piperdriver

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

piperdriver

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.

67mustang302

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.

piperdriver

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.

barnett468

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.

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67mustang302

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.

barnett468

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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.




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



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Gun Jam

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

-Gun

piperdriver

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.

barnett468

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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.

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