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Discussion Starter #1 (Edited)
Setup: 1st Gen 350 with stock bottom end, Patriot 2168 angled alum heads, NGK R5171A-7 plugs (0.045 gap), Holley 750 DP wth Proform main body, airgap intake, headers, hyd flat tappet Comp Cam 292H, MSD 6AL with MSD pro-billet distributor. Approximately 10:1 CR
Have AEM O2 sensor, sensor is in collector, have short exhaust with magnaflow muffler. 14 initial timing, 36 total. 93 non-ethanol pump gas.

I used same carb as on previous 350 which was same cam, same ignition, but small valve double hump heads, 11.5:1 CR, used 110 gas.

Carb jets started out at 74 front, 84 rear. Plugs showed lean, looked like brand new, with O2 sensor showing rich, ranging from 11.5-12.5 at 3,500 rpm. Jetted all the way up to 80/86 till I got a tan color. O2 sensor showed a little richer as I jetted up so I believe it’s working but cannot explain the near white plugs at pretty large jetting compared to previous setup. Folks I’ve talked to don’t have that much jetting on even hotter setups. 71/80 is stock jetting on this 750.

Any ideas on how plugs read lean while sensor says rich?

- Location of sensor, short exhaust, too hot of a plug? Patriot (now PROMAXX) tech line recommend the NGK plug, but their spec card calls for a colder plug, equivalent to a R5672A-9, 2 heat numbers colder.
Thx for any ideas
 

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Discussion Starter #2
AEM O2 gauge shows AFR “rich” at 11.0 or less. Green range is from 11.0 to 13.5, so maybe I’m not really all that rich around 12.0-12.5. Was thinking I needed to be closer to 13.5.

But would expect plugs to show some color before jetting all the way up to 80 on the primaries. Plug too hot?

What jets sizes are you guys running on a 750 DP?
 

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Setup: 1st Gen 350 with stock bottom end, Patriot 2168 angled alum heads, NGK R5171A-7 plugs (0.045 gap), Holley 750 DP wth Proform main body, airgap intake, headers, hyd flat tappet Comp Cam 292H, MSD 6AL with MSD pro-billet distributor. Approximately 10.1 CR
Have AEM O2 sensor, sensor is in collector, have short exhaust with magnaflow muffler. 14 initial timing, 36 total. 93 non-ethanol pump gas.

I used same carb as on previous 350 which was same cam, same ignition, but small valve double hump heads, 11.5:1 CR, used 110 gas.

Carb jets started out at 74 front, 84 rear.
74/84 stock jetting for a Quickfuel 750 mechanical with the Proform body. They don't run a pv in the rear so usually that way is a 6-8 size jet up, 10 in their case for a starting point. I have that setup in my buddies 383, AFR 195 and a healthy roller cam and it works very well.
 

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Discussion Starter #5
74/84 stock jetting for a Quickfuel 750 mechanical with the Proform body. They don't run a pv in the rear so usually that way is a 6-8 size jet up, 10 in their case for a starting point. I have that setup in my buddies 383, AFR 195 and a healthy roller cam and it works very well.
Thx! That's what jets I was running on the old engine. My heads are 185 cc intake runners. Anybody using a carb spacer on their RPM Air Gap intakes? I am using a 1/2" spacer to clear the intake.
 

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Discussion Starter #6
Thx! That's what jets I was running on the old engine. My heads are 185 cc intake runners. Anybody using a carb spacer on their RPM Air Gap intakes? I am using a 1/2" spacer to clear the intake.
74/84 stock jetting for a Quickfuel 750 mechanical with the Proform body. They don't run a pv in the rear so usually that way is a 6-8 size jet up, 10 in their case for a starting point. I have that setup in my buddies 383, AFR 195 and a healthy roller cam and it works very well.
What spark plug are you running with the 383? I believe I've got too hot a plug, NGK R5171A-7.
 

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Unleaded gas doesn't color the plugs the same way a leaded race gas will.
 

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Discussion Starter #8
Unleaded gas doesn't color the plugs the same way a leaded race gas will.
Hmmm, good point. I’m running 93 non-ethanol. Just always been told to believe your plugs...
I was running a R43S, maybe a R45S before with iron heads.
What are the msjor considerations for choosing spark plug heat range? Compression, WOT or cruising, head design, etc?
 

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Discussion Starter #9
Unleaded gas doesn't color the plugs the same way a leaded race gas will.
Hmmm, good point. I’m running 93 non-ethanol. Just always been told to believe your plugs...
I was running a R43S, maybe a R45S before with iron heads.
What are the msjor considerations for choosing spark plug heat range? Compression, WOT or cruising, head design, etc?
Dumb question but I’ll ask anyway:
Is it possible that too hot of a plug will burn off any deposits on the ceramic so it’s hard to tell rich or lean?
 

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I run a set of Profiler 195 cc heads on my 383 based engine. I'm not sure if the patriot heads require a extended reach plug but most aftermarket aluminum heads do. I've been building and in a different lifetime racing small block engines for over for over thirty years. I run autolite 3923 or 3924 plugs depending on the heat range I want. You don't need a super hot plug. You may have to put a few more miles on a plug with unleaded gas but the plug will show color. If the plug is too dark lean it and if it is too white add fuel. One of the reasons I run the autolite is money. It is a good plug that is fairly cheap. When racing I always carried spares. The sparkplug is the window to the inside of the engine.

I'm certainly not going to try and tell you that QuickFuel doesn't know about jetting a carburetor but I've never found a need to have a ten jet difference between the primary and secondary side. I have ran them as tight as a three jet size difference front to rear and on race carburetors without power valves have ran many squared up.
 

