Rpm related vibration

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pjc360
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Re: Rpm related vibration

Post by pjc360 » Mon Feb 26, 2018 6:34 pm

So this afternoon I finally had some free time on my hands.
I pulled all the spark plugs out and cylinder 6’s plug is carbon fouled, black soot looking substance on it that was dry, almost how the inside of a chimney looks.
So I’m sure that was causing the roughness I was feeling under load going down the highway.
I put a brand new plug in it and checked all the other 7 plugs and they all looked fine.
So like I said before I forgot to check all the plugs before taking the crane ignition box out and putting the new Msd box inn.
I’m hoping that plug was fouled prior to installing the new Msd box and now that it has a brand new plug everything will be good.
I’ll know tomorrow morning when I take it to work and get to drive it on the highway.
If it still feels the same then I’ll be convinced there is an issue on cylinder 6 somewhere, valve or valve spring.
But compression is a solid 155psi on cylinder 6 and on all the other 8 cylinders.
So I’m realky hoping this fixes it.

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Re: Rpm related vibration

Post by BigBlocksOnTop2 » Mon Feb 26, 2018 7:35 pm

pjc360 wrote:
Mon Feb 26, 2018 6:34 pm
So this afternoon I finally had some free time on my hands.
I pulled all the spark plugs out and cylinder 6’s plug is carbon fouled, black soot looking substance on it that was dry, almost how the inside of a chimney looks.
So I’m sure that was causing the roughness I was feeling under load going down the highway.
I put a brand new plug in it and checked all the other 7 plugs and they all looked fine.
So like I said before I forgot to check all the plugs before taking the crane ignition box out and putting the new Msd box inn.
I’m hoping that plug was fouled prior to installing the new Msd box and now that it has a brand new plug everything will be good.
I’ll know tomorrow morning when I take it to work and get to drive it on the highway.
If it still feels the same then I’ll be convinced there is an issue on cylinder 6 somewhere, valve or valve spring.
But compression is a solid 155psi on cylinder 6 and on all the other 8 cylinders.
So I’m realky hoping this fixes it.
Was that spark plug dropped and then installed. What happens is the porcelain will crack and you will have spark jump. I would check the wire and the terminal on no.6 for issues. Check the wire with an ohm meter and compare to other wires. I think you got it!

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Re: Rpm related vibration

Post by pjc360 » Mon Feb 26, 2018 9:56 pm

BigBlocksOnTop2 wrote:
Mon Feb 26, 2018 7:35 pm
pjc360 wrote:
Mon Feb 26, 2018 6:34 pm
So this afternoon I finally had some free time on my hands.
I pulled all the spark plugs out and cylinder 6’s plug is carbon fouled, black soot looking substance on it that was dry, almost how the inside of a chimney looks.
So I’m sure that was causing the roughness I was feeling under load going down the highway.
I put a brand new plug in it and checked all the other 7 plugs and they all looked fine.
So like I said before I forgot to check all the plugs before taking the crane ignition box out and putting the new Msd box inn.
I’m hoping that plug was fouled prior to installing the new Msd box and now that it has a brand new plug everything will be good.
I’ll know tomorrow morning when I take it to work and get to drive it on the highway.
If it still feels the same then I’ll be convinced there is an issue on cylinder 6 somewhere, valve or valve spring.
But compression is a solid 155psi on cylinder 6 and on all the other 8 cylinders.
So I’m realky hoping this fixes it.
Was that spark plug dropped and then installed. What happens is the porcelain will crack and you will have spark jump. I would check the wire and the terminal on no.6 for issues. Check the wire with an ohm meter and compare to other wires. I think you got it!
I can’t tell if the porcelain was cracked or not, I really need to get a wideband o2 sensor on this engine and get my carburetor dialed inn a lot better.
But my settings aren’t all that rich at all for what I’m running and the altitude I’m at and the timing I’m running.
Carbon fouled plugs are normally caused by too rich of an air fuel mixture, a specific problem with an individual cylinder as in bad valve, broken valve spring, ect ect, but I get good compression on that cylinder and every other cylinder.
So it’s hard to say there is an issue with one individual cylinder.
I know the crane cams box I was running prior to the msd box I’m running now would miss fire like crazy at a steady load or rpm.
I found out I had my trigger wires and my coil wires running to close to each other and I think that damaged the box from all the false triggering it was getting from interference.
So what I’m hoping happened is since I didn’t check every plug prior to installing the msd box that number 6 was already carbon fouled and now that it has a new plug everything is going to be ok.
I’m really really hoping that.
If it ends up carbon fouling again then I’m going to have to get a wideband on there and dial the carb in a little better and if I still have issues after that then I’m going to have to look a little closer at that specific cylinder and see what’s going on.
I’ve been wanting to get new heads forever now, a set of Eq’s with the better chambers and bigger valves, and then put in a better camshaft while I’m at it.
Maybe I should put that higher on the to do list.

