• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

Spark plug reading and Edelbrock RPM manifold

dcala84141

Well-Known Member
Local time
6:09 AM
Joined
Jul 11, 2015
Messages
178
Reaction score
76
Location
New York
I running an edelbrock rpm performer intake manifold on a stroke 383 to 450 with aluminum heads with an 830 quick fuel carb. At first out of the box engine was running very rich so I changed the idle fuel restrictors a few sizes smaller and main jets from 78 to 71 .My idle AFR at idle is 14.5 My AFR at cruising ( 2,800 to 3,000 ) is 13.3 My issue now is that spark plugs 1,7,4,6 which are on the low runners are rich and plugs 3,5,2,8 on the high runnings are good
 
0CE503FD-993D-4C47-94DC-B9F322153E84.jpeg
 
I have a 500" stroker and just did a plug change myself and noticed some of the cylinders were on the richer side and some leaner.....Maybe its a characteristic of the dual plane high rise design? Not quite sure what to think myself as the jetting seems about on in my application. I was wondering if a TM7 intake I have laying around would distribute fuel better with all 8 runners being pretty equal in the single plane design. I will be following this thread to see what comes of it for advice.
 
If the afr is ok, i would run a hotter plug. The plugs look like a pretty cold heat range.
 
Last edited:
What number plugs are those and what’s your compression ratio



Better pictures or clearer of the ground strap and porcelain

Timing
 
First off....those look short on the reach? How long are the threads? Maybe it's just the picture or the angle you're holding them at? Aluminum heads need a 3/4" reach plug.

That said, those both look pig rich from here....did you install new plugs after getting the idle AFR in the 14s? But, if the plugs are too short......

Dropping 7 jet sizes is a lot on an 830 carb with 450 cu in, but again I don't know that we can be getting an accurate read or making any suggestions if the proper reach plugs aren't installed.
 
Last edited:
If those ARE the correct plugs and I'm just seeing an optical illusion, IMO a good thorough tune-up is needed starting with the timing, transfer slots and float levels.

I'm also seeing oil on the base of those plugs. Could be from external weeps like valve covers, but if your plugs are the proper length, your AFR is mid 13s-14s cruise/idle and new plugs got that black that soon, next time you pull the carb off take a looky down the runners for evidence of oil in the chambers. A small telescoping mirror and flashlight can get you a peek at the valves. Excess getting sucked through the PCV line, rings not sealing well enough, or oil getting sucked underneath the valley pan (if you're using a factory-style pan) can get oil in there.
 
I’m running NGK V power #7 , initial timing set at 14 total 35 all in at 3,000 rpm. I see no leaks at all anywhere.plugs are what engine builder put in and engine was dyno at 530hp. With the 76 mains my AFR at cruising was 11.4 I changed main jets to 73 AFR went to 12.4 at cruising, then I went to 71 on main to bring me to the present 13.4 Now I just put in 69 main jets only on the lower runners side. Have yet taken car out for test. Floats are set to middle sight glass as per instructions. I called Holley and they told me that the carb is to big for what my set up is. PS I kind of doubt that, but I can b wrong. Anyway it makes me nervous that I’m droping my main jets so much, but I’m relaying on my plug reads and AFR guage although the AFR guage is giving me an average of both the high and low runners. Will know later on how I make out. Left secondary mains at 84
 
The guys that already posted know WAY more than I do, but IF it is just half the engine running rich, and IF your intake is a "split plenum" (term?) where the upper/lower intake runners are split left/right, then your idea makes complete sense to me. Your checking air/fuel mixture about as good as you can...it is what it is & nothing says you have to have the same jets left/right. Makes sense to me.
 
Like mentioned already from Frank and Beans



Now that we know your running a NGK 7 Series

To COLD of a plug for 10.3 Compression

Next start with a NGK 6 Series and go from there first
 
IMO...WHY NOT stagger jet the carb's low runner passages leaner? There is no iron clad law that requires both metering control orifices/devices to be the same. What you are observing is a classic example of mixture velocity discrepancies and how it effects cylinder to cylinder fuel delivery. Just lean the mixtures SLIGHTLY to approach the other cylinder group,then look at spark plug coloration, regardless of the brand of plug used, if you're satisfied with the spark advance characteristics (initial and rate). Get the fuel mixture distribution fixed first.
BOB RENTON
 
I run #7's in my turbo engine, wouldn't think you should ever need those with your combo, I agree try #6.
I also run the rpm perf but efi so the manifold is dry, but I have perfectly even distribution, it actually surprised me.
I wouldn't stagger jet yet, go to fresh #6 and test. But stagger is ok if that's what it needs
 