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What spark plug are you running with the 383? I believe I've got too hot a plug, NGK R5171A-7.
AC 41-629 I believe. Those are the one's I do have in my solid roller 383 with the AFR 210 heads. I believe I put the same plug in my buddies 383. Both run Quickfuels, but I run their 850 Q.
 

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Thx! That's what jets I was running on the old engine. My heads are 185 cc intake runners. Anybody using a carb spacer on their RPM Air Gap intakes? I am using a 1/2" spacer to clear the intake.
Assuming your air-gap is a dual plane manifold you will want to run a 4-hole spacer and not an open spacer.
 

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I ran the Autolite 104. I believe the 3924 is just a .750 reach instead of the .708 reach. I tried a few different brands and heat ranges that's what I settled on. Inexpensive plug that works good. I was a tad under 10:1 CR. I have a 1" open spacer on an Air Gap. Don't think I ever ran a 4 hole to compare it to though. Had to dig through an old thread of mine to find what it did when I put it on:

6th-August-2006

There it is!! Had to resort to the 1" carb spacer, took a little away on the bottom end, but gave me about 1 mph on the top end, good enough. Battery died after that and I was done for the night, no complaints here though.

Last run.

60' - 1.67
1/8 - 7.58
MPH - 89.76
1/4 - 11.98
MPH - 111.33
 

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On a dual-plane, an open spacer allows any particular cylinder (runner) to "see" all four barrels of the carburetor. This turns a two-plane manifold into a very crude single-plane manifold and eliminates the purpose of the plenum dam.
Not only does this cancel-out the strong points of a two-plane manifold, mainly mid-range, it also raises the carb boosters away from the cylinders and reduces signal. This is not helping your plug reading issue.
A carb spacer on a single-plane can be a tuning device but should not be used on a dual.
 

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Discussion Starter #15
AC 41-629 I believe. Those are the one's I do have in my solid roller 383 with the AFR 210 heads. I believe I put the same plug in my buddies 383. Both run Quickfuels, but I run their 850 Q.
Thx, these cross to a Autolite 3924, which should be about the same as an -8 heat range in the NGK type (0.75" reach) I'm trying; R5671A-7.
Found this cross reference link: https://www.dragstuff.com/techarticles/spark-plug-cross-reference.html These Autolite #s should be the same, except non-resistor plugs.

Autolite NGK
AR3931 R5671A-10
AR3932 R5671A-10
AR3933 R5671A-9
AR3934 R5671A-8


Update: I put in a set of R5671A-8 plugs and drove it around a bit at various RPMs and the plugs are dry sooty black... Thought that the -8s might be just right, but now I'm still confused, will keep tweaking with jets.
 

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Discussion Starter #16
Assuming your air-gap is a dual plane manifold you will want to run a 4-hole spacer and not an open spacer.
On a dual-plane, an open spacer allows any particular cylinder (runner) to "see" all four barrels of the carburetor. This turns a two-plane manifold into a very crude single-plane manifold and eliminates the purpose of the plenum dam.
Not only does this cancel-out the strong points of a two-plane manifold, mainly mid-range, it also raises the carb boosters away from the cylinders and reduces signal. This is not helping your plug reading issue.
A carb spacer on a single-plane can be a tuning device but should not be used on a dual.
Thx! good insight, as I am using a 1/2" OPEN spacer... I can't clear my linkage without a spacer. Will try a 4-hole spacer and see what that does.
 

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Discussion Starter #17 (Edited)
I ran the Autolite 104. I believe the 3924 is just a .750 reach instead of the .708 reach. I tried a few different brands and heat ranges that's what I settled on. Inexpensive plug that works good. I was a tad under 10:1 CR. I have a 1" open spacer on an Air Gap. Don't think I ever ran a 4 hole to compare it to though. Had to dig through an old thread of mine to find what it did when I put it on:

6th-August-2006

There it is!! Had to resort to the 1" carb spacer, took a little away on the bottom end, but gave me about 1 mph on the top end, good enough. Battery died after that and I was done for the night, no complaints here though.

Last run.

60' - 1.67
1/8 - 7.58
MPH - 89.76
1/4 - 11.98
MPH - 111.33
Thx for the input, I need a 1/2" spacer to clear the linkage. As Alf recommended, I'll try a 4-hole spacer.
 

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Before you go changing out spacers I want you to look into something. Going all the way back to the late 60's Chevy made a dual plane intake for the big block Chevy. It was found that by milling down the divider wall it made more power. And it still holds true today. Look at the base Edelbrock performer rpm intake, and then look at the airgap. The airgap comes factory with the divider wall milled down. Using a open spacer on a dual plane does the same exact thing as milling the wall down. This is NOT a bad thing for a dual plane intake, especially on a race car.
 

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Before you go changing out spacers I want you to look into something. Going all the way back to the late 60's Chevy made a dual plane intake for the big block Chevy. It was found that by milling down the divider wall it made more power. And it still holds true today. Look at the base Edelbrock performer rpm intake, and then look at the airgap. The airgap comes factory with the divider wall milled down. Using a open spacer on a dual plane does the same exact thing as milling the wall down. This is NOT a bad thing for a dual plane intake, especially on a race car.
The reason the plenum divider in some dual-planes does not go all the way up to the base of the carburetor is to provide a little air mixing at high RPM's.
Many airgaps have about a 3/8" depression cast in the plenum divider for that reason.
If you use a 4 hole spacer the center of the spacer will maintain the original size of the plenum opening cast into the manifold.
In general, no spacers on dual-plane manifolds but if you need the half inch for clearance use a 4 hole.
Doubters should call Edelbrock.
 
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