Random Misfire while cruising

[Mark Maras]

Take a look on YouTube. There are a lot of excellent videos. Search "240Z valve seals".

[Dave WM]

13 minutes ago, jalexquijano said:

Ok. Dday is tomorrow. Im removing the valve cam and connecting an air compressor to cylinder number 4 to keep the valve from falling. Any caution or consideration i should take in the process. I really need a guide in order to get this thing right change the seals and avoid opening any pandora box, sort of a step 1 , 2 and 3 guide.

You may find the air pressure to hold the valve closed does not work. You can use some rope stuff in the cylinder as well. If you do try the air pressure, make sure the car is in gear and the Emergency brake is set.

[jalexquijano]

Finally took the car to a friend's garage to change the valve seals on cylinder 4. He argues that its not oil but gasoline on the spark plug ring and that its due to the lack of vacuumn at such cylinder. This doesnt make much sense. Nevertheless, we will change the seal to see whats going on inside.

[jalexquijano]

5 hours ago, jalexquijano said:

Finally took the car to a friend's garage to change the valve seals on cylinder 4. He argues that its not oil but gasoline on the spark plug ring and that its due to the lack of vacuumn at such cylinder. This doesnt make much sense. Nevertheless, we will change the seal to see whats going on inside.

Just changed but seals. Lets see the outcome. I did not noticed the seals broken.

[Patcon]

It probably wouldn't be broken. Could have a little nick or slight cut. That's all it would take...

[jalexquijano]

Same plug still black. So i lost time and money. Time to pull the cylinder head out?

[Patcon]

9 minutes ago, jalexquijano said:

Same plug still black. So i lost time and money. Time to pull the cylinder head out?

What would pulling the head accomplish?

[Mark Maras]

 A couple of questions? Did you use a straw over the valve stems to prevent damaging the new seals? Are those plugs a new set? Don't pull the head yet. You need a good reason to do that. If you're absolutely positive you didn't damage the new seals, I'd do another compression test, dry and wet.

[jalexquijano]

3 hours ago, Mark Maras said:

 A couple of questions? Did you use a straw over the valve stems to prevent damaging the new seals? Are those plugs a new set? Don't pull the head yet. You need a good reason to do that. If you're absolutely positive you didn't damage the new seals, I'd do another compression test, dry and wet.

Yes and Yes. Car does run better and no backfire in 2nd and 3rd gear when lifting the gas pedal. The owner of the shop who assisted knows a good deal of mechanics and used to race car a modified 240z in the 80s with triple solex carburetors. He  recommends the following:

1. The damper must be changed to determine if the timing is correct. 12 BTDC was recommended. 

2. There is something going on in the balance tube which causes the 4th cylinder to turn out black compared to the other ones. Still to be determine swapping the vacuum hose to other port in the balance tube, if possible. 

3. There is some slight play on the links that connect both carburetors together. He believes it should have no play and when you accelerate both carburetors should open exactly at the same time. 

Any recommendation on your behalf is highly appreciated. 

Regards,

Jorge

[jalexquijano]

On 8/14/2021 at 5:36 PM, Mark Maras said:

 A couple of questions? Did you use a straw over the valve stems to prevent damaging the new seals? Are those plugs a new set? Don't pull the head yet. You need a good reason to do that. If you're absolutely positive you didn't damage the new seals, I'd do another compression test, dry and wet.

Hi Mark,

I did noticed yesterday while cruising the car and then keeping the car in idle for 15 to 20 minutes that temperature raised to 3/4 of the temperature gauge. Mechanic argues that water should run through the rear of the carbs. This is non sense as i dont have the heater installed any longer and the manifold valve. This was shut down to reduce heat.

Should in purchase and install a new damper and verify timing is at 12 btdc? I still wonder why plug is turning black at idle.

[Mark Maras]

 I wouldn't worry too much about the rise in heat after that much idling. The carbs don't need to be connected to the cooling system. The early four screw carbs had no cooling connection. I'd determine if the dampener has slipped first before buying one. You'll need to determine the exact position of TDC on #1 cyl. There are a number of ways of doing this. A M14-1.25 bolt screwed into the #1 spark plug hole (when the piston is down) will act as a piston stop. SLOWLY bring the piston up until it bumps the bolt. Mark the dampener at the timing tab TDC mark. Turn the engine the opposite direction (piston will go down and come back up) until the piston LIGHTLY bumps the bolt again. Mark the dampener at the TDC timing tab again. Actual TDC is halfway between your two marks which should be the same as the factory TDC mark on the dampener. That's a brief explanation. I'm sure there are plenty of videos on Youtube that explain it much better. I don't see how the #4 fouled plug can be balance tube related or carb related. That's why I'd do yet another dry, wet compression test. Don't ask why, it's an ugly subject and I sooo want to be wrong.

[jalexquijano]

5 hours ago, Mark Maras said:

 I wouldn't worry too much about the rise in heat after that much idling. The carbs don't need to be connected to the cooling system. The early four screw carbs had no cooling connection. I'd determine if the dampener has slipped first before buying one. You'll need to determine the exact position of TDC on #1 cyl. There are a number of ways of doing this. A M14-1.25 bolt screwed into the #1 spark plug hole (when the piston is down) will act as a piston stop. SLOWLY bring the piston up until it bumps the bolt. Mark the dampener at the timing tab TDC mark. Turn the engine the opposite direction (piston will go down and come back up) until the piston LIGHTLY bumps the bolt again. Mark the dampener at the TDC timing tab again. Actual TDC is halfway between your two marks which should be the same as the factory TDC mark on the dampener. That's a brief explanation. I'm sure there are plenty of videos on Youtube that explain it much better. I don't see how the #4 fouled plug can be balance tube related or carb related. That's why I'd do yet another dry, wet compression test. Don't ask why, it's an ugly subject and I sooo want to be wrong.

I pulled out plugs 4 5 and 6.