Need help identifying pinging sound cylinder 3 Options

[nditiz1]

I recently adjusted my vales .06/.08 and also rebuilt my carbs. They were done separate and driven in between. I'm now noticing a pinging sound coming from the carb barrel on cylinder 3. If I put my hand over the carb throat it makes the sound go away. It is doing this at idle of ~950. I will load a video of the sound. What do you all think?

[nditiz1]

Here is the link to the clip:

https://youtu.be/aSAyKNDn2y0

[DM_2000]

I'm on a slow internet connection at the moment so can't call up the vid.

Do a leakdown test and see if air is leaking past the intake valve.

[nditiz1]

Let me know what you think after you watch the video. I just ordered a leak down tester as I didn't have (IMG:style_emoticons/default/rolleyes.gif)

[DM_2000]

In the mean time, are you getting fuel standoff above the # 3 carb inlet? Fuel standoff is when a cloud of fuel vapor is above the carb inlet. This is caused by pressure under the carb rather than vacuum. ( leaking intake valve , sticking valve stem )

When you do the leakdown, actual numbers are less important than where the air is leaking out. I use the hose from my compression gauge after I've removed the check valve and apply ~ 100 PSI not caring about numbers. In your case, compare numbers to the good cylinders.

Starting with a new engine and working towards many miles

Past rings into the crankcase, normal , gets worse with more miles

Past exhaust valve , signs of wear but still acceptable if slight, gets worse with more miles, tight valve adjustment.

Past intake valve, this will reduce power / lead to a rough idle and is a sign of trouble that needs fixed.

Into the cooling system ( radiator ) needs attention ( yes I know it is AC motor)

Leaking between head and cylinder, the AC version of a cylinder to cooling system leak

[porschetub]

Read this and thought the noise may be lean spitback from that cylinder,you mentioned putting your hand over the throat it go's away,by doing that it points to a lean condition in that cylinder,have you balanced the carbs and set the mixture screws correctly....just asking.
Valve setting and timing is it correct also ?
Good luck.

[nditiz1]

I did adjust the mixture screws and synched the carbs. That carb barrel did need me to set the idle air screw where the other 3 did not. I did this to sync carb barrels on the same carb with each other. The other 3 carbs have the screws bottomed out per Weber adjustments.

I did not check the timing as it was set before I adjusted valves and rebuilt carbs. I will double check it is still correct.

[nditiz1]

After doing some more reading, I realized I did not properly set the air bypass screws to the recommended 1/2 half turn out from bottom. I will reset those and redo all the sync and mixture again to see if that changes anything. In theory if I did create a lean condition on cyl 3 richening the mixture would create the pinging to go away. Thoughts?

[nditiz1]

Readjusted the idle bypass screws on all to 1/2 turn out. Cylinder 3 carb barrel still needed more turns to have both barrels in sync. Resynced left and right and readjusted mixture screws. Still no change with the sound. It also seems to be more prevelant when the motor is warmed up. Next need to do a compression check and a leak-down test.

[rhodyguy]

Check the flow on each Venturi with all 4 air by-pass screws fully seated. The CB suggests the same partial backing off seated but my previous suggestion takes human error out of the equation. A little goes a long way. If you still see a drop on #3 it might also be a vacuum leak. How old are your gaskets, head-intake-carb, and how many times have they been reused? For better flow meter readings turn the idle speed up to the point where pulses don't bounce the needle all over the place. For my Uni-syn I spin the turnable rubber base and make sure the side ports are sealed. You should be doing this with the linkage disconnected.

[nditiz1]

@rhodyguy

I originally had all seated. I then noticed #3 sync was not in line with #4. #3 needed the idle bypass to be adjusted out to bring it up to the level of #4 barrel.

Then, I redid everything with 1/2 turn out on all. Again #3 needed more to get it in sync with #4.

I am using the snail syncrometer. I have a uni-syn if that might be better.

I did just check the compression with a cold motor, plugs removed and at WOT. Here are the results:

Cyl 1 - 130
Cyl 2 - 138
Cyl 3 - 132
Cyl 4 - 138

Maybe I'm just paranoid and the sound in the engine bay coming from cly 3 is normal. I will double check my valve adjustments as well since the carbs and the valves were the only things I touched on this motor since purchasing the car.

[rhodyguy]

Try going the other direction. IN not out. It's an air by-pass. Backing a screw out will lower the needle reading. My snail type is the STE brand what is what I was referring too. I should have been more specific.

[nditiz1]

Try going the other direction. IN not out. It's an air by-pass. Backing a screw out will lower the needle reading. My snail type is the STE brand what is what I was referring too. I should have been more specific.

I'm confused.

With all 4 bypass screws set to 1/2 half turn out #3 was showing 4.5 flow and 1,2,and 4 were showing ~4.9. I adjusted #3 out to bring it up to 4.9. I guess I could adjust #4 bypass in to bring it down to 4.5 and then adjust the idle speed on 1+2 carb down. This is only possible if I can go in far enough on #4 bypass to get it down to #3 barrel.

[porschetub]

You mentioned the carb on the effected side won't balance as the other setting wise,its not your compression because that is great.
Appears you have a weaker vaccum signal on that side ...must be leaking air ?.
I have never liked the thick gaskets supplied with some of the air cleaner kits,they tend to settle in and the manifold can leak @ the head,worth checking?
Good luck.

[nditiz1]

You mentioned the carb on the effected side won't balance as the other setting wise,its not your compression because that is great.
Appears you have a weaker vaccum signal on that side ...must be leaking air ?.
I have never liked the thick gaskets supplied with some of the air cleaner kits,they tend to settle in and the manifold can leak @ the head,worth checking?
Good luck.

Maybe I do have an intake leak. When I rebuilt the carbs I put new carb to intake gaskets on, but not new intake to head gaskets. The reason being I never removed the intake manifold. I will first put the plugs back in and check for in intake leak with some starter fluid around the number 3 manifold. Anyone know if the gasket for the head to manifold is included in the Redline weber rebuild kit?