Cleaning intake valves on the engine

[djwarner]

Actually I have down draft Webers.

 

I replaced the manifold gasket when I de-carbed the cylinders, cleaned the mating surfaces and torqued to factory specs when re-assembling.

 

I may be a bit anal regarding the intake valve cleaning, but the rest of the head is super straight.

[Sam280Z]

Can you describe what you did for this repair more explicitly? It sounds like you attempted to remove the head to clean the carbon and tried not to disturb the cam position.

Have you verified the cam timing, or are you trusting that it went back together correctly?

Did you take the same approach with the carbs?

When was the last time the carb bowl covers came off?

Are they clean inside? Just because they were working before doesn't mean that they will work now. You have moved them around, possibly disrupting the linkage or loosening dirt that can clog a passage.

How have you verified the carb balance?

Are your spark plugs tight / wires seated and good?

Have you checked your ignition timing?

Leakdown test?

 

These are all things I would check based on 25 years of my experience screwing stuff  up...

 

If you want to mechanically clean the backs of the valves, the only proper way is to disassemble.

 

Sam

[djwarner]

Hi Sam,

 

Removed the head by the book (Chilton and Haynes) using the jam stick for the timing chain. Took pictures of everything including the cam sprocket before dis-assembly. Since I was working alone and did not have and engine hoist, I removed the manifolds before manhandling the head. Sprocket and timing chain were marked with a magic market before dis-assembly to assure proper re-installation.

 

Carbs were rebuilt by the prior owner about 15 months ago and were running fine before this. Carbs were maintained vertical sat on a table without moving while they were off. Idle throttle and mixture were reset after re-installation per instructions of the US Weber distributor. I don't have a method or a gauge to check their balance. Upon re-installation, linkage was adjusted to remove any free play and assure equal alignment between units.

 

3 months old plugs were cleaned and gapped while out. Timing was spot on and confirmed after re-assembly. I always use a torque wrench for spark plug installation - especially with aluminum heads. Leads and distributor cap were removed as a unit, inspected and placed on the table next to the carbs. Cap, rotor, and distributor base were marked to insure proper re-installation and alignment.

 

De-carbing was performed with a steel brush on a drill after nothing I had seemed to chemically attack the deposits. The pictures in the last post should give you an idea how bad the deposits were.

 

As for a leak down test, I haven't visited Harbor Freight to pick up the tool yet.

[Sam280Z]

You really need to make sure that the carbs are balanced - both at idle and at speed. This requires a couple of tools - the flow meter and an adapter:

http://www.alamomotorsports.com/weber/synchronizers.html

http://www.alamomotorsports.com/weber/STE_40.jpg

 

Out of balance carbs can make one bank run really lean as it attempts to get additional air from the other carb through the balance tube. Air makes it through easily while the fuel has a harder time. THis is most prominent at small throttle openings.

 

You can get an idea of if the carbs are close to balanced by listening to the air flow through a piece of heater-hose held perpendicular to each throat and comparing the sound. I do not trust this method for actually setting the balance. The hose can also be used to listen for vacuum leaks at all joining surfaces.

 

I personally do not have a high opinion of the DGV carbs as they are fitted on the L6. I have had better luck with SUs.

 

Did you rotate the cam to close the valves before you took the wire brush to the head? I doesn't sound like it, but this is what you should have done to protect the valve seats of the open valves. Leak down  test will tell you if there is a problem there. You might want to put a vacuum gauge on it as well to make sure you are still getting steady vacuum.

 

I also strongly recommend getting the following books and using their methods for verifying the cam timing:

http://www.amazon.com/Rebuild-Your-Nissan-Datsun-Engine/dp/1931128030

http://www.amazon.com/Modify-Your-Nissan-Datsun-Engine/dp/1931128049/ref=pd_bxgy_b_img_y

[Xnke]

Doubt all you want, but the evidence I found was clear...the Shell gasoline cleaned away a LOT of deposits when used in the engines I work on. The grungier the engine, the longer it takes, but it will clean 'em up!

 

I would suspect that anything you could get at with a steel bristle brush but not a brass bristle brush is going to require removal of the valves to properly clean off. I might run a quality fuel with a good additive package or an available additive specifically for cleaning for a few weeks before trying to clean them up first.

[djwarner]

When I changed the head gasket the first time, I purchased a Felpro. I was a little suspicious when I pulled the head and found a Felpro already installed looking identical to the new one. I had decided to change the head gasket because of an oil leak between #3 and #4 cylinders and high compression reading indicating excess coke.

 

Sure enough, the new head gasket leaked in exactly the same place, and now I had the lumpy idle.

 

After a lot of research, I finally decided to spring for a NISMO head gasket. I even ended up talking directly with Nissan Motor Sports to determine I needed one with an 85mm bore and 1.2mm installed thickness. PN 11044-E4620.

 

Before pulling the head for the second time, I decided to do another compression check. #3 came in at 125 psi while every other cylinder came in at 190-195 psi. Obviously I had a valve seating problem. With the head off, I removed the intake and exhaust valves and cleaned up the valves and seats.

 

With the re-worked valves and new head gasket, the engine fired right up and idled almost 300 rpm higher. I had no idea just how bad the cylinder was affecting the idle. She now purrs like a kitten. Sam280Z thanks for pointing me in the right direction.

 

Just a heads up for our fellow Z car owners. Nissan Motor Sports indicated they now only had 42 of these head gaskets left in stock. Considering the legacy status of these cars, I suspect there will not be another production run when these are gone.