4000 RPM surging/stumble

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savethe2strokes
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4000 RPM surging/stumble

#1 Post by savethe2strokes » Fri Oct 13, 2017 10:40 pm

I seem to have a slight surge or stumble under 1/4 throttle or less in the 4-5000 RPM range. I have been trying to tune it out by richen up the mixture. I have tried changing needle position(little effect), I have also tried having the air screw at 1/2 a turn out.

Bike: Stock displacement, Stage 2 portjob, Mikuni TM28 flatslides (280 MJ, 27.5 PJ, 1.5 Turns out), Uni pod filters, and Toomey exhaust.

The bike runs great outside of that range. My next thought was going to go up on Piliot jet and then lean out the air screw for idle and off idle. I purchased a 2 into 1 filter setup as I saw some people said that helps with this stumble.
Last edited by savethe2strokes on Fri Oct 20, 2017 8:12 am, edited 1 time in total.
85 RZ350-Messed up wiring

hondaror
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Re: 4000 RPM surging/stumble

#2 Post by hondaror » Fri Oct 13, 2017 11:13 pm

You are asking for near impossibility. The bike is not meant to be dogged, down there. Ride it hard. Accelerate hard. That is how a 2 stroke should be operated. These bikes are not cruisers.
The porting creates low end issues, ruining street ridability. Your larger carbs also affect the low end. You have done everything to boost upper end power and wipe out low rpm function. Jet for hard running. If you want smooth linnear power band, ride a 4 stroke.
Rory
2 1984 RZ350s
1985 RZ350
9 1986-1990 RZ350s
2000 ST2
2005 749
2005 749 with 999 engine

MK
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Re: 4000 RPM surging/stumble

#3 Post by MK » Sat Oct 14, 2017 5:29 pm

Usually a combination of needle position and idle air screw can positively influence this nasty habit. You may try harder or get some help by experienced staff ?

Second or additional possibility:
Try an adjustable ignition and get a RGV throttle cable splitter with a TPS.
I run ignition curves with very low advance at low rpm and TPS. As low as 7 deg for example at around 10% TPS and 3500 rpm.
That cures even more of the issue but at the cost of additional air/fuel ratio fiddling as the ignition also influences the combustion, so re-adjustment in that range was required. Generally speaking a high AFR of around 15 to 15.5 was required here (aka lean).

BTW: I run EFI with 38 mm throttle bodies, ported cylinders and DIY pipes that peak at 9.5k. So yes, it is possible to run this on the road.
Bye
Martin

savethe2strokes
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Re: 4000 RPM surging/stumble

#4 Post by savethe2strokes » Sun Oct 15, 2017 10:22 pm

MK wrote:Usually a combination of needle position and idle air screw can positively influence this nasty habit. You may try harder or get some help by experienced staff ?

Second or additional possibility:
Try an adjustable ignition and get a RGV throttle cable splitter with a TPS.
I run ignition curves with very low advance at low rpm and TPS. As low as 7 deg for example at around 10% TPS and 3500 rpm.
That cures even more of the issue but at the cost of additional air/fuel ratio fiddling as the ignition also influences the combustion, so re-adjustment in that range was required. Generally speaking a high AFR of around 15 to 15.5 was required here (aka lean).

BTW: I run EFI with 38 mm throttle bodies, ported cylinders and DIY pipes that peak at 9.5k. So yes, it is possible to run this on the road.
I have adjustable ignition. Not sure if that would help or not. I might try dropping the needle again. I feel like it is too lean right at 4500 rpm at light throttle.

How did you setup fuel injection?
85 RZ350-Messed up wiring

hondaror
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Re: 4000 RPM surging/stumble

#5 Post by hondaror » Mon Oct 16, 2017 12:54 pm

Dropping the needle, is going the wrong way if you are too lean. Drop the clip to raise the needle.
Rory
2 1984 RZ350s
1985 RZ350
9 1986-1990 RZ350s
2000 ST2
2005 749
2005 749 with 999 engine

savethe2strokes
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Re: 4000 RPM surging/stumble

#6 Post by savethe2strokes » Mon Oct 16, 2017 2:17 pm

hondaror wrote:Dropping the needle, is going the wrong way if you are too lean. Drop the clip to raise the needle.
That is what I meant to say. I do that everytime when talking about messing with the needle. I mean dropping the circlip down on the needle to raise the needle and make it richer.
85 RZ350-Messed up wiring

MK
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Re: 4000 RPM surging/stumble

#7 Post by MK » Mon Oct 16, 2017 3:15 pm

A good method that'll always work is hypothesis and experiment.

In your case your hypothesis is "it's too lean at 4k and x% throttle"
An appropriate experiment would be to richen that range.
The lower throttle range is influenced by the idle jet, the idle air screw, the slide cutaway and the combination of the needle jet inner diameter and the outer straight diameter of the needle.

Raising the needle will influence the slightly higher throttle positions, so you may try idle air screw and idle jet first.

