Air/Fuel experts please help
Thread Starter
Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Air/Fuel experts please help
This has been driving me nuts for a while now. It seems when I go WOT my A/F readings go very lean instead of the other way. I have a 825 Race Demon on my 406ci and it is jetted 78 pri. and 92 sec. The 92 was put in because of the the lean problem but it still goes lean.
Every other graph I have seen from other cars shows the A/F being pulled low at WOT, but mine seems to go the other way. I have 30cc pumps and I also changed the squirters to 35 from 31.
Don't pay attention to big spike on the end, it is noise.
As you can see from the graph, the RPM's for first ( shortshifted ) and second and third ( only going to about 4000RPM now as I have been doing this so many times ) the A/F seems to follow the RPM's insted of being pulled low and staying there.
Any help from anyone with experience with A/F and wideband 02 sensors is appreciated.
Every other graph I have seen from other cars shows the A/F being pulled low at WOT, but mine seems to go the other way. I have 30cc pumps and I also changed the squirters to 35 from 31.
Don't pay attention to big spike on the end, it is noise.
As you can see from the graph, the RPM's for first ( shortshifted ) and second and third ( only going to about 4000RPM now as I have been doing this so many times ) the A/F seems to follow the RPM's insted of being pulled low and staying there.
Any help from anyone with experience with A/F and wideband 02 sensors is appreciated.
Racer
Joined: Apr 2006
Posts: 477
Likes: 43
From: Millbrae California
my first guess would be fuel volume, what is the fuel pressure at the same time it is going lean. if you are using the stock gas tank system the vent system may not be up to the job. try it without the gas cap or log the fuel pressure with a pressure transducer and see what the fuel pressure is under load. FYI a wide-band or any o2 sensor can give a false lean reading from misfire and that is why i like a 5-gas analyzer when the readings do not make sense
i hope this helps
ps i make no claims of being a expert i am just a person having fun while making a living
i hope this helps
ps i make no claims of being a expert i am just a person having fun while making a living
Last edited by olescarb; Apr 19, 2006 at 07:01 PM.
Race Director
Joined: Sep 1999
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From: Plano TX
Looks like a fuel delivery problem but could be that the carb is busted. You may be able to tell by going about 4000 rpm steady and then punch it. If it starts rich and then gradually leans out I'd say fuel delivery is a pretty sure bet. If it imediately goes lean I would try another carb.
Thread Starter
Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
I think I'll put my old Holley 750DP back on and see what it does. If I remember correctly though, I had the same lean condition with the Holley too.
It really bugs me not to be able to figure this out. I have jetted the secondary up to 92 and changed to larger squirters and it does nothing to bring the A/F down. I am going to see if I can get a pair of 39 or 41 squirters tommorrow, and maybe a 50cc seconday pump.
The A/F goes lean as soon as I go WOT. I don't think it is fuel delivery either as it runs like a scalded ape with no misfires or hesitaions. I am lost here.
It really bugs me not to be able to figure this out. I have jetted the secondary up to 92 and changed to larger squirters and it does nothing to bring the A/F down. I am going to see if I can get a pair of 39 or 41 squirters tommorrow, and maybe a 50cc seconday pump.
The A/F goes lean as soon as I go WOT. I don't think it is fuel delivery either as it runs like a scalded ape with no misfires or hesitaions. I am lost here.
Racer
Joined: Apr 2006
Posts: 477
Likes: 43
From: Millbrae California
the wide band systems can and will read a false lean reading if the sensor sees a misfire or a exhaust leak. if you have access to a exhaust gas analyzer you can confirm the mixture since they look at the 5 main gases in the exhaust. a oxygen sensor determines the a/f mixture by analyzing the need for oxygen to consume the "un-burnt combustibles" (very poorly put but in a few words the best i can do)
Race Director
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From: Waterloo ontario Canada
I hope you are running power valves? Also how does the squirers help? They only help when the throttle is punched and add nothing while running at a steady state or in your case with the throttle down.
Power valves also have metering channels in the carb that can be drilled. A power valve opens and closes yes but the metering channel controls how much fuel it flows.
I find that under any hard throttle conditions my normal 15-1 fuel ratio drops immediatley to 13-1
Power valves also have metering channels in the carb that can be drilled. A power valve opens and closes yes but the metering channel controls how much fuel it flows.
I find that under any hard throttle conditions my normal 15-1 fuel ratio drops immediatley to 13-1
Thread Starter
Race Director
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
As far as the squirters go, if I put a bigger one in the secondary and a I see just a momentary drop in the A/F reading then at least I will know I have done something, up to now nothing has worked, different carbs, bigger jets etc.
Someone mentioned one time something about big cams with lots of overlap screwing up the air fuel readings, can't remember exacty something to do with reversion, but cam is a street cam,fairly big but I can't see that as the problem but I am not going to rule anything out yet.
