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Message started by DavidOfMA on 11/14/13 at 14:01:17

Title: Eliminating shutdown gunshot pop?
Post by DavidOfMA on 11/14/13 at 14:01:17

Now that I've got the decel backfiring tamed, I'd like to get rid of the shutdown gunshot pops that happen about 50% of the time. I can get rid of them by stalling the engine, but that is tedious and also, I imagine, not great for the clutch. And they went away when I tried a bigger pilot jet and also when I replaced the TEV spring with a much weaker spring, but neither of those was a good overall solution. Since all carbureted motorcycles don't do this, there should be a fix, but I'm out of ideas. What might work?

My understanding is that they happen because some fuel gets pulled into the cylinder after the spark has been shut off and then, unburned, gets pumped out to the exhaust pipe, where it is then ignited. Is that correct? If so, why don't most other bikes do this? And how might I emulate what other bikes do to avoid shutdown cracks on the S40?

Thanks for any ideas.

Title: Re: Eliminating shutdown gunshot pop?
Post by old_rider on 11/14/13 at 14:33:45

Dave I stated before, that all I did to the wife's savage was to take off the top of the carb, clean up the air piston/slide which ever you call it and replaced the white spacer with a couple of small washers, then adj. the screw under the brass cap (air/fuel mix?) till it sounded good.

She has never had a pop on shut-off yet and that's been about 5 months ago.

Title: Re: Eliminating shutdown gunshot pop?
Post by DavidOfMA on 11/14/13 at 15:03:19

Did that quite a while ago. Improved midrange performance but had no effect on backfiring or shutdown pops.

Title: Re: Eliminating shutdown gunshot pop?
Post by engineer on 11/14/13 at 18:12:49

I did the white spacer mod and turned the air screw out to optimize idle speed but still had a pop after shut down. Following the advice of members of this forum I opened the idle screw another quarter turn and the pop nearly disappeared. It only happens now if I have been running it hard and shut it down very quickly. So the final solution is to let it idle for about 30 seconds before I turn the ignition off and then it is OK.

There was a thread on this same topic not too long ago, last spring maybe. If you can find that "after fire" thread there is lots of information.

Title: Re: Eliminating shutdown gunshot pop?
Post by engineer on 11/14/13 at 18:37:05

Dave of MA, I found that thread and it is very informative:
http://suzukisavage.com/cgi-bin/YaBB.pl?num=1366007765/0#0

Here is a quote from it by Verslagen that took care of my afterfires:

Re: Backfire theory...
Reply #2 - 04/15/13 at 10:28:47
S'bot, I've always found the biggest bang on shut down was when the idle mixture was set at optimum. And all it takes to get rid of it is an 1/8 to 1/4 turn richer.

Title: Re: Eliminating shutdown gunshot pop?
Post by old_rider on 11/14/13 at 18:40:11

That is probably what happened with me then.... I spent a good hour and a half playing around with the screw settings.

I'd run it down the road feeling throttle twist vers accel, then hit the house, let it idle for about one min., shut it down and listen. Then adjust more and rinse and repeat.

Title: Re: Eliminating shutdown gunshot pop?
Post by DavidOfMA on 11/14/13 at 19:13:50

Well... I've had the idle mix turned out 1/4 turns ranging from 1.5 to 3 turns (where it is now), which helped eliminate the decel backfires, but it had no effect on the shutdown pop. I'll look through that other thread to see if there are any additional gems, but adjusting the idle mix didn't do it.

EDIT: Read the other thread. I can play with the idle speed and mix some more, but as I've done a lot of that with no improvement, I was hoping there was some variable I'd missed. Maybe not.

Title: Re: Eliminating shutdown gunshot pop?
Post by Steve H on 11/15/13 at 03:25:56

I have nothing to back it up but I think part of the pop problem is the large volume of fuel mix and the relatively small volume of the exhaust system. It helps keep the mixture in the burning range where a smaller cylinder's mix would have proportionately more exhaust and be diluted, no pop.

