Carb too small ?
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Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Carb too small ?
I have a 750 Holley DP with a Proform main body on my 406ci right now. It seems to run fine but the A/F measurements I have been taking with the LM-1 don't seem quite right. At idle the A/F is around 12. Under part throttle/cruising it varies between 12 and 13 and under full load/wide open throtlle the A/F fuel will jump to 14-15. It seems like it goes the opposite way
I have 75 primary jets and 88 secondary and don't know what to do here. Do I put in 90 or 92 jets or what, that's getting pretty big. I bought a 825 Race Demon but have not put it on yet, I wanted to get the Holley tuned first but now I'm thnking maybe it is just too small for the motor ? What jets do you you use in big blocks ?
I have 75 primary jets and 88 secondary and don't know what to do here. Do I put in 90 or 92 jets or what, that's getting pretty big. I bought a 825 Race Demon but have not put it on yet, I wanted to get the Holley tuned first but now I'm thnking maybe it is just too small for the motor ? What jets do you you use in big blocks ?
Pro
Joined: Jan 2004
Posts: 535
Likes: 1
From: Darwin, Australia NT
First up I'm not a Holley Expert so all I can do is post some more questions to keep the ball rolling on this one.
Im running a 454 with a HP950. I dont have an AF meter (next purchase) but my motor seems to want small jets ie 93/96 and I'm running low density avgas! My 950 has power valves on primary & secondary sides but a previous 750 only had one power valve. Obviously things like that can make a difference. Does your 750 have two power valves?
Im running a 454 with a HP950. I dont have an AF meter (next purchase) but my motor seems to want small jets ie 93/96 and I'm running low density avgas! My 950 has power valves on primary & secondary sides but a previous 750 only had one power valve. Obviously things like that can make a difference. Does your 750 have two power valves?
Thread Starter
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
I only have the one power valve, so you say 93/96 is small, how much HP do you figure you are making. Just a ballpark would be fine as I am just trying to an idea here of where I should be with regards to carb size and jet size, the purchase of the 825 Race Demon is starting to look like a good idea
Pro
Joined: Jan 2004
Posts: 535
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From: Darwin, Australia NT
Originally Posted by MotorHead
how much HP do you figure you are making. Just a ballpark would be fine as I am just trying to an idea here of where I should be with regards to carb size and jet size
Thread Starter
Race Director
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
I keep meaning to check the fuel pressure under WOT and the rear float level. I remeber a long time ago I was having trouble with the motor dying under hard braking and cornering. I raised the front float level and still the same problem.
Someone said he had the same problem and it turned out that he actually fixed it by lowering the rear float level. I have no idea how lowering the level in the rear bowl would fix the cutting out/stalling problem but it did. I never actually looked to see just how low it is so there is a chance it is too low and it is getting drained at WOT
Someone said he had the same problem and it turned out that he actually fixed it by lowering the rear float level. I have no idea how lowering the level in the rear bowl would fix the cutting out/stalling problem but it did. I never actually looked to see just how low it is so there is a chance it is too low and it is getting drained at WOT
Drifting
Joined: Nov 2002
Posts: 1,665
Likes: 1
From: Ontario
My buddy's Holley was giving him lean conditions at lower RPMs (measured with A/F meter and his idle & off-idle really sucked).
He found a problem with the base plate; the blades did not line up to the transfer slots like they were supposed to. He got a different base plate that had the blades in correct position in relation to the transfer slots. It made a huge difference. I will get the link off him and post it of the diagram he used to figure it out.
He found a problem with the base plate; the blades did not line up to the transfer slots like they were supposed to. He got a different base plate that had the blades in correct position in relation to the transfer slots. It made a huge difference. I will get the link off him and post it of the diagram he used to figure it out.
Race Director
Joined: Oct 2000
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From: central FL, near the beach.
Originally Posted by MotorHead
I have a 750 Holley DP with a Proform main body on my 406ci right now. It seems to run fine but the A/F measurements I have been taking with the LM-1 don't seem quite right. At idle the A/F is around 12. Under part throttle/cruising it varies between 12 and 13 and under full load/wide open throtlle the A/F fuel will jump to 14-15. It seems like it goes the opposite way
I have 75 primary jets and 88 secondary and don't know what to do here. Do I put in 90 or 92 jets or what, that's getting pretty big. I bought a 825 Race Demon but have not put it on yet, I wanted to get the Holley tuned first but now I'm thnking maybe it is just too small for the motor ? What jets do you you use in big blocks ?
