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Author Topic: Carbs. Am i on the right track?  (Read 1476 times)

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Offline Gizmo

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Carbs. Am i on the right track?
« on: February 22, 2017, 10:08:29 am »
Looking for tips;
73 CB 750
bored out to 836cc
K&N pods
4-1 exhaust
130 mains
40 idles
needle clip 4th from top
mixture screw currently 1 full turn out


Having synced the carbs, i started playing with the mixture screws to compensate for the excessively rich state it is currently sitting in. Despite having backed all mixture screws out in 1/16 increments to the point where they were all 2 full turns out the bike is still excessively rich. My instinct is to change needle position to choke some of the flow of gas.

thoughts?


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Offline calj737

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Re: Carbs. Am i on the right track?
« Reply #1 on: February 22, 2017, 10:11:45 am »
Are you idle jets stock? Check to see if someone drilled them open before you. Needles won't make a difference at idle...

Are you operating the choke in reverse by chance?  8)
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"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline Gizmo

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Re: Carbs. Am i on the right track?
« Reply #2 on: February 22, 2017, 10:13:47 am »
The carbs were rebuilt with brand new jets straight from Honda. Are you suggesting i go down a size on the idle jets?
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Offline calj737

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Re: Carbs. Am i on the right track?
« Reply #3 on: February 22, 2017, 10:19:48 am »
No, I'm suggesting you did not provide sufficient information to make an informed decision.

With the pods and 836 kit, you should be lean (all things equal). Under what circumstances did you determine the bike is running rich? Is this from a new, clean set of fresh plugs and an idle chop? If not, perform that test.

Then when you know you have the idle jetting correct, use fresh plugs and go for a spirited ride and perform a WOT plug chop. Jet to that condition.

Fine tune with needle or screw changes as required.
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"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline ekpent

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Re: Carbs. Am i on the right track?
« Reply #4 on: February 22, 2017, 10:20:56 am »
Won't hurt and easy to change main jets if actually rich with the clip on bowls. Do you have the higher 10+ comp ratio 836 or one of those flat dome E-bay kits ? 120's may be a good starting point also, lets see who chimes in with someone close to your set-up.
 We need a side picture of your Bad Boy first though before we can really tell anything  ;)
« Last Edit: February 22, 2017, 10:26:09 am by ekpent »
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Offline PeWe

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Re: Carbs. Am i on the right track?
« Reply #5 on: February 22, 2017, 10:24:55 am »
My engine with 836 and pods (+cam + ported head) and K6 carbs and rather open and loud 4-1. Pilot std 40, needle 4 ( 1 step richer than std), main jet wopping 147.5!! Less flowing exhaust need smaller mains.
Bike ran really well, not sooted plugs either. Engine did not like any other plugs than NGK D8EA or DR8ES-L. (Bosch Silver and NipponDenso did not survive long, sooted to death shortly. It's still strange for me

Many km on Autobahn with luggage and 2 younger and slimmer persons where the main jets had to work when it went around 100 mph +/-10 all day long! I had noticed too rich :)
The floats however were not checked. Clear tube is the only way to verify OK or not.

Correct in mm when open and set floats will not say if fuel level will be OK, 2-4mm under the float bowl gasket. Float migh be stucked, valve leak...
« Last Edit: February 22, 2017, 10:28:27 am by PeWe »
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CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
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Offline flybox1

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Re: Carbs. Am i on the right track?
« Reply #6 on: February 22, 2017, 10:28:49 am »
Quote from: Gizmo on February 22, 2017, 10:13:47 am
The carbs were rebuilt with brand new jets straight from Honda. Are you suggesting i go down a size on the idle jets?
Honda doesnt make jets  :-\
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Offline calj737

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Re: Carbs. Am i on the right track?
« Reply #7 on: February 22, 2017, 10:29:43 am »
Quote from: flybox1 on February 22, 2017, 10:28:49 am
Quote from: Gizmo on February 22, 2017, 10:13:47 am
The carbs were rebuilt with brand new jets straight from Honda. Are you suggesting i go down a size on the idle jets?
Honda doesnt make jets  :-\
I was gonna let that go, Fly...for now  ;)
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"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline Gizmo

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Re: Carbs. Am i on the right track?
« Reply #8 on: February 22, 2017, 10:51:24 am »
Unfortunately there is still snow on the ground in my home town. So riding it is not an option at the moment. I currently have it in my car port and was in the process of de-winterizing it. It was only towards the end of last years riding season that i was able to put any road time on it, so i am picking up where i left off in carb tuning and shake down rides.

I felt that it was excessively rich based on the smell of the exhaust, the amount of fuel it is sucking back during synchronization, and the lag when i rap the throttle from idle.

@ekpent- the 836 kit is the wiseco one, 10.25-1 ratio
@ PeWe- I am also running the NGK 8EIX with dyna ignition

« Last Edit: February 22, 2017, 12:52:08 pm by Gizmo »
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Offline 754

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Re: Carbs. Am i on the right track?
« Reply #9 on: February 22, 2017, 06:24:55 pm »
I think it would be happier with 125 mains..
 And raise the needle one above stock. I think stock is near or in the middle.
« Last Edit: February 22, 2017, 10:45:51 pm by 754 »
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Offline Keith

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Re: Carbs. Am i on the right track?
« Reply #10 on: February 22, 2017, 07:29:17 pm »
I may be way off, but isn't cranking the mixture screws in lean, and turning them out rich?
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Offline Gizmo

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Re: Carbs. Am i on the right track?
« Reply #11 on: February 22, 2017, 08:37:57 pm »
Hey frank, thanks for the input. I head out of town on saturday for 2 weeks of work. I may order up a set of 125s to have on hand for road testing when i get back.  Couldn't hurt to have another set of mains to test out.


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Offline scottly

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Re: Carbs. Am i on the right track?
« Reply #12 on: February 22, 2017, 08:39:15 pm »
The idle mixture screws are just that; they are for fine tuning the mixture at idle, with the throttle closed.
Tuning from scratch, assuming the floats are set correctly and the carbs, especially the emulsion tubes, are clean, is best done starting with the main jet at full throttle, with the needle set in the middle position (this will allow adjustment of the needle for fine tuning at mid throttle conditions). 
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Offline Gizmo

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Re: Carbs. Am i on the right track?
« Reply #13 on: February 22, 2017, 08:44:04 pm »
@keith
I could be wrong, but the 73 keihin 657B carbs lean out as the screw is turned out.
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Offline Vin750

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Re: Carbs. Am i on the right track?
« Reply #14 on: February 23, 2017, 02:28:03 am »
Quote from: Keith on February 22, 2017, 07:29:17 pm
I may be way off, but isn't cranking the mixture screws in lean, and turning them out rich?

Round top carburettor have air screws, out let's more air in so leaner.
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Offline Keith

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Re: Carbs. Am i on the right track?
« Reply #15 on: February 23, 2017, 04:12:14 pm »
Thanks...I'm a former auto tech, so was thinking in terms of auto carbs!
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Offline Gizmo

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Re: Carbs. Am i on the right track?
« Reply #16 on: June 16, 2017, 03:20:56 pm »
I am in the process of fine tuning some carb settings. Quick question, I noticed in the parts fiche for my bike that the idle air screws have no o-rings, and yet the head of the screw has a recession cut in it. on latter/earlier models were o-rings used? Thanks!

PS a friend of mine is a commercial photographer, he took some amazing shots of the bike and i thought I'd share them
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