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SuzukiSavage.com
/cgi-bin/YaBB.pl General Category >> Rubber Side Down! >> Backfire theory... /cgi-bin/YaBB.pl?num=1366007765 Message started by Serowbot on 04/14/13 at 23:36:04 |
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Title: Backfire theory... Post by Serowbot on 04/14/13 at 23:36:04 Actually,... after-fire theory,.. (most often referred to as "backfire")... A real "backfire",... goes backwards... blowing up through the carb... (this is most often caused by a valve/spark timing issue)... What we refer to as backfire, (the gunshot out the tailpipe),.. is correctly called an after-fire... This is caused by a hot pipe, igniting unspent fuel in the exhaust... Why does it happen?... 1. you have unspent fuel in the pipe... (this could be too rich... or too lean... either will cause inefficient burning in the cylinder... one, will leave the cylinder with not enough fuel to give complete combustion, and the other will have too much)... 2. the hottest engine, is the one with the best combustion.... and that will expel heat into the pipe that can reach combustive levels... but, it shouldn't cause a big one,... because there shouldn't be that much fuel in there... 3. an air leak... in the exhaust pipe will introduce fresh oxygen into the pipe, and this will make a nice secondary combustion chamber... What are some clues?... a. if it's mostly upon shutdown... there must be unspent fuel in there... a "poof" could just be the final cycle of the engine after the ignition is shut off... That would be normal... A huge "Kappowww!" at shut-off should indicate rich... (there's too much fuel in there).... b. if it's more upon decel and downshifts,... but not so much on shutdown,... I'd guess lean...(a lack oxygen in the combustion chamber is causing inefficient burning in the chamber and when you close the the throttle there is a momentary richness, caused by the throttle plate blocking air before the fuel flow is reduced, causing a boom in the pipe on throttle chops)... c. all the time... (yer' so screwed)... jest kiddin'... You are probably rich,...the gunshot afterfire is still indicating an excess unspent fuel... Basically,.. what I'm proposing, is that judging rich or lean, by your pipe is not a good way to go... It's very popular... but, often wrong... Rich, or lean, can cause popping, afterfire, and blueing... ...and, when your engine is working best, it is burning hottest... (at least in the combustion chamber)... The best indicator of your engine's tuning/jetting is the environment... If popping/afterfire increases in... thinner air hotter air more humidity higher altitude ... go leaner... ...if popping/afterfire increases in... thicker air cooler air less humid lower altitude ... go richer... If your bike is generally disturbed more when the weather/or conditions are unusual in either direction,... then it stands to reason, that it is jetted for best performance in normal conditions,... and this is where you want it... ... and, if you live in Florida... where it's hot, humid and flat...all the time... yer' jest so screwed... I guess you just gotta' change jetting, up and down to alter the mix conditions, because nothing else ever changes... ;D ;D ;D... Same rules apply though... you'll just have to alter your engine conditions, rather than observe the environment... ... and... I'm probably wrong here somewhere... but, it's my theory... and it's roughly based on science... and I'm sticking to it... :-?... So,.. take advantage of the next muggy, or humid,.. or hot or cold, or mountain or valley ride,.. and observe your bike's behavior... If popping, and backfire/afterfire increase, that's a good sign... You tune your bike for what is usual... ;)... |
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Title: Re: Backfire theory... Post by Dave on 04/15/13 at 04:45:51 Serow.....Good anaylysis. I have had the opinion for a long time tnat jetting with the goal of eliminating the backfiire is wrong, as it may have a negative impact on the power and economy of the engine. The goal of jetting should be to get the best running engine you can - then maybe tweak it a bit to help reduce backfires......which may not be possible. I installed an O2 sensor on my bike and was surprised to learn that the exhaust shows a lean condition when the throttle setting is closed and you are coasting. I guess what is happening is the high vacuum can pull air past the butterfly and slide - but the fuel delivery is mostly shut down. That would explain why the TEV (Throttle Enrichment Valve) is supposed to add fuel when the throttle is closed. By opening the idle air screw and increaseing the size of the pilot jet you may be able to get some additional fuel into the engine when you are coasting or have just let of the throttle - but the downside of that is the engine may be running rich at idle and low throttle settings. |
