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Old 07-30-2013, 09:50 PM   #1
shifflett
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Default carb gurus please come inside

Hey guys, I have a problem I hope you all can help me with? im running dual carter yfs on my 235 and they have a stumble of idle, they have been rebuilt with auto parts store generic kits (I didnt know any better at the time). I have synced them with a snail style vacuum gauge, im running 5/16th line which is just a hair smaller than stock. it idles pretty good and runs really well at higher speeds. if anyone could maybe point me in the right direction it would be greatly appreciated!
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Old 07-30-2013, 10:17 PM   #2
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Default Re: carb gurus please come inside

If you have a stumble coming right off idle, there is probably a lean condition right at that point. This can happen if the butterflies are open slightly too much to reach the desired idle speed exposing the little holes in the main circuit. If you advance the initial timing up say 5-10 degrees and close the butterflies some to get your best idle speed and the stumble goes away, that is the problem. If not, you may need a larger shot of fuel from the accelerator pumps. This is assuming you don't have a vacuum leak anywhere in the intake system. Hope that helps.

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Old 07-31-2013, 07:46 AM   #3
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Default Re: carb gurus please come inside

I wee bit more information would be helpful.

What YF's are you using?

If you have a basically stock 235 and trying to use two YF's originally designed for a 235, then the stumble may be very difficult to eliminate.

However, often a stumble right off idle is a direct result of having the idle set too rich, resulting in puddling in the intake. When the throttles are opened, the puddles are swept into the cylinders creating an instantaneous OVERRICH condition, hence the stumble. This condition often is caused by using too large a carburetor(s).

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Old 07-31-2013, 08:47 AM   #4
Ricks Garage
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Default Re: carb gurus please come inside

Bog ?....or.....hesitation?
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Old 07-31-2013, 09:58 AM   #5
shifflett
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Default Re: carb gurus please come inside

My motors a 54 235 bored 60 over with dual exhaust and dual carbs the head is a 53 235 with hardened valve seats the rest is stock. The carbs are carter yf 964, if I slowly accelerate its ok but if I try and give it any gas quickly it almost dies then comes to life.
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Old 07-31-2013, 02:10 PM   #6
carbking
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Default Re: carb gurus please come inside

The 964s should work well.

Before doing ANYTHING to the carburetors, make certain your distributor is advancing correctly, and the dwell and timing are as they should be.

Then, acquire a Uni-Syn (brand name I am familiar with, there are probably others) and synchronize the carbs:

http://www.thecarburetorshop.com/Dual1barrelcarbs.htm

If these two procedures do not eliminate the problem (or at least minimize it to an acceptable level), it may be necessary to redo the idle circuits on the carbs. If so, give me a call at the number and hours posted below. Will be glad to explain. Or this link may help:

http://www.thecarburetorshop.com/Idlemixturescrews.htm

Jon.
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Last edited by carbking; 07-31-2013 at 03:26 PM.
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Old 07-31-2013, 07:16 PM   #7
shifflett
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Default Re: carb gurus please come inside

The vacuum advance moves just not sure how much it should move also I have synced the carbs with a gauge not to sure how well the motors timed
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Old 07-31-2013, 09:01 PM   #8
carbking
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Default Re: carb gurus please come inside

Just a suggestion:

Set the dwell first, then the timing, then think about the carbs.

Jon.
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Old 07-31-2013, 09:17 PM   #9
shifflett
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Default Re: carb gurus please come inside

Alright ill give that a try could those really be that serious of a problem?
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Old 07-31-2013, 09:19 PM   #10
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Default Re: carb gurus please come inside

That funky 235 Combustion Chamber need lots of advance, try it with manifold to the vacuum advance. Here is a 59 235 Timing Spec, 3-5 initial. Dont mess with the carbs until you have the advance right, thats opinion.

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Old 07-31-2013, 09:49 PM   #11
shifflett
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Default Re: carb gurus please come inside

How much should the vacuum advance move when accelerating
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Old 07-31-2013, 10:03 PM   #12
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Default Re: carb gurus please come inside

Listen to carbking for the carbs to work wright the engine needs to be in proper tune and the engine needs to be sound.
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Old 07-31-2013, 10:08 PM   #13
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Default Re: carb gurus please come inside

The dist sould move around 8 to 14 deg
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Old 07-31-2013, 10:10 PM   #14
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Default Re: carb gurus please come inside

Quote:
Originally Posted by shifflett View Post
How much should the vacuum advance move when accelerating
There's no way to answer this.

Under light throttle ( incidentally you said yours runs good) the vacuum can should advance the timing proportionately to the amount of vacuum being generated by the engine. There is plenty of vacuum at light throttle. Look at hoops chart.

Under heavy throttle ( incidentally where you are having trouble) there is very little vacuum ( high load) so the can does little to zero on the advance. You are relying solely on the mechanical curve built into the distributor. This curve is based off of rpm and is very adjustable by changing weights and springs. Again look at hoops chart.

Picture this-
You are sitting at a light, the ignition is advanced via vacuum.
Light turns green and the pedal goes down, throttle plates open, vacuum goes to zero, all vacuum advance drops out and your relying on mechanical advance only. But since it falls on its face the Rpms go down instead of up and you loose your mechanical advance too. This really compounds your problem.

Ever hear the saying " 90% of carb problems are of an electrical nature"

Your distributor may very well be perfect and you may very well have a carb problem. But you need to make sure that your distributor is working as it should or tuned to your engine first. It's the hierarchy that reduces the tail chasing. You can ignore the tried and true hierarchy but you will get dizzy chasing your tail.
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Last edited by 31Vicky with a hemi; 07-31-2013 at 10:16 PM.
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Old 07-31-2013, 10:29 PM   #15
shifflett
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Default Re: carb gurus please come inside

Alright ill get all the timing stuff situated and go from there
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Old 08-01-2013, 07:59 AM   #16
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Default Re: carb gurus please come inside

And always address the dwell first, as changing the dwell will change the timing. If you set the timing first, and then change the dwell, you will modify the timing.

Like 31Vicki stated, doing things in order can eliminate a lot of chasing your tail.

Jon.
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Old 08-01-2013, 09:10 AM   #17
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Default Re: carb gurus please come inside

Quote:
Originally Posted by shifflett View Post
How much should the vacuum advance move when accelerating
You need a "MytiVac" pump or similar. You use the chart above and apply vacuum to the cannister and make sure it is inside the spec. So at 12" of vacuum you should advance 16-21 degrees.
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Old 08-11-2013, 08:11 PM   #18
shifflett
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Default Re: carb gurus please come inside

Ok timing has been checked and seems right on, when the pointer is on the bb in the flywheel then rotor points to cylinder number 1, points are gapped to .018 is this correct? If so the car is still acting the same.
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Old 08-11-2013, 08:16 PM   #19
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Default Re: carb gurus please come inside

If you can adjust the accelerator pump. I think it needs to come in earlier...

You got good vacuum right?

There is that moment when the throttle is opened. That split second the manifold vacuum is gone, and your engine needs fuel. The accel pump makes up that fuel deficit.... or perhaps your pump is bad all together.
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Old 08-11-2013, 08:40 PM   #20
shifflett
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Default Re: carb gurus please come inside

Vacuum at my carbs? If that is what you mean when I synced them it was 20 inches of vacuum. Im no mechanic by any means so some of this doesn't make alot of since to me so mind my ignorance
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