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How do you tune a carb?

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Old 05-27-04, 07:27 PM
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How do you tune a carb?

these almighty yet mysterious things are pissing me off, but im to poor to go F.I. so i know people know I just dont know any of them, and im sure its probably been gone over before, so links are fine,
So how is a carb. tuned, mine runs but its alittle out (was a multi-month trip through the land of agravation), where do you start.
I recently had timing trouble and killed my plugs, with new ones it starts fine, but the engine still doesnt run smooth, even if the timing is at TDC..... Im cofused. i dont even really know what my problem is, but im confodent its with the carb not being in tune, the A/F mixture is at 3 turns out ( i had to reset it after messing it up thinking i had a '79 carb)


any insight or direction to someone who knows would help, thank you

Dustin
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Old 05-27-04, 08:01 PM
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Idle SPEED screw...located under the linkage on the right side, very low, and hard to see. Requires a very long flatblade driver.

Idle MIXTURE screw...located dead center driver's side of the throttlebody (cast iron lower part of carb).

Run engine and get it warm. Fuel behavoir is subject to temperature. Tuning a cold engine does no good when it becomes warm.

You are tuning the IDLE only. Tuning the idle has no effect on anything over 2000 rpm.

Adjust the idle SPEED to anything below 1000 rpm.
If you cannot get below 1000 rpm, you have a vacuum leak...a cracked hose, dried-up gasket, that sort of thing- It's letting air come in to the idle circuit, and bleeding off the signal to the fuel source.

Next, adjust the MIXTURE screw by turning it clockwise till it wants to stall. This is leaning the idle mixture.

When it starts to "hunt", or stumble, turn it back 1/2 turn.

Redirect your attention to the idle SPEED screw, and turn it counterclockwise untill you are idling at 750 rpms.

Return to the idle MIXTURE screw, and adjust it the same way as before.

Check the idle SPEED again, and if necessary, repeat both steps.

Your done.

A vacuum leak will only cause an incorrectable high idle.
It will not cause a stumbling.
If you are experiencing stumbling during idle no matter how many times you adjust it, and if you can hear it as you slowly rev the engine, then this is most likely an ignition problem.
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Old 05-27-04, 09:19 PM
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Thats some good info Sterling.
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Old 05-27-04, 10:10 PM
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Great explaination Sterling! It's so good that I've added this thread to the FAQ.
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Old 05-27-04, 10:39 PM
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Thats the best explination I have seen on tuning a carb to date. Deffinately worth putting in the FAQ.
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Old 05-27-04, 10:58 PM
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Thanks guys.
It's just a flowchart kind of thing.
Don't proceed to # 4 untill # 3 is accomplished, that's all.

Carburetor tuning should never be intimidating to a sportscar owner. If it is, all it takes is one success to cure the frustration.
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Old 05-28-04, 01:03 AM
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thank you sterling(i figured you or carl would help me), Ill do that first thing tommorow.

thats awsome inittab i thought about saying " i need an explenation good enough for the FAQ" HA

Thanks again sterling

Dustin
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Old 05-28-04, 07:31 AM
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Good explanation Sterling.
I'll just add this. A symptom of a vacuum leak is runs/drives fine but wont idle or idle is very lumpy depending on the size of the leak.

Small leak = higher than normal idle
Medium leak= hunting/lumpy idle
Big leak= wont idle at all at a normal idle speed, just stalls. But will run fine and stay running if you keep the idle high enough with the gas pedal.
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Old 05-28-04, 04:21 PM
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Adjust the idle SPEED to anything below 1000 rpm.
If you cannot get below 1000 rpm, you have a vacuum leak...a cracked hose, dried-up gasket, that sort of thing- It's letting air come in to the idle circuit, and bleeding off the signal to the fuel source.
Is this only for stock carbs, or does this apply to modified carbs aswell?

I have a 4brl Holley and my car idles between 1200-1500 rpm. Althought it's probably not tuned right.
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Old 05-28-04, 06:19 PM
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This applys to most carbs that have a idle mixture and speed adjustment.
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Old 05-28-04, 07:15 PM
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It was meant for the Nikki carb.

Some carbs have very different transistion circuits, etc.
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Old 05-28-04, 11:59 PM
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the use of a vacuum gauge can help a lot as well I have found with all of my carbs.
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Old 05-29-04, 12:57 AM
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bbmac, would u care to explain?
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Old 05-29-04, 02:19 AM
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I guess i have a vacum leak ( i have no clue where) i did as was explained and couldnt get the idle below 1k, it just wouldnt go and the engine just doesnt sound right ( even thoug im not sure what its supposed to sound like, it has a huge exhaust leak)


thanks again
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Old 05-29-04, 11:59 AM
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if you still have your rats nest you may want to invest some time...go to the "mazspeed" tutorials and take off that piece of garbage
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Old 05-29-04, 12:00 PM
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that may take care of your vacume problem and free up a few HP
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Old 01-03-05, 06:37 PM
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Originally Posted by Rx7carl
Good explanation Sterling.
I'll just add this. A symptom of a vacuum leak is runs/drives fine but wont idle or idle is very lumpy depending on the size of the leak.

Small leak = higher than normal idle
Medium leak= hunting/lumpy idle
Big leak= wont idle at all at a normal idle speed, just stalls. But will run fine and stay running if you keep the idle high enough with the gas pedal.
One thing I'll add from my experience: big enough vacuum leak = car won't idle but also won't run properly. When I had the pipe that goes from the manifold to the AAV#1 I think it is, inadvertenly opened where there was supposed to be a cap, the car would not idle, but also would have less power than usual, and sound different, kinda like it would sound and perform when the leading ignition goes. Capped that pipe again - problem solved, idle perfect and power back.
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Old 05-17-05, 10:31 PM
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How would i go about finding this big vaccum leak since i cant get the car to idle? should i pray around with carb cleaner while the choke is on to see if i can find the leak?
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Old 07-11-06, 11:28 PM
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To lower the idle, do you turn the screw clockwise or counterclockwise? Sorry for the newbish question but I haven't been able to find an answer.
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Old 07-11-06, 11:33 PM
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In lowers, out raises, On the air mix, in leaner, out richer. Do the checks I posted in your other thread and visit the FAQs sticky at the top of the forum. There are links to troubleshooting carbs posted by Sterling. Very good info. Read first, play later. Nice job on the search.
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Old 07-11-06, 11:54 PM
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I have read most of the threads in the FAQ. It is good info.
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Old 07-12-06, 12:31 AM
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you turn it clockwise to raise the idle. In other words...screw in the screw to raise your idle.
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Old 07-12-06, 01:26 AM
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Originally Posted by chedda_j
you turn it clockwise to raise the idle. In other words...screw in the screw to raise your idle.
Damn, your right, first mistake in 2 weeks. Turning it in pushes against the arm and raises the idle. You're getting better there kiddo.
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Old 07-12-06, 03:24 AM
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My car likes the idle better around 960. Is that good? I mean that is after a full warm up and a good run.
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Old 07-12-06, 03:35 AM
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Might be worse on the wallet becuase of fuel consumption, but I like my idle around 1200-1400rpm. mainly because anything less than that going wot suddenly will cause a stumble because of it not being tuned and having huge jets in it. maybe if id properly tune it i could lower it. but i still like a slightly higher idle
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