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Old 02-28-2011, 10:09 AM   #16
anotherguy
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The bike has CV carbs does it not? If so the air temperature is negated by the carbs. The reason the numbers increase by the run is efficiency. The hotter it gets inside more of incoming charge is evaporated thus an increase in amount burned. If you have copies of the earlier runs watch the A/F ratio change as the runs go on.

If it were mine I'd go down a step on the main. Then again if it ain't broke.............
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Old 02-28-2011, 10:14 AM   #17
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If it were mine I'd go down a step on the main. Then again if it ain't broke.............
There's the debate... she runs great..fires up everytime...no issues.. But I always strive to make it more efficient... Hmmm...

BTW..I'm in Olympia, WA ... PacNW.. elevation of maybe 100msl. On a trip to central OR a year or so ago...had to turn up the idle a hair just to keep it running (about elevation 3500msl)
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Old 02-28-2011, 10:36 AM   #18
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None of the readings mean much at all as long as your fresh air plumbing is hooked up, no?

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Old 02-28-2011, 10:41 AM   #19
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Wide open AFV is only part of the story. Good dyno tuners will set the brake at cruising speed. Then check what the AFV is at 1/4, 1/2 and 3/4 throttle. As in rolling along, pulling up a hill and pulling up a really big hill against the wind. This will tell you if you're getting best gas mileage without detonation and burning down the motor.
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Old 02-28-2011, 10:46 AM   #20
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Not aware of CV carbs negating air temp? Very interesting. Even if there were some "warming" contribution to the IAT (Inlet Air Temp) at idle or part throttle by the slides, I don't see how it would differ at all from a non-CV slide. The point is moot, however, since this test is a WOT test and the slides/needles are fully raised. Inlet air temp is definitely a factor.

You won't change the "efficiency" at all by re-jetting. The Brake Specific Fuel Consumption (BSFC = lbs fuel/hr/HP = efficiency) is already fixed by the combustion chamber design, CR, pipes, runners, etc. You'll only change the amount of fuel thrown in the fire (and out the pipe). Of course, higher temps will better vaporize fuel, and higher temps will come with leaner operation. That's one reason why leaner operation contributes to detonation.

BTW, your main jet doesn't contribute to your idle change at altitude; your pilots do. But understand the effect exists across the band.

If you were really interested in making a change to your main jet, which you can probably do with just the data you have now and not worry much, it would be great to have some part throttle A/F data. It might be wiser to change the needle and leave the WOT fuel (main jet) alone. you don't run at WOT all that often, and when you do...protection is good!

Where I live in Mid-Michigan, my GS actually runs better with the needles raised a notch (richer through the middle). Also, I ride down into the 20sē F. Lots of oxygen in the air at that point here at ~ 230 msl. The stockk jetting works pretty well.
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Old 02-28-2011, 10:48 AM   #21
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Lornce - correct. I commented on that. To get specific readings you would need to cap the injection ports.

Benjava - +1 on that. See above.
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Old 02-28-2011, 10:50 AM   #22
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Originally Posted by Rob Farmer View Post
I'm covinced they need to run rich for road use. My last dyno run showed mine running slightly rich just off tickover and midrange. I dropped a jet size on the needle jet and it needed the choke on for several miles and even then it felt like it was straining.
Rob, you can't go buy what a dyno says just off idle. They usually don't even start reading until close to 3k rpm and then it still takes a bit for them to settle into a reading.

Needing to have the choke on until the engine is good and hot can be a sign of getting the jetting right. I have worked with many bikes that are jetted to the tits for most power and they are VERY cold natured. My bike is cold natured too but I don't really care how it runs cold. You can't if you want to get it running at it's best when it is hot. If you are trying to get the best out of your engine when it is hot, it won't run right when it's cold. How could it? It's two different worlds with one carb. That is what chokes are for!

