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Discussion Starter #1
I know this topic has been beaten to death, like the oil topic but ....
I took my bike out for a spin and the exhaust was popping a lot while engine braking. I didn't have any popping while accelerating and didn't notice any loss in power. The whole trip was around 4 miles so I would have to say the bike was cold.
Am I running too rich or too lean?
 

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Either one. Both can cause backfire. Backing out the screws is bad advice. There are no air/fuel screws on the SV carbs. Those screws are idle mixture screws. Backing them out makes the idle richer. Backing them out 2 1/3 turns without knowing what you're doing is a mistake. Many people back them out to make up for low to midrange mixture. A better solution (if that's the problem) is adding a shim under the slide needle.

First determine what the problem is. Warm the bike up and when approaching home get it up to about 5,000 RPM for a few seconds. Declutch, hit the stop switch and coast in. Pull the rear plug (easier) and see what it looks like. Typically SVs are lean. Do what the plug tells you is needed.
 

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andyauger said:
Either one. Both can cause backfire. Backing out the screws is bad advice. There are no air/fuel screws on the SV carbs. Those screws are idle mixture screws. Backing them out makes the idle richer. Backing them out 2 1/3 turns without knowing what you're doing is a mistake. Many people back them out to make up for low to midrange mixture. A better solution (if that's the problem) is adding a shim under the slide needle.

First determine what the problem is. Warm the bike up and when approaching home get it up to about 5,000 RPM for a few seconds. Declutch, hit the stop switch and coast in. Pull the rear plug (easier) and see what it looks like. Typically SVs are lean. Do what the plug tells you is needed.
That's good advice, for someone that knows how to read plugs, which is really more of an art, than a science.

One easy test to do for the lamen, is go out on a long stretch of road, free from any traffic and hopefully police, get the bike going and really wind out forth gear...roll full on the throttle, and back off just a hair. If you start to accelerate when you roll of that hair, you are/might be rich.
 

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I disagree Andy. When you are throttling off you are going to idle fuel delivery, you are relying on the idle mixture more so than anything else. I do agree with the spark plug checking.
 
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Discussion Starter #7
Well I guess it's time to pull the spark plugs again. I pulled them the first time after 500 miles on the jetted carbs and they were fine. Also back then I didn't get this popping problem or the low idle problem.
 

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Either rich or lean can backfire

Rich conditions can cause raw fuel vapor to build in the exhaust pipe due to incomplete combustion. When the next cylinder fires the hot exhaust can ignite the vapors resulting in backfire.

Lean conditions can cause misfire in the cylinder, again putting raw fuel vapor in the tailpipe. This vapor cools rapidly as it passes through the exaust valve into the exhaust pipes. This can make the resulting mixture rich enough to burn when the next cylinder fires.

I've experienced both lean and rich backfires from a variety of vehicles I've worked on over the years. My experience is that lean conditions cause more backfire problems than rich conditions.

Reading plugs is not an art at all. Anyone can tell the difference between wet and black, sooty and black, black, brown tan, white, and white with ash. The biggest trick is figuring out how far you can coast to your driveway after you shut the engine off.
 

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Take the air filter out and run your fingers along the area where the rubber boots meet the carb throats.
Mine was doing the same thing you described and I found a small gap between the rear carb boot and carb.
Also check the rubber boots where the carbs meet the engine.
If any of these areas are sucking air, it will backfire.
 

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Low idle? You didn't mention that in your first post. What toadstranglerx suggests is very good. Slow and lean can mean a vacuum leak. Check both ends of both boots. Be careful not to overtighten the clamps.
 

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Andy,

There are no air/fuel screws on the SV carbs.


Huh? I just installed a jet kit this weekend, and sure enough there were A/F screws under a cap.
 
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Discussion Starter #12
toadstranglerx said:
Take the air filter out and run your fingers along the area where the rubber boots meet the carb throats.
Mine was doing the same thing you described and I found a small gap between the rear carb boot and carb.
Also check the rubber boots where the carbs meet the engine.
If any of these areas are sucking air, it will backfire.
Thanks for the advice, I'll check that the next time I'm off.
 

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Wrong name, jakeway

Those aren't air/fuel screws. Adjusting them does not admit more or less air. They are idle fuel mixture screws. It may be semantics, but air/fuel screws, in my long experience, refer to adjustable air bleed passages. Backing them out makes the mixture leaner. Backing out fuel mixture screws makes the mix richer.
 

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Idle screws then (have been referred to as AF on here for many years) are what control off throttle fuel mixture. Yes the needle plays into it too. But not as much as we would like to think. That needle works more directly and influences more so the Main jet and Pilot jet for 1/4 to full throttle.
 

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More semantics

We may be talking a different language, technically. The idle mixture screws are what smooths out the idle, that is with no throttle. Properly done the idle screws should be adjusted for the correct speed and on the lean side, with the slides as far closed as possible. This is best done with an EGA, but can be done by ear with a bit of experience. The procedure is to back off the idle speed adjuster and screw in each screw until the idle gets lumpy, back off until it smooths out. Do this back and forth a couple of times on each cylinder until you have a nice, smooth idle.

Once you've got that, test the off-idle response. If the bike balks off idle then you can adjust the slide needles. Once that's done then you can worry about the high speed jets. It takes a bit of back and forth to get everything just so. There is a good deal of overlap between the circuits as you open the throttles.
 
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Discussion Starter #16
Hi,
I am having the same problem, back fires when I decelarate or slow down and my idle got really low (1000 rpm) what did you do to fix your issue ?
 

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1. Sounds like a different problem. Try to adjust the idle back up. Make sure both cylinders are firing. It almost sounds like you've lost one cylinder. Could be a fouled plug, a loose wire, a bad coil, etc. If you can't get a smooth idle at around 1,300 RPM then it's time to do some more investigation.

2. It's a bad idea to take another person's solution to a problem that sounds similar and applying it to your situation unless you do some checking.
 
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Discussion Starter #19
yes back fire big time.
feels like I am at the fiering range practice.

today, I adjusted the idle and it went a little better yet still there.

3rd gear, 3000 rpm, taking a slow turn, it's like vietnam war, it backfires like hell.
 
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