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Idle mixture screws
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videoguy009
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PostPosted: Sun Mar 01, 2015 1:02 pm    Post subject: Idle mixture screws Reply with quote

Found an issue with the popping carb on my dual 40 idf carbs.the idle mixture screws have no effect when turning them.If I turn them in all the way the carb stays running strong.I have checked for vacuum leaks and there are none.I can get a good idle and a good sync but when I try to adjust the mixture screws to richen it up to take the popping out they do nothing.The linkage is right on as I followed the correct procedure,unhook for syncing and reattach and listen for change in idle.No change.I am running 55 idles and 135 mains.
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leaker
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PostPosted: Sun Mar 01, 2015 4:04 pm    Post subject: Reply with quote

Could be plugged idle jets, idf's plugging up is common, although I have never had all four plug up at the same time.
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MURZI
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PostPosted: Sun Mar 01, 2015 4:42 pm    Post subject: Reply with quote

Before you adjust the mixture screws make sure that the throttle stops are not screwed into far. If you expose the progression ports in the carburetor by having the throttle blades in the wrong position the carb will run rich....

Disconnect your linkage at each carb, set the idle mixture screws at 1.5 turns from seat. now start the car and adjust "snail" to about 4 or 5....the engine will be idling super slow. Now adjust your mixture screws.
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Casting Timmy
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PostPosted: Sun Mar 01, 2015 4:48 pm    Post subject: Reply with quote

The fuel has to be coming from somewhere if the screws don't affect idle.

Do Murzi's suggestion as most likely it's comingfrom the progressive circuit as the throttle shafts are too far open. YOu can also use a vacuum gage to adjust your throttle plates if you have a distributor vacuum advance port on your carbs. Simple adjust the plates until you see vacuum and then unscrew until the vacuum goes away.

You can also check fuel pressure to make sure it's not over 3.5 psi and pushing the fuel thru the carbs.

Are these carbs new to you? Checking float height would be my third even if the carbs were new.
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videoguy009
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PostPosted: Sun Mar 01, 2015 5:04 pm    Post subject: Reply with quote

I am running a mechanical dist.The carb is 3 yrs old and the other is new which runs like a charm.They are new jets in them but i know that doesn't mean there isn't a blockage somewhere in the idle circuit.One thing I did notice today is when the car was running the exhaust from that side would go hot then cold intermittently.I guess that would mean plugged idle circuits?Sorry,never thought to mention that!
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PostPosted: Sun Mar 01, 2015 8:21 pm    Post subject: Reply with quote

I don't see it as a blocked idle jet. The progressive circuit feeds off the idle jet as well as the idle screw.

On the exhaust, was that during the same throttle setting or what else was happening then?
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videoguy009
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PostPosted: Sun Mar 01, 2015 8:30 pm    Post subject: Reply with quote

It was on the same throttle setting.When I used this carb as a centermount one jet was plugged and I noticed the heat riser was cool.I pulled the jet and their was debris on it.This is what makes me think one or two jets are plugged as the exhaust runs cool.When I rev the car for the mains to kick in their is no problem as it runs strong.I have not been able to get back to the car today to check things.
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MURZI
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PostPosted: Sun Mar 01, 2015 8:59 pm    Post subject: Reply with quote

videoguy009 wrote:
It was on the same throttle setting.When I used this carb as a centermount one jet was plugged and I noticed the heat riser was cool.I pulled the jet and their was debris on it.This is what makes me think one or two jets are plugged as the exhaust runs cool.When I rev the car for the mains to kick in their is no problem as it runs strong.I have not been able to get back to the car today to check things.


There is your problem....using a center mount as in dual carb application. My bet is your emulsion tubes are different and the throttle plates have holes drilled in them. (hence the no response from idle mix screws)
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PostPosted: Mon Mar 02, 2015 5:37 am    Post subject: Reply with quote

The butterflies on the single carbs do have holes drilled in them from the factory.Could this be my big issue.
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PostPosted: Mon Mar 02, 2015 6:27 am    Post subject: Reply with quote

Yes. also check emulsion tubes. You need f11's.
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PostPosted: Mon Mar 02, 2015 7:40 am    Post subject: Reply with quote

Do the emulsion tubes hold the main jets.Is that them?
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PostPosted: Mon Mar 02, 2015 8:17 am    Post subject: Reply with quote

Yep
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PostPosted: Mon Mar 02, 2015 8:19 am    Post subject: Reply with quote

With the holes drilled in the butterflies,does this cause the carb to bypass the idles curcuits or something similar to that.
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PostPosted: Mon Mar 02, 2015 8:21 am    Post subject: Reply with quote

Yeah. Like a vacuum leak. Put a piece of tape over the holes... Temporary... Youll see a difference right away.
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PostPosted: Mon Mar 02, 2015 8:25 am    Post subject: Reply with quote

Thanks for everything.I have been chasing a dead horse for the past week.I will have to purchase an identical carb to the one I bought and everything should go well.Thought I could save money by using my old carb,but guess what.Lol.
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PostPosted: Mon Mar 02, 2015 8:32 am    Post subject: Reply with quote

Just loctite some small screws and nuts in the holes. No biggie. Emulsion tubes can be bought too. F11 is what you need.
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PostPosted: Mon Mar 02, 2015 8:40 am    Post subject: Reply with quote

Will do and save some money.Thanks for the advice.
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