1956 312 Spark plug carbon build up quickly

[albjerryg]

I have a 1956 Tbird with a 312 engine, I just changed the plugs out yesterday and pulled the plug on number 1 cylinder and noticed quite a lot of carbon on the plug. I have a Eledlbrock carburetor and it is a weber 8867 which I think is a 1407 and it is 750 cfm. Could this be the cause of the rapid build up on the plugs?? Thanks for any help. Jerry

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[55blacktie]

Considering the problem is affecting just one spark plug, I would rule out the carburetor. Start by checking/replacing the plug wire.

 

[Ward 57]

If it's only one plug I wouldn't blame the carburetor. Ignition and valve adjustment checks would be in order. The cylinder is running cold. Right plugs? Proper gap? Possible intake manifold leak?

 

[57tbird57]

I have a 1956 Tbird with a 312 engine, I just changed the plugs out yesterday and pulled the plug on number 1 cylinder and noticed quite a lot of carbon on the plug. I have a Eledlbrock carburetor and it is a weber 8867 which I think is a 1407 and it is 750 cfm. Could this be the cause of the rapid build up on the plugs?? Thanks for any help. Jerry

alot of carb for a stock 312. 600 cfm would be better. if you have any vacuum leaks it will cause carbon build up as well

 

[albjerryg]

If it's only one plug I wouldn't blame the carburetor. Ignition and valve adjustment checks would be in order. The cylinder is running cold. Right plugs? Proper gap? Possible intake manifold leak?

It all the plugs getting carbon build up quickly. Jerry

 

[thessler]

Sure it's carbon and not oil fouled ?
As mentioned, that carburetor is about twice the size of what is needed. Even a 600 is to big !
Just some thoughts.

 

[thessler]

Just another thought, are they black and kind of a velvet texture? That would indicate extremely rich condition.
Fast carbon build up would be quite rare

 

[Bruce Hardy]

I have a 1956 Tbird with a 312 engine, I just changed the plugs out yesterday and pulled the plug on number 1 cylinder and noticed quite a lot of carbon on the plug. I have a Eledlbrock carburetor and it is a weber 8867 which I think is a 1407 and it is 750 cfm. Could this be the cause of the rapid build up on the plugs?? Thanks for any help. Jerry

Maybe two problems to check. Check or replace your spark plug wire on that cylinder. And replace that Carb. If it is a 750,you are way over carburated. Large carbs only help high RPM. Your engine is probably maxed around 5000 RPMS. You should be around 600CFM and you'll be very happy .

 

[Jim P]

I have a 490 CFM Holly on my 57. Even that is a but much for the 312 motor. 700 is a lot of air movement.
Look over your gap and wires. Switch out the points for a Pertronix....
You won't regret it.

 

[jack-in-sac]

Check spark plugs and wires including terminals. check rotor and cap.
Check compression on #1 and al others to determine overall engine condition.
Get a carburetor that matches your engines need. Case inpoint; HiPo 289 engines had a less than 450cfm carburetor to produce 271 h.p. Granted GT350 had 715cfm carburetors. However those engines had a higher h.p. rating and to be able to feed fuel needs at wfo. Additionally 390 FE engines often had less than 450cfm carburetors. You have way too much carburetor unless you are doing race only at full throttle. I suggest your problem solution should focus on that health of that single cylinder that is not functioning as it should.
consider the following formula: engine size × max rpm × efficiency ÷ 3456 = carburetor size
312c.i. x 5000 x 78% at best / 3456 = 578+
This would be a maximum need, Actual need more like 415c.f.m comparing the y-block to later more efficiently designed engines.
This would even be a optimistic efficiency of 245(h.p.) / 312c.i. the actual efficiency is less for the 312.
Have fun, ohn

 

[albjerryg]

alot of carb for a stock 312. 600 cfm would be better. if you have any vacuum leaks it will cause carbon build up as well

Maybe two problems to check. Check or replace your spark plug wire on that cylinder. And replace that Carb. If it is a 750,you are way over carburated. Large carbs only help high RPM. Your engine is probably maxed around 5000 RPMS. You should be around 600CFM and you'll be very happy .

The Carb was on the car when I bought it, and I am pretty sure it is carbon, rubs off real easy. I can change the Carb and see what happens. Thanks for the input.

Check spark plugs and wires including terminals. check rotor and cap.
Check compression on #1 and al others to determine overall engine condition.
Get a carburetor that matches your engines need. Case inpoint; HiPo 289 engines had a less than 450cfm carburetor to produce 271 h.p. Granted GT350 had 715cfm carburetors. However those engines had a higher h.p. rating and to be able to feed fuel needs at wfo. Additionally 390 FE engines often had less than 450cfm carburetors. You have way too much carburetor unless you are doing race only at full throttle. I suggest your problem solution should focus on that health of that single cylinder that is not functioning as it should.
consider the following formula: engine size × max rpm × efficiency ÷ 3456 = carburetor size
312c.i. x 5000 x 78% at best / 3456 = 578+
This would be a maximum need, Actual need more like 415c.f.m comparing the y-block to later more efficiently designed engines.
This would even be a optimistic efficiency of 245(h.p.) / 312c.i. the actual efficiency is less for the 312.
Have fun, ohn

I went out this AM and rechecked everything and the carbon is velvety but it is carbon and not oily, it is mainly on #1 spark plug, I realize the carburetor is too big it is what came with the car. I went in the attic and found an older Holley I think is a 600 cfm but did not have the time to check the numbers. I had to leave the camp where my garage is and get home think might be getting the coronavirus vaccine Monday. Thanks for the help i let you know the progress on Thursday.

 

[albjerryg]

It look like the spark plug wire was pulled out of the cap on the distributor end . Replaced it and seem fine. Thanks

 

[Ward 57]

Ockham's razor is in effect here. The most simple solution is usually the problem. We tend to chase down rabbit holes when the solution is right on the surface. I'd go with the old Holley and have it rebuilt. In years past the owners found it easier to replace than repair and that's where you have found yourself. Plus some thought bigger is better also new is better. Not so much with old technology If you understand it. Many like to replace with new technology stuff and as you can read on this site you end up with new problems. It's tough to meld new technology to systems that are over 60 yo. Old parts in the attic? You may have a gold mine up there.

 

[jack-in-sac]

As I often say; "When I hear hoof beats, I look for horses first not zebras.