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5 and 8 Spark plugs run lean

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Old 08-29-2014, 12:28 PM   #1
piperdriver
 
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Default 5 and 8 Spark plugs run lean

What would cause my number 4 and 8 sparks plugs to run lean? 5 and 8 is a typo in the title. The other plugs look normal but 4 and 8 are squeaky clean and white. If I put a tan plug into 4 or 8 it turns white after a 25 min drive.

I'm running a cobra intake with a holly 600. I checked the jets and they aren't plugged. If it was a carb issue wouldn't the other plugs indicate lean as well?


My engine runs warm and I think the lean cylinders are causing the problem. I checked compression and a cylinders are the same aprox 150
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Last edited by piperdriver; 09-02-2014 at 12:28 PM.
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Old 08-29-2014, 12:39 PM   #2
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I forgot to add that the engine runs great in all ranges. Idles good and doesn't hesitate. It doesn't smoke and starts fine.
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Old 08-29-2014, 02:48 PM   #3
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The intake. That's what happens on engines with flow imbalances between cylinders. single plane intakes provide the best balance of flow, but don't work well on mostly stock engines.
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Old 08-29-2014, 04:21 PM   #4
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Originally Posted by 67mustang302 View Post
The intake. That's what happens on engines with flow imbalances between cylinders. single plane intakes provide the best balance of flow, but don't work well on mostly stock engines.
I don't think it's the intake. I've been running this intake for a couple of years without this issue. It was only after the rebuild that it started to happen. Same set up as prior only thing different is the roller set up.
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Old 08-29-2014, 07:02 PM   #5
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I don't think it's the intake. I've been running this intake for a couple of years without this issue. It was only after the rebuild that it started to happen. Same set up as prior only thing different is the roller set up.
your particular prob is NOT caused by the design if a dual plane intake in and of itself, no way, no how.

a dual plane ford intake can tend to run slightly lean on one entire bank of the intake. this is possibly because the runners fed by the drivers side carb holes are basically on the bottom of the intake where it is hotter and the ones fed by the passenger side carb holes have more exposure to the outside air.
remove ALL vacuum fittings to intake including brake and pcv etc..

lower idle

gently spray flammable carb cleaner at the carb base and where the intake meets the head. if your rpm increases, you have an air leak.

if you do not have an air leak, put tan plugs in the rear holes and drive it with the vacuum fittings plugged and then inspect them.

if they stay tan, your vacuum fitting location is causing the prob.

if they are white you may have a leak on the valley side of the intake on those cylinders.

.

Last edited by barnett468; 08-29-2014 at 11:53 PM.
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Old 08-29-2014, 10:09 PM   #6
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It's a common problem with dual plane intakes because of their design. Half the cylinders get their charge from a larger plenum and half from a smaller plenum. In each half, half of those cylinders (2) get their charge from short runners and half from long runners. It's why single plane intakes generally run smoother and the cylinders ru at more consistent temps and fuel ratios. It's also why on EFI, everything is single plane these days, much better power distribution (they don't need a dual plane because metering signal is a non issue).

It's typically not a major issue, since the balance isn't too far off; especially in the newer dual planes that are designed with computational fluid modeling. But it is always there, regardless of whether you have a problem or not; which also means that even a minor vacuum leak into the larger plenum or a runner fed by it can be very noticeable.

In a SBF dual plane, cylinders 5 and 8 are both fed from the driver side plenum, which is the larger plenum. As are cylinders 2 and 3 (which are the shorter pair, 5 and 8 are the longer pair). It's not uncommon to cross jet a carb, putting 1 jet size larger (or more, or a combo with air bleeds) into the carb barrels that are over the larger plenum. There's no rule that says the jetting has to be symmetric.

What do the 2 and 3 plugs look like? What do they look like compared to the remainder (1, 4, 6, 7)?