I have 75 primary jets and 88 secondary and don't know what to do here. Do I put in 90 or 92 jets or what, that's getting pretty big. I bought a 825 Race Demon but have not put it on yet, I wanted to get the Holley tuned first but now I'm thnking maybe it is just too small for the motor ? What jets do you you use in big blocks ?
88 is .104 as i recall it was very rich until 5000? post new chart after pri. change and fuel psi test. which side is the 02 sensor? Why are u tuning a 750 if u bought an 825 for it? 750 is better thru 6000. 825 better 6k UP.
.
Last edited by Matt Gruber; Jun 19, 2005 at 11:34 AM.
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Thread Starter
Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
[QUOTE=
i can't wait to see u buy a dental drill. that will make u a dentist, right? u can fill yor own cavity
[/QUOTE]
If that's some stupid crack about my mechanical abilities it is not very well recieved.
i can't wait to see u buy a dental drill. that will make u a dentist, right? u can fill yor own cavity
[/QUOTE]If that's some stupid crack about my mechanical abilities it is not very well recieved.
Race Director
Joined: Oct 2000
Posts: 15,009
Likes: 79
From: central FL, near the beach.
Originally Posted by Matt Gruber
75 is .082", i'd try 71 primary .076"
88 is .104 as i recall it was very rich until 5000? post new chart after pri. change and fuel psi test. which side is the 02 sensor? Why are u tuning a 750 if u bought an 825 for it? 750 is better thru 6000. 825 better 6k UP.
.
88 is .104 as i recall it was very rich until 5000? post new chart after pri. change and fuel psi test. which side is the 02 sensor? Why are u tuning a 750 if u bought an 825 for it? 750 is better thru 6000. 825 better 6k UP.
.
Thread Starter
Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
It goes lean as soon as I go WOT, here is a graph which shows WOT for first, second and third gear only up to about 4500RPM though, but you can still see the A/F goes up not down. THis is what I am trying to figure out. If the A/F goes up then you would think you would jet up, I already went from 84 secondary to 88 and it made no difference. I checked the rear float leve and seems OK, I have yet to run the fuel pressure gauge to the windshield so I can see it but if the fuel pressure was so low it was going lean then you would think it would be running a little worse than it does at WOT.
RPM is the black plot under the A/F plot
PS if the dentist crack is in reference to my mechanical abilities, I can assure you if I can build a perfect runnnig high performance motor that puts 400HP/400TQ to the rear wheels in my basement then mechanicking ain't that hard. You can't just buy a calculator and get a job in my engineering department.
RPM is the black plot under the A/F plot
PS if the dentist crack is in reference to my mechanical abilities, I can assure you if I can build a perfect runnnig high performance motor that puts 400HP/400TQ to the rear wheels in my basement then mechanicking ain't that hard. You can't just buy a calculator and get a job in my engineering department.
Last edited by MotorHead; Jun 19, 2005 at 01:30 PM.
Race Director
Joined: Oct 2000
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From: central FL, near the beach.
i suspect the af goes up 'cause the fuel flow is inadequate. paradoxically, leaning the jets should help, but i would confirm WOT(4500-5000) fuel psi 1st. what fuel pump do u have? af sensor location?
Melting Slicks
Joined: Dec 2001
Posts: 2,025
Likes: 1
From: Billings MT
Hey Wayne, it looks to me that your running out of fuel. What pump/line size are you running?
I was going pretty lean friday at the track, it was breaking up around 6500rpm, plugs were bright white with the blue all the way to the end of the ground strap
I was going pretty lean friday at the track, it was breaking up around 6500rpm, plugs were bright white with the blue all the way to the end of the ground strap
Thread Starter
Race Director
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Could be, I gotta find some way to look at my fuel pressure gauge when driving, your motor could be doing the same thing. I dontt understand it though as soon as I go WOT the A/F starts to go high, you would think it would go low like it is supposed to then go high if it runs out.
I am using the Carter M6900 SB-Chevy Street Super Fuel Pump it is a mechanical pump, they are supposed to flow 120gph.
