Seem to still have a bit of a rough idle 9though way better than the previous carb we had on it)....

If I pull each spark plug wire while the engine is running, should each plug being "missing" cause the idle to significantly change on a slant 6?

I dont see why it would be any different but figured I would ask....

I did this for all 6 cylinders and all 5 net a noticeable change except for cylinder #6.

Pulled the plug for cylinder #6 and it is firing, as evidenced by the plug looking like the rest.....there is spark getting to the plug as well, as evidenced by my feeling the shock...LOL

Next step I guess is a compression check to confirm that all is ok.....wondering if our valve setting hasn't changed since the last time we set it.

On any engine you will notice a drop in rpm, rough running, etc... if nothing changes then the cylinder may have other issues (no spark, low-no compression, etc....)

Another easier way to do this test without getting 'lit', is to use a circuit tester, put the clamp on a ground (body or engine), then use the point to 'probe' into the sparkplug boot... once it makes contact with the wire or plug retainer it will short and act like you pulled the plug wire... a lot faster and less shocking than pulling and replacing wires on a running engine....

Just did a compression check ........

about 190 on all cylinders....... one cylinder is 186, a couple are 192 ish. So all about the same. Cylinder 6 and 5 are the same at 190.

Plug 5 is tan, all the rest are sooty and black..running rich....why would only cylinder 5 be tan?

We swapped in a new set of wires, a cap and rotor yesterday.....

I just tried swapping plug 5 and 6 and no change in idle , still when I pull 6, but there is when I pull 5.....so not the plug. I did notice when I pulled the plug in 5 (6s plug) back out after running it, the side electrode was starting become not so black.....unless I did that putting it in....

That's an excellent result.
Uneven mixture distribution if the black soot is dry and crumbly on the other plugs. If the soot is oily/gummy/shiny, that's oil, not excessive fuel.
If that soot is dry and powdery, it's the fuel system that needs your attention.

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Yeah its definitely fuel......not oil.......dry and powdery.....

So why is cylinder #5 "normal" looking and the rest rich?

And why , when pulling #6 wire, do we not get a drop in rpms or any noticeable changel? Definately spark there....and as I mentioned we swapped the plugs between #5 and 6 and it worked fine in 5.

We do still have the plastic limiter caps on the BBS carb......were running it at as far rich as it would go.......changed it to about halfway in the range which dropped the rpms very slightly.....maybe smoothed it out a tiny bit but not much.

Maybe with the cam we need to pull the caps? Do they just pop off ? Can we do it on the car? Or does the carb need to come off?

Wondering, too, how rough an idle is "normal" with the cam we got? The idle "note" is even and repetitive but "hard"...feel it in the body of the car...especially at idle in gear.....idle is set at 750-800 rpm in park.....drops to 5-600 in gear.

Also wondering -- the ecu box on it now.....wondering if that could cause what we are seeing, with the new cam and all? We just cleaned that up and remounted it, and provided a new wired ground to the firewall...no change, but didnt hurt.

It sounds to me like the reason you don't get a noticeable difference when you pull the #6 wire is it is not firing, so no difference. It may be getting spark, but perhaps not sufficient spark, it is being suppressed, likely drowned by too rich a mix, fouled out quickly. Like you said, it's not the plug, when you switched #6 plug to #5 it worked fine.

Sounds like you have a fuel distribution issue. I wouldn't worry so much about #6 per se, it sounds like you're running really rich mix and that needs to be addressed. I believe that #5 just happens to be a bit less rich than the rest (which may be a concern or not). When I was fattening up my AFR, I went to way too big jets and the plug fouling was quick and random for any one to a mix of plugs. Get that fuel mix leaned out some and #6 may no longer be an issue.

Brian

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Could have a vacuum leak on #5 at mating of intake runner and head. This would lean out A/F mixture in that one cylinder leaving the others rich.

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Dan is correct, look to fuel system first, with all those carbon deposits...too rich.

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Thanks Dan and Brian

Gonna have to recheck the torque settings on the manifold bolts.....maybe we'll luck out and its "just" loosened up bolts.

Have never been a carb guru....I figured with a OEM style brand new BBD we should be OK carb wise....

Its a rough idle but hard to describe.....very even...versus a rough and lumpy idle like with a big cam. But rough.

Here's a shot of plug 6 and 5 (left to right) before switching....



And here they are after running for a few minutes with 5 and 6 plugs swapped..... but put back into correct spot in plug holder block.

Yes, those look pretty sooty, too rich a mix. I wouldn't worry about which one in particular, just get the fuel mix leaned out and then recheck.

If #5 is by chance consistently leaner than the rest, you can check for a leak at the intake manifold #5 runner, maybe it's getting more air. #5 looks fine (brown color) now, perhaps a bit rich still, but better than the others (way too rich).

One sign of a too lean condition is a greenish tinge to the plug electrode, but that's not gospel. I've seen it, others have commented similarly.

Brian

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As an FYI... on a 1 bbl Slant 6, # 2 & 5 run the leanest.
Is there a vacuum fitting in the #6 runner? If so, plug off the connection(s) to see if that helps.
Have you plugged the PCV "air bleed" to see if that changes anything?
DD

Ours is a 2bbl now....

Yes we have that vacuum "tree". We had to do a repair to it, so wondering if maybe that is contributing. I have a new one ready to install, albeit with two more "branches" than needed.

Guess its time to spend some time with the carb cleaner.....there and around the intake again.

We are using a brand new out of the box BBD slant 6 carb with the plastic caps still on the mix screws.....figured that with a brand new carb we would have to worry about running lean or rich outside of the mixture screws.....of course that doesnt take into consideration that we have the cam in it.

One thing to note though.....I had pulled those plugs right after a warmup that happened right after a failed start....not not quite "flooded", but too many pumps on the gas when cold....so blew some black smoke....and we are still running the standard style choke pulloff.....seems like the car is slow to warm up and come off the high idle on its own.....it used to do that nice and fast on its own....so maybe time to change to the electric choke mentioned elsewhere? The sooty plugs COLD be due to all that.....so maybe I will have to clean and then recheck them after driving normally a bit.

But checking for a vac leak at 5 seems to make a lot of sense to me....thanks all!

In another thread the topic of the air pump stuff came into play.

We have an air pump on ours, with two big hoses coming off it....one goes to the metal tube that connects to the head and one hose isnt connected to anything....that is how we got it.

WOndering now if the air pump system could somehow be playing into this issue? The tube that connects to the head connects at the rear of the head...right above cylinder #6......

??????????

I think Doctor Dodge is onto something. Plug the vacuum tap on the 6th runner and see if you are still getting the same result when you pull the plug wire off...i had a vacuum leak in the power brake booster which caused a miss on #6. It's a stupid problem but hard to diagnose.

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Greg loves his daily driver.
1973 Swinger 225 /6 Seats 6