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Title: Re: Backfire theory... Post by verslagen1 on 04/15/13 at 07:28:47 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. |
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Title: Re: Backfire theory... Post by Bubba on 04/15/13 at 07:57:09 I just got the bike kinda ready for summer here in Denver. I cleaned/reoiled my K&N drop in filetr, changed the oil, aligned my belt AND removed the snorkle.... After de-snokling it I rode around for a little bit...weird but everytime I shut down there was no poof, pop, kepow...whereas previously it was always there. I noticed that my 1st to 2nd shift had a definate pop that I don't remember but I plan to mess with my airmix a bit to see if that can be eliminated. Anyone else have afterfire disappear after removing the snorkle? |
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Title: Re: Backfire theory... Post by Digger on 04/15/13 at 18:35:01 4E796E6E6D3D3C343F0C0 wrote:
Snorkle? :question |
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Title: Re: Backfire theory... Post by Bubba on 04/15/13 at 19:01:33 oh yah, probably should 'splain myself.. What I refer to is the rubber piece that fit's over the airbox intake under the seat...on mine it has a lip that takes up room as well as a split piece of rubber that runs vertically down the middle of it... Just seems like it is there to direct flow in the airbox but it just reduces flow i think???? maybe I'm wrong.... :-/ |
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Title: Re: Backfire theory... Post by markbacon on 04/15/13 at 20:02:58 Vacuum at the jet is produced by air movement, in either direction, past the orifice in the carb throat. The orifice for the idle jet (which is what you tune with the air screw) is located right where the edge of the butterfly rests when closed. Because of valve overlap, etc, when the engine's at idle air is pulsing back and forth past the edge of the butterfly. So the idle jet sees more than one suck for each intake and hence is tuned lean to deliver a correct mixture. On the overrun the butterfly's closed but higher revs smooth out pulsing in the carb: the idle jet's now seeing only one suck per intake and is delivering a lean mixture. When a couple of other variables line up, particularly revs dropping past a critical point, this produces banging and crackling as unburnt fuel ignites in the exhaust. The "temporary enrichment" valve on the side of the carb is supposed to address this - it gets sucked open by high vacuum on the engine side of the carb, opening another circuit to the low speed jet. Cleaning this out can help. Going one size up on the low speed jet reduces backfiring, so long as you've also ensured there are no leaks in the exhaust system for air to get in. Another step is to alter or replace the spring on the temporary enrichment valve diaphragm - a softer one means the valve stays open longer - but it sounds as if this has upset other aspects of the carburation. |
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Title: Re: Backfire theory... Post by engineer on 04/16/13 at 16:42:39 My bike always rewards the neighborhood with one big massive Ka-boom about half a second after I shut off the ignition. The only exception is when I just run it a mile or two and it isn't up to full temperature. So I took it out on a ride yesterday after tightening the manifold which was slightly loose. I mostly wanted to test the new EBC brake pads I put in, they are wonderful compared to the OEM ones that were on it. No noise from the brakes. Anyway I got it good and hot but I let it idle down and got a bang but not a Ka-boom. Next time I'll shut it off quickly and see if it was tightening the manifold down or letting it idle that made the improvement. After that I'll try Verslagen's method and richen it up a bit to see if I can eliminate it altogether. |
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Title: Re: Backfire theory... Post by B.J. or Jo on 04/17/13 at 08:36:26 My wife had a 86 and 96 Savage and they had a heavy putt sometimes but our 09 sounds like a canon anytime you "decelerate" from 5th. to 4th. or 4th. to 3rd.!!! :o I'd be really HAPPY to get the backfire down to a less decible level for sure. It's bad when your riding a poker run with a bunch of "straight piped" Harley's and our supposedly to quite Japanese scoot get's the "Evil Eye"! :-[ |
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Title: Re: Backfire theory... Post by Bubba on 04/17/13 at 09:12:11 Have you messed around with the air mix screw at all?...that got rid of most of the backfire for me (well and jetting too). Where I really notice it is on mountain descents...since we're at altitude the lean/rich conditions change pretty rapidly...you can control that somewhat with the air mix screw but I usually don't wanna stop and do that so i just snap/crackle/pop on the way down the backside of the passes...lol |
