Lean at High RPM - Chevelle Tech
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post #1 of 23 (permalink) Old Mar 25th, 20, 7:51 PM Thread Starter
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Lean at High RPM

I have a 496 with a mild cam. 10:1 compression. Rebuilt Holley 850 double pumper. I am getting popping/misfiring above 5000rpm and high load. Getting sufficient fuel pressure with my electric pump.

I have a wideband in one bank of the exhaust. It is going lean at higher RPM. It's hard to tell at WOT, but at part throttle, it gets to almost 15:1 above 4500rpm. At lower RPM, it is around 12-13.

Went up 4 sizes on secondary jets. Did not seem to help.

Running 6.5 power valves. Will going to 4.5 help?

What else should I try?

Thanks!

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post #2 of 23 (permalink) Old Mar 25th, 20, 7:52 PM
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Vacuum leak ?

The PV # is when it will open
(“manifold vacuum)
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post #3 of 23 (permalink) Old Mar 25th, 20, 8:34 PM
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Kerry
 
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Re: Lean at High RPM

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.
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post #4 of 23 (permalink) Old Mar 25th, 20, 11:06 PM
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Re: Lean at High RPM

Change spark plugs before you get too far. Donít get too carried away with plug gap.

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post #5 of 23 (permalink) Old Mar 25th, 20, 11:07 PM
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Re: Lean at High RPM

What are the high speed air bleeds ?

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post #6 of 23 (permalink) Old Mar 25th, 20, 11:24 PM
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Re: Lean at High RPM

Could the popping be from valve float?
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post #7 of 23 (permalink) Old Mar 26th, 20, 3:38 AM
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Re: Lean at High RPM

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
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post #8 of 23 (permalink) Old Mar 26th, 20, 10:15 AM
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Re: Lean at High RPM

Quote:
Originally Posted by rlchv70 View Post
I have a 496 with a mild cam. 10:1 compression. Rebuilt Holley 850 double pumper. I am getting popping/misfiring above 5000rpm and high load. Getting sufficient fuel pressure with my electric pump.

I have a wideband in one bank of the exhaust. It is going lean at higher RPM. It's hard to tell at WOT, but at part throttle, it gets to almost 15:1 above 4500rpm. At lower RPM, it is around 12-13.

Went up 4 sizes on secondary jets. Did not seem to help.

Running 6.5 power valves. Will going to 4.5 help?

What else should I try?

Thanks!
I would try a different carburetor. The Holley DP's are known for part throttle issues, most suffer from being to fat. Maybe soak the metering blocks and blow out the passages real good. Raise the float level to see if that helps.


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post #9 of 23 (permalink) Old Mar 26th, 20, 10:46 AM
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Yes what carb list number and what jetting now You say power valves are you still running a pv on secoundary side ??? If so remove and plug and jet up 8 sizes from the as built jets

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post #10 of 23 (permalink) Old Mar 26th, 20, 10:48 AM
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Re: Lean at High RPM

I would go up on the primary jets, and try an 8.5 power valve. Part throttle 4500 you probably are not into the secondaries yet.

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post #11 of 23 (permalink) Old Mar 26th, 20, 11:06 AM
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Bill
 
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Before you start changing parts

Until you tell us carb list number and current jetting hard to help first ensure the float levels are correct is it a float level plug older carb or a newer site glass newer version. Where is float set now ??
I would Pull carb apart and make sure nothing plugged blow air thru all Openings
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post #12 of 23 (permalink) Old Mar 26th, 20, 11:11 AM
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Bill
 
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Last thing I would change is the hi speed air bleeds a 850 dp the bleeds are fine for 5000 rpm going lean at 4500 With a 850dp with factory jets is unuasual what do the plugs look like

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post #13 of 23 (permalink) Old Mar 26th, 20, 11:28 AM
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Gene
 
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Re: Lean at High RPM

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.

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post #14 of 23 (permalink) Old Mar 26th, 20, 11:32 AM
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Re: Lean at High RPM

Start by checking plugs, wires & the rest of the ignition, maybe the valve springs. Misfiring will show as lean.
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post #15 of 23 (permalink) Old Mar 26th, 20, 11:34 AM
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Re: Lean at High RPM

YUP! So if the air bleed blast and float check dont solve it... I think Jim is onto something.

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