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PostPosted: Fri Sep 22, 2017 3:25 pm 
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Adventure Rider
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Location: City of Angels
@DesertRogue, 2.8 or 1.8 gallons before reserve?

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PostPosted: Fri Sep 22, 2017 3:42 pm 
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Location: Mackinaw City, MI, USA
Kommando wrote:
91 Octane is a waste in a low-compression naturally-aspirated engine, especially with a freer-flowing exhaust installed.

:s_thinking OK, splain the last part... I'm with ya on the first part. :s_dunno

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PostPosted: Fri Sep 22, 2017 4:02 pm 
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DesertRogue wrote:
I have a Cali bike also. Our bikes have 2.8 gallons before it goes on Reserve.

My bike has slightly bit more modifications than yours does…side open air box as well as top of the box cut out; Full exhaust (FMF Powerbomb & Leo Vince X3 muffler. The BST40 has been jetted to 155 main.

On a normal 300 mile romp across the canyons, mountains, and freeways of Socal I’ll get an average of between 47 mpg to 52 mpg. That’s riding it at 80+ on the freeways and quite a few WOT passes in the canyons. The lowest I’ve seen was 46 and the highest 64 mpg when I was forced to ride like a H-D! LOL!

So your bike is most certainly running somewhat rich and wasting fuel somewhere.

Start with your idle. After riding it until the engine is up to normal temp, adjust the idle to around 1500-1600RPM. Then adjust the Pilot Mixture screw by turning it in until the engine begins to run rough. Then back it out until it smooths out. Back it out just a smidgen again (maybe 1/8 of a turn). After that, test the bike out at least two tankfulls of normal mixed riding and see what average you come out to be.


Wouldn't I have to have a special instrument to know the RPMs? Since our bikes don't have a tach?
I thought the pilot mix and setting idle were the same thing?

Also, if this is that easy to adjust, why do people have to go in and adjust the needle?


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PostPosted: Fri Sep 22, 2017 4:07 pm 
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650Blackout wrote:
DesertRogue wrote:
I have a Cali bike also. Our bikes have 2.8 gallons before it goes on Reserve.

My bike has slightly bit more modifications than yours does…side open air box as well as top of the box cut out; Full exhaust (FMF Powerbomb & Leo Vince X3 muffler. The BST40 has been jetted to 155 main.

On a normal 300 mile romp across the canyons, mountains, and freeways of Socal I’ll get an average of between 47 mpg to 52 mpg. That’s riding it at 80+ on the freeways and quite a few WOT passes in the canyons. The lowest I’ve seen was 46 and the highest 64 mpg when I was forced to ride like a H-D! LOL!

So your bike is most certainly running somewhat rich and wasting fuel somewhere.

Start with your idle. After riding it until the engine is up to normal temp, adjust the idle to around 1500-1600RPM. Then adjust the Pilot Mixture screw by turning it in until the engine begins to run rough. Then back it out until it smooths out. Back it out just a smidgen again (maybe 1/8 of a turn). After that, test the bike out at least two tankfulls of normal mixed riding and see what average you come out to be.


Wouldn't I have to have a special instrument to know the RPMs? Since our bikes don't have a tach?
I thought the pilot mix and setting idle were the same thing?

Pilot mixture and idle speed settings are two separate functions. Idle speed simply adjusts how far the butterfly valve is open at idle. Pilot mixture (pilot screw, low speed screw, etc) controls the air fuel ratio at idle and slightly above.

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PostPosted: Fri Sep 22, 2017 7:00 pm 
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Welcome to the forum 650Blackout! If you'd like the owners or service manual in PDF format PM me for the links. :s_yes

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PostPosted: Sat Sep 23, 2017 12:03 am 
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WoodRat wrote:
Welcome to the forum 650Blackout! If you'd like the owners or service manual in PDF format PM me for the links. :s_yes


Thank you! I just found it in a pile of stuff the guy gave me. Looking it over now


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PostPosted: Sat Sep 23, 2017 12:04 am 
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Location: Elkhart, IN
There's no reason to warm anything up for 5 minutes, unless you're living above the Arctic Circle. Once it's idling with the choke off, you can ride it in a sane fashion (I have to get out of my subdivision before I hit a road where I can go faster) for a minute or two, then ride it as hard or easy as you like.


Last edited by bongodave on Sat Sep 23, 2017 12:38 pm, edited 1 time in total.

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PostPosted: Sat Sep 23, 2017 7:41 am 
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650Blackout wrote:
Also, if this is that easy to adjust, why do people have to go in and adjust the needle?

There are several different "circuits" (not electrical) in your carby. Various passages, jets and needles. Each circuit looks after a different rev range/throttle opening. For instance, your main jet is about your only consideration for WOT. The needle covers the lion's share of your mid-range revs/throttle opening. It's essential that the fuel/air mixture is kept optimal throughout the rev range/throttle opening range, and this is how they do it. The circuits (I think there are 4 plus the starter (choke) circuit in the BST40) do overlap a little and everything is a compromise.

The BST40 is a CV (constant velocity) style device is actually quite a simple carby. Most CVs are. The Keihin FCR MX series of carbies are pretty much the apogee of bike carbies, and are commensurately complex. Lot's of blokes use either them or a Mikuni TM40 as a popular performance upgrade.

There are lots of really good resources on the intergoogle with line drawings and animations showing how they work. It won't take long for you to become quite conversant with the whole deal.

Btw, I also subscribe to the get-on-and-ride-it warm-up method. As soon as it'll idle OK without the choke get going. However, for the 1st few miles just be sane and gentle so that the oil can warm up before undue stress is applied, particularly high ring pressures.

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PostPosted: Sun Sep 24, 2017 12:35 am 
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WoodRat wrote:
Welcome to the forum 650Blackout! If you'd like the owners or service manual in PDF format PM me for the links. :s_yes


I would like the service manual since you offered. I only have the basic user manual. I tried a couple times to PM you but the forum kept saying "SQL error". Not sure why??
Thx for the offer!


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PostPosted: Sun Oct 01, 2017 2:21 am 
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Location: Modesto, California
I grabbed one of these https://smile.amazon.com/gp/product/B01FZ100K8 to set idle.


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