Cylinder sucking noise

[sgd37]

Hi all,

After going back and forth with this engine I discovered that my number 4 makes a sucking noise when cranked which the others don't seem to make. This is also present when running. I took a compression test and it is similar to the others. Could it be my intake valve not closing properly?

I took a video of cranking it by hand
https://youtu.be/5lNVgVJgoUU

Would love to know what the experts think

Cheers

[kevski]

Your video uploading sucks, so cannot comment on it, but i have heard this type of noise before on various engines from lorries to bikes and never had any running problems.

[sgd37]

Sorry I had it as private initially. I changed it to public, so should be fine now. Good to know that it is common to other engines (this is my first one , but please have another look.

Thanks

[sgd37]

Also is it normal to have some air blowing back out the intake?

[Trigger]

Also is it normal to have some air blowing back out the intake?

It needs to suck to draw fuel and air mixture from the carbs  It should not blow back, did you lap all the valves in and do a leak test while you had it stripped ?

[kevski]

Now that doesn't sound quite right, check tappets to start with, then leak down test.

[K2-K6]

There is an overlap period on camshafts with both inlet and exhaust open at the same time that could do this.

You don't usually hear much with carbs fitted,  it could be normal but most people don't usually listen to them without.

[Trigger]

There is an overlap period on camshafts with both inlet and exhaust open at the same time that could do this.

You don't usually hear much with carbs fitted,  it could be normal but most people don't usually listen to them without.

Yes, there is a overlap but, strange as he says it is only on one cylinder. Did have a head in once with a chipped valve and it whistled 

[sgd37]

I put a flame next to each inlet and they all seem to have some blow back.

So I took the head cover off and refit the cam sprocket just to make sure it was timed correctly. While the rockers were off I covered the top of the valve with wd40 and compressed the cylinder, and no bubbles were visible so given the excellent condition of the valves (plus lapped and leak test) when the engine was stripped I assume it's ok. Readjusted the tensioner, because the manual said that a loose one can change timing, and reassembled. She sounds better now with four distinct sucks and from what I can tell no audible blows, so hopefully that's shook the bugs loose. Might have been sticky valves or tight clearances, I don't know.

I'll put my carbs back on and try to sync again. By the way is she supposed to eat a lot, she went through liter quite fast at idle.

Thanks for all your advice