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General Category >> Rubber Side Down! >> What actually cases a bike stall on deceleration?
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Message started by B-Will on 10/27/21 at 06:58:18

Title: What actually cases a bike stall on deceleration?
Post by B-Will on 10/27/21 at 06:58:18

So what actually causes the bike to stall when doing a rapid deceleration? For example, let's say you're cruising at 50mph, you pull in the clutch, the rpms start to lower to the 'idle' point and then the engine stalls.

If I'm understanding the Mikuni's design correctly, during this scenario the carb is rapidly transitioning from using the main jet, to the needle jet, to the idle jet (with some overlap in there where it's using multiple jets as to transitions). So, a stall is probably the needle jet not functioning correctly. Right?

My theory is that the movement of that needle is somehow being restricted--it's catching on something or having a hard time sliding. I know the needle itself moves, but does the black cylinder the needle lives in also move up and down a bit? Seems like that diaphragm allows it to move a little. If either of those are restricted, that would cause the bike to stall during deceleration, right?

This is all assuming the air/fuel is tuned properly (idling fine, accelerating fine, little to no backfiring, etc.)

Thanks for the insight!

Title: Re: What actually cases a bike stall on decelerati
Post by B-Will on 10/27/21 at 07:29:44

I might have answered my own question. I found a great animation on the Mikuni:

https://youtu.be/wyspAHrMbb8

The carb in the animation is a little different, but it shows the basics of how it works. Anyhow, I'm guessing that transition from the main jet, closing the throttle slide, to the pilot jet is where the problem is. The bike is 20 years old so the throttle slide probably needs some TLC.

Title: Re: What actually cases a bike stall on decelerati
Post by Dave on 10/27/21 at 10:52:35

When the butterfly closes and the slide drops - you create a situation where there is a very high engine vacuum....and the only fuel circuit that is open is the idle circuit. The high engine vacuum pulls a lot of air past the butterfly and slide - but only a small amount of fuel can come out of the idle circuit.....therefore the fuel/air mixture goes very lean, and it is so lean the spark plug cannot ignite the mixture and create the power pulse in the engine.

The stock carb has a small diaphragm that opens when the vacuum gets high, and it is supposed to add enough fuel to correct the lean fuel mixture when the throttle is closed. Unfortunately the system doesn't seem to work very well. A low idle speed can also make this situation worse, as can a dirty pilot or one that is too small, or a low float bowl level, etc.

Title: Re: What actually cases a bike stall on decelerati
Post by B-Will on 10/27/21 at 12:13:51

Thanks for the insight Dave. Ok I'll start by turning my idle up. From your info, sounds like a low idle chokes the engine on decel.

Title: Re: What actually cases a bike stall on decelerati
Post by justin_o_guy2 on 10/27/21 at 18:06:03

The engine Really does not like low rpm. The oiling isn't that great. The book calls for11 or 12 hundred rpm, I don't remember. But the Blop, Blop, Blop cool sounding idle is a death sentence for it.

Title: Re: What actually cases a bike stall on decelerati
Post by B-Will on 10/29/21 at 12:20:40

Ok so I read on another post that you can use a metronome set to 500bpm to get a sense of what 1000 rpms sounds like. I'll use that trick to make sure the idle is at least that, maybe faster.

With regard to idling on the kick stand, is it pretty safe to put the bike on my motorcycle lift, raise it just to it's vertical (and not leaning on the kick stand), then let it idle for 10 or so minutes at a time while I dinker with idle stuff? I'm guessing oil doesn't really flow properly until the RPMs are a bit higher, so I wouldn't do this for very long.

Lastly, I took the rear carb boot off and had a peak inside the carb while it was running (on the motorcycle lift). I learned a few things and highly recommend this for other newbs:

- The slide actually vibrates a bit with the RPMs of the bike (at least, mine does). For some reason I figured it would just be sitting there until you throttle it. Nope, it looks like a little sewing machine needle in there vibrating up and down a little while the bike is idling. That was cool to see.
- When you increase throttle, you can see the slide move up and the fuel come up and out of the main jet. If you have a sticky slide I bet you can see it pretty clearly doing this.
- You'll see some fuel in there squirting around and really get a sense of how vacuum operated the carb is. Pretty neat to see.
- When you do this, you see exactly the interplay between pilot, needle and main jet and it really all makes sense. You can also get a sense of how vacuum issues can really degrade the carb's performance. Everything in that component operates on vacuum power. It's really fascinating to see and an engineering marvel to be honest.