Author Topic: Setting the Air/fuel mixture on a Demon carb, 1965 fastback  (Read 4693 times)

naika

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Setting the Air/fuel mixture on a Demon carb, 1965 fastback
« on: February 21, 2010, 02:47:38 PM »
Hi guys,

I took a look at the car today, cleaned a few things, took time to check the carbs since the car runs really rich and I have a question.

I have a demon carb, and the engine is a Code A, so I assume it's a 4V. I'ma ttaching a few pics.

I checked the setting of the air screws on the 4 corners of the carb, and they are all set to more or less slightly less than 3/4 of one revolution open. It seems to me that it would be really rich and that's how the car feels.

Can anyone share experience on settings of their car, and maybe a way I could use to fix this? The sparks are black, and the car has some miss when i don't clean them really often, so I assume it needs leaned down, but I'd like an idea of how many revolutions, or approximately so i can have a good starting point to fine tune it.

Thanks for your help

Franck





Soaring

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Re: Setting the Air/fuel mixture on a Demon carb, 1965 fastback
« Reply #1 on: February 21, 2010, 02:52:12 PM »
Not sure about the Demon,  but most carbs have a setting of between 1 1/2 to 2 1/2 turns out.  My Motorcraft is set at right at 2 1/2 turns.  Try tweaking each screw out about a 1/4 turn at a time to find that sweet spot.  3/4 of a turn just seems too little to me, and therefore you are running rich. 

naika

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Re: Setting the Air/fuel mixture on a Demon carb, 1965 fastback
« Reply #2 on: February 21, 2010, 02:56:27 PM »
Thanks ;)
I'm going to try adding a 1/4 turn out, starting at 1 full turn and see if that's better.

What's the best way to check the quality of the combustion? spark color closer to a brown? or is there another way?

Thanks again

Franck

Soaring

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Re: Setting the Air/fuel mixture on a Demon carb, 1965 fastback
« Reply #3 on: February 21, 2010, 03:11:34 PM »
The tips of your spark plugs should be a light gray in color.  You will be able to tell if they are either wet, or black, and that indicates not enough air.  Get back after you have reached that sweet spot and let us know how it turned out.  Your engine will tell you when you reach that sweet spot, because it will idle up a bit and seem to run smoother. 

66GTKFB

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Re: Setting the Air/fuel mixture on a Demon carb, 1965 fastback
« Reply #4 on: February 21, 2010, 03:33:50 PM »
Slow down a bit. The only way to adjust fuel air mixutres is to change the main jets (secondaries included in final tuning), the venturi diameter or on some carburetors, the emulsion tubes. Most carbs do not have replacable venturis so it's the main jets that are changed. If you are running rich, one or two sizes smaller (aka leaner) should work. As you are in France, replacement jets might prove to be difficult to locate. In the USofA, jets are about $6.00 each (I can remember when they were a quarter). You need two. The first thing is to determine what main jets you have (open up the carb and have a look-see).
Screw adjustments are for the idle, although some carbs also have jets as well. The normal procedure for idle adjustment: warm up the engine; back off the throttle screw on the main butterfly plates (it's the large hex screw on the lever where the arm from the accelerator pedal rod goes); screw in the idle screw on the primaries (do both sides independently and them together) until the engine misses (or dies, same thing); back off 1/4 to 1/2 turn; use the throttle plate screw to get the idle speed up to spec. Most instructions require the vacuum advance to the distributor to be pluged.
Jim

Soaring

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Re: Setting the Air/fuel mixture on a Demon carb, 1965 fastback
« Reply #5 on: February 21, 2010, 04:33:03 PM »
Yeah, Jim is right about full throttle, but I thought we were talking about idle.  If you are talking about full throttle, then the jets will need to be downsized.  I doubt that is the problem, but it could be.  Try adjusting the idle adjustment screws first, and if that doesn't solve your problem, then get back to us. 

naika

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Re: Setting the Air/fuel mixture on a Demon carb, 1965 fastback
« Reply #6 on: February 21, 2010, 04:39:26 PM »
It is actually  an idle problem, the car idles correctly but spits a lot from the exhaust while idling. it full throttle, it might be too rich, but I haven't had any signs that made me think so.
I'll check the screws and see if that makes a difference, if not, then I'll take it to a carb guy to do the job ;)
Thanks, I'll keep you posted

Soaring

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Re: Setting the Air/fuel mixture on a Demon carb, 1965 fastback
« Reply #7 on: February 22, 2010, 01:42:29 AM »
Tweaking the idle adjustment screws should solve your problem.  That's all a carb guy will do, and charge you a ton of money. 

 

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