Gs425 sputtering mid range issue

[Hokano12]

New problem popped up on my 425 over the weekend. Have had the bike about 6 months and haven’t really had any issues. The engine has started to sputter at about 1/4-3/4 throttle in all gears. When this happens the dash lights/headlight start to flicker a bit as well. Carbs have been dipped recently and I replaced spark plug boots. The CV Carbs have 132 mains and 127.5 pilots. I have K&N pod filters as well as a Mac 2-1. Pulled spark plugs after this started happening and surprisingly they were a nice chocolate/tan throughout the whole thing. Today I replaced the intake boots w new o rings and new gaskets for the spacers. Tank is very clean, petcock is clean as well and gas flows smoothly. Bike has pulled smoothly the whole time I’ve had it up until now.
Not sure where/what to tackle next. Could it be the coils failing? I cleaned up all my connections recently as well as replacing the old r/r with and shindigen sh775 r/r which has been working great and charging at 13.8v-14.1v.
Any pointers appreciated thanks guys.

[Hokano12]

*27.5 pilot jets sorry haha

[Rob S.]

When this happens the dash lights/headlight start to flicker

Sounds like electrical might be the first place to look. See if turning the bars end to end causes anything.

[Hokano12]

Turning lock to lock hasn’t done anything from what I’ve seen so far, just when throttle is at about 1/4-3/4, is when I get the sputter/flicker of lights. The vacuum line from petcock to carbs is old so maybe I will replace that and see if there’s any cracks

[Gorminrider]

It does sound electrical because of the light flicker and engine sputter....check for tightness at the battery terminals and ignition components. Having the motor apparently die completely for a second or so sounds like the spark stopped . Have you got points? Bad condensors can cause this I think.

I just recently found a kink in a petcock vacuum tube and it was more a decline of power as the gas mix got leaner unless I tweaked the throttle to make a stronger vacuum pulse that'd get past the kink so I can't see that that'd be your problem. But by all means look at it. My vacuum line was a plastic one -more prone to kinking especially if warmed,and routed badly I'm guessing

Report back- we all learn from solutions.

[Hokano12]

Thanks for the response, I'll check the battery terminals today. I have a dyna s ignition right now with stock coils so no condensors. Checked the wires coming from the coils going into the harness and cleaned those up as well. Specifically what ignition components should I be looking at? Would the actual turn key ignition be somewhere to start? Gonna go ahead and put in new spark plugs today as well as the ones in there are from the PO.
I also read another thread where someone had a similar issue to mine and once he replaced the stator the issue went away. Will definitely do that down the line after some more troubleshooting.

[Hokano12]

Also another thing I did test recently was compression on both cylinders....92psi on both cylinders.... When I poured a half oz of oil in both sides they jumped up to 120psi. Not sure if this could be a cause of the sputtering but I am definitely going to be doing piston rings this winter along with a cylinder hone then valve clearances.

[Gorminrider]

aah. a new ignition system...I'm especially not helpful there Are there different models? if so, you might need to get into that with specifics as to which one you used so someone else might know it better.

Does the Dyna use the original ignition sensors? and mechanical advance? Have a look and make sure your mechanical advance weights are not getting jammed by a loop of wire.

Sparkplugs? worth a try, though I usually clean them test them and put em back in unless they are notably bad. With the OEM coils still being used, You might want to have a look at the plug caps and wires for any damage. Some people have discovered trouble (in a dark place) with sparks coming from the now-ancient plug leads.

I've read similar per stator ...I guess it might be a wire from it to the regulator arcing from a chafe point. But I'm not sure of the exact effect of AC leaking to ground..but I do know that running the bike without a battery properly in place generally causes choppy running and a flickering headlight at low rpm so if you have that going on, it's a clue.

I'm trying to avoid carburettors as an issue...a kink in the throttle cable, sticking CV carb slides, torn diaphragm all that crap.

[Gorminrider]

Also search the forum with words like "sputtering" "mid-range" and all. Including the "Old Technical Info" section. This is a long-lived forum and just about everything that can happen has been solved by someone. I have never posted but once with a question, because I always "find" it.

[Hokano12]

Awesome I’ll keep digging and troubleshooting and report back with any success.

[John Park]

Also another thing I did test recently was compression on both cylinders....92psi on both cylinders.... When I poured a half oz of oil in both sides they jumped up to 120psi. Not sure if this could be a cause of the sputtering but I am definitely going to be doing piston rings this winter along with a cylinder hone then valve clearances.

If you use a rubber tip type compression tester the 3/4" hole will become part of the combustion chamber volume. This doesn't make much difference on a big V8 but can drop maybe 20 lbs on a small bike cylinder. With a screw in type with a 3/4" reach you will get an accurate reading and usually about 140 lbs. Probably, by now, the rings will be somewhat worn but if the oil consumption is reasonable the performance won't suffer much. Be sure to open the throttle wide when testing. An ounce of oil is way too much and can affect the reading; just give a good squirt of WD40 or similar to wet the rings.

If you decide to do the top end be sure to get at least the gaskets to put it back together before taking it apart. If you can't find any, ask on the Twins Forum as somebody may have some left over. Also the availability of rings, probably standard. Cmsnl.com can be helpful here. Many parts for the roller bearing twins are getting scarce whereas the 450 motors continued on as the 500 until a decade ago. I had a 425 that served me well and proved hard to kill - and I certainly tried. Great memories of 7000 rpm days on the backroads of Mexico.

[Hokano12]

I already have a nos gasket set and piston ring set lined up for when I decide to tear the top end down. I have the screw in type with the rubber o ring on it. It does consume a little more oil but it never has an issue starting and definitely has a nice powerband! Cruises at 80mph at 6000rpm. Love the bike and thanks for the input!!

[Hokano12]

Just wanted to update this thread. I ended up switching from the stock coils over to new coils that are 5ohm along with ngk spark plug caps. After this, the sputtering went away and the idling seemed to be healthier. Bike no longer hesitates past that 1/4-3/4 throttle position and pulls hard past 5000rpm. The dyna s ignition calls for the 5ohm coils to be used with the system. I guess my stock coils just gave out after riding hard for the past 6 months. Thanks for all the previous input everyone 👍🏻

[Gorminrider]

Good news, Hokano!