, or rich because the slide blocks air and the increased vacuum on the engine side still pulls fuel through.

[bogan]

That seemed to do the trick anyway, no stumble at cruise, bugger all carbon on my exhaust louvers this time, and plug came out cleaner than it started!

Will wait for a nicer day then go find a hill to do a needle plug chop on I reckon, just check that is all good still.