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Author Topic: Carb Jetting  (Read 2025 times)

Chaz

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Carb Jetting
« on: February 07, 2008, 03:53:35 PM »
I want to address owners who have at least 1,000 miles on their bikes, because up until now, you haven't been using the high speed circuit of your carb.  I installed the after-market classic exhaust with the connical K&N filter.(I'm assuming all of you have stripped off the emmissions devices!)  I should also mention I live at sea level, as this will affect jetting.  I am running a 130 main jet and have the needle in the highest position, i.e. C-clip in the lowest position.  Keep the stock pilot jet and turn the air screw out one turn from seated.   Change spark-plug to one heat range hotter, a BR7ES.
The bike idles great, does't pop when descelerating, and gives perfect plug readings!
For those of you making winter modifications, this should get you in the ball-park.

Thumper

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Re: Carb Jetting
« Reply #1 on: February 07, 2008, 07:00:29 PM »
Welcome Chaz, good to have you onboard!

Good to hear you got yours spot-on.

MIne is too except that mine does pop on rapid deceleration.

No exhaust leaks, so I've always assumed that it's because I'm running a tad rich and the unspent fuel is igniting.

However, I read an interesting analysis a couple months ago that indicates that this is an almost unavoidable lean condition. The logic is that:  a) you are at speed, b) you just chopped the throttle and it's now operating soley on the idle circuit, so c) there's not enough fair/fuel mixture going into the engine that is adequate for it's current speed.

Made sense to me, especially because I've always had trouble eliminating the popping from my various performance set-ups.

Matt

Chaz

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Re: Carb Jetting
« Reply #2 on: February 07, 2008, 07:19:36 PM »
Hello Thumper,  Thank you for the welcome.  Usually, if bikes are jetted too lean at an idle, they will tend to search up and down in the RPM range and you will generally have an inconsistent idle as the engine is searching for gas.  With that said, have you done the old "sustain 50MPH ,pulling in the clutch and hillting the kill switch, and immediately check your plug?"  (I'm hoping you've pulled over to the side of the road)   I think your bike is too lean, I had the same problem.  I was running a 125 main and the are screw turned out 1 and 3/4 turn and the needle lowered one notch and when it got warm the bike would pop on deceleration.  Jetting is certainly a black art, but at least I have a great collection of jets ranging from 112 to 130.  Maybe I'll make my wife some earrings!!

Thumper

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Re: Carb Jetting
« Reply #3 on: February 07, 2008, 07:31:03 PM »
Hello Thumper,  Thank you for the welcome.  Usually, if bikes are jetted too lean at an idle, they will tend to search up and down in the RPM range and you will generally have an inconsistent idle as the engine is searching for gas.  With that said, have you done the old "sustain 50MPH ,pulling in the clutch and hillting the kill switch, and immediately check your plug?"  (I'm hoping you've pulled over to the side of the road)   I think your bike is too lean, I had the same problem.  I was running a 125 main and the are screw turned out 1 and 3/4 turn and the needle lowered one notch and when it got warm the bike would pop on deceleration.  Jetting is certainly a black art, but at least I have a great collection of jets ranging from 112 to 130.  Maybe I'll make my wife some earrings!!

No, mine's definitely not too lean at 50mph (although I've never chopped the throttle because I've had no need to). The slide valve cutaway is one cut richer than the one that came with the Amal kit. I don't know what size the main jet is, but it's designed for the freer flowing aftermarket header and muffler. The idle circuit on the Amals does affect other circuits more than I'm used to (on Kehins and Mikunis). When it's adjusted a tad lean then I do get more popping on rapid decel.

Regardless, it's running killer strong. I did get  just a little more out of the top rev range with the influence of the leaner valve cutaway - almost as much as if it were running a leaner main jet. But I don't operate at WFO very much and the advantage of the cooler running rich mixture is more important to me for 95% of my driving.

