Adjusting mix and temperature

[izzy]

Since the temperature started dropping my pre has started to afterfire/backfire. Can temperature affect the mix, and should I try and adjust for a lean/rich mix? Also, it has always surged...could the whole thing be caused by needing a carb sync?

Cheers

[hardlydangerous]

Colder the air temps...the leaner the bike will run.
poping on decel will usually get a little worse and will longerwarm ups and smooth riding for the first couple KM.

The only time you need to make an adjustment for cooler weather is if the performance is suffering even when the bike is hot.
usually a bike will perform better in the cool weather because of the leaner mixture

[scottynoface]

+1, my bikes have always loved october. It's a teaser before you put it away!

[izzy]

OK. Thanks. I still get the popping after a long ride, so I must need to adjust the mix.

[The CyberPoet]

Colder the air temps...the leaner the bike will run.

But only when cold. Because the gas coming out of the carbs doesn't vaporize as well (the tiny droplets tend to cling in bigger bunches). As the engine heats up, it warms up the carbs, heating the gasoline in it, and the problem should disappear -- typically within a couple miles. This is also the reason there's a choke on your bike -- to help you get over this too-lean-when-cold phenomena by dumping more fuel into the intake stream long enough for you to get a steady idle going (if it takes more than 30 seconds on choke above about 45-50 degrees F, kill the choke and use the throttle to keep the RPM's around 2k, and consider turning up your idle adjustment).

Cheers,
=-= The CyberPoet

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[hardlydangerous]

air has mass and density, and cold air is much denser than warm air. That's why us racers take time to set up the jetting on race bikes for each race. a 15 degree air change can change the outcome of a race. humidity also plays a role as well.
That's why bikes run so much richer in the summer weather and much leaner in the winter

[The CyberPoet]

air has mass and density, and cold air is much denser than warm air.

CV carbs, such as those on the Katanas, should automatically adjust for any changes in air density (incl. altitude changes & temperature changes), since the vacuum operating the diaphragm slides operates in correlation to the ambient air pressure (the primary advantage of constant velocity carbs). This allows the same carbs (if jetted correctly to begin with) to operate the bike at sea level on a hot day and at 5,000 feet elevation at near freezing without the need to rejet -- provided the engine is warm.

That's why us racers take time to set up the jetting on race bikes for each race. a 15 degree air change can change the outcome of a race. humidity also plays a role as well.
That's why bikes run so much richer in the summer weather and much leaner in the winter

Race bikes normally operate throttle-slide carbs (or more rarely, butterfly-only carbs), which do not change on their own in relationship to altitude, temperature or other factors which affect air density -- rejetting for the actual race conditions is the key to getting best performance out of them at any point. On the other hand, these forms of carbs present more direct throttle feedback and less lag under throttle changes (the reason they are favored for racing applications).

Cheers,
=-= The CyberPoet

______________________
CyberPoet's KR Specials
Suzuki Stratosphere - 6 Cylinders, the new Katana?
The Best Motorcycle Metal Billet Tire Valves in the World, plus lots of motorcycle & Katana (GSX600F / GSX750F) specific help files.

[hardlydangerous]

True CV carbs are some what a compensating carb design and about as close as you can get to an FI system... smoother, slower reacting and more accurate over wider temp ranges but with fixed air and fuel passages for the pilot and main circuits they are still not as accurate as people often seem to think..
Thats why so many people install pipes and filters and don't rejet because the bike still runs smooth and shows no real sings of being lean..
Thats also why dyno testers request a ride after testing because cruise speeds and difference in air temps inside and outside can make a difference in the final tune of the engine and overall performance.

I agree and thats why I said the only time you need to re-jet is if the performance is still suffering when hot.

The only real way to see the difference is by running an EGT gauge or by PROPERLY reading spark plugs. NOT just removing them at any random time and looking at them. That is so inaccurate I can't amd won't even try to explain. 90% of most street riders and even vet riders with decades of riding can't tell the slight differences between dropping a jet size or two... Thats why most people that re-jet only adjust the needle height, mixture screws and main jets and ignore the many other adjustable settings that are needed to be changed to dial in the jetting perfectly on any performance or race bike.
Even with a CV carb if you were to rejet a bike this time of year in the cold weather then drop you main in the summer and you will feel a big differenece... easily 150 degrees on the EGT between summer and winter. An engine tuned to 1250 in the summer could be dangerously lean in the winter. usually not a problem though..most people park the bikes when it gets that cold

back to the original post.. his bike started to "afterfire/backfire"

Is this due to the colder wetaher... YES
Almost all bikes will decel pop much more with the cooler weather...
Even with the throttle shut, the air mixture is still slightly leaner.

and extra 1/4 turn out on the mixture screws will 99.9% solve the issue, but because it causes not damage or long term issues there is no need to adjust the jetting or mixture screw settings for the cooler weather unless the bikes performance is suffering when hot.

[The CyberPoet]

I fully agree that an Exhaust Gas Analyzer is always the best way to determine where your mixture rates across the RPM spectrum, esp. in conjunction with a dynometer (as a "rolling road" to provide real-world resistance as the engine spins up). I always run my bikes over a dyno with an EGA after jetting/exhaust/etc. changes to make sure the mix is at the place I want it to be -- targetting about 13.1:1 for max power in the main torque band and somewhere around 14.2 - 14.3:1 for idle**.

Footnote **: Used to be I'd jet for 14.7 - 14.9 (lean) at idle to get a nice, fast warm-up, but since they started putting 10% alcohol in gasoline (E10) and 10% alcohol drops the stoichiometric rate down to around 14.1 - 14.2:1, I shoot a lot lower for the same effect. A bike that runs ideally at 14.7:1 (perfect burn) with test-quality gasoline will run particularly lean with E10 because of that change.

Note that after-fires when the engine cold can be from both too lean or too rich -- too rich tends to get you hard bangs, while too lean tends to get you more of a burbling (neither is absolute).

Cheers,
=-= The CyberPoet

______________________
CyberPoet's KR Specials
Suzuki Stratosphere - 6 Cylinders, the new Katana?
The Best Motorcycle Metal Billet Tire Valves in the World, plus lots of motorcycle & Katana (GSX600F / GSX750F) specific help files.

[BarMatt80]

man these threads makes me want to

i sure do love my FI...