Carb tuning mystery

[Skinner]

So I've been trying to tune the carbs on my Katana. It's running well except when at idle and not fully warm. The carbs were stripped and dipped, new o-rings, float levels good, new intake and exhaust boots and clamps, bench synced then synced on the bike with a Morgan Carbtune. The mixture screws were adjusted while using the carb tune to obtain the highest idle. The airbox lid is sealed with foam. And I have a brand new Suzuki petcock with Suzuki fuel line installed and no fuel filter.

The bike starts immediately while on choke, and only requires about 1/2 choke at that. It does take some time - 2 or 3 minutes - before it will idle off choke. Until the engine is completely warm, the idle will drop quite low when I stop at an intersection, enough that I have to give it some throttle to prevent a stall. But after it's completely warmed up idle rises to where it should be - 1100 rpm or so - and stays there. I have no hanging idle issues at all - the idle drops back down immediately when the throttle is released.

I'm assuming it's a mixture screw issue but it could probably be something else as well. I'm going to pull the plugs tonight and see how they look. Couldn't do it over the weekend because of Mothers Day festivities. The bike runs great and pulls hard to redline. It's only the idle issue I'm chasing.

K

[Nessism]

Sounds close to normal. How long do you have to ride before the idle will hold increase to 1000 rpm or so? It wouldn't surprise me if it took a good 5-7 mins or riding before reaching that point.

[chuck hahn]

The heat inside the jugs helps increase idle performance on carburetored bikes. Until theres some good heat established they tend to act like this. Sounds like its ok to me too.

[Skinner]

Sounds close to normal. How long do you have to ride before the idle will hold increase to 1000 rpm or so? It wouldn't surprise me if it took a good 5-7 mins or riding before reaching that point.

That sounds about right Ed. My old 650 didn't act this way and it's my only frame of reference.

[Skinner]

The heat inside the jugs helps increase idle performance on carburetored bikes. Until theres some good heat established they tend to act like this. Sounds like its ok to me too.

Thanks Chuck.

[Gorminrider]

computerised fuel injection has spoiled us. Every carburetted vehicle I've owned has needed awhile to warm up.

(Who else can remember the troublesome heated springs that operated the choke and redirected manifold exhaust until the vehicle warmed up? argh

[mmattockx]

The bike starts immediately while on choke, and only requires about 1/2 choke at that. It does take some time - 2 or 3 minutes - before it will idle off choke. Until the engine is completely warm, the idle will drop quite low when I stop at an intersection, enough that I have to give it some throttle to prevent a stall. But after it's completely warmed up idle rises to where it should be - 1100 rpm or so - and stays there. I have no hanging idle issues at all - the idle drops back down immediately when the throttle is released.

If this is in moderate summer weather then you are lean on the mixture screws. How many turns out are you at? My 1100E with pods and 4-1 exhaust is pretty close on everything and it will start with part choke and idle on its own in 30 seconds or less. The cold idle is around 800rpm and it warms up to an 1100rpm idle inside of 2-3 minutes. It will never die at a stop when cold or require fiddling with the throttle to keep it going.


Mark

[Skinner]

If this is in moderate summer weather then you are lean on the mixture screws. How many turns out are you at? My 1100E with pods and 4-1 exhaust is pretty close on everything and it will start with part choke and idle on its own in 30 seconds or less. The cold idle is around 800rpm and it warms up to an 1100rpm idle inside of 2-3 minutes. It will never die at a stop when cold or require fiddling with the throttle to keep it going.
Mark

Not sure how many turns. I started with them all the same - about 3 from loosely seated - then adjusted them individually for the best idle. Your description of starting from cold is what my 650 used to do. I'll take another crack at it. Lots of rain in my forecast so it's a good time to do it.

[mmattockx]

Not sure how many turns. I started with them all the same - about 3 from loosely seated - then adjusted them individually for the best idle. Your description of starting from cold is what my 650 used to do. I'll take another crack at it. Lots of rain in my forecast so it's a good time to do it.

FWIW, I never use the best idle method. I have seen far too many stories of problems like yours to think it is a good way to go. I set them all the same and test ride, then adjust all of them based on the test ride results. I am currently at 3.25 turns out on my 1100E, but that is with pods and a 4-1 pipe. If yours is stock as your sig indicates I would start about 1/4 turn farther out than whatever your manual says the stock setting was.


Mark

[Skinner]

If yours is stock as your sig indicates I would start about 1/4 turn farther out than whatever your manual says the stock setting was.

Thanks Mark.

Yes, it's stock. My manual states "PRESET" for the mixture screw setting. My bike has BS34 carbs and the manual for the GSX1100 (which also has BS34 carbs) says 3.5 turns. That said, the manual for the GSX750, which has BS32's, also says 3.5 turns. I'll give this a shot and see how it goes.

Thanks.

[mmattockx]

Yes, it's stock. My manual states "PRESET" for the mixture screw setting. My bike has BS34 carbs and the manual for the GSX1100 (which also has BS34 carbs) says 3.5 turns. That said, the manual for the GSX750, which has BS32's, also says 3.5 turns. I'll give this a shot and see how it goes.

Hmmm. My factory manual for my 1100E says 'preset' as well for the mixture screws. I would say that 3.5 turns out is too far for a stock set up. That is pretty much wide open for the mixture screws and you would normally be stepping up a pilot jet size to get back under 3 turns and gain some adjustability back. Check where all your screws are at and post it. For a stock airbox/exhaust/engine I would start the screws at 2.5 turns out and see how it idles and runs then. Adjust in 1/4 turn increments from there until it responds worse and then back up a bit.


Mark

[Skinner]

I would say that 3.5 turns out is too far for a stock set up.

Mark

Mark,

I thought that 3.5 turns seemed excessive as well. That said, my manual is for the European version of the bike so their emissions requirements may have been different than those in North America. All of the NA manuals for most bikes seem to say "PRESET" for the mixture screw settings. I can recall reading on the carb rebuild tutorial that 2.5 turns out is a good place to start and you work either way from there. Regardless, I'll try setting them all the same and see how it goes.

I checked around a bit and found a 1983 GSX1100SD Supplemental Manual (uses BS34's as well). The carb chart is broken down into various countries and the specs say "PRESET" for all of them, like what we see in our manuals. However, in brackets next to "PRESET" it has 2.5 for some countries and 3.5 for others, so I guess it's an emissions thing.

K

[red1100]

My GS1100E running ok now except at idle,now occasional pop out the pipes. No sparks to ground that I can see. Any ideas?