Matt

Chaz

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Re: Carb Jetting
« Reply #4 on: February 07, 2008, 07:55:30 PM »
Wow, throw out what I just said since you're running an Amal.  I sold my  two BSA's last year and I must admit I'm a little rusty on jetting Amals, but I'm glad yours is running strong.  Gotta go, LOST is on!!

LJRead

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Re: Carb Jetting
« Reply #5 on: February 08, 2008, 11:46:26 AM »

Lot of good info in this thread.  Question, if I was to lower input restriction to the carb by simply switching over to the K & N type air filter, would I need to go to larger jet?  Also, If i did go to a larger jet and the air filter conversion, would I need to open up the exhaust.  In other words are all the elements of the open air modification required for it to work right?

Thumper

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Re: Carb Jetting
« Reply #6 on: February 08, 2008, 01:04:53 PM »

Lot of good info in this thread.  Question, if I was to lower input restriction to the carb by simply switching over to the K & N type air filter, would I need to go to larger jet?  Also, If i did go to a larger jet and the air filter conversion, would I need to open up the exhaust.  In other words are all the elements of the open air modification required for it to work right?

You don't always have to rejet when you replace the stock airfilter/box with a K&N. If it doesn't run too lean because of the change then you're good.

If you change the A/F and change to a freer flowing exhaust then carb adjustment (pilot A/F mixture, needle position and/or main jet) is almost certain.

You are right, they are all part of one system - a big pump. Air/fuel in and exhaust out. It's when the flow gets impeded on one end or another that a change is required.

Matt

Chaz

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Re: Carb Jetting
« Reply #7 on: February 08, 2008, 01:56:52 PM »
I'm totally with Thumper on this one...  These bikes come into the country with a 110, that's painfully lean.  My dealer had already re-jetted mine to a 120 before I bought it.   The plug looked a little rich.  My first modification was the classic exhaust, at which point the plug looked slightly better, but not by much.  Then, I switched to the K&N fillter which made the plug run way hot. I changed the jet to a 125 , the plug looked better (grey), but popped during deceleration.  I went to a 130 jet which produced a real nice tan-brown plug.  But the bike a 45MPH seemed to search for gas (a slight surging feel), so I raised the needle to its highest position and switched to a BR7ES plug.  You might think at this point I'm going through lots of gas, but not true, I still get 70MPG two-up!

LJRead

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Re: Carb Jetting
« Reply #8 on: February 08, 2008, 02:32:13 PM »
Good clear explanations, from both of you.  Problem I might have is 1) knowing which jet I'm starting with since mine is the 350 cc 1AVL (Thunderbird) and 2) which jet to go to, since I think the jetting might be a bit lower on the 350 AVL, as it is one the 350 iron.  The reason I asked about the exhaust is it may take months before I get in the new exhaust system (muffler really) while the jets and air filter will be here in a few weeks(One by sea, others by air)

Thanks again,

Larry

Chaz

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Re: Carb Jetting
« Reply #9 on: February 08, 2008, 04:09:52 PM »
Larry, 
To figure out what jet you have you need to remove your float bowl, you can leave the carb attached to the intake.
1)  Remove the screws and slide float bowl straight down, take care not to bump your float.  If it's stuck a little tap with the back end of your screwdriver will suffice.
2)   You will see your main jet between the two float halves.  Use a stubby flat-head screwdriver and carefully remove jet.  There is a brass washer that may or may not come with the jet, don't loose this.  A rag on top of the engine case will prevent you from losing any dropped parts in the crevices of the motor.
3)  Once your jet is out, you'll be able to see the stamped size on the face of your jet.  If you're like me, reading glasses or a magnifier will help greatly.

Buy at least the next three larger sizes, i.e. 112.5,115,117,5 etc.  Don't rely on what the owner's manual says your jet size is, someone(dealer, factory) may have switched it to meet a particular countries emmission standard.

LJRead

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Re: Carb Jetting
« Reply #10 on: February 09, 2008, 12:06:05 PM »
Thanks, I'll do that today.  I was just going to order a 115 and let it go at that - I don't need a magnifier, I need new eyes!