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Re: Rpm related vibration

Post by pjc360 » Tue Feb 27, 2018 7:43 pm

Put the brand new plug inn on cylinder 6 and it’s still missing like a sob.
I’m convinced now there is an issue with cylinder 6 somewhere possibly cylinder 8 as well.
I just don’t know how to diagnose the problem.
I’m thinking ether a valve or valve spring or a broken rocker arm or push rod.
Something mechanical has failed on the passenger side cylinder head by cylinder 6 and possibly 8.
I get 155 psi on compression test.
So what do I do know?
Pull the head and see if I can find anything wrong?
Bring it to a machine shop to test the valves and valve springs?
I’ve been wanting to get new heads and I’m seriously thinking now is the time.
It’s not drivetrain, it’s not engine balance, it’s not ignition primary or secondary.
Something is causing cylinder 6 to become carbon fouled and something is causing cylinder 8 to be a tad wet with fuel around the base of the plug.
These factory magnum heads are known to crack.
I’m thinking I just bite the bullet and order some EQ magnum heads or spend a little less on a pair of re-built factory magnum heads from a reputable cylinder head shop.

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Re: Rpm related vibration

Post by pjc360 » Wed Feb 28, 2018 9:15 am

What are people’s thoufhts on it possibly being the distributor/ oil pump drive gear bushing?
I’ve read online these can go out and cause a lot of issues with ignition.
I’m starting to consider that as a possibility seems how every last piece to the ignition is brand new and I have good compression on every cylinder but I still have this awful missfire under load.

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Re: Rpm related vibration

Post by pjc360 » Sun Mar 04, 2018 2:20 pm

I believe I finally figured out what’s going on!!
It’s not ignition, it’s not mechanical, it’s fuel related!
I think my quickfuel carb is giving cylinder 6 too much fuel to where it’s so rich it’s causing a missfire.
Has anyone else encountered this before?
I realized that sometimes if I step on it super hard on the highway and then back off and remain a steady rpm of 2500 it would clear up quite a bit.
Another guy that’s been trying to help me over on moparts said that sure sounds like a carb/fuel issue to me that you can get it to clear up a little bit by hammering on it.
Yesterday I pulled number 6’s plug and it was carbon fouled again, I had just put that plug in a couple of days ago.
So I put a new plug and a new plug wire on cylinder 6, then I removed the valve cover on the passenger side and fired it up and let it idle as I watched all the rockers and valve springs.
The rocker arms and valve springs on every cylinder on the passenger side appear to be fine and moving in sync with each other.
None of the rocker arms were loose, I couldn’t see any broken valve springs, all the push rods felt tight.
So I put the valves cover back on and I lowered my float level from being half way up the sight glasses to the very bottom of the sight glasses.
This should lean out when the main jets start to pull fuel from the bowls.
I’m going to see if that makes a difference, if it gets better then I’ll know I’m on the right track and I’ll need to lean out some of the other settings.
Right now I’m running 65 main jets and 75 secondary jets.
Idle feed restrictors are .031’s, idle air bleeds are .065’s.
High speed air bleeds are .033s.
Power valve is a 9.5hg, and pvcr’s are .049’s.
Floats are now at bottom of the sight glasses.
Does this set up sound excessively rich?
I am at 3500 feet above sea level here in Montana.
Should I increase the high speed air bleeds a little more to keep the main jets from pulling too early and de-crease the idle feed restrictors down to .028’s or .030’s?
And maybe de-crease main jets from 65s to 64s or 63s?

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Re: Rpm related vibration

Post by ozyfordman » Sun Mar 04, 2018 2:38 pm

Maybe time for cable tie spivvy on the booster above #6 to direct fuel away from that cylinder.

