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Old 09-28-2013, 06:37 PM   #1
boxer15
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Default how to tell if running lean

Hi,

I have cobra straight pipes on my vulcan 800 and had backfiring and popping and the exhaust was blue and purple, so I set the fuel mixture to 2.5 turns, and I removed the afterburner valve and vacuum capped accordingly. Approximately how much run time is needed before I can check the plugs to see if the mixture is still running to lean. I checked after 30 minutes of idling and I jacked the bike up in the rear and shifted up into to 5th gear a couple of times to listen for backfiring and they looked fine. Should I drive a few hundred miles and then check them again?
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Old 09-29-2013, 07:34 AM   #2
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Quote:
Originally Posted by boxer15 View Post
Hi,

I have cobra straight pipes on my vulcan 800 and had backfiring and popping and the exhaust was blue and purple, so I set the fuel mixture to 2.5 turns, and I removed the afterburner valve and vacuum capped accordingly. Approximately how much run time is needed before I can check the plugs to see if the mixture is still running to lean. I checked after 30 minutes of idling and I jacked the bike up in the rear and shifted up into to 5th gear a couple of times to listen for backfiring and they looked fine. Should I drive a few hundred miles and then check them again?
at idle speed, your plugs are firing 15 times a second. Trouble is if you remove them after idling, you swill only see what your idle mixture is.

Spark plug testing is not a very good way of checking lean. You would need to take a spark plug spanner with you, ride at full throttle and then kill the engine. Coast to stop and remove the plugs. that is the ONLY way to get an instant result. other than that, it takes a few thousand miles for the plugs to permanently change with build up of deposits.

Gunson make a fuel lean /rich testing system. or, you can use the seat of your pants.
If it runs well and does not get stupidly hot, youre on the button.
backfiring on acceleration = too lean (or an inlt air leak), backfiring on over run, too rich (or an exhaust air leak)
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Old 09-29-2013, 09:27 AM   #3
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are you talking about the colortune? I was thinking of this but can it be seen with how deep our plugs go?
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Old 09-29-2013, 10:24 AM   #4
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1. Kickstand tuning does not work.
2. WOT/chop only tells what you have at WOT.
3. Idle plug readings only tell you what you have at idle.
4. It is everything in between that really counts.
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Old 09-29-2013, 10:33 AM   #5
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1. Kickstand tuning does not work.
2. WOT/chop only tells what you have at WOT.
3. Idle plug readings only tell you what you have at idle.
4. It is everything in between that really counts.

so, should I just drive it around and take note of what happens at what range?
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Old 09-29-2013, 05:33 PM   #6
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What you want to look for is:

1. Engine performance. That includes idle quality, start-up hot/cold, backfires, etc.
2. Fuel mileage.
3. Seat of the pants. Down on power, etc. Would not pull the hat off of your head.
4. Spark plug readings.
5. Driveability issues including power sags, surges, etc. at any/all road speeds.

If any issues are noted, then they can be addressed.

One thing that can happen is if an engine is over-jetted. It seems to have lots of power, runs fine, but gas mileage will tank and excessive combustion chamber carbon could result. Might even foul plugs.

It is a delicate balancing act to dial it in correctly.
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Old 09-29-2013, 05:37 PM   #7
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As an add, I did some tuning on a Buick V6 to which I added a wide band O2 sensor.
You would not believe how far the A/F ratio can vary before one notices an issue, BUT where it showed up is in gas mileage, big time!
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Old 09-30-2013, 08:04 AM   #8
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If you don't care about gas mileage, error on the rich side is safer. Sure you could build up carbon in the combustion chamber and on the piston effectively raising the compression ratio which could lead to problems. This would take some time, a lot if your opening it up with much frequency. It could foul plugs too, but again slowly if your opening it up very often. A lean condition can produce problems much quicker, like a holed piston or burned exhaust valve.
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Old 09-30-2013, 09:23 AM   #9
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I just bought my 800 and it has jardines pipes on it. Close to the head my pipes got the bluing going on and it backfires when I leave off the throttle. These are signs of the mixture being to lean correct? The bike runs it's ass off and the gas mileage is in the 45-50 range. Does take the choke on to start in the morning for about 30 seconds and then I can let it idle on its own. Do I need to do some tuning? I plan on putting a cone intake/filter on in the future too
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Old 10-02-2013, 10:27 PM   #10
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Well, I never owned it while it was running right so hard for me to say if there are any power lags. As far as idle, even at 90 degrees I need to hold the choke out or about 30-40 seconds at maximum then I push it all the way in and it will idle fine otherwise it will start and idle really slow and stall out after a few seconds. I also noted that my mpg's aren't all that great only getting like 35 miles per gallon right now. Checked spark plugs after 30 miles and they are clean except for some carbon at the ring. I removed the afterburner but the bikes still gets very hot. The air filter cover starts burning my leg after a while. I seems to run lean at idle and WOT, however it runs good at mid rpm's. The bike only had drag pipes when I got it but the previous owner had the a/f mixture plug at 4 turns and shimmed that needle in the piston with 2 shims and the plugs were sooty as hell. I turned it down to 2.5 and now it seems to run better.
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