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Bad Backfire coasting downhill

This is a discussion on Bad Backfire coasting downhill within the Engine forums, part of the Technical category; I have 450 with the intake snorkeled out under the hood. It was running fine. I did a header wrap on it last week. Now ...


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Old 02-01-2009, 02:11 PM
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AZ Bad Backfire coasting downhill

I have 450 with the intake snorkeled out under the hood. It was running fine. I did a header wrap on it last week. Now when I am coasting downhill it backfires alot everytime. If I give it just the slightest little bit of gas on the downhill it does not backfire. Is this an issue with the idle screw adjustment? To lean or to rich?

Thanks in advance
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Old 02-01-2009, 02:28 PM
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Sounds like an exhaust leak to me..........
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Old 02-01-2009, 02:57 PM
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It's probably an exhaust leak. I Had a ALBA head pipe crack just inside the flange area on both pipes where they bolt up. I think it was due to using the header wrap. It gets the pipe too hot and under stress it is weaker and subject to failure. The header wrap also is not good if you are going to be getting it wet alot. My stainless pipe had rust on it from the header wrap getting wet alot which could have also led to the cracking. I had to buy a new headpipe because ALBA wouldn't warranty the cracked one. good luck and keep us posted on the outcome.
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Old 02-01-2009, 04:24 PM
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Decel backfires are caused by two things, too lean on the idle circuit or exhaust leak.

Kinda doubt the wrap would cause it to go lean.
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Old 02-01-2009, 09:14 PM
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How does your spark plug look. Lean/Rich
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Old 02-01-2009, 09:28 PM
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Quote: Originally Posted by hunterworks View Post
Decel backfires are caused by two things, too lean on the idle circuit or exhaust leak.

Kinda doubt the wrap would cause it to go lean.
The header wrap could cause the headpipe to be weakened and crack causing a leak.
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Old 02-01-2009, 09:30 PM
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AZ

Thanks for the input. I checked the exhaust for leaks/cracks and didn't notice any when I reinstalled the pipe after the wrap. I will try turnig out the idle screw a little and see if that helps. Probably will not get a chance to try it again until next weekend. I will let you know what happens.

Thanks again for the input.
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Old 02-01-2009, 09:39 PM
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The idle screw will not help ya.........the wrap causes the pipes to crystalize, and it probably cracked. Did you put the exhaust donut back when you installed the header pipe?
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Old 02-02-2009, 10:21 AM
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Absolutely adjust the idle screw for backfiring and popping...

Actually you need to tune the carb from top to bottom...

All those kinds of problems go away with a well tuned carb and consistent fuel delivery...

Assuming you don't have a cracked header...

Even still...the tuning is woth it...

Also...snorkels totally dork the operation of the stock carb...tuning is much more difficult...and can be impossible to get to fully work depending on HOW the snorkle is done...
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Old 02-02-2009, 04:32 PM
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AZ

Quote: Originally Posted by nvs2none View Post
The idle screw will not help ya.........the wrap causes the pipes to crystalize, and it probably cracked. Did you put the exhaust donut back when you installed the header pipe?

Donut is back in. I checked the exhaust for leaks after the re-install. I put my hand as close to the pipe as I could feeling for escaping exhaust. Is there any other way to check for exhaust leaks. I find it hard to believe the pipe would crystalize and crack in the first 5 miles after the install. The first hill I coasted down it was backfiring. I'm thinking that the extra heat may becausing it to detonate and backfire. I am going to try working with the idle mixture screw first and see if that will fix it. That will be this coming weekends project.
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Old 02-03-2009, 11:04 AM
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Quote: Originally Posted by 1stRhino View Post
Donut is back in. I checked the exhaust for leaks after the re-install. I put my hand as close to the pipe as I could feeling for escaping exhaust. Is there any other way to check for exhaust leaks. I find it hard to believe the pipe would crystalize and crack in the first 5 miles after the install. The first hill I coasted down it was backfiring. I'm thinking that the extra heat may becausing it to detonate and backfire. I am going to try working with the idle mixture screw first and see if that will fix it. That will be this coming weekends project.

