what causes carbon on intake valve?

3069 Views 15 Replies 7 Participants Last post by alfaparticle, Mar 25, 2015

alfaparticle

Discussion Starter · Mar 24, 2015

I removed the intake manifold from my hot Spider engine in preparation for the switch to 45DCOE's and I observed carbon buildup on the back of the #4 intake valve. The other three valves are pretty clean. I have good compression on all 4 cylinders. I have no idea what is causing it.

Has anyone else experienced or observed this and has a good idea of what may be going on?

Thanks.

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ljayr

· #2 · Mar 24, 2015

Has anyone else experienced or observed this and has a good idea of what may be going on?

You can rule out direct injection as a cause.

On a carbed vehicle you are typically left with things like PCV and EGR ports near the cylinder allowing un-burned hydrocarbons back in. Maybe a loose or leaking vacuum line near the port. Poor quality gas can also be a culprit, but seeing how its only one cylinder you might be able to rule that one out.

Are you seeing the carbon build up on top of the piston as well?

Gubi

· #3 · Mar 24, 2015

ljayr said:
On a carbed vehicle you are typically left with things like PCV and EGR ports near the cylinder allowing un-burned hydrocarbons back in.

That was my initial thought as well, but it has Webers. Valve seal on that one leaking a little more oil than the others, perhaps?

alfaparticle

Discussion Starter · #4 · Mar 24, 2015 (Edited)

Maybe a loose or leaking vacuum line near the port.

The vacuum booster is plumbed into #4 port. How would a leak result in carbon?

Valve seal on that one leaking a little more oil than the others, perhaps?

The motor smokes a little. It is hard to notice unless you are right behind the car.

The engine is revved hard and I suppose that it is possible that I have a little valve bounce.

Gordon Raymond

· #5 · Mar 24, 2015

I believe that is oil carbon Ed. Fuel carbon is usually a smoother coating. Plenty of old (pre'68) V12 Ferrari intake valves look like that. They did not use stem seals on the older 12's. Most have seals installed on rebuild, so not a lot of the older 250's and 275's are running around trailing a blue cloud anymore.
It seems to do no harm in my experience.

euro1750

· #6 · Mar 24, 2015 (Edited)

Maybe there is a brake fluid leak in you booster, and the "carbon" is in fact some brake fluid getting sucked in and "grilled" on the intake valve? This would at least be unique to cylinder 4.

ljayr

· #7 · Mar 24, 2015

How would a leak result in carbon?

Lean conditions also cause incomplete combustion, thus leaving un-burned hydrocarbons to form carbon.

A quick check of the spark plug will rule out any lean or rich conditions. A vacuum check might also be a good idea to do to make sure there isn't something else going on. If everything checks out you may have a previously mentioned valve stem seal issue.

Whatever the case the old water trick should remove any carbon build up once the root cause has been found and fixed.

Alfar7

· #8 · Mar 24, 2015

Brake fluid

pull the vacuum line off the booster and run a strip of cloth in to the bottom. If there is fluid in the reservoir, suck it out, and replace the brake MC.

That could both explain the smoke and the carbon.

alfaparticle

Discussion Starter · #9 · Mar 25, 2015 (Edited)

pull the vacuum line off the booster and run a strip of cloth in to the bottom. If there is fluid in the reservoir, suck it out, and replace the brake MC.

There is a little fluid in the booster - see the picture. The level in the brake reservoir has not gone down enough for me to notice anything.

A quick check of the spark plug will rule out any lean or rich conditions.

#4 plug looks exactly like #2 & #3 - a nice light brown. Only #1 is a little darker.

I believe that is oil carbon Ed.

So the intake valve gets hot enough to burn the oil!

It seems to do no harm in my experience.

The motor is running very well. I was surprised to see the carbon. The motor has only about 4k miles on it but it sees high rpm very frequently. The cams are 12.8 lift and Richard had me check for clearance between the keepers and the seals before I installed it. It has only been in a few weeks. Prior to that I ran 12.3mm cams.

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gigem75

· #10 · Mar 25, 2015

Is it possible the guide seal on that cyl has been disturbed in some way? If you take the tappet out and look down the side of the spring you might see pieces if it has.

alfaparticle

Discussion Starter · #11 · Mar 25, 2015

Whatever the case the old water trick should remove any carbon build up once the root cause has been found and fixed.

Do you mean squirting some water into #4 weber intake while the engine is running?

alfaparticle

Discussion Starter · #12 · Mar 25, 2015

I mopped up the fluid from the booster. There was only a small amount and it was nasty. I think that it had been in there a long time, so the current master cylinder is most likely OK. I think that Gordon is right and that it is oil.

gigem75

· #13 · Mar 25, 2015

From a failed guide seal

you will know for sure next time you tear it down, which will be in the distant future. Bottom line is the motor runs like stink (that's good) so keep on truckin!

ljayr

· #14 · Mar 25, 2015

Do you mean squirting some water into #4 weber intake while the engine is running?

Yes.

I use a 2 liter pop bottle and punch a small hole in the cap. Make sure the car is up to temp and with the engine running squirt a steady stream of water into the intake. I prefer to inject the water behind the carbs thru an open vacuum port. The more water the more throttle you will have to give it.

On engines that have a heavy build up of carbon the exhaust will be quite black when doing this. Just keep repeating until the exhaust color clears up.

You more than likely may not see too much color variation in the exhaust as the carbon build up is not yet excessive.

Alfar7

· #15 · Mar 25, 2015 (Edited)

valve

Ed, since the valve is seated use a little WD 40 and MEK and scrub and wipe it off with a bent screwdriver and cotton cloth.

Water isn`t going to remove carbon on the outside of the valve. Only helps with large crusty deposits in the combustion chamber.

alfaparticle

Discussion Starter · #16 · Mar 25, 2015 (Edited)

Ed, since the valve is seated use a little WD 40 and MEK and scrub and wipe it off with a bent screwdriver and cotton cloth.

Two of my go-to products.

I have added a picture of the valve after the MEK/WD40 treatment. IT WORKS!

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