4kRpm stutter

[rpgoerlich]

After getting the new carb bots installed, cleaning/fixing carbs, I took it for a short run. It has a stutter right around 4k or so then it picks back up. It cruises fine. But when you lay into it it stutters until it clears up. Float heights set @ 25.7mm. Had cleaned and re-gapped plugs before the ride. Added Fuel Filters to both feeds(no octy), Petcock to On. Has a 4 into 1 header.
Pulled the Air Filter and found this:



I have some Uni filter media leftover from another filter project I'm going to wrap it in.



Before I cleaned the carbs I noted that the center 2 carbs had different emulsion tubes and carb hats. These stayed with those carbs. I'm not sure which carbs hats should be where, the ones with the deeper hats are on the center 2. And the emulsion tubes with more holes in them are with the center 2 as well. Is this correct? All I could find was a part number on diagrams for these and there's no part number on the parts...

Thanks,

[rpgoerlich]

Before I did the air cleaner I went and bought a new set of plugs. Then did a plug chop after a 2 mile 60mph ride. From left to right 1-4.
2 & 3 fouling like they are was why I was asking aout the carb hats and emulsion tubes.

[DiverRay]

If you have the extra money, and IF YOU PLAN ON KEEPING THE BIKE, go with a K&N filter. It's not worth playing with the old one!
If you do, remember the dirty air is on the INSIDE of the filter, so the "new" element should be wound the inside of the mesh, NOT the outside.

[renegade_xs11g]

Just throwing the dice on this one...

but 4K studder, that's when switching to the mains, so....
you may need to double check that the mains are clean

[prometheus578]

Not familiar with the different carb "hats". Being deeper?
I'm used to seeing a difference on the inner two vacuum diaphram covers(caps), in that the outer ones are chromed, the inner ones aren't.
I'd have to go downstairs and try to compare different banks of different year carbs.
The holes in the emulsion tubes allow air to mix with the fuel before getting sucked into the carb throat.
Now, the way these carbs are set up... due to the inner cylinders not having the same amount of cooling fins, etc, they have the inner carbs run richer to help keep the cylinders cool. That's why the inner main jets are a larger size than the outer ones on some years of carbs.
With that in mind... the inner emulsion tube should have LESS holes... which would give those carbs a richer mixture.
Of course... you already have plugs showing that something is making those inner cylinders run rich.
HHmmmm.
Now then.. let's look at other possabilities, shall we?
Now... around 3,500/4000 is when the vacuum advance kicks in. The advance mechanism moves the timing plate on which your ignition pick-up coils are mounted. Due to the constant flexing of the pick-up coil wires over the years, they have been known to break internally.
So.. when the advance kicks in, the plate moves, the break in the wire separates and you get no signal to the magic box to tell the plugs to spark. Cylinders #2 and #3 work offa the same pick-up coil.
You may have a broken pick-up coil wire. This would explain the hesitation around 4,000rpm... resulting in two plugs not firing and getting fouled.
As for the emulsion tube issue... Sure, it was designed to use two different types of tubes, but the difference is slight. same as the set-up using two different sized main jets. Many members just replace them and have all the jets the same size. Not much of a difference: the early models had the same size jets all across the board.
It's doubtful that you'll ever ride the bike in a manner where you'd notice the difference, even if you could.

[Ivan]

If you have the extra money, and IF YOU PLAN ON KEEPING THE BIKE, go with a K&N filter. It's not worth playing with the old one!
If you do, remember the dirty air is on the INSIDE of the filter, so the "new" element should be wound the inside of the mesh, NOT the outside.

I would keep your receipt if you do get a KN. I have one that the rubber on the ends gets old and shrinks, then they don't fit. Seen a few others with a similar problem. I have never been able to get them to make good on their warranty either.

[rpgoerlich]

If you have the extra money, and IF YOU PLAN ON KEEPING THE BIKE, go with a K&N filter. It's not worth playing with the old one!
If you do, remember the dirty air is on the INSIDE of the filter, so the "new" element should be wound the inside of the mesh, NOT the outside.

Took a look at a stock filter. It appears the PO pulled out the paper/media that went around the inside of the filter. I have enough UNI filter media to do just that.

[renegade_xs11g]

From left to right 1-4.
2 & 3 fouling

at first glance you have an issue with #2 & #3, but
just to clarify, you have the plugs layed out and labeled 1,4,2,3
assuming you labeled them correctly you have a problem on 4 and 2

I'd suggest double checking

I hope you labeled them wrong, if you did refer to prom's post, paying close attention to the p/u coils

if labeled correctly, you may have a couple of issues, again refer to prom's post, but looking into the jets and emulsion tubes,

if that doesn't solve the problem, more info is needed
like what kind of plug boots are you using....NGK?, are they the resistor type,
some other kind? do they test OK? do the ignition coils check out OK?

I'm sure I'm forgeting some, but you get the idea

[rpgoerlich]

Yea, layed out 1,2,3,4.
I had already re-checked the float levels and re-installed the carbs. Same results with new plugs and carb sync.
Going to check the pick-up coil wiring next.
Been a little busy with the XJ550

[DiverRay]

Richard,
I would check the spark plug caps FIRST! Three looks a LOT worse than two, so the resistors in the caps may be bad. You may also have issues with the spark plug wire, and need to trim about 1/4" off the end when you reinstall the caps. I run the NON RESISTOR caps, with the resistor plugs, in my bike. keeps the EMI down, and I still get a good spark.

[rpgoerlich]

Well, #3 had a loose connection in the cap. #2 was a bad resistor which I replaced with a spare I had laying around. Both check out at 5k ohm now. I cut off 1/4" and re-installed the caps. Still need to check valve clearances. Dark now, Headlight just burned out, so a test ride will have to wait until the weekend. 30mph in the hood just wont tell. Got to get it on the highway.

Thanks,

[SG-Olli]

I would keep your receipt if you do get a KN. I have one that the rubber on the ends gets old and shrinks, then they don't fit. Seen a few others with a similar problem. I have never been able to get them to make good on their warranty either.

If the rubber of the K&N element gets harder over time and shrinks, just heat it up with a hairdryer or a heatgun (carefully).

[rpgoerlich]

Took some time today to look at the pickup coil wires... Both had 1 wire(gray) broken. Repaired both. Runs ok, need to swap out the plugs as they are still fouled out from the previous run. Should know more after the plug swap.