I’m running NGK V power #7 , initial timing set at 14 total 35 all in at 3,000 rpm. I see no leaks at all anywhere.plugs are what engine builder put in and engine was dyno at 530hp. With the 76 mains my AFR at cruising was 11.4 I changed main jets to 73 AFR went to 12.4 at cruising, then I went to 71 on main to bring me to the present 13.4 Now I just put in 69 main jets only on the lower runners side. Have yet taken car out for test. Floats are set to middle sight glass as per instructions. I called Holley and they told me that the carb is to big for what my set up is. PS I kind of doubt that, but I can b wrong. Anyway it makes me nervous that I’m droping my main jets so much, but I’m relaying on my plug reads and AFR guage although the AFR guage is giving me an average of both the high and low runners. Will know later on how I make out. Left secondary mains at 84
You're NOT wrong. Holley's talking out their *** as usual. Even stock 440's run great with 850 carbs, and you've got a higher flowing intake and heads.
Is it a mechanical or vacuum secondary carb?
To dyno at 500+ I'm guessing the camshaft is NOT close to a stocker. I'd get the initial timing up to18-20 BTDC. Just be wary of the total advance; you'll need to shorten it when you add initial.
I realize you had it built and dyno'd by a shop but that doesn't mean it's right for your driving environment. If that were the case your tune should be a lot closer. As stated above NGK heat range of 7 is too cold for 10.3 compression. Personally I'd use a 5 and watch the threads. If more than three from the top get discolored from the heat then go to a 6.
And it wouldn't hurt to measure the threaded portion of a plug just to be sure they're correct. They still look short to me in the picture but like I said it's probably an optical illusion.

To have to be dropping that much jet something's way off.
To start with I would pull the carb and verify that the transfer slots are correct. More or less 'square' when you look at them. Your idle mixture screws should be the same on all four corners, or very close to the same.
Make sure the power valve is properly seated in it's bore and the gasket is good.
If there's any internal leakage at that point it goes straight down the throat at all times.
 
Last edited:
You're NOT wrong. Holley's talking out their *** as usual. Even stock 440's run great with 850 carbs, and you've got a higher flowing intake and heads.
Is it a mechanical or vacuum secondary carb?
To dyno at 500+ I'm guessing the camshaft is NOT close to a stocker. I'd get the initial timing up to18-20 BTDC. Just be wary of the total advance; you'll need to shorten it when you add initial.
I realize you had it built and dyno'd by a shop but that doesn't mean it's right for your driving environment. If that were the case your tune should be a lot closer. As stated above NGK heat range of 7 is too cold for 10.3 compression. Personally I'd use a 5 and watch the threads. If more than three from the top get discolored from the heat then go to a 6.
And it wouldn't hurt to measure the threaded portion of a plug just to be sure they're correct. They still look short to me in the picture but like I said it's probably an optical illusion.

To have to be dropping that much jet something's way off.
To start with I would pull the carb and verify that the transfer slots are correct. More or less 'square' when you look at them. Your idle mixture screws should be the same on all four corners, or very close to the same.
Hi mechanical secondary. I spent better part of a week with the idle and transfer slots adjustments.I know they are rite . Car idles nicely at 800 rpm it’s a 4 speed car. I agree something not rite though. Lower runners running rich
 
Do you know your vacuum at cruise? I ask because if your power valve is open all the time at cruise, you're dumping fuel when it's not needed and that would explain you overly rich cruise trouble. It wouldn't affect idling mixture unless it was blown or leaking. It's basically like having a larger jet.
 
Just got back from about 40mi. Cruise I’m going to pull out the plugs in the morning, AFR at about 2800 rpm was 13.8, 13.9 so I’m expecting a better looking plug. Regardless I’m going to try a hotter plug. Vacuum at idle is 7lbs. Cruising vacuum is 10. Cam has a large overlap which I think is contributing to my problems. My power valve is 3.5
 
I have a small stagger in mine, but get everything else right first.
 
Just got back from about 40mi. Cruise I’m going to pull out the plugs in the morning, AFR at about 2800 rpm was 13.8, 13.9 so I’m expecting a better looking plug. Regardless I’m going to try a hotter plug. Vacuum at idle is 7lbs. Cruising vacuum is 10. Cam has a large overlap which I think is contributing to my problems. My power valve is 3.5
And you're putting new plugs in at least one or two holes before testing, right? Sorry if that's a dumb question....but once they're black, that's it..chuck 'em and put in clean ones. Get some miles on them before making a judgement.

For reference. Now our gas out here is a little "special":rolleyes: and it doesn't color up quite like other places' fuel does, and guys might say these are lean but after lots of testing/clean cuts/etc they're actually pretty dead on with what we have, and the AF meter agrees:
fyuio.jpg


rtifyui.jpg


From my solid cammed, Edelbrock-headed 10 to 1 actual C.R. 505" with an old 850 Demon which is actually comparable to a Holley 1000 in cfm terms. These were RC12YC Champs but I currently run NGK BKR5E and they look basically the same. I get 9" vacuum at idle, cruise runs from 13-19" depending on engine speed and throttle position. 10.5 power valve. Lots of street miles, runs beautifully, monster response, no hints of pinging or stuttering/bogs etc. Any richer slows me down. Going much leaner starts to weaken the response as well.
You'll get it dialed. It takes quite a bit of work to get the tune nailed, but that's how it goes.....anyone that truly has a carb give a good curve OOTB needs to go buy a lottery ticket!
 
C6941882-1092-4AD7-93FA-2536E22E6992.jpeg
3593FBEB-67FD-4846-9BE5-3DC4389C0EFC.jpeg
Just to update. Took out the NGK plugs and went with as per suggestion to a two step hotter plug (RC 12Y) only put about 30 miles on them, pulled two plugs one from high runner one from low runner on my duel plane and they look much better than the NGK7 that I had in earlier
 
Auto Transport Service
Back
Top