I am running a power valve in the primary side, not the secondary and I have checked it with a vacuum gauge under load to make sure I have the correct one.
I am going to throw my Holley 750 back on and see what I get
Someone mentioned one time something about big cams with lots of overlap screwing up the air fuel readings, can't remember exacty something to do with reversion, but cam is a street cam,fairly big but I can't see that as the problem but I am not going to rule anything out yet.
I am running a power valve in the primary side, not the secondary and I have checked it with a vacuum gauge under load to make sure I have the correct one.
I am going to throw my Holley 750 back on and see what I get
Race Director
Joined: Nov 2000
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From: Waterloo ontario Canada
Originally Posted by MotorHead
I am running a power valve in the primary side, not the secondary and I have checked it with a vacuum gauge under load to make sure I have the correct one.
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Race Director
Joined: Sep 1999
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From: Plano TX
Originally Posted by norvalwilhelm
Why not in the secondaires?? If you want to richen the mixture up at wide open throttle why not add power valves to the secondaries??
Race Director
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From: Waterloo ontario Canada
Originally Posted by zwede
If he has a progressive linkage it doesn't make much sense to run a secondary power valve. Easier to just jet up the secondaries 6 sizes.
I feel if holley felt they weren't necessary they would have left the holes out.
You can also jet up to 10 sizes bigger to make up for the lack of a power valve
Race Director
Joined: Sep 1999
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From: Plano TX
Originally Posted by norvalwilhelm
I run 4 power valves in my 2 850 double pumper holleys and it seems to work for me.
Burning Brakes
Joined: Jul 2005
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There is no harm involved with running two PVs (dont believe the PVs are as fragile as glass crap!!)...........
I'll give you a perfectly good reason to run two PVs on any double pumper......engine draw isnt opening to secondaries, his right foot is.
Progressive or not, the engine is just about guaranteed to run more efficient with both PVs.
I'll give you a perfectly good reason to run two PVs on any double pumper......engine draw isnt opening to secondaries, his right foot is.
Progressive or not, the engine is just about guaranteed to run more efficient with both PVs.
Instructor
Joined: Aug 2005
Posts: 209
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From: Hollywood FL
Originally Posted by MotorHead
Someone mentioned one time something about big cams with lots of overlap screwing up the air fuel readings, can't remember exacty something to do with reversion, but cam is a street cam,fairly big but I can't see that as the problem but I am not going to rule anything out yet.
Burning Brakes
Joined: Jul 2005
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Originally Posted by zwede
Why? With a progrssive linkage the secondaries stay closed until about 1/2 throttle. At half throttle the power valve will be open so what's the point of having it?
The power valve will not automatically be open at half throttle.
Its different on every engine......but unless the carb is MASSIVELY oversized for the engine, the power valve wont be open unless the car is accelerating hard (and the engine is "catching up to the big hole in the top of the intake", or the throttle is more than 80% open (give or take).
There is absolutely no detriment to running a secondary PV, and they dont open unless accelerating hard, or the throttle is WAAAAAAAAYYYYY open......meanwhile, that engine he built is going to need a lot more than the primaries at high cruise RPMs......the secondaries will be open for highway use, especially for non-overdrive setups.
Safety Car
Joined: Jul 2002
Posts: 3,770
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From: St Louis MO
I agree with BigBlokk, put some smaller secondary high speed air bleeds in it. This will make the secondaries flow sooner provided the secondary butterflies are open. However, you need to try jetting up on the secondaries 2 numbers at a time, ie 92s to 94s. If you don't see improvement in the WOT mix by the time you get to 98, you probably have a bad carb or some partially blocked passages.
I bought a Proform carb body, proform metering blocks and an air bleed kit once. It ran too lean everywhere on the scale. No matter what I did, it would not richen up. I ditched all that stuff and went back to the Holley units.
Let us know if you try the 750 and how it works.
Brett
I bought a Proform carb body, proform metering blocks and an air bleed kit once. It ran too lean everywhere on the scale. No matter what I did, it would not richen up. I ditched all that stuff and went back to the Holley units.
Let us know if you try the 750 and how it works.
Brett
Thread Starter
Race Director
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Holley 750DP seems to do the same thing, I am going to give up here. I remember last year I went thru the same thing and gave up. Motor runs fine and made 550HP so I will just drive it now. Maybe a 550HP small block behaves different than a stock or mild small block with regards to A/F I don't know.
THey don't even have an answer on the LM-1 forum
THey don't even have an answer on the LM-1 forum
Burning Brakes
Joined: Jul 2005
Posts: 787
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I'd keep looking.....maybe at the rest of the fuel system.
I've read many times that with inadequate fuel delivery (tank to carb) the engine wont usually ping or anything, just never produce the power it could.
Burned pistons would be bad.....
I've read many times that with inadequate fuel delivery (tank to carb) the engine wont usually ping or anything, just never produce the power it could.
Burned pistons would be bad.....