Still check for vacuum leaks though, but if you can't find a source just try adding jet to the left side of the carb.
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Old 08-29-2014, 11:26 PM   #7
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.
hello pipedriver;


The length of the runner has little affect on the lean vs rich conditions in a dual plane manifold because the next fuel charge is still in all the runners once the valve closes. Now one can try to get into pulses vs runner length but it has little affect in this regard. If a pulse pushed the fuel charge all the way back up to the carb then an engine with 3” long intake runners would not run well if at all, but since it does, this “theory” doesn’t apply to your condition. if the runner length "theory" held water, your 2 and 3 plugs would be much lighter than your 6 and 7 plugs on your cobra intake.

in addition, the distance in runner length between all the cylinders is no more than around 3/4” on a stock 4v iron intake. they are definitely much different in length on a shelby intake however.

Also, since you have a cobra intake, if you wanted to make cylinders 2-3-5-8 richer, you would increase the jet size in the passenger side of the carb and NOT the drivers side as was suggested.

factory iron intake = 1-4-6-7 = fed by driver side venture which are all upper runners.

Click the image to open in full size.


factory Shelby intake = 2-3-5-8 fed by passenger side venturi.

Click the image to open in full size.

.

Last edited by barnett468; 08-30-2014 at 03:33 AM.
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Old 08-30-2014, 12:00 AM   #8
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I had this problem.

I found the issue to be that I had not used the exhaust gas cross over block plates when installing the intake.

The heads are designed to allow exhaust gas to cross under the carb when using an intake that allows for crossover. My edelbrock rpm airgap has no such passage. When installing the intake gasket I should have used the metalized block off wafers that came with the intake gasket. They fit poorly so i did not used them.

Because I left the block off wafers out hot exhaust gas cut the intake gasket open after a few 1000 miles allowing air and exhaust gas to enter cylinders 2,3 and 6,7. the symptoms were decrease in vac reading then uneven plug color then audible exhaust leak sound, futher decrease in vac, and finally visible intake gasket deformation between head and intake with black soot streaks near cylinder 6 and 3

Look for these indicators.

Happy hunting

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Old 08-31-2014, 08:07 PM   #9
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I misspoke it's actually the 4 and 8 spark plugs that are lean/hot.. I have moved around spark plugs (put the tan ones in 4 or 8) and they always come out white.

Since I have run this intake on this engine for over two years and prior to the rebuild without any issues I am going to exclude the set up as the problem.

I only have one vacuum line coming off my engine and that's for a boost gauge. The dizzy is a mech advance only so I will investigate a vacuum leak in and around the intake gasket as the possible source.

Thank you for all of the input, please keep them coming if anyone has other ideas.....I will post my findings.
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Old 08-31-2014, 08:34 PM   #10
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.

you do NOT have a pcv valve?

both 4 and 8 are fed by different runners on both manifolds. they are also far away from the ex heat crossover, therefore, you most likely have a vacuum leak.

.

Last edited by barnett468; 08-31-2014 at 08:37 PM.
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Old 08-31-2014, 10:50 PM   #11
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This possible vac leak could somehow occur from inside the valley under the manifold (internal vac leak) making its detection much more difficult.
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Old 09-01-2014, 05:01 AM   #12
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What intake gaskets are you using?
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Old 09-02-2014, 12:26 PM   #13
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I do have a PCV, doesn't appear to be leaking.

The intake has Felpro gaskets, I'm not sure which ones as the builder installed them.

Is there a way to tell in the vacuum leaking is coming from under the intake?
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Old 09-02-2014, 12:52 PM   #14
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The only real way to know is if you had a baseline vac reading before the problem started

example was 14.5 at idle 850rpm hot now its 12.0 at idle same temp and engine rpm.

However!!! if the issue is sever enough you would be able to positive ID a vac leak the same way.

Engine at run temp rpm at reasonable idle say 850 to 900

Hook vac gauge into full manifold vac line off carb the lowest one typically not the one the distributor plugs into.

if the gauge reads 12s or lower at 850 hot then you almost certainly have a vac leak (assuming non radical cam)

if its in the 14 to 13s thats debatable depending on cam.


I love felpro gaskets with the exception of the soft intake gaskets they make I cant remember but it might have been the 1250??? This is a soft rubber like gasket with a blue outline that seals extremely well but has no crush resistance. Some people think they are awesome but I think they suck bad and thats because literally the weight of the intake manifold + 5 foot pounds on intake bolts over about 6 months crushes these gaskets to the point where they split and leak particularly at the water jackets. I have tried multiple times and came to the conclusion that maybe if you only hand tighten the bolts using fingers only or just left the bolts out it would work for a while. What a waste of time these were.

