Off-Idle Bog/Hesitation/Stumble: Let's figure this out with Science!

Maintenance, repair, troubleshooting, etc.
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radare
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Mon Jun 26, 2017 12:28 pm

This is a problem I've had with both my Scrambler and my black/red XJ. When they warm up, I get this off-idle bog/stumble/hesitation. Essentially, it idles well, runs well, cruises well and accelerated well. But when sitting idle at a light, when the light turns green, I have to be very careful with my throttle and clutch timing. If it let out too much clutch while twisting the throttle, it'll idle down, feel like its going to die. Continuing to twist the throttle causes more bogging for a brief moment and then it accelerates like normal. This gets worse as it gets warmer. If I rev it up around 2500 to 3000 rpm and slip the clutch, I don't have any problem.

I routinely tune the pilot screws using a wideband A/F gauge and decided it was time to mount that up and see what the bike is doing during routine riding. Here's what I've observed: While idling, the A/F is around 13 to 13.5. Cruising at 60mph in 6th gear, the A/F is around 11.9 to 12.5 but seems to run around 12. When accelerating, it is in the high 11's. When accelerating from a stop, when the bike is warm and when I've been sitting for a while, the A/F drops to the mid to high 10's for a moment and then bumps up to the mid 12's.

Graphically, it looks like this (based on observations at different throttle positions as best I could, for Cylinders #1 and #2):

Image

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Jimbo
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Tue Jun 27, 2017 5:37 am

I am not as high tech as you with your wide band equipment, but it does graphically show you have a lean spot just off idle. I know your up in the thin air of the mountains so typically you run richer at that altitude. That being said maybe it would be better to go with the stock pilot jetting but raise the needle? I am thinking that with the needle up it can draw the fuel up more easily with better atomization when the vacuum changes upon opening the throttle with a smaller jet vs. a larger opening of the #20 pilot jet with the needle farther down and more seated. Reducing the chance of a lean spot off idle? That's my hunch.

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Tue Jun 27, 2017 7:50 am

Jimbo wrote:
Tue Jun 27, 2017 5:37 am
I am not as high tech as you with your wide band equipment, but it does graphically show you have a lean spot just off idle.
Do you mean rich-spot here, Jimbo?

I think its rich across the board but is going excessively rich just off idle. At 1/8 throttle, cruising, it tends to be okay and then tapers sharply as I twist the throttle. I didn't observe any instances where it went lean except when dropping the throttle closed after a WOT run and only for a moment. I expected the chart to look a lot more like the black-line. Does that seem right?

Image

What I wonder is, am I seeing an interaction between the pilots and the transition ports? Specifically, an overly rich pilot that, when the throttle opens and the transition ports add fuel, becomes overly rich to the point of causing combustion issues. What do you think?

I plan to pull the float bowls and install a set of 17.5 jets and see how it runs and what the A/F ratios are. I'll check the float heights, wet, at that same time. We can revisit then. I'm wondering if I need to drop to a #100 main. I have that jet size in my other XJ and it's always felt like it pulls harder during acceleration.

The joys of living a mile high.

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Wed Jun 28, 2017 4:57 am

Yes, I meant rich. I think the smaller pilot would restrict the fuel delivery overall but especially at initial throttle opening. Also it would allow for better atomization which would help it burn more efficiently.

I think its odd that cylinder #2 (green) is richer at 1/2 throttle and leaner everywhere else when compared to cylinder#1 (red)

Last, what does the black curve represent?

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radare
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Wed Jun 28, 2017 10:30 am

Jimbo wrote:
Wed Jun 28, 2017 4:57 am
Yes, I meant rich. I think the smaller pilot would restrict the fuel delivery overall but especially at initial throttle opening. Also it would allow for better atomization which would help it burn more efficiently.

I think its odd that cylinder #2 (green) is richer at 1/2 throttle and leaner everywhere else when compared to cylinder#1 (red)

Last, what does the black curve represent?
The black curve is what I expected to see.