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Title: Re: Backfire theory... Post by Beagle on 04/17/13 at 11:29:12 Good explaination on it. I just keep a little screwdriver with me and depending on the weather, I might have to adjust the carb screw a half turn either way to fix this problem for me. |
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Title: Re: Backfire theory... Post by B.J. or Jo on 04/17/13 at 13:31:01 6E594E4E4D1D1C141F2C0 wrote:
No I haven't I'm pretty much "all thumbs", isn't that the screw that's "covered" by a metal plug from the factory?? I wouldn't care it it "poped" a bit on deceleration but it's just ONE very loud "BANG" and that's not good. According to the dealership service dept. it won't "hurt" the motor and their getting with the National Suzuki Service Rep as there's suppose to be a "factory service bulletin" fix to at least "tame" the backfire. We will see what they come up with. :-/ |
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Title: Re: Backfire theory... Post by Bubba on 04/17/13 at 14:00:15 Yep, that's the one...it really isn't a big deal to remove the plug...just make sure ya read up on it and don't go all crazy, otherwise you'll mess up the screw head that lies under it. After hearing the horror stories about the dealerships I wouldn't let them touch it...I'd just ask what the "fix" is and do it myself... |
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Title: Re: Backfire theory... Post by smthbh on 04/18/13 at 09:00:02 I had a problem with after-firing on my savage too. After it got warmed up, if I shut the bike off there would be a two second delay and then BOOM! An enormous explosion from the pipe. I adjusted the valves to see if that would fix it, and it still backfired the same as before. After that, I lowered the idle speed (it sounded like it was idling too fast after it was warmed up) and adjusted the screw under the EPA plug to make it richer at idle, and it's completely fixed the problem! No backfire on turn off at all now! I'm not sure whether it was the idle speed or the mixture screw that fixed it though, so I'm going to try adjusting the mixture screw back to default and see if the backfire comes back. If not, then it was just the idle speed. |
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Title: Re: Backfire theory... Post by Bitterswede on 07/31/13 at 16:27:54 There is tons of discussion about the Savage backfire issue and I'd like to add my experience in case it might be helpful. :) I have a 2001 Savage with low miles that has been reconstructed as a cafe racer (photo attached). It has a fairly free-flowing air filter and a very free exhaust but otherwise stock. The exhaust header is 1-5/8" ID, 31" long and flows into a 9" long glass-pack muffler which empties into a 3" dia turn-out. ID in the glass-pack is about 1-1/4". This bike is fairly loud and now revs way more freely under WFO than when stock. So I'm pretty happy with the main jet. I reside in Phoenix which is about 1200 ft elevation. In reading all the guides on jetting, I started with a 155 main, 52.5 pilot and cut the thickness of the white spacer on the needle by half. But the bike would backfire on almost every shift from first through third and anytime if you chopped the throttle. Two ways I found to stop the backfire were to roll off the throttle slowly or to be on the gas hard between shifts. The slow roll-off is cumbersome, counter-intuitive, and just BS and you can't always be on the gas, like in heavy traffic. So POW. POW, POW. Some guys like the backfire, I do not. Ryca uses a 55 pilot at sea level, so I though I'd try that and also bought a 57.5 pilot. Started making one change at a time; the 55 almost immediately stopped the backfiring. After a good warm-up and pilot screw adjustment, it was still popping from time to time, so I installed the 57.5, readjusted the pilot screw and took a long ride. With the 57.5 pilot it backfired just as badly as the 52.5 and now it would stall after down shifting to a stop light, perhaps TOO RICH now. Overall, worse than the 52.5 pilot by far. I did some more reading, one thread suggested throwing out the white washer but that opinion was contested pretty heavily and to my thinking, when you chop the throttle, you're almost instantly back to the idle circuit anyway, so I re-installed the 55 pilot, rechecked the exhaust for leaks, warmed up the bike, and re-set the pilot screw. But this time I turned the pilot screw out till the idle began to stumble and then just turned it back in until it picked up and smoothed out. Only about 1/2 turn in and presto, almost no back fire at all. I've taken a couple of 10-mile rides and made sure the bike was completely warm, (it was 107-deg ambient), and it is now very hard to get a back fire. Only did it once in 20-miles. Now when I shift and/or chop the throttle, no pops, no back fire, just the same kind of sound I get on my