Author Topic: Please Help, I'm carb dumb  (Read 2724 times)

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Offline 03rangerxlt

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Please Help, I'm carb dumb
« on: June 03, 2012, 01:51:51 AM »
Just like the title says, I'm carb dumb.

I had my carbs cleaned and rebuilt by a local shop (one of only two services I've farmed out in this project of mine.)

I don't have a clue about carbs, and I know I will need to rejet whats in there.  Right now it has size 100 jets.

I will be running velocity stacks (with screens and possibly a foam cover) and a Moto GP Werks Yoshi replica exhaust (pretty wide open exhaust I'm thinking).

Thanks for any help you might be able to provide.
1972 Honda CB500 Cafe "The War Eagle"

Offline Dimitri13

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Re: Please Help, I'm carb dumb
« Reply #1 on: June 03, 2012, 01:55:41 AM »
Standard questions: What year, what bike, what model carbs. This will help you narrow down your options.

Standard answer: There's no standard jetting. It's all trial and error.

Offline dave500

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Re: Please Help, I'm carb dumb
« Reply #2 on: June 03, 2012, 01:58:27 AM »
is it a quadrajet?or a holley?

Offline 03rangerxlt

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Re: Please Help, I'm carb dumb
« Reply #3 on: June 03, 2012, 02:02:30 PM »
Wow, thats what I get for posting around 4am, before I go to bed.  Pretty important info left out.

The bike is a 1972 CB500 with stock keihin carbs.  I've got velocity stacks on the carbs, and the Yoshi replica 4 into 1 exhaust that looks pretty wide open.

I've found a place that sells the jets (http://jetsrus.com/a_jets_by_carburetor_type/jets_keihin_99202-601_main_secondary.html), and I figure I will probably have to order a couple of different sets.  I just don't know where to start...
1972 Honda CB500 Cafe "The War Eagle"

Offline lucky

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Re: Please Help, I'm carb dumb
« Reply #4 on: June 03, 2012, 03:45:03 PM »
What we know so far.

1972 CB500.
Intake stacks with screens and foam covers.
Exhaust. 4 into 1 Yoshimura.
Main jets #100mm (What was the stock size?)
Idle jets #? (What was the stock size?)
needle clip position from the top??? (Stock position?)
mixture screw???


A lot of gaps still.
Is there any info on the invoice from the local shop??? There should be.
« Last Edit: June 03, 2012, 03:46:45 PM by lucky »

Offline lucky

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Re: Please Help, I'm carb dumb
« Reply #5 on: June 03, 2012, 03:47:25 PM »
is it a quadrajet?or a holley?

Good one Dave!!!

bollingball

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Re: Please Help, I'm carb dumb
« Reply #6 on: June 03, 2012, 03:57:27 PM »
For a person that says he is carb dumb. You would be a lot better off going to parts for sale a start looking for the airbox that came with your bike they come up all the time. A person that is carb savvy would still have a hard time making this work. You do know Vstacks are normally tuned for just WOT not for street driving don't you? Where to start is easy Start where you are now.

Ken

Offline 03rangerxlt

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Re: Please Help, I'm carb dumb
« Reply #7 on: June 03, 2012, 08:16:33 PM »
Well this isn't good sounding!  Check out the picture of the bike, and you'll see why!



The bike came sans airbox, so I de-tabbed the frame and relocated the electrics and battery to under the bum stop.  So because of lack of knowledge on the front end of thsi project, this is where I start learning about carbs!

Questions:
-What is WOT compaired to street driving?
-Where on the carbs are the idle jets?
-The needle clip?
-Mixture screw?

I've only ever messed with the two barrel motorcraft off my old '68 F100 shorty.  Fun but aggravating!
Oh, no invoice from the local shop, just an envelope of small nasty bits and parts they took off before cleaning and rebuilding.
1972 Honda CB500 Cafe "The War Eagle"

Offline cgswss

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Re: Please Help, I'm carb dumb
« Reply #8 on: June 03, 2012, 08:56:00 PM »
Not everyone would do it this way, but I always tune from main jet back.  To do that, first you need to do a plug chop.

DO you have a long lonely road near by?  you need your plug wrench  and somewhere you can get away with a little racy driving.  The chop is simple- run the bike up thru the gears at WOT.  Pull in the clutch and turn off the motor.  hold in the clutch and coast to a stop.  now pull each plug and look the the insulator.  It would be a real good Idea to take pics you can post here.  That way we can give you some idea where to go on the main jet.  Belive me, this is the easy part.

Do you have a number drill set?  You may have problems finding certain sizes of jets and if you simply have a spare set we can suggest drill sizes to get where you need to be

Once we have you running "right" at WOT we can move back to the needle settings


Offline cgswss

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Re: Please Help, I'm carb dumb
« Reply #9 on: June 03, 2012, 09:17:58 PM »
You really need to download a shop Manuel so you will know what we are talking about.