Title: Re: Eliminating shutdown gunshot pop?
Post by DavidOfMA on 11/15/13 at 07:04:05

I guess that makes sense, although then I'd think bikes with twin cylinders that feed the exhaust into one pipe would also backfire and pop on shutdown. I have never owned such a bike, but I've seen a couple on rides I've been on and didn't notice any backfiring from anyone but me!

Also mysterious is why some Savages/S40s, even at the same elevation, backfire and pop on shutdown while others don't, with the same modifications to the bike. Unless it's just very sensitive to small changes, and manufacturing tolerances and variations would explain it. Frustrating!

Title: Re: Eliminating shutdown gunshot pop?
Post by Gyrobob on 11/15/13 at 13:44:33

I sort of like the pop after turning the key off. With my particular jetting and exhaust, though, it sounds more like a paper bag being popped inside a small closet than a gunshot.

When I am around someone who might be annoyed with the sound of a gunshot, I shut it off this way:
-- move the raptor to off.
-- let it idle for ten seconds or so in gear with the clutch pulled in.
-- let the clutch out a tiny bit,... just enough to put a little drag on the clutch,... by just enough, I mean the rpm reduces by about 50 rpm.
-- then turn off the motor by using the kill switch.
-- make sure to put the transmission back in neutral. (I removed the safety switches)
-- turn the key to off.
-- move the kill switch back to on.

With the drag of the partially engaged clutch, the motor stops spinning almost instantly, probably reducing the amount of unburned fuel pulled into the combustion chamber and exhaust pipe.

This cancels out the POP about 90% of the time.

Title: Re: Eliminating shutdown gunshot pop?
Post by DavidOfMA on 11/15/13 at 13:59:17

Thanks. I also use the clutch drag method, though not as gently as you; I 'll try that, but it would still be nice to find a mechanical fix. Tried turning in the idle mix screw from 3 to 2, which seemed to reduce the volume, but got more decel pops (though not very loud). So it seems having the mix rich enough to kill the decel backfires is too rich for a quiet shutdown.

Title: Re: Eliminating shutdown gunshot pop?
Post by Dave on 11/18/13 at 05:43:30

Whenever you shut off the ignition......the engine pumps air and unburned fuel into the hot exhaust system. A lean mixture will burn faster than a rich mixture......so jetting will have some affect on the loudness of the shutdown pop. A lower engine speed will pump less fuel into the exhaust......so that is what a dragging clutch helps to do....slow the engine down and fewer piston pulses after the ignition is turned off results in less unburned fuel in the exhaust.

Some lawn mowers have a solenoid in the carb that shuts of the fuel flow when the ignition is turned off.....as lawn mowers are subject to this same backfire when the ignition is turned off. This backfire also is avoided on fuel injected engines which stop fuel flow as soon as the ignition is turned off.

It may be that our double wall pipe also allows the inner pipe to stay hot longer than a single wall pipe would.....so there is more tendency for the unburned fuel to ignite after the shutdown.

Title: Re: Eliminating shutdown gunshot pop?
Post by DavidOfMA on 11/18/13 at 05:59:30

So, a slower, richer idle should, on average, result in quieter shutdown pops, but it will still pop once the exhaust pipe is hot?

So why, I wonder, does old_rider's wife's bike not pop on shutdown? She's at roughly the same conditions in Florida as I am near Boston, as far as air density goes.

Title: Re: Eliminating shutdown gunshot pop?
Post by Dave on 11/18/13 at 07:04:42

5C5A5757535A51565C5D4A380 wrote:

There are lots of things that could be different....including riding style. If Mrs. Old-Rider does not go as fast, takes her time getting the bike parked and allows a bit of idlng time before she turns off the ignition - the exhaust might have cooled down enough that it does not ignite the fuel. I know that my bike will occasionally pop if I shut it down soon after getting off the highway (gas station, etc.) - but it never pops after I have driven slowyly down my gravel driveway.