XR200 between shifts, a sweet little rumble. So, my observations seem to support the conversations you can find here and elsewhere that the back fire is the ignition of unburned fuel in the exhaust which may happen if the mixture is too lean OR too rich. This seems to be my experience. My conclusion is that you can stop the backfire but that it is a very sensitive issue; get the pilot jet and pilot screw adjustment JUUUUUST RIGHT, and you're cool. I thought that Lancer's notion of going up on the pilot jet when you screw out the air screw more than 2-turns was a good idea and a good guide for sizing the pilot jet. It really seemed to help me. Anyway, now the bike is a sweet ride, sounds like a thumper should, with a good rumble between shifts or when you chop the throttle. I'm not an expert tuner but I've been wrenching on bikes for about 5 decades and I have to say, this was a head-scratcher. I can't guarantee that my experience will dictate yours but my hope is that it may help someone out there. My recommendation is use Lancer's logic on the pilot jet selection and then turn the pilot screw out till the idle stumbles and just back in till it picks back up. Try that. God knows it won't work for some one but it worked for me and it may be worth a try. Best of luck. Bitterswede. |
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Title: Re: Backfire theory... Post by WD on 07/31/13 at 16:36:08 You have GOT to do a write up on that blue bike, in its own thread. Phenomenal! |
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Title: Re: Backfire theory... Post by Steve H on 07/31/13 at 19:37:47 That's the way I've always adjusted the pilot circuit. Learned that with lawn mowers and dirt bikes. |
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Title: Re: Backfire theory... Post by apache snow on 08/01/13 at 11:20:00 When screwing in my pilot fuel screw the idle begans to drop at 1-1/2 turns from seated. When screwing out the idle begains to drop at 4-1/2 turns out. I have a 52.5 pilot jet with evelation at 500 feet. What do the experts say I should be at. |
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Title: Re: Backfire theory... Post by Bitterswede on 08/01/13 at 11:34:44 4A5B4A48434E5845445C2B0 wrote:
A lot of the experts say to adjust to right between 1-1/2 and 4-1/2. I found that closer to the second position worked. Lancer would suggest a larger pilot jet if you go past 2-turns out. Luckily, this is easy to experiment, (although some many insist this is like running with scissors) put a little screw driver in your pocket and go for a ride and stop every now and then and play with the pilot screw. At 4 turns out, I'd be worried about losing the screw. Swede |
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Title: Re: Backfire theory... Post by apache snow on 08/01/13 at 11:36:56 At 3-1/2 turns most all the pops are gone. I don't know that I am comfortable at running with that many turns out. |
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Title: Re: Backfire theory... Post by Bitterswede on 08/01/13 at 11:40:04 4F5E4F4D464B5D4041592E0 wrote:
Pilot jets are only a few bucks, I'd be inclined to try a 55 and re-adjust the pilot screw. Cheers |
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Title: Re: Backfire theory... Post by justin_o_guy2 on 08/01/13 at 11:45:21 If youre concerned about spring pressure, look at a ball point pen spring or a flint spring from a bic liter,, one mite do it, IDK.. |
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Title: Re: Backfire theory... Post by apache snow on 08/01/13 at 12:04:03 I have two #55 pilots on hand. I will not be putting a ball point pen spring on my carb. :-? Oh yeah, It runs just fine on 2-1/2 to 3 turns out. It just pops a lot on decell and when up shifting. |
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Title: Re: Backfire theory... Post by MarcosS40 on 08/01/13 at 14:53:39 445544464D40564B4A52250 wrote:
Get a bottle of White-Out and put a small drop at the juncture of screw head and carb body after you have the adj. screw where you like it. It acts as a stop to keep it from backing all the way out. I prefer this to nail polish, but that will work too. The W-O is easier to remove. |
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Title: Re: Backfire theory... Post by LANCER on 08/01/13 at 16:33:53 5259444455424347555455300 wrote:
A lot of the experts say to adjust to right between 1-1/2 and 4-1/2. I found that closer to the second position worked. Lancer would suggest a larger pilot jet if you go past 2-turns out. Luckily, this is easy to experiment, (although some many insist this is like running with scissors) put a little screw driver in your pocket and go for a ride and stop every now and then and play with the pilot screw. At 4 turns out, I'd be worried about losing the screw. Swede[/quote] 3 turns actually |
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