I switched to the edelbrock fiber like gaskets with the black outline torqued the bolts to 20ftlbs and they dominate! I got to inspect one after about 8months of service life when it failed due to that exhaust gas leak the water jacket ports and intake ports looked perfect 100/100.


Click the image to open in full size.


you can see the deformation of the gasket around water jacket. Looks like I might have used about 40ftlbs.....actually it was about 12 or about half of the standard tq rating attempting to use less gave the same results its almost like they melt and then split over about a 6 month period.
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Old 09-02-2014, 02:53 PM   #15
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I asked specifically because of the 1250's. Tons of people have issues with them tearing.

I've never been able to get a pair to last longer than about 12-18 months.

The 1250-S3 is more like an OEM type gasket though, same as the 1250 but with a steel laminate core so it doesn't distort like that.

If you can find a shop with a smoke machine, they can check to see if you have a leak, and where it is.
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Old 09-02-2014, 04:41 PM   #16
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I do have a PCV, doesn't appear to be leaking.
OMG...ok, i have a headache now. i said ALL vacuum ports. a pcv is connected to the intake and is nothing more than a HUGE vacuum leak.

plug the thing on the intake or carb or wherever it is connected to. CAREFULLY and THOROUGHLY redo your leak test with FLAMMABLE brake cleaner only. do NOT use carb cleaner or wd40 etc. spray it at the pcv hose where it connects to the intake or carb also.

leave it plugged and do your plug color test and maybe also tell us where it is plugged into the intake or carb as i previously asked.


Quote:
Originally Posted by piperdriver View Post
The intake has Felpro gaskets,
well are tge the hard metal gaskets or the soft flexible ones?


Quote:
Originally Posted by piperdriver View Post
Is there a way to tell in the vacuum leaking is coming from under the intake?
yes, take it off and replace the gasket.

Last edited by barnett468; 09-02-2014 at 04:45 PM.
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Old 09-02-2014, 05:01 PM   #17
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hello gun and poiperdriver

i tourque the intake bolts in stages and use the factory tprque pattern.

after i have torqued the last bolt to the required spec.

i start at the first bolt and torque them again. the bolts are always loos after the first final torque.

i do this a total of 3 times.

after this, i run the car for maybe 30 minutes then torque rhem again and will then run it for a few days and torque them again. they are rarely loose at this point.

if the bolts are "stretched" they need to be replaced.

grade 8 bolts can be torqued higher.
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Old 09-02-2014, 10:14 PM   #18
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That's an excellent method Barnett....unless its a 1250.

If you used factory tq spec on at 1250 you would probably experience almost instantaneous failure especially if you re-torqued them.

I know it looks like hell but that photo is of a brand new 1250 with less than 6mo time and about 12ftlbs using the recommended tq pattern. Recomended tq is 18 to 20.

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Old 09-02-2014, 11:06 PM   #19
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That's an excellent method Barnett....unless its a 1250.

If you used factory tq spec on at 1250 you would probably experience almost instantaneous failure especially if you re-torqued them.

I know it looks like hell but that photo is of a brand new 1250 with less than 6mo time and about 12ftlbs using the recommended tq pattern. Recomended tq is 18 to 20.

-Gun
hello gun jam.

i'm not quite sure how to say this because i like you, lol, but i have actually installed around 10 zillion 1250 fel pros and never had a leak ever...never

your prob must have beem from irregular gasket surfaces?

fel pros just don't like you?

at just 12 ft lbs, the bolts are definitely going to loosen up.

this is not to say that there are not better gakets out there by any means.
.
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Old 09-02-2014, 11:22 PM   #20
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I gave it a really good shot...Apparently Im not the only one.

I would have to based on my experience not recommend them to anyone for any reason.

Did you see the above photo I posted? What is your analysis of the failure?

-Gun
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Old 09-02-2014, 11:22 PM
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