Low rpm misfire under load

[RWhitehouse]

So over the past few weeks I've been noticing an issue with my '98 tour developing what feels like misfire at low rpm under load. Namely, setting off from a stoplight, or anything but the most gentle roll-on from less than about 2000rpm. If you lug the engine too much, it seems to suddenly start misfiring, feeling as if several cylinders cut out. Gentle acceleration is fine, and above 2k any throttle, any rpm runs great. Pulls cleanly to redline with zero stutters.

It has Cobra exhaust but otherwise airbox, filter, and carbs are stock, aside from pilot screws at 3 turns.

Bike has about 55k on it. Carbs rebuilt/ultrasonic cleaned and new intake o-rings around the 45k mark. Nothing changed since.

I tried replacing the oem air filter, it really didn't look too bad but put a Hiflo-filtro replacement in. No perceptible difference better or worse.

Plugs (iridium's) have about 20k on them and all six looked very similar with a light tan color, no red flags there.

I tried waving an unlit torch around the carbs and intake with it idling to see if I could detect a vacuum leak, again, no luck there. Idle is rock steady at 900rpm.

Any suggestions where to look next? I find myself having to give it way more revs than seems necessary to cleanly get off the line, and running it a gear lower around town to be able to cleanly accelerate without the stuttering.

Thanks for any suggestions or help.

[Chrisj CMA]

I would go back to NGK (stock plugs) and stop lugging the engine

[WintrSol]

It has Cobra exhaust but otherwise airbox, filter, and carbs are stock, aside from pilot screws at 3 turns.

That seems very rich, if it is a Standard or Tourer; only a bit rich for an Interstate. Have you tried adjusting them for best idle after warm up?

[98valk]

if you have the cobra (aka dyno jet) needles, lower needles by raising clip one notch. your too rich.  if using ethanol gas plug color is indicating too rich.

[hubcapsc]

I think nothing's changed on his bike for 10,000 miles, and the problem started showing up
in the last few weeks... I wondered about a vacuum leak, I don't know whether OP's test
was a good one or not... if desmogged, there's only about one vacuum hose and some caps
on the back of the intake runners... if not desmogged, it is only a little harder to change
the vacuum lines... if the bike has sat a lot this summer, maybe it is gummed up again...?

-Mike

[RWhitehouse]

It has been desmogged, all six intake nipples are capped off, manual petcock. Regular use all summer, not sitting around, never goes more than 2-3 days without being ridden. The caps and o rings were new at the carb rebuild.

The needles are OEM and all only have 1 shim as stock, no extras. It has stock 100 mains as well.

I'm not slamming it WOT from idle in top gear or otherwise excessively lugging the motor, it shows up in just normal around town driving.

I do notice the problem doesn't occur on a cold engine, it's only after 5-10 min when it's fully warmed up which would seem to go along with it being too rich. I'll try giving the pilot screws a 1/2 turn in and see if the issue improves.

[WintrSol]

I do notice the problem doesn't occur on a cold engine, it's only after 5-10 min when it's fully warmed up which would seem to go along with it being too rich. I'll try giving the pilot screws a 1/2 turn in and see if the issue improves.

Another indication the idle is too rich. Warm it up, then adjust the pilot screws to best idle.

[Ricky-D]

A misfire under load is a symptom of a weak ignition.

I'd suggest to install new spark plugs for a start.

If that doesn't correct the problem, then I would suggest to change the plug wires. I'd only change two wires going to one coil. If that doesn't correct the problem I'd change those wires to the next coil, and so on.

Finally, if you're still having the problem, I get a new coil and do the same as with the plug wires.

I feel pretty positive this should correct your problem.

Hopefully, with the new spark plugs.

***