WOT is Wide Open Throttle.  That means you twist you right wrist as far as it will go

All the jets are inside the the float bowl of the carb.  You have to remove the float bowl to get at them.

The needles are inside your slides  (the parts that slide up and down when you twist the throttle)

THe main jet controls the mixture from about 1/3 throttle to full throttle.

The needle slides down inside the main jet.  It reduces the gas coming thru the main jet under about 1/2 throttle.
If you are running "lean" when you roll on the throttle.  You can lower the clip (raise the needle) to give it more gas at low rpms.

then things get real tricky Idle and low speed.  Your biggest problem will be in the off idle to about 1/3 throttle.  This is a side effect of the pods.  So lets strat with the easy stuff-the main jet
 

Offline JimJamerino

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Re: Please Help, I'm carb dumb
« Reply #10 on: June 03, 2012, 09:46:41 PM »
+1 on a shop manual.  Download them from the forum, or buy them.  Study them.  Become one with them.  Read them on the crapper.  I say "them" because it's a good idea to use more than one, since a completely comprehensive manual doesn't yet exist.  Use those to build your knowledge base so that our responses make some sort of sense.

We on the forum are a good resource, but for many questions you're going to get a lot of responses that say something to the effect of "Check your manual."

Also, "WOT" (wide open throttle) compared to "street driving" is the difference between drag racing and regular riding.  Think of a drag race car.  It's built to be run with the pedal to the metal, in a straight line, for a very short length of time.  Drag race cars are not designed to be taken for a leisurely drive on the curvy back roads.  Can you make a drag race car into an everyday car?  Sure... but it'll take work. 

The same principle applies to putting velocity stacks on your bike.  They're designed for race bikes, intended to feed engines that are designed to go very fast for a very short amount of time.  Can you put them on your basically stock bike and make them work?  Sure... but it'll take work.  And knowledge, much of which you will acquire through manuals and the Google Custom Search bar under the orange Home button at the top left of the screen (ignore the obvious search bar at the top right for now).

There are people who will tell you stacks will ruin your engine and it'll be impossible to tune and yadda yadda yadda.  It's your bike, do with it what you want.  Just be prepared to work to make it work.
~ Jim

formerly "emperorspartacus"

There are old bikers.  There are dumb bikers.  There are no old, dumb bikers.

My CB650C rebuild http://forums.sohc4.net/index.php?topic=74212.0

Offline Dimitri13

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Re: Please Help, I'm carb dumb
« Reply #11 on: June 04, 2012, 12:01:31 AM »
While stacks won't completely kill all your streetability, it will be worse off than the airbox.

I've made my setup work decently enough. I have stacks and a modern two brothers racing exhaust (aftermarket, came from a newer cbr1000rr). It still stumbles slightly, and very momentarily (and not just because I don't have an accelerator pump) between closed and 1/4 throttle, but after that, it pulls adequately then really takes off in the top end. Sure I could have a smooth and better throttle response of I went back to the airbox, but stacks look great and I've made them perform decently.

Settings:
Velocity stacks
Two Brothers Racing exhaust
110 main jets
38 slow jets
Pilot screw 1-1/4 turns out
Needle in stock position
Engine internals stock

Offline 03rangerxlt

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Re: Please Help, I'm carb dumb
« Reply #12 on: June 04, 2012, 09:11:45 AM »
Good news, I have a Clymers and a 70's Honda shop manual in the garage.  I didn't consider looking at them though because I was kinda pissed off at them after the issues I had building the brake master cylinder.  I guess I'll be giving them a second chance though for this carb stuff!

I appreciate the replys so far.  Some have actually helped me understand what lies before me in figuring this stuff out.  I'm trying to be resistant to farming this out to a shop so that I can add one more "I did that" to my list of things to be proud of with my project.
1972 Honda CB500 Cafe "The War Eagle"

Offline Tugboat

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Re: Please Help, I'm carb dumb
« Reply #13 on: June 04, 2012, 10:54:14 AM »
Nice lookin' bike btw.. dig that exhaust!
If it's worth doing, it's worth doing twice.

Offline lucky

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Re: Please Help, I'm carb dumb
« Reply #14 on: June 04, 2012, 05:20:31 PM »
While stacks won't completely kill all your streetability, it will be worse off than the airbox.

I've made my setup work decently enough. I have stacks and a modern two brothers racing exhaust (aftermarket, came from a newer cbr1000rr). It still stumbles slightly, and very momentarily (and not just because I don't have an accelerator pump) between closed and 1/4 throttle, but after that, it pulls adequately then really takes off in the top end. Sure I could have a smooth and better throttle response of I went back to the airbox, but stacks look great and I've made them perform decently.

Settings:
Velocity stacks
Two Brothers Racing exhaust
110 main jets
38 slow jets
Pilot screw 1-1/4 turns out
Needle in stock position
Engine internals stock

Good choice on the mainjet.
You might consider going up on the idle jet also if it still has poor throttle response.