Hi Guys, been a while since I have posted, but I have got a gremlin again.....
I finally after a long wait, got some more Main jets to play with, my little GS400e is now pulling nicely at WOT. But, after I installed the smaller mains (Went from a 130 to a 125), I now have a massive flat spot/stumble when I try to accelerate at 1/2 throttle from 3500 - 4500 rpm. I can reduce the stumble slightly by going to WOT quicker, but it is not ideal for riding in traffic to have to gun the bike constantly.
I did not have the issue with the 130 mains, however, the 130s were too rich up top. Needle clip is set to the lowest position (Richest), so I presume from deduction it is a lean issue as it started after I moved down to 125 Jets, so I might have to add some washers to richen the mid-range more?
Things to note:
- Yes it is a 400, not a 500, so the standard jetting matrix does not work (Tried it)
- I am running pods (Never got the stock airbox with the bike when bought, they are impossible to find in South Africa and K&N lunchbox filters are also impossible unless you import them at the cost of your firstborn)
- I am running a very open Airage exhuast slipon
- Pilots are size 40, 2 & 1/2 turns out
- I also went down from the stock 15T front sprocket that the 400s come with to a 14T front sprocket when changing jest, but I doubt that would have anything to do with the flat spot (Correct me if I am wrong)
I know, pods are the devil and all, but that is what I have to work with, so any advice other than returning to stock would be highly apreciated.
mine had a stumble on the same range, part throttle. It was rich. Took a lot of work to figure it out. I would not assume it's lean.
Quote from: mr72 on May 10, 2021, 04:15:45 AM
mine had a stumble on the same range, part throttle. It was rich. Took a lot of work to figure it out. I would not assume it's lean.
Rich on the needle or on pilots? I have not dropped the needle past the mid clip position for fear of going lean, will try going more lean on the needle thank you
Quote from: mr72 on May 10, 2021, 04:15:45 AM
mine had a stumble on the same range, part throttle. It was rich. Took a lot of work to figure it out. I would not assume it's lean.
I just realized now as well, I tried using the choke to get the flat spot to go away, it did not make it better, so I think you might be on to something with the rich issue.
Take your air filters off and go for a short ride. Better? that would make the mixture leaner and indicate which way to go.
In my case it was rich on the main jet. Smaller main jet and shimming the needle fixed it.
Jetting is related to throttle position. Not RPM.
Post back with throttle position.
Cool.
Buddha.
c'mon buddha, you know that's not exactly true on CV carbs...... :-)
Update, I shimmed the needle, stumble is nearly gone. I am planning on adding another washer on Saturday to see if it cures the stumble completely, otherwise, I will just live with it, I am tired of taking off my tank and disassembling the carbs every week. :hithead:
Quote from: herennow on May 13, 2021, 12:34:14 AM
c'mon buddha, you know that's not exactly true on CV carbs...... :-)
Yea, its only true from idle on up to redline, only at steady state (where rpm is stable) and its also not linear. Its not true below idle. LOL.
However it is more true in a CV carb than a smooth bore or other non cv variety LOL.
Cool.
Buddha.
The traditional carbs such as old brit bikes have a direct relationship to throttle opening and twist grip.
As you say on CV carbs the relationship only holds at steady state. All the other times when accelerating or decelerating you have no control over the throttle opening, it is all done by the vacuum. Not sure about the other folks, but my flat spots were always when accelerating hard with so that's when you need to troubleshoot. 😉
OMG - you're correlating throttle opening to twist grip.
Not the jets to throttle opening.
Ofcourse it has to be steady state rpm else you're not in whatever jet dictated by twist grip.
Your problems accelerating hard are likely high float level or needle lifted too high or sometimes slide rising too fast. But you still ahve to see if it is 1/4 steady state to 1/2 steady state and then it is float level or 1/2 to 3/4 again steady state.
Jets being used are related to throttle opening even in a cv carb once it is at steady state. On a smoothbore throttle opening will correlate to jet being used without steady state rpm.
Cool.
Buddha.
For me "Throttle position" as you stated = twistgrip position :2guns:
Throttle valve is what I think you are referring to ;)
My flat spot / stumble accelerating hard at 5k was too large main jet. It's perfect now, although at 50 000 km I suspect it would be even better with new needle and needle jets.
Twist grip = butterfly position, since you cant see that you have to go off the grip that you mark with white paint or something.
I have no idea what a "throttle valve" is.
The slide will rise when rpm has steadied and that is related to the butterflies. So basically at steady rpm the twist grip has a direct correlation to both butterflies and the slide regardless of rpm. So twist grip position is used as the proxy for the slide which really is what controls the jetting.
Cool.
Buddha.
Valve =
common vernacular for that part is "slide".
hilarious screen shot obviously from BikeBandit who seem to be incapable of getting OEM parts. Amazing they are still in business. I've been waiting 3/4 of a year for some Triumph parts that should be easy commodities.
Mr72, you're killing me.
Honestly I thought valve was the butterflies ... sorry I dont do troubleshooting from a parts fiche.
Cool.
Buddha.
Yeah, if someone said "throttle valve" I would have also thought of the butterfly. Surprised by that parts diagram terminology. No wonder people think carburetors are complicated.