Lean at High RPM - Page 2 - Chevelle Tech
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post #16 of 24 (permalink) Old Mar 26th, 20, 1:10 PM Thread Starter
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Re: Lean at High RPM

Quote:
Originally Posted by pockets View Post
What is your fuel system set up? Hose size, fuel pump model, tank and vent set up.

Check that the high speed bleeds are the correct size, are not blocked. Float levels center of glass or bottom of plug when taken out (if you have plugs). What is the list number on the carb, and what size jets now.
Don't think it's fuel supply. I installed a fuel pressure gauge and watched it all the way up and it stayed at 7psi. For the record, it is stock tank with vented cap. Walbro 255 pump regulated down with a Mallory regulator. 1/2" fuel line with 3/8" return.

Float level is to bottom of plug.

List number is 4781-5.

Jets are 78s on the primaries and 84 on the secondaries.

Quote:
Originally Posted by DragRacer View Post
Change spark plugs before you get too far. Donít get too carried away with plug gap.
Quote:
Originally Posted by 77 cruiser View Post
Start by checking plugs, wires & the rest of the ignition, maybe the valve springs. Misfiring will show as lean.
Quote:
Originally Posted by suntreemcanic View Post
Could the popping be from valve float?
If ignition or valve float, wouldn't it go rich instead of lean? It is a whole new ignition system with new distributor, wires, and plugs. Retarding the timing did not help.

Quote:
Originally Posted by DragRac69 View Post
What are the high speed air bleeds ?
Factory, non-changeable.


Quote:
Originally Posted by swampy 6x6 View Post
HI
Assuming fuel supply system ok
[pump lines pressure all correct ]

Change secondary high speed air bleed for smaller . This will cause more engine vacuum/suck on jet ie richer mixture

What is the cruise mixture ie on primaries light constant throttle ???
vehicle speed and Engine rpm at cruise ??

The theory about fuel CURVE
fuel mix taken every 500rpm
1/ flat horizontal straight line up and down =main jets
2/fuel flat curve angled either gradually up or gradually down
=low speed bleeds to shift start of diagonal flat curve up/down
=high speed bleeds to shift end of diagonal flat curve up/down
3/primary jets = light throttle cruise 40mph to 55mph
4/ secondary jets = 3/4 throttle
5 / power valve start to engage at 50--55 mph or equivilant engine vacuum ,drill out to provide correct mixtures .at 100% throttle.

If the fuel curve is actually either U or upside down U =emulsion jets

During cruise, it is around 13:1 at about 2700rpm. During accel, it starts rich around 11:1 at 2k rpm. It gradually gets to 14+:1 by 5k rpm.


Quote:
Originally Posted by 427L88 View Post
I'm late but will re-iterate what Bill said, ensure fuel level is correct, as a low level will lean it out top of pull for sure.

my "race" 850 is dumb simple old skool ( no adj bleeds) and works mint.... 77 w/085 PV primary, 80's w/045 in rear.

Same config run on dyno, AFR was perfect thru the pull. Jets wont matter in a assymetrical AFR problem like this.

ALSO, again shirt tailing on Bill, before you pull it apart, blast some carb cleaner down the orifices in the top of the air horn to ensure the air bleeds are clear.

So blast and check float HOT b4 any parts changes, which I dont think you need.
If the air bleeds are restricted, won't that make it run rich?

As a test, can I add a restriction to the high speed air bleeds?

Randy Johnson
70 Chevelle
454
12.449[email protected]<
1.766 60 ft

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post #17 of 24 (permalink) Old Mar 26th, 20, 2:19 PM
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Re: Lean at High RPM

Misfire shows as lean because the sensor in measuring oxygen in the exhaust.
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post #18 of 24 (permalink) Old Mar 26th, 20, 2:36 PM
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Bill
 
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Jetting sounds close esp if your not reving past 5000 yes if bleeds plugged would go rich but if carb not seeing a strong signal wont pull fuel Maybe up the sec float a tad if car stumbles under hard braking its too high
But also time to ck other issues Dont rely 100% on O2 readings ive chased issues because I believed the numbers

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post #19 of 24 (permalink) Old Mar 26th, 20, 5:14 PM
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Re: Lean at High RPM

List number is 4781-5.

Jets are 78s on the primaries and 84 on the secondaries.

You are a fair amount leaner than factory jetting on the primary side. Go back to factory jetting, but plug the secondary power valve, and up the mains 8 sizes.

Factory spec is 80/80 and 6.5 PV primary and secondary. Go 80 primary, 88 secondary and secondary remove the PV and install a plug.

What do the plugs tell you when you kill it after a full throttle run?

Any miss in the engine will show lean reading. Tighten the plug gap to .040".
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post #20 of 24 (permalink) Old Mar 27th, 20, 11:53 AM
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Russ
 
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Re: Lean at High RPM

re: post #17 -- If you're suffering an ignition problem, like a weak coil, causing the missing, that could show up as a lean mixture due to unused oxygen in the exhaust.

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post #21 of 24 (permalink) Old Mar 29th, 20, 10:26 AM
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Re: Lean at High RPM

You will need a good ignition system for this beast. If you are running old-style points and coil it may miss if the plugs aren't brand new. Upgrade to a Pertronix or something similar to be sure.

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post #22 of 24 (permalink) Old Mar 29th, 20, 8:42 PM Thread Starter
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Re: Lean at High RPM

Running a new Proform 66941R distributor.

Randy Johnson
70 Chevelle
454
[email protected]<
1.766 60 ft

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post #23 of 24 (permalink) Old Mar 29th, 20, 8:58 PM
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If your running Around 10.1 cr And not reving engine much past 5500 you dont need a big race ignition
Points or an electronic module ( which is just a switch like points ) electronic module does little to increase spark its all about the coil so good coil proper heat range plugs gapped .035/.040 max does it misfire Under load At top of 2, 3 rd gear underload or just in high gear over 4500-5000

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post #24 of 24 (permalink) Old Apr 7th, 20, 9:56 PM
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Re: Lean at High RPM

Is the fuel system OK? Not running out of fuel supply?

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