Sputter right at 2000 rpm

My XJ recently started to sputter when coming off idle. I noticed that it's almost exactly at 2k and it's worse when it's warmed up. I don't notice it at all when the engine is still cool.

The bike starts easy, idles nice and smooth, pulls strong and steady at highways r's. It just has this sputter at 2000. The plugs have a nice tan color. One more thing, it's usually worst after idling at a traffic light for several minutes.

Any ideas?

 

Oh well, no opinions. I thought someone may have had a similar issue.

If I find the cause and cure, I'll post it.

 

tuff one man, was it jetted for mods? if so maybe the fuel pilot's a tad to large. or could just be dirty emulsion tubes, when was the last time they were cleaned? You sure all the plugs are tan?

 

Yep, the plugs all look nice and tan. No mods - all stock. They were cleaned VERY thoroughly last fall and since I'm getting pretty good at pulling and installing the carb rack, I'll yank 'em and check everything again. Maybe some crap got by the fuel filter.

The one thing I haven't done yet, only because the slides clunk as Rickomatic says, is polishing the bores. Might as well do that while the rack is off.

It's just so odd that it only happens at 2k. Is that some transition point in the carbs?

 

Isn't that when the pilot jets stop doing their business and main jets take over?

From what I've read, the pilot jets work from idle to about 1/4 throttle... might be worth looking into.

 

yeah that's why i mentioned the emulsion tubes. as the needle comes up more holes get exposed and allow fuel flow. I guess that could happen if the lowest holes are clogged

 

Thanks and noted.

I was reading about carb tuning on another site and it said that if low speed stumble worsens as it gets warmer, then the pilot circuit is rich and if it lessens, it's lean. In my case it's worsens. I don't even notice it until I've gotten it good and warm.

If the weather keeps on being so nice, I might not get around to looking into it.

 

i had a similar problem with my bike - chalked it up to the aftermarket 4-into-1 exhaust and upped my pilot jet (after i had already upped the main jet and shimmed the needles).

after i increased the pilot, the stumble went away.

my theory is that 2000 rpms (or, in that neighborhood) is where the main jet hasn't really come into play and your engine is calling for everything the pilot circuit can give it.

with that said, and since you said it just started happening, i'd chalk it up to partially blocked pilot circuit.

temperature dependent symptoms could point to electrical issues, but - in my experience - electrical problems are usually worse when cold and better when warmed up. however, ohm-ing out coils is easier than removing carbs . . .

 

yup, since there's no mods the jetting should be spot on. with the Pilot Fuel and Pilot Air jets both removed you should be able to get guitar string out the other hole. Could be a little aggrivating but worth it, it just takes 2 or 3 minutes but it's all worth it when you feel you've broken through something