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Old 02-28-2011, 11:02 AM   #23
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Needing to have the choke on until the engine is good and hot can be a sign of getting the jetting right. I have worked with many bikes that are jetted to the tits for most power and they are VERY cold natured. My bike is cold natured too but I don't really care how it runs cold. You can't if you want to get it running at it's best when it is hot. If you are trying to get the best out of your engine when it is hot, it won't run right when it's cold. How could it? It's two different worlds with one carb. That is what chokes are for![/QUOTE]

True dat. Remember that before FI cars (always under more scrutiny for driveability than bikes) used to pre-heat the inlet air using the exhaust manifold in addition to a choke to minimize the cold stumbles.

Sorry...guess I don't know how to use the "Quote" feature. Quoting Supershaft above.
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Old 02-28-2011, 11:04 AM   #24
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Negatory, Supershaft. Extra fuel is how you avoid detonation.

Of course, carbon buildup can be the source of detonation. Most "carbon" (or more accurately, combustion deposits) you find in your ring grooves, on your piston crown, and on your head/valves are from the components of the oil, not the fuel.

And remember, "Old's Cool" factory jetting will always be conservative to protect the engine from timing scatter, low-grade fuel, hot running, etc. No dispute that more power can be safely made by leaner jetting if, for example, you ride where the air is thin, never load it in the dirt or two-up, etc.

When dyno tuning a carbureted engine for everyday use (motorcycle or otherwise), tuners often find the A/F ratio that yields LBT conditions (Lean Best Torque), then add ~ 5% fuel for safety. With an injected engine, the game is a little different because sensors can tell you a lot more about the actual operating conditions. I've seen conditions with A/F ratios as low as 8.5:1 at certain operating conditions on production cars to extend the life of pistons, converters, etc.
What's the difference between LBT and BT?
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Old 02-28-2011, 11:09 AM   #25
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It's used to differentiate between fuel v. spark. MBT pertains to spark timing (Mean Best Torque).
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Old 02-28-2011, 11:14 AM   #26
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I don't have much experience playing with carbureted engines...but if I plug the air injection system on the PD..I suspect the air/fuel will go even richer? right?..

So maybe go down to the 130 and then plug the air inj system would still be safe and give a bit more power?
This is what so many people do not understand about jetting. In order to jet your bike you have to buy a bunch of jets and try them. There is no other way to do it! A 130 might not work. You don't know until you try it! Sometimes different combo's of jets gets different results than just changing one at a time. What works best will work best. Personally, I have every Bing main in steps of five from 125 to 170. I have every needle jet in steps of two from 260 to to 274. Different idle jets too. While you are testing jets, you also have to be careful and not melt your engine! IMO, it's not for the faint at heart. It takes money AND time. Not to mention some wherewithal.

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Old 02-28-2011, 11:16 AM   #27
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Originally Posted by anotherguy View Post
The bike has CV carbs does it not? If so the air temperature is negated by the carbs. The reason the numbers increase by the run is efficiency. The hotter it gets inside more of incoming charge is evaporated thus an increase in amount burned. If you have copies of the earlier runs watch the A/F ratio change as the runs go on.

If it were mine I'd go down a step on the main. Then again if it ain't broke.............
That's what I am trying to say AND warn about!
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Old 02-28-2011, 11:21 AM   #28
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Originally Posted by Weaverman View Post
It's used to differentiate between fuel v. spark. MBT pertains to spark timing (Mean Best Torque).

How would the crankshaft know the difference? Other than it all being interrelated, what's the point?
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Old 02-28-2011, 11:25 AM   #29
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It depends on which parameter you're tuning. When developing an engine, the first variable you start analyzing is MBT, since it impacts losses directly. Fuel mapping comes later.
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Old 02-28-2011, 11:28 AM   #30
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Originally Posted by Weaverman View Post
It depends on which parameter you're tuning. When developing an engine, the first variable you start analyzing is MBT, since it impacts losses directly. Fuel mapping comes later.
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