spark plugs burning black

[67hat34c]

Mike, what are the reasons for plugs to be burning black. we have the 454 model 350 crusaders. plugs are running black. they are the correct ones according the the book.

 

[MikeP]

Since we know that the plugs are the proper ones for the engine...

First question, does the engine "run OK?" I realize that's a very general question but it can eliminate a bunch of possibilities. With no other info, I'd suggest that the black plug indicates a rich fuel mixture. If that assumption is correct then the next question is, where in the operating range does this richness occur - Idle, mid-range, WOT, all? You can't know for sure without reading the plugs but reading spark plugs is quite involved and cannot be done by the naked eye. Does the engine get up to normal operating water temp - around 180 degrees? If not, the "correct" plugs might be too cold for the engine. This is not unusual for marine engines which often run much cooler than car engines.

ASSUMING THE IGNITION SYSTEM IS WORKING PROPERLY, in general, a black spark plug tip indicates either an overly rich (mixture or oil burning depending on the appearance of the black stuff. If it is more of "flat" black, it is probably too rich; if it is a wet black, it is probably oil. There are a lot of variations though so a generalization is not that reliable. The problem is that the blackness could occur at idle, because the idle mixture is wrong; at midrange, because that mixture is wrong; or at WOT because that mixture is wrong. Conversely, it could be right at any one setting and wrong at another. Carburetors can be adjusted to correct any of the areas. (Obviously, misfiring ignition from whatever cause will result in fuel-fouled plugs)

Some questions I'd ask : is it all the plugs showing black or just some of them? Both engines the same? If it's a mixture problem, all plugs in the same engine will normally look pretty much look alike. If it's oil burning then they may not.

One of the biggest problems with reading plugs is that no one - I mean NO ONE - can read a used spark plug. If anyone tells you they can, laugh and leave. So first it requires a new set of plugs. Then you have to make a WOT runs, shutting the engine off WHILE it is at full throttle. Then you pull the plugs and read them. You have to look all the way into the plug to where the insulator meets the metal base of the plug. This requires a magnifier and a light - of course they make tools specifically for this purpose!

Generally, you read the tip of the plug for idle mixture, the midpart of the insulator for midrange mixture, and the base of the insulator for WOT mixture. You have to do the WOT reading first, then a mid range, then an idle. Ideally the insulator should have a light pinkish-brown color with no deposits of any kind. Frankly, I wouldn't recommend worrying about all this at the moment.

The carbs, unless not original, are probably Q-jets and the leaking castings and fuel-logged floats that I mentioned in posts re Jackman's set up apply here. They will cause an over-rich mixture, especially at idle. In my experience, I would say that barring major screwups, it is the idle circuit that is most common culprit. First however, check to be sure the choke is completely off (open) when the engine is warmed up. If it is not, than it is probably the cause of the problem. Once this is checked, do the following:

An easy way to set the idle mixture is with a vacuum gauge. (THIS PRESUMES THE CARB IS CLEAN AND ALL PASSAGES ARE CLEAR.) Again - be sure the choke is fully open. Connect the gauge to the carb. Turn the mixture screws all the way in until they just bottom lightly - do not force them - and back them out 1.5 turns. Start the engine and adjust the idle to the correct RPM. Then adjust the idle mixture screws to obtain the max reading on the gauge. This may require turning them in or out. Do not be concerned with the specific number the gauge is actually reading. After doing this, re-adjust the idle RPM, then readjust the mixture again because it may have changed with the rpm adjustment.

Adjusting mid and WOT mixture is more involved and unless the carbs were modified, should not be necessary.

 

[67hat34c]

thanks Mike.

No clue if carbs have ever been off. both engines burn black. we may be getting some oil in cyl but no smoke. most likely guides leaking . just changed plugs and wires and have not re checked them. i also tried to adjust the slow speed jets however it was difficult as i only had a hand held tach, will try vacuum next time. i guess you plug the vacuum into the port where the fuel pump over flow hose goes. Idol speed does vary, have to knock it down some times. plan to pull the carbs and have them rebuilt as i am guessing they are the main problem

All that aside, the engines do run fine. running temp 170 to 180

have Pertronix ignitor, their coil, no balast resistor, msd wires, new plugs and rebuilt distributors. opened the gap to .040 from .035.

anything wrong with wider gap? should i go back to .035?

also i noted that the dwell reading using the Pertronix ignition is no longer within the range of breaker ponts. is this a problem? I called pertronix and they claim that it is not.

 

[MikeP]

Re the plug gap - If Pertronix suggested a wider gap, I would set them at that gap; if they do not, I would set them at the engine manufacturers recommendation - .035, I assume. But I wouldn't worry about the .005 difference - it's not enough to cause a problem regardless.

Don't worry about dwell readings for breakerless distributers. You can't adjust it anyway. You can toss the dwell meter if you'd like!

On most carbureted engines there is a large vacuum fitting at the base of the carburetor. That is the best place to plug in the gauge. It is also the common vacuum fitting location for the engine's PCV valve (if it has one). The Intake manifold may also have one or more vacuum fittings - they are also suitable. Other vacuum sources on the carburetor are not reliable for this purpose.

 

[67hat34c]

will do that next time.

by the way, when checking compression, hot engine or cold.

 

[MikeP]

Correct compression checks are done with the engine warmed up and the throttle set at wide open. Lots of people prefer doing it cold because it is a pain (literally) to pull those hot spark plugs! However, cold checking will result in inconsistent readings. Pull all the plugs, ground the coil-to-distributer spark wire, and check each cylinder in turn. Crank the starter to obtain 6 "pumps" into the gauge per each cylinder.

As you probably know, the difference between the cylinders is the critical measurement, not the actual compression reading compared to specs. Re that - and here's the limitation of the compression check - don't be surprised if your engines show a higher gauge reading than the book specs, ie book says compression is 145 PSI, you show 154. Carbon buildup on the pistons over the years will cause this. The problem is the carbon buildup can raise the compression but ring wear lowers it. They can offset each other! You can show a satisfactory compression check while the engine is actually not sealing as well as indicated. I have seen engines clearly blowing oil that show OK by a compression test. This highlights the reason that single diagnostic tests, though useful, can be misleading. A cylinder leakdown test, in conjunction with a compression test will quite definitely show what's going on.

Unfortunately, a cylinder leakdown test is relatively difficult compared to a compression test. If the engine is already warmed up a compression test can be done in 20 minutes or less per engine. A cylinder leakdown test will probably take 1+30 (also warm, per engine) and requires a source of compressed air and appropriate gauges.