Image

In any case: If the experiment was successful, go on into that direction until it's good enough.
If it got it worse, invert the hypothesis i.e. try the opposite (i.e. richer in the interested region)

Do the same kind of experiment with strongly reduced ignition advance in the problem area.
Bye
Martin

tankslap
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Re: 4000 RPM surging/stumble

#8 Post by tankslap » Wed Oct 18, 2017 5:59 pm

"A good method that'll always work is hypothesis and experiment."

I do this and it works. Think about what is happening, conjour up a way to test your idea, make the change, test again and prove the theory right/wrong or no change.

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savethe2strokes
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Re: 4000 RPM surging/stumble

#9 Post by savethe2strokes » Fri Oct 20, 2017 8:18 am

Update:

I tried to go richer on the air screw and it helped with the surging but off idle felt terrible. Stumbling and has to really rev the bike to get going.

I leaned it out to 1.75 screws out and that helped clean up off idle, but the throttle wasn't as snappy. It felt dull in the midrange. The needle was on the 2nd richest setting. My next run I turned it the air screw 2 turns out. Ran well but was still fouling plugs. I moved the needle to the 2nd Leanest setting and the bike was crisp and plug color looks great. Now the surging is back under certain throttle positions. Overall the bike runs great. I just have that slight surging from 4-5k RPM. Everywhere else is great.

I bought a 2 into 1 intake to see if that helps vs. the pods.
85 RZ350-Messed up wiring

justind97
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Re: 4000 RPM surging/stumble

#10 Post by justind97 » Fri Oct 20, 2017 11:01 am

4-5k, that's right on the transition of the powerband to getting on the pipe. You will be hard pressed to get rid of the stumble and have the bike running perfect throughout the entire RPM range unless you start playing with the advance/retard on the ignition at that point.

The question becomes, how much time are you willing to spend on this stumble and will it significant depreciate your happiness while riding the bike if it is there?

savethe2strokes
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Re: 4000 RPM surging/stumble

#11 Post by savethe2strokes » Fri Oct 20, 2017 4:00 pm

I have a programmable ignition already so there is no additional cost.

https://youtu.be/AeZEHtgkrzo" onclick="window.open(this.href);return false;

Around 15 seconds into that video you can hear the engine a bit with the "surging".
Last edited by savethe2strokes on Mon Oct 23, 2017 9:27 am, edited 1 time in total.
85 RZ350-Messed up wiring

hondaror
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Re: 4000 RPM surging/stumble

#12 Post by hondaror » Fri Oct 20, 2017 11:17 pm

I still say, you won't be able to get rid of that stumble due to porting changes. There's no such thing as perfect...only what's good enough for what you want. You haven't stated what you want, other then perfect by trying to get rid of the stumble. Your port work tells me you want top end performance. What exactly are you looking for?
Rory
2 1984 RZ350s
1985 RZ350
9 1986-1990 RZ350s
2000 ST2
2005 749
2005 749 with 999 engine

savethe2strokes
Posts: 513
Joined: Tue Jan 24, 2006 6:35 pm
Location: Kansas City, MO

Re: 4000 RPM surging/stumble

#13 Post by savethe2strokes » Sun Oct 22, 2017 11:10 am

I use this bike to tool around town as well as go for a good blast once in a while. I'm not using it for touring, but I do like to cruise the highway not at WOT all the time.

Overall the powerband is perfect IMO. Great off idle characteristics, midrange seems to pull cleanly, and top end punch. It is just under light throttle and that 4-5k RPM range that seems to be an issue. If I richen up the mixture where off idle sucks or is too rich, the surging goes away. It sounds like I could maybe retard the timing a tiny bit to help with that. I don't know where to really start there though.
85 RZ350-Messed up wiring

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T.RexRacing
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Re: 4000 RPM surging/stumble

#14 Post by T.RexRacing » Sun Oct 22, 2017 1:42 pm

Start with MK's advice. It costs ya nothing. Retard timing from 4-5K RPM a couple degrees at a time until it goes away. It will be interesting to see how it pulls through that area under load though. The TPS is the real key to a cure.
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MK
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Re: 4000 RPM surging/stumble

#15 Post by MK » Sun Oct 22, 2017 2:45 pm

"A few" deg likely wont do the job. As I mentioned I run as low as 7deg in that low rpm / low tps region.
If you don't have a TPS, the mod may yield to poor performance on WOT in that region.

And 2ct on jetting:
Checking plug color when fiddling with idle or partial settings is BS. It's a test to solely rate WOT air fuel ratio and that is solely affected by main jets and power jets (if existing).

Changing one component and then making one rpm/load point better but another one worse is the curse of a carb. In that case it's not enough to change one component, but to try combination of two like idle air jet and needle position (that's what I optimised in stockish 31K engines fitted with the F2 pipes which required richer partial load setup)

BTW: Can't see the video. YouTube says not available.
Bye
Martin

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