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Re: Rpm related vibration

Post by Aukai » Mon Mar 05, 2018 2:35 pm

I have not read every post, but have you tried a hotter heat range plug in that cylinder? Wondering if it would make a temporary difference.
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Re: Rpm related vibration

Post by modok » Tue Mar 06, 2018 1:30 am

Don't know anything about the induction setup.....maybe not good distribution? bad carb? could be

Being very RPM related.....IMO it would probably NOT be a head problem, but possibly a lifter problem.
I don't want to send you chasing your tail, but is possible for such things to happen.
Poorly ground cam lobe or timing chain slack to cause a lifter to pump up at certain RPM bands.
If signs start pointing THAT direction, maybe you could rule that out with adjusting to zero lash....instead of pre-load?
Glen Urban

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Re: Rpm related vibration

Post by Aukai » Tue Mar 06, 2018 7:27 am

I have heard of aluminum heads having the spark plug not grounding in the threads due to too much anti seize, or helicoils. A long stretch I know, but seems that's where you are now, looking for hay stacks.....
Mike Lewis street 572
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"You know you have the power, and if you use that power bad things are going to happen"

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Re: Rpm related vibration

Post by Firedome8 » Tue Mar 06, 2018 9:38 am

Look for a shop that has a chassis dyno and an engine analyzer.

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Re: Rpm related vibration

Post by Geoff2 » Wed Mar 07, 2018 4:00 am

Put another carb on it [ & not a POS brand H or clone{S} ]!!!

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Re: Rpm related vibration

Post by pjc360 » Mon Mar 12, 2018 11:07 pm

I think I may finally figured out the issue, or at least I hope I have.
I called and spoke with a guy at AED performance carburetors because I was inquiring about a set of metering blocks they have because Initially I was thinking there was a manufacturer defect with my quickfuel metering blocks.
Anyways I was explaining how I have an awful missfire around 2500 rpm and that when I lean the carburetor down it seems to help but doesn’t eliminate it and that cylinder 6 and cylinder 8’s plug are the ones showing the issue.
The guy asked right away are you running a carburetor spacer? And I said yes and he said what kind and I said a 2 inch 4 hole spacer and he said take that spacer off and try a smaller open spacer.
He said I think that 4 hole spacer is causing a significant fuel distribution issue to cylinder 6 and cylinder 8 and that’s causing the engine to load up and miss while your cruising down low around 2500 rpm and that’s what’s causing your plugs to foul in those cylinders.
So I’m really hoping an open spacer does the trick.
I’m going to get one and try it and I’ll report back afterwords.

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Re: Rpm related vibration

Post by Geoff2 » Tue Mar 13, 2018 4:36 am

I'll say a Hail Mary for you but I don't think it will do any good....

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Re: Rpm related vibration

Post by pjc360 » Tue Mar 13, 2018 11:05 pm

What else could is possibly be? Compression is great on all 8 cylinders, 155psi on the dot on every cylinder.
Engine vacuum is steady at 20hg idling, I just don’t understand what else it could possibly be.
It’s not ignition, it doesn’t appear to be a mechanical issue inside the heads with such good compression numbers, I’m at a loss as to what else it could possibly be besides a fuel issue.
This is a brand new Quickfuel super street series 680 vacuum secondary carburetor.
I’ve leaned out every circuit, idle feed restrictors, main jets, I’ve lowered the floats, I’ve had the carburetor apart and don’t see anything wrong with it.
The fact that I can get this issue to sometimes go away when I hammer on it and then ease back into a steady throttle position has got to be the smoking gun to figuring this out, and I’ve noticed some days it does better then others, and I’ve always noticed cylinder 6 and cylinder 8’s plugs are always darker then the other 6, always.
So if it’s not a fuel issue it has to be in the heads somewhere.
I found a guy that’s a couple of hours away and he’s really good with small block Mopars and carbureted 360 magnums, he’s built a ton and he’s got a good reputation over on moparts as being knowledgeable, if taking the 4 hole spacer Away doesn’t solve the issue then I’m throwing in the towel and bringing it over to him to look at cause I will have exhausted all my resources short of replacing the heads.
I’ve had different carburetors on it in the past and it’s done the same dam thing with an Edelbrock and with this quickfuel.
It’s just so frustrating to battle something for so long and no one has any idea on what it could be.
The strange part to me is how it runs great below 2500 rpm, idles good no missing firing there, cruises good around 1500-2000 rpm in every gear but the sells you get to 2500 it starts missing.
What kind of carburetor do suggest I try on it, what’s wrong with the quickfuel carb? I know of lots of guys that run this same carburetor with great success.

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