Main air bleed screw 7040263

[Grych]

Greetings

In Cliffs book I discovered the small screw under the triangular shaped metal cover over the front vent.  I have this adjustment on a 7040263 carb. I am curious as to what affect the adjustment of this screw will have on a 7040263 carb.  My LM-2 indicates that I am cruising at 15.1-1 with a 70/35 jet/rod combination. My next available combination is 72/37 cruises at 14.2-1.  I do not have a 36 rod for a 71/36 combo that would put me at roughly 14.6-1.

My thinking is: If the screw allows more air to the main air bleed circuit, I could lean the 72/37 combo slightly to reach a 14.6-1 or so cruise ratio.


Thank You
Mark   

[Cliff Ruggles]

Mark, the tiny screw behind the vent on the 7040263 carburetor leads to passages under the upper main airbleeds.  Unless fully seated, it will add additional air to the main system.  I would gently seat the screw and do some A/F ratio testing.  There should be no reason to run such small primary metering rods in relationship to the jets, provided everything else with the carburetor is up to par?.....Cliff

[Grych]

Hi Cliff

My compliment to a great book on quadrajets!  I had checked that little screw and it is seated.  You have me a little curious when you stated that I should not need that small of a primary needle..... I understand the typical spread between jet and needle is roughly 30  (70/40 as an example)  which is very close to the 70/39 that I started with.  Using the LM-2 AND with the way the car ran the 70/39 combination just too lean.

I felt that my application was the reason for the rod/jet spread.  My apllication is a pontiac long rod 413 CI, Stock edelbrock heads, 7040263 carb, ramair IV camshaft with rhodes lifters, 1.65 rockers, ram air manifolds and a 7530t MSd box. roughly 11 to 1 compression.  Warm Idle is roughly 14 in vac.

The 7040263 was a stocker that was modified many years ago based on Doug Roes book.  It was just recently put into operation.

CARBURETOR PARAMETERS 9-21-08
 Primary Needles                   35
 Primary Jets      70
 Power Jet Spring                    Medium tension
 Idle Down Jets      .035
 Idle Bleed Jet      .053
 Idle Bypass Air      .090
 Channel Restriction   .062
 Secondary Hanger   K
 Secondary Needles   AY
 Base idle speed      700 RPM
 Solenoid Idle speed   900-1100 RPM
 Idle Mixture Screws   1-1/4 Turns Out

The car runs fairly well, but is lean off idle and during the tip in when cruising down the road.  Maybe with my application and carb information you would suggest something different?  I did not have your book at the time this carb was modified otherwise I would have used one of your recipes.

Thank You
Mark

[comp]

you have enough acc. shot ??

[Grych]

Accelerator pump shot is good.  No stumble. Just slightly lean in the cruise area. with the 70/35 combination.  I have run the 72/37 combo and the LM-2 shows that I am slightly rich.  Dont take this wrong as the car runs good through out the entire range with this combo, its just slightly rich.

My concern is based on Cliffs comments whereas the small primary needles should not be necessary. Do I have a problem within the carb or is it my application?  I will likely go back to the 72/37 combo and experiment with the adjustable main air bleed.

Mark

[comp]

  let us know

[Cliff Ruggles]

Mark, looking at the combination, I'd go to .073 or even .074" main jets, and .041" primary metering rods.  The aluminum heads, big cam, and the fact that modern fuel is lacking some in energy content, diluted with ethanol, etc usually requires more fuel clear across the engine rpm/load range.  We have all the jet numbers, and .041" rods new and used in the shop.  I'd also replace the power piston spring with a new one, we offer an assortment of them for custom tuning......thanks....Cliff

[Grych]

Greetings,  my last test session on Sunday I was already leaning (no pun here) towards your suggestions with larger jetting.  Thanks Cliff.  I used a 73/39 combination which provided slightly more fuel during part throttle and it provided mixed results.

Part throttle,  which to me is 30 mph down main street the lm-2 was indicating 13.5 to 14.0 AFR.  Hwy Speed,  which was 55-60 mph the LM-2 was around 14.0-14.4 AFR.  The car ran smoother as the surging was now gone.

However,  upon decceleration from either hwy or part throttle cruising the LM-2 goes rich around 12.0 and the warm idle is roughly the same 12.0 AFR  with an rich exhaust smell.  It was set to 14.0 before leaving for the test drive.  At this time I can only assume that the power piston spring is too stiff or I do not have enough idle by pass air.  More testing.

I must say, another down side to using rhodes lifters beside the noise is the amount of change from warm idle vacuum to a hot idle vacuum.

Thanks
Mark

[comp]

 cool 

[Hotrodrobert]

Where are you taking the LM1 reading from?  If you have the tailpipe tool, they tend to read LEAN.  Seems they get some contamination from the end of the pipe.
Also, you can get the best mileage usually from a slightly leaner that stoic mixture as high as 15.5:1.
No lean surge, good plug readings, test for best mileage will tell.