I have got the LM-1 sorted out now, I had to run the sensor wire under the car and up through the hole in storage box. The LM-1 is pretty sensitive to the MSD and other electrical sources but now that it is working it is a pretty neat tuning tool that I am going to get some good use out of.
Here's a log of cruising at aroind 2000RPM then idle at stop sign then first, second and third.
I am using the Carter M6900 SB-Chevy Street Super Fuel Pump it is a mechanical pump, they are supposed to flow 120gph.
I have got the LM-1 sorted out now, I had to run the sensor wire under the car and up through the hole in storage box. The LM-1 is pretty sensitive to the MSD and other electrical sources but now that it is working it is a pretty neat tuning tool that I am going to get some good use out of.
Here's a log of cruising at aroind 2000RPM then idle at stop sign then first, second and third.
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
Sounds like the power valve is leaking. Are you sure it is screwed in tight? Is there supposed to be some sort of gasket under it? Have you back fired through the carburetor lately? Could be a split diaphragm.
BigBlockk
Later.....
BigBlockk
Later.....
Melting Slicks
Joined: Dec 2001
Posts: 2,025
Likes: 1
From: Billings MT
It should drop when the pump shot fires, are both of them working?
As far as fuel pressure, you could go the cheap way and get an inexpensive gauge, and run a line up to the windshield and tape it there or you can get the expensive isolater and run a gauge in the car.
I am tired of "reading" plugs, I may be ordering the LM-1 here pretty soon.
Also, have you tried cleaning out the air bleeds? You could have a clogged high speed air bleed.
As far as fuel pressure, you could go the cheap way and get an inexpensive gauge, and run a line up to the windshield and tape it there or you can get the expensive isolater and run a gauge in the car.
I am tired of "reading" plugs, I may be ordering the LM-1 here pretty soon.
Also, have you tried cleaning out the air bleeds? You could have a clogged high speed air bleed.
Safety Car
Joined: Jul 2002
Posts: 3,770
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From: St Louis MO
So your plan is to rack your brain getting the 750 tuned, then take it off do it again for the 825? You must love pain!
With your head/intake/cam combo, the 825 should run quite well. Put that one on and forget the 750.
Brett
With your head/intake/cam combo, the 825 should run quite well. Put that one on and forget the 750.
Brett
Race Director
Joined: Oct 2000
Posts: 15,009
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From: central FL, near the beach.
Originally Posted by Brettmc
So your plan is to rack your brain getting the 750 tuned, then take it off do it again for the 825? You must love pain!
With your head/intake/cam combo, the 825 should run quite well. Put that one on and forget the 750.
Brett
With your head/intake/cam combo, the 825 should run quite well. Put that one on and forget the 750.
Brett
also remove and discard both brass inlet fuel filters.
Pro
Joined: Jan 2004
Posts: 535
Likes: 1
From: Darwin, Australia NT
Originally Posted by MotorHead
I am using the Carter M6900 SB-Chevy Street Super Fuel Pump it is a mechanical pump, they are supposed to flow 120gph.[/IMG]
Someone suggested the power valve and thats definately worth checking.
Thread Starter
Race Director
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
I am using the stock fuel line from the tank. I have one power valve in the primary that came with the carb new and as far as I know it is functioning but I will check that out too. I have nothing but time here and tuning the Holley is good experience and besides it is running remarkablely good just not tuned to it potential yet. There is something fundamently wrong here. From what I know when you give the motor gas that should make the motor run richer no ? So for ballpark figures say ~13.5 to 14 for cruise and idle and down to around 12 for WOT.
I seem to be going completely opposite like 12 A/F at idle and cruise and up to 14 at WOT, can't figure it out
I am not even sure the Race Demon is going to work very well anyway it is the drag race version so some modifications are going to nessessary anyway.
Heres first, second and third at WOT, only to around 4 grand as I have been doing it quite a bit lately. Purple is the air fuel and black is the RPM as you can see when I get on it the A/F goes up. Disregard the big spike it is noise
I seem to be going completely opposite like 12 A/F at idle and cruise and up to 14 at WOT, can't figure it out
I am not even sure the Race Demon is going to work very well anyway it is the drag race version so some modifications are going to nessessary anyway.
Heres first, second and third at WOT, only to around 4 grand as I have been doing it quite a bit lately. Purple is the air fuel and black is the RPM as you can see when I get on it the A/F goes up. Disregard the big spike it is noise