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reading plugs ran into strange issue
#1
Alright all you fine folks im hoping one of you knowledgeable people can point out the obvious for me. Today i finally got to tuning my Holley 670 for my car. Its a 351C 4-V with a little pocket work on the quuence heads with a lunatic mild cam (https://www.lunatipower.com/Product.aspx...21&gid=287) cam specs. edlebrock performer manifold hooker headers comp cams roller rockers electric fan conversion whole engine hs only about 15k since build maybe 20k hopefully i didnt forget something if i did ask me and i will do what i can.

Problem:
i replaced to plugs to aid in being able to read new ones vs old ones. Upon removing them i noticed a few were cleaner than others figured hey whatever. I put the new ones in after setting float levels and dialing in the idle circuit with a vac gauge and setting idle. Put in 70's from the factory 68s knowing that it was running lean already. Drove down the road and cruised it at 2500 for a couple mins then shut it off and pulled into an orchard (luxury of living in the country) and pulled cyl 1 and it looked just like i had put it in. pulled number two and it looked lean. Threw in some 72's and repeated and pulled #1 and again looked brand spankin new not lean i mean like it was shielded entirely from everything in the cylinder. Pulled #2 and it looked a little closer but still lean. put in 74's and ran down to the shop. Let it cool down a bit and pulled the whole bank out and 2 and 3 looked PERFECT and 1 and 4 look completely new. like i just took them out of the box.

Any thoughts? The thing runs great idles good pulls good
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#2
Since you say it runs great, I tend to doubt this is the issue, but if the engine has coolant leaks into the cylinder such as cracks in the heads or block or blown head gaskets, the water/coolant will steam clean the plugs.

Without smoke in the exhaust or a loss of coolant or coolant in the oil this seems most unlikely.

[Image: 1_01_07_15_8_53_18.png]

"I love my Hookers!" and "Get some Strange" probably have a different connotation to non automotive enthusiasts!
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#3
I think the issue may be that with only a 670cfm you are always going to run a bit lean. I have the 73 OC heads with small valves and a mild cam and I am running a 750cfm vacuum secondary, only had to drop jets from 72 to 70 and presto, perfect on the dyno. I'm certainly no expert, but that carb may be a bit small. My engine builder suggested my carb. Maybe someone else can weigh in?
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#4
too large a carb will make you run lean. many people do not realize that. it is because the vacuum signal drops off and the engine starts requesting less fuel when it really needs more. you can chase your tail thinking a larger CFM means more fuel. it doesn't it can flow more air because the ventris are larger, all it really means is more higher end rpms you basically shift your power curve up the band.

The Jets only come on under WOT or cruise.
you can adjust the power valve to kick on earlier and overlap a lean condition.
same with the accelerator pump circuit.


to set the power valve, take vacuum at idle, divide by 2 and usually add 1 for safety.

so if you make 17" at idle, (17/2) + 1 = 9.5hg,, Stock a holley usually has a 6.5HG power valve.

what that means is power enrichment only starts to come on at 6.5HG.
so when you floor it the Vac drops the valve kicks on when it reaches 6.5HG.
lets say you are lean, the mistake is people put in a 4.5HG valve thinking smaller number means more fuel.

what is happening now is enrichment must wait longer to kick in which leaves the engine even leaner then before.
you kick the enrichment sooner by raising the number.

now you can also buy dual or triple stage power valves, so if you have an engine that only runs well with a 12.5 HG valve but you are sick of 2MPG, then you can go to a dual or triple stage valve that brings on enrichment but doesn't just pour a gallon of fuel down the throat.

Lean at cruise will have the engine surging, rpm and vacuum will go up and down,

checking the plugs you want to see if you are too hot or too cold, you look for discoloration on the threads you want to see 2-3 threads on the plug discolored that indicates good temperature. if the plug is showing 5 threads of color then come down a heat range. too hot can show the engine running lean. too cold the engine will be rich and show no color.

make sure your timing isn't too advanced, that will make the engine run lean. you want to see the discoloration on the plug strap half way.
if the discolor goes past half towards the threads then you are kicking too much timing. the otherway is too little timing.

if you threw on a mr gasket light spring set on the distributor you might want to think about raining the timing in a little, too much timing too soon will make the engine lean, then you need to compensate, more accelerator shot, more power valve. you don't want to go crazy on the jets as you will be way too rich at cruise speed. you can also try kicking back the initial timing 1-2 degrees and retest. also you can tune your distributor advance but that is fine tuning usually at the end. Kicking back the mechanical advance is an option as is installing stoppers to limit total timing.

how does the engine behave when it is cold, does she; Studder then go, or does she; go, studder, then go again.
if the engine runs normal when cold and hot then you don't have a problem. if you test the engine under heavy load does it ping or detonate?
you find a steep hill go like 20-30 up it load that engine up and listen to the engine and exhaust did it change sound like marbles rolling around inside the mufflers then you are lean otherwise fine.

put some miles on the new plugs then recheck, drive the car and see if you find it does something you don't like and you can reproduce it easily.

then you can tune it some more.
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#5
Are you trying to fix a problem or just curious? Reading spark plugs is usually done by making a WOT, all gears, run and shutting the engine off and then "reading" the plugs. Using unleaded fuel makes it more difficult to reach any accurate conclusions other than gross rich conditions. It is nearly impossible to achieve perfectly equal fuel distribution with an intake using a common plenum, single or dual plane. The plugs will read somewhat differently from cylinder to cylinder. If you feel the plugs look "too clean" install plugs that are one heat range colder and see what happens. If you really what to spend a lot of time you can experiment with stagger jetting and using different heat range plugs in different cylinders to optimize combustion. Chuck
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#6
Chuck is correct, the runner lengths change spark plug readings, with the longer runners reading leaner. This is especially noticeable in straight sixes and eights.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#7
Thanks everyone for the quick replies. I am just curious and want to get the best performance possible. Long story longer the car used to have a 770 and gave me nothing but problems and i picked this 670 up cheaply enough and rebuilt it and it ran ok but was certainly lean and didn't pull as hard as the messed up 770 that was on it. Ill put some mileage on it and read them again.

72H code thank a whole lot im going to re read all of that when i get some slow time in the shop today. The only thing it does consistently is if i accelerate semi hard not WOT just a hard acceleration then roll out of it rather than just dropping the pedal back it almost kind of hiccups or shudders a little bit then goes back to normal operation. I shouldn't complain heck the thing drives good and pulls as hard as a 3.00 : 1 rear end car pulls.

Also i did use the Mr.Gasket timing springs but only installed one of them and put in a limit pin bushing to get the timing set. It is currently 16 initial with 36 total all in centrifugal no vac advance hooked up. full advance in the 2600-2800 range. and i run premium with a little marvel and haven't had it ping yet.

Ill report back in a few days after i get some miles on these plugs
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#8
The factory calibration on the Street Avenger series carbs is ridiculous.
Jets are 65/68. The one I had, on a similarly built 351C 4V, was terrible. It didn't do anything well. I changed the primary jets to 69 and secondary to 76. I also changed the secondary spring to the long yellow spring from the plain one. Set the primary transfer slot to expose about .025" of the slot, secondary slot barely covered. IIRC the mixture screws to ended up at about 1.250 turns CCW. Let us know how it all turns out. Good luck. Chuck
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