I did reset the fuel levels to 5mm below line and installed the 17.5 pilot jets. I did notice that I had #100 main jets already (I thought I had installed 102.5's) so the chart above is with #100 mains.

Here's what I saw after the change. I only looked at cylinder #1. The off-idle bog is still there, when hot, and I notice its still dropping to the high 10's the moment the throttle is opened. What I suspect is that the pilots screws need to be set a bit more lean, say, 14.1 or so A/F. I'm thinking that the bike is a bit rich at idle, then when the butterfly is opened and the transition ports kick in, she's going overly rich. I'm going to adjust those and see what impact it has.

Image

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radare
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Wed Jun 28, 2017 11:34 am

As mentioned above, I'm suspicious of an interaction between the transition ports and the pilot circuit. I don't fully understand how the transition ports work, where they get the pressure that drives fuel flow, or how to adjust the pilot screws to account for the immediate rich condition when I open the butterflies. Any ideas on good sources to read or online resources that might help?

Image

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Jimbo
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Sun Jul 02, 2017 4:24 pm

I found this link which describes the function of the "transition holes". It is for Keihin carbs but the theory holds for all carbs with these ports. It appears it is part of the pilot circuit and is for better atomization through many fine holes rather than through a single larger port/jet. The draw or vacuum is the same as for the jets through the carb venturi. I am guessing these fine ports are cast into the pilot circuit and for fuel. Maybe a test would be to pump air or fuel through the temporarily sealed pilot jet and see if the air or fuel emits from these transition holes?

http://www.performanceindian.com/Keihin.html

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radare
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Wed Jul 05, 2017 10:26 am

After a good ride with Ed this past weekend, the problem persists though better. I am still getting an off idle bog, the exhaust still smells of unburned fuel and the bike runs worse at altitude.

So, my friends, I'm going to do it. #15 pilot jets. That's right. #15. Yes sir. We shall see.

Once I get this sorted out, I'm going to persuade Ed to follow suit. His bike is even more rich than mine (after following him for 200 miles, I know, trust me).

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radare
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Wed Jul 12, 2017 12:39 am

My new set of #15 pilots came in today. I'm going to pair them with a new set of #100's I have and just install new jets in the whole thing. More to come on success or failure of smaller pilot jets.

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radare
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Thu Jul 13, 2017 10:41 am

I installed the new #15 pilot jets in the Scrambler and took it for a ride to warm it up for tuning. When cold, it starts up with choke after a couple of cranks. It was 80F, overcast and humid yesterday; in these conditions, it took about 3 minutes for it to warm up and run right without enrichment.

I rode it for about 30 minutes to warm it up and then set the pilot screws using my wideband a/f gauge. I set each to roughly 14.7:1 at idle. Pilot screws were all in the 3.5 to 5 turns out range. At this setting, the exhaust smelled slightly of unburned fuel and the engine bog was slightly present off idle.

I then rode the bike for another half-hour, adjusting the pilot jets inward and noting the affects. With each 1/4 turn inward, the bog would be a bit less and the throttle response would improve. I ended up with pilot screws set at between 3 and 4 turns from seat. I will reconnect the A/F in the next few days and see what my final idle A/F ratio is. I suspect it'll be in the lean range, 15.5:1 or so.

Throttle response is greatly improved. You can twist the throttle, let out the clutch, and the bike goes. No bog, no hesitation, no thinking-about-it, it just goes. Like it should.

Low speed riding is also greatly improved. Low gear light-throttle jerkiness is almost completely eliminated and the bike responds well when applying the throttle in a downshift, say, dropping into second, letting the clutch out and taking a turn.

The engine power feels flat from 3k to 5.5k where it then picks up in a woosh and carries well all the way to red. The liveliness of the engine in this RPM band is reduced and you can feel the difference over the more rich setting.

General rideability is greatly improved

I think the next steps are to take a look at the low-end power band and see what can be done to improve fueling in that range. Right now

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