Idle air screws

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Idle air screws

Post  tconnection on October 30th 2014, 12:11 am

If my motor wants less than 1 turn out, what's to be done to jetting and air bleeds?
Thanks

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Re: Idle air screws

Post  aquartlow on October 30th 2014, 7:57 am

Before any mods are done to the carb, like enlarging the air bleeds, make sure the carb's baseline is correct. If you purchased the carb used, try contacting the carburetor's manufacturer to get orifice dimensions so you can check your carb against those measurements to see if it is in it's stock form. Primary throttle blades/transfer slots, secondary throttle blades/transfer slots, float levels, fuel pressure and etc. are set correctly. That said, if you only need 1 turn out or less on the idle mixture screws,(and the baseline of the carb checks out correctly) it may indicate the IFR's are too large or the IAB's are too small(or a combination of both if someone "tweaked" the carb before you). Make sure the ignition timing, both initial and total are correct, before making any drastic changes to the carburetor. A wideband will help immensely with tuning your carb. Good luck. Todd
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Re: Idle air screws

Post  rmcomprandy on October 30th 2014, 10:04 am

tconnection wrote:If my motor wants less than 1 turn out, what's to be done to jetting and air bleeds?
Thanks

Idle screws needing to be less than one turn out is not any problem. It only is a problem if they are completely seated and you still can not make it lean enough.
There MAY be other problems but, not the idle screws.

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Re: Idle air screws

Post  kim on October 30th 2014, 11:48 am

Im with rmcomprandy. Most street stuff I've tuned has come in at about 3/4 to 7/8 off seat. High manifold vacuum at idle does a hell of a good job at pulling fuel. If you have a good idle speed and mixture is set with anything off seat... its within that's carbs current design. Press on to having fun or other problems to solve.

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Re: Idle air screws

Post  Calypso on October 30th 2014, 12:10 pm

Agree with guys above. That itself is not an issue. However, it's possible to have IFR and IAB combo which is way off, while idle screws are in the ballpark.

As an example: I have an old 850DP with 2corner idle on 460 with performer intake (and 4-hole spacer). Original IAB was .073 and IFR .036. Idle screws were useable, but it was pig rich in cruise. I'm .059IAB and .028IFR now, with idle screws in the ballpark (IFR area is 60% of the original). It's much cleaner and more responsive at part throttle, but I still don't need choke at freezing temperature (can't remember Fahrenheit scale). I'm going to try even smaller IFR...

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Re: Idle air screws

Post  tconnection on October 30th 2014, 3:13 pm

This carb that I have is a 1250 quickfuel that came off of a 545 scj, xr292 cam I think 671 lift. It has 96's in the front and back.
I got it on my 580 ported some, aheaded, 839/780 lift cam, solid drag only. If any adjustments need to b made, about what would they be?
Thanks

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Re: Idle air screws

Post  kim on October 30th 2014, 3:22 pm

original engine displacement to cam, damn near street. If anything with your cam, might need a little more on the main jets. As your still about 300+ cfm light, its going to have a pretty stout pull on the fuel side of things and shouldn't really need a whole bunch of anything. When your running in excess of 13" manifold vacuum you can take a 650 cfm carb off a 302 and put it on a 460. The fuel will be correct up to the limitations of the air flow of the carb. As we meet or exceed the engine air flows demands with the volume the carb can flow is when drastic tunning measures come into play. Take that 1050cfm off the 460.. and try and put it on a 302, jet surface area would have to increase exponentially to move fuel at the low vacuum demand at the boosters the 302 would make.

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Re: Idle air screws

Post  tconnection on October 30th 2014, 10:12 pm

Kim, maybe what you are saying is all within what im about to ask. Forgive me but with the little amount turned out on the idle air screws, would bigger air bleeds allow more turn out on the screws allowing more fuel.
How bout this? At one turn out im rich as crap. Would bigger idle air bleeds lean it up some?
Thanks

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Re: Idle air screws

Post  Lem Evans on October 30th 2014, 10:25 pm

tconnection wrote:Kim, maybe what you are saying is all within what im about to ask. Forgive me  but with the little amount turned out on the idle air screws, would bigger air bleeds allow more turn out on the screws allowing more fuel.
How bout this? At one turn out im rich as crap. Would bigger idle air bleeds lean it up some?
Thanks

It'd depend on the carb style and aplication...but...historically I have, when the 'screws' want to be 3/4 ish out, is open the idle air bleeds .002"
And hope to get the screws 1-1/4 - 1-1/2 turns off the seat.