Title: Re: Eliminating shutdown gunshot pop?
Post by Gyrobob on 11/18/13 at 09:27:12

07010C0C08010A0D070611630 wrote:

We don't want a slower idle, though. It's a cam oiling issue.

Title: Re: Eliminating shutdown gunshot pop?
Post by Serowbot on 11/18/13 at 09:28:44

132D263B363B36540 wrote:

We don't want a slower idle, though. It's a cam oiling issue.

That'll fix the pop issue permanently... ;D...

Title: Re: Eliminating shutdown gunshot pop?
Post by DavidOfMA on 11/18/13 at 10:01:49

Yes, I know that. Not many variables to work with.

Title: Re: Eliminating shutdown gunshot pop?
Post by Serowbot on 11/18/13 at 11:05:04

The best thing to do,.. is jet the carb for best running performance/economy and don't try to fix an exhaust problem with the carb...
A freer flowing muffler, Harley Shorty, will allow the unspent exhaust to escape easier and reduce the shut-down pop...
Using a clutch kill, or idling for 5 or 10 seconds before shutting down, will help...
Ultimately,.. hot pipes are a fact of life in a large air-cooled thumper... and so are pops...
If you can't live with them,.. you may have the wrong bike...

Title: Re: Eliminating shutdown gunshot pop?
Post by bobert_FSO on 11/18/13 at 11:11:02

There must be some additional factors at play here, since we all seem to have different experiences with afterfires and shutdown explosions.

My experience has been mostly free from the problem. Only an occasional pop when shifting. A nice burble when deccelerating as the RPMs approached idle (correct idle speed). A VERY occasional soft poof (not a gunshot) on shutdown if shutting down immediately after running hard. I would shut the engine off with the key.

I was running a dyna muffler with a 1/2" hole in the baffle plate for a little extra noise. Jetting was left over from running the same too-loud/too-open Emgo muffler as Ryca uses. I forget the numbers, but I had stepped up one size main jet, idle jet and a white spacer mod.

I know this was a little rich, as I would slightly soot up the rear brake arm. I also didn't need to pull out the enricher valve (choke) to start until temperatures were in the 40's F. Even then only half out on the enricher. Full out was too much. The idle mixture screw was left alone, as I broke off a screw in the cover plug while trying to get it out. However, set this way, it ran great and never starved for fuel. My mileage was about 46 MPG.

I think what worked for me was the slightly rich condition and an exhaust system with NO leaks. I sealed the header/muffler joint with some asbestos cloth I inherited from my grandfather. I don't know if you can still get that stuff (Thanks, Grandpa). The asbestos created a soft gasket like the soft sealing ring on the stock exhaust.

Title: Re: Eliminating shutdown gunshot pop?
Post by DavidOfMA on 11/18/13 at 19:20:15

495F48554D58554E3A0 wrote:

Bike was given to me by my brother to induce me back into riding (it was the bike he used to get back into riding; now rides three other bikes). It would not have been my personal choice, but now that I have it, I'm interested in seeing if I can iron out some of the glitches. I don't mind pops. It's POPS I mind.

Title: Re: Eliminating shutdown gunshot pop?
Post by old_rider on 11/18/13 at 22:54:26

I too am on the boat about, some will, some won't, no matter what you do.
Albeit I have only been on this site for only about 9 months now, but there have been folks just adj. the air/fuel screw, some that took air/fuel screw + washer, some that did jet change and air/fuel screw...ect....

I don't think one thing is the same on EVERY bike... I've even added the dyna muffler, then adj. the air/fuel screw and now her bike don't even POOF at shut off, now matter if its shut down after a minute of idle or shut off after just a few seconds when she stops and puts down the kickstand, it still doesn't poof or pop....

I will be buying me one here soon, so we shall see... :)

Title: Re: Eliminating shutdown gunshot pop?
Post by DavidOfMA on 11/19/13 at 06:31:34

I'll be interested in how your bike rides. According to my brother, it backfired and POPped when he first got it, new, even after he adjusted the idle mix.