Carb tuning help

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Nov 26, 2017 | 11:03 PM

olblue77

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Joined: Nov 2012

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From: Nashville, Tennessee

Carb tuning help

I have a quick fuel carb part # BD-1957 and I'm having the worst time trying to tune it. It's sitting on top of a 302. I have the fuel bowls set to read in the middle of the sight glass as per instructions but when tuning the idle mix screws using a vacuum gauge I adjusted to the highest manifold vacuum until the idle would increase, then back it down to where the idle was back to normal. It will idle alright, accelerate fine, but cruising it is erratic and will spit and backfire unless I give it gas then it runs fine. First time dealing with any carburetor and I'm stumped.

Nov 28, 2017 | 12:39 PM

Beechkid

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Joined: Aug 2002

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From: Southern California

I'm not familiar with QF carbs but....... it sounds like you have a circuit issue.....needs to be adjusted...IMHO, needs to come in sooner....if it comes in too soon it could also caused this as well....if the carb has an acel pump on it, it likely needs to come in sooner....typically during the transition from cruising to accel, a shot of fuel is needed to overcome the lean condition which occurs.....

Dec 2, 2017 | 08:35 AM

lee

Laughing Gas

Joined: Jul 1999

Posts: 1,047

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From: Lacey, WA

Hi,

Sounds like you are adjusting the idle mixture correctly except,.... don't back the idle speed back down with the idle mixture screws.... adjust the idle speed screw on the side of the linkage with the idle speed screw.

Adjusting the idle mixture for highest Vac reading is correct.... sometimes this causes the idle speed to increase... if this increase is to high the carb starts running off of the "run" circuit and not the "idle" circuit. So,.. adjust mixture for highest Vac reading while also adjusting idle speed screw to keep in idle circuit ~600 - 800rpm.

Dec 2, 2017 | 09:26 AM

Tedster9

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Joined: Apr 2002

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From: Waterloo, Iowa

Thing to keep in mind is each carb circuit is tuned or adjusted independently of the others. If there's a problem in the cruise or power circuit it's not made up by going back and buggering with idle mixture again. Set the idle mixture about as lean as it will go consistent with a good smooth idle (highest steady vacuum) and forget about it. If idle mixture is too rich it will load up the plugs and won't idle very well. It may need slight re-adjustment seasonally, or for altitude.

Then get the steady highway level ground cruise mixture dialed in. Hardly any load under this situation. This determines whether you get 10 mpg or 20 mpg. Finally make sure it isn't leaning out too much under load or acceleration. You don't want that, engines do not like going lean under load or power. Think cutting torch - notice how hot the flame gets when you start feeding in more O2?