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Re: Idle air screws

Post  tconnection on October 30th 2014, 10:51 pm

At idle should I have black plugs?
I haven't ran this thing yet.
Lem I bout got it ready for a test pass. Its been a long long haul.
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Re: Idle air screws

Post  kim on October 30th 2014, 11:13 pm

No you shouldn't have black plugs. Have you checked that its not pulling fuel out the booster at idle?

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Re: Idle air screws

Post  tconnection on November 2nd 2014, 11:25 pm

im gonna double check my float level. With a return reg, does the engine still need to b running or can I check with it off?

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Re: Idle air screws

Post  kim on November 2nd 2014, 11:54 pm

running

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Re: Idle air screws

Post  tconnection on November 5th 2014, 10:51 am

I think im about to figure this engine out. Im thinking I been having my float level to high and using to light of an oil for low tension oil ring. Any thoughts, suggestions and tips would b great.
Thanks

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Re: Idle air screws

Post  bbf-falcon on November 5th 2014, 10:55 am

I prefer running when setting floats.

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Re: Idle air screws

Post  tconnection on November 5th 2014, 11:07 am

Qft 1250, where r u guys setting your level in the sight glass? And with .003 on the main bearing and 21/2 on the rod, what weight oil and what kind? I got access to some Brad Penn for 6 bucks a qt.
Thanks
Th

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Re: Idle air screws

Post  tconnection on November 5th 2014, 11:16 am

Qft 1250, where r u guys setting your level in the sight glass? And with .003 on the main bearing and 21/2 on the rod, what weight oil and what kind? I got access to some Brad Penn for 6 bucks a qt.
Thanks
Th

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Re: Idle air screws

Post  kim on November 5th 2014, 11:20 am

Never use sight glass. Use brass plugs. At idle set the float so that the fuel will trickle down the side of the carb if you bump the car. Otherwise no trickle...

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Re: Idle air screws

Post  rmcomprandy on November 5th 2014, 3:02 pm

tconnection wrote:Qft 1250, where r u guys setting your level in the sight glass?
Thanks
Th

Experiment with setting it a bit higher and lower, (as long as the is no sign of booster dripping), until the vehicle runs the best.
Then mark it and set THAT combination there, all the time.

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Re: Idle air screws

Post  kim on November 5th 2014, 3:08 pm

Randy's advice is best.. I typically didn't have the luxury of days of testing. It was carb rebuild and bolt on... So the quickest way to establish a "decent" tune was to use a preset fuel level, and then tune around it. The fuel level in the bowl is the "fuel pressure" for the various circuits for the carb. It is another "tune-able" option. One size does not fit all. Instead of pulling jets and playing with air bleeds, the simple twist of the float setting can net gains or losses.

Ensure you don't have fuel siphon at idle through the booster. Ensure your not creating a huge fuel slosh issue up vent tubes... but the beauty and simplicity of the carb, is the detriment that you have to play with it ALOT to get everything "perfect". And then perfect is only really perfect for that combination, AND atmospheric condition.......

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Re: Idle air screws

Post  tconnection on November 6th 2014, 12:33 am

I lowered my floats down. The fuel is only showing about an 1/8 or so above the bottom of the sight glass. So far when I rev on it it's alot more clear. I gonna see about getting a video running it down the road tom and post it in my build section.
Thanks

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Re: Idle air screws

Post  res0rli9 on November 6th 2014, 1:32 am

tconnection wrote:I lowered my floats down. The fuel is only showing about an 1/8 or so above the bottom of the sight glass. So far when I rev on it it's alot more clear. I gonna see about getting a video running it down the road tom and post it in my build section.
Thanks

We need videos of it on the back wheels @ about 45* Cool

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Re: Idle air screws

Post  rmcomprandy on November 6th 2014, 10:41 am

tconnection wrote:I lowered my floats down. The fuel is only showing about an 1/8 or so above the bottom of the sight glass. So far when I rev on it it's alot more clear. I gonna see about getting a video running it down the road tom and post it in my build section.
Thanks

Free reving it will tell you little ... driving it and racing it will make a difference.

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