More carbon buildup in #1 + #8 cylinders than the rest....Why?

[48 New Yorker]

A few years ago, when I had the head off my ’48 New Yorker 8 cylinder I noticed that #1 and #8 pistons had a heavy layer of carbon scale buildup  — the remaining pistons had much less. Compression is low, but even for all 8 cylinders...(possibly to stuck rings), and none of the plugs were oil fouled, but sooty…  (See pics below)
 
Question: What would account for the greater amount of piston top carbon buildup on #1 and #8 only?  Kind of goes against logic if  #1/8  have the leanest mixture due to their distance from the carb.

[Bryan]

The only thing separate are spark plugs, each wire, valves and rings.   Whatever timing or mixture all should have in common.  One thing I noticed on mine (6 cylinder) are the water passage holes are different near the cylinders, and the further spaced cylinders (from each other) have a water passage space between them in the block.  The closer ones are joined.

[kencombs]

Just a guess, but the operating temp may be lower on those two since one side is not touching another cylinder..

[greg g]

 Try running a heat range or two spark plugs in one and six.  Also get an inexpensive point and shoot thermometer and see if there is a big difference in exhaust manifold Temps for 1 and 6 compared to others. Might want to do a compression check also.  If those two a re low, you are not getting the same fuel burn as other cylinders.

 

Not familiar with the porting on the straight 8, how are 1 and 6 ported? Do they share ports with another cylinder, or do they have to share with a neighbor?

Edited April 12, 2022 by greg g

[48 New Yorker]

Bryan & kencombs:

I think you both are on to something.  When I first got the '48 (years ago) it ran hot.  I discovered the water distribution tube had rotted out toward cyl. 7 and 8....so,(with much effort) I replaced it.  

 

Still have more thinkin' to do to figure out this puzzle.

 

BTW .....wondering if the wire loom (see stock photo- not my engine) could cause ignition "flash over" causing a misfire.  All my wires are copper core and sound, but still, they're all bunched up in that encasement.

 

[greg g]

How are you using your car?  If you are just doing a 30 mph average speed run to a cruise in or ice cream shop, it may not ever get to properoperating temp.  A long jaunt at highway speed might be necessary for good engine health.  Drive it like you stole it a couple times a month.  You mentioned spark plug wires, might be a good thing to get new ones along with new plugs.  Speaking of the plugs how do they look across the set? Are 1 and 6 looking different from the rest?  Swap the plugs around, among all cylinders go for a 70 mile jaunt at highway speed, pull them and have a look to see how 5hey are burning in different homes.

Edited April 12, 2022 by greg g

[Dodgeb4ya]

27 minutes ago, 48 New Yorker said:

Bryan & kencombs:

I think you both are on to something.  When I first got the '48 (years ago) it ran hot.  I discovered the water distribution tube had rotted out toward cyl. 7 and 8....so,(with much effort) I replaced it.  

 

Still have more thinkin' to do to figure out this puzzle.

 

BTW .....wondering if the wire loom (see stock photo- not my engine) could cause ignition "flash over" causing a misfire.  All my wires are copper core and sound, but still, they're all bunched up in that encasement.

 

I have never had any misfire because of the metal loom issue in any of my eight cylinder cars.

But I use modern  Hypalon 7 mm plug wire.

Edited April 12, 2022 by Dodgeb4ya

[Sniper]

Someone mentioned 1 and 8 being the farthest from the carb, true but manifold design can still play a part.  A nice long sweeping bend to 1 and 8 is less restrictive than a short run with a couple sharp turns.  That might be part of it.  Don't forget that 1 and 8 are farthest from the manifold heat source as well.  Might be a case of fuel condensing out as well.  Lot's of interesting possibilities here.

[Dodgeb4ya]

If you can stand a nickel on edge on the eight cylinder head while idling or higher rpm the engine is in good shape tune wise.?

[Los_Control]

Oh boy, here I go spouting Uncle Tony again ???

 

He was explaining how he was tuning his slant 6 race engine he was building. By reading the spark plugs.

As others have said above previously, because of the length of the intake runners to reach the furthest cylinders, he was changing the heat range on those spark plugs.

 

How he tested them was at the track. He would install a brand new set of plugs in the pits, send it down the 1/4 mile track wide open. Then back in the pits and pull the plugs to read them. ..... First thing he found was he needed to change the jets in the carb. Take it home make adjustments then next race day when the 4 middle plugs were reading correct. He then started to adjust the heat range on the further plugs to get them to burn correctly ... each time he used fresh new plugs for the 1/4 mile test to get a proper reading. Hard to imagine how many passes he made and how many new plugs he used .... He has plenty of future spares 

 

Just saying to adjust your procedure as needed. I personally would just clean mine & re-use them for the next test. I think it depends on the individual and how involved you want to get.

[Bryan]

How does heat range on a plug affect how much carbon builds up on the piston?  The plug burns hotter or colder based on the range, but that only affects the plug.   It doesn't affect the mixture.   I shouldn't even say burns hotter.  It retains more heat and has less deposits. The spark is the same.

Edited April 12, 2022 by Bryan

[Los_Control]

I think Tony did a fair job explaining it here.

I think it is pretty accurate and explains what & why we do what we do.

Just explaining why we are trying to get the perfect burn on the plugs.

Seriously, if just going out for Hot dogs. coffee, Ice cream .... Nobody cares how clean your plugs are running.

 

@Bryan This older video gives a pretty good explanation about it ... He later in 2022 did a video specifically on this engine & why he was changing what he did.

You can find it if interested.

@48 New Yorkeryou are not asking something foreign, kinda a thing all car guys go through and deal with different options.

 

[Sniper]

I would guess and this is just a guess that fouled plugs cause combustion issues in the chamber.