If the header was cracked you would see it...hear it...especially if you took the pipe out to inspect it...

I have tuned out this and similiar problems too many times to count and if you don't take my advice you deserve what you get...
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Old 02-03-2009, 11:58 AM
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Quote: Originally Posted by tyrap View Post
Absolutely adjust the idle screw for backfiring and popping...

Actually you need to tune the carb from top to bottom...

All those kinds of problems go away with a well tuned carb and consistent fuel delivery...

Assuming you don't have a cracked header...

Even still...the tuning is woth it...

Also...snorkels totally dork the operation of the stock carb...tuning is much more difficult...and can be impossible to get to fully work depending on HOW the snorkle is done...
Agree!!
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Old 02-03-2009, 01:21 PM
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I was only kidding about the get what you deserve part...

But... it's true...

Actually...

It takes more time and effort to try to fix it w/o doing it right and with piss poor results...

In the end...it pays to get what you need and learn to optimize every element of the fuel delivery system...but especially the carb it's self...

And, believe it or not... A restrictive air intake is your worst enemy when in high temps, high elevation, hill climb/decent and throttle response in general...especially with the stock carb...
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Old 02-03-2009, 01:25 PM
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Believe this or not, most snorkels make the engine run lean and it is because the opening in the end of the snorkel is larger than the stock opening and you lose some of your vacuum needed to operate the slides correctly and meter fuel correctly.

If this is your case you can start taping off the opening a little bit at a time until it runs right.
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Old 02-03-2009, 01:59 PM
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Quote: Originally Posted by hunterworks View Post
Believe this or not, most snorkels make the engine run lean and it is because the opening in the end of the snorkel is larger than the stock opening and you lose some of your vacuum needed to operate the slides correctly and meter fuel correctly.

If this is your case you can start taping off the opening a little bit at a time until it runs right.
I agree that the snorkles...including the factory one causes leaner operation...but, worst of all...it causes lean stumble when you stomp the throttle from a dead stop...and kills throttle response...the slide doesn't lift as it should but opening the throttle let's in a huge amount of air w/ no gas to go with it...

But it's not the opening size...and the stock snorkle opening is smaller than the airbox opening...so just tossing the factory snorkle is a huge improvement...running 04 style with proper tuning helps tremendously...and that is basically what WW did with his raise tube...which works better than stock but not nearly good enough for me and puts noise more noise in the cab...

Taping off the opening is like putting a turniquette on your leg for a scraped knee...

People say backfiring....to describe the symptom...

However I have seen it many times in two forms...

1. Any time you let off the throttle and it starts "popping" like a .22 rifle...that is lean idle a/f and typically the pilot jet in the stock carb is way too small...making it somewhat difficult to tune out with a/f screw alone...if the vehicle is bone stock...try removing the snorkle and see if helps...then try removing the airbox lid...that will almost certainly help....careful not to let the filter come loose...

2. At warm/hot operating temps...when the motor is settling down towards idle...especially after a hard wot run...you may get a loud KA-BOOM...one time...more like a 12ga shotgun blast...that is usually the result of a combination of high idle rpms, needle postition too high, rich idle a/f...if the vehicle is bone stock...again opening the air box will help with immeadiate relief as will lowering the idle rpms...lowering the needle postion is quick and easy and done from the top of the carb...

IN ALL CASES...your best performance will come from completely tuning the carb...and in all cases...your best throttle response will come with the highest possible flowing air intake....which is a big-ass k&n with no airbox...

It's not always practical to run no air box and there is no debating w/ me that the k&n let's in super fine dust where a well oiled (spray-on is best) 2 stg foam filter will keep the air intake charge absolutely clean as a whistle...
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