Donman

Engine- 1986 Mercury 420, stock.

Port engine popped through carb when I was getting on plane. I left off, it idled fine. I took off again and was running steady 3500 rpm and started popping again. I stopped. Engine still runs great, idles smooth, but now making a ticking sound on right bank, not a rocker or lifter, but deeper, like a wrist pin or valve. Just a tick, not a knock. I drove in on starboard engine and finally got a chance to look at today.
Constant ticking noise at idle. Gets louder / faster, consistant with rpms. Engine runs fine otherwise, no miss or rough idle. Revs up good.
I revved engine hard several times trying to listen / diagnose. As I was doing this, ticking noise went away, but now has popping through carburetor. It has a constant pop-pop-pop through carb at idle and gets worse as you rev it.
The engine seems to idle and run fine, but this popping is constant, like somebody is hitting underside of intake with a hammer.
Is this a broken valve spring ???

I need help. Give me your thoughts please.

TwinTurboStroker

Possibly..

You can compression test that bank to isolate the cylinder.

I'd pull the VCs to get a closer look.

Popping through carb is a valvetrain problem (so long as firing order is in check).

Donman

I`m planning on pulling the valve covers tomorrow. I gotta get the manifolds out of the way first....

I figure if it`s a valve spring, I`ll replace the whole set and probably do the other engine too.

Before removing parts, is there anything else I should look for ?

boatnt

you can check it with a vacoum gage

Donman

I`ve got a vacuum gauge. What should I look for ? Bouncy reading ?

John B

You may have a bent or broken valve. Check the height of all the valve stems when the valves are closed. I believe the 420's have dual valve springs, one should close the valve at slow speeds. A compression test done and repeated with the rocker arm removed should indicate a problem requiring the head to be removed. It could be a valve seat problem.

A vacuum test should show a fluctuating reading. Let us know it you see this.
JB

boatnt

bouncy reading at idle

Griff

Could also be a flat cam lobe.

stevesxm

if you THINK it is a valve spring... if that even OCCURS to you... do not turn the enginge again for any reason... forget all this vacuum gage nonsense and just pull the vc and check....

think about it.... IF it is a spring and it IS broken... you have pieces floating around and the REAL possibility that the VERY next time you rotate it the you will lose the keepers and the valve will fall in ...WHY in gods name take the risk ? there was a post here a couple weeks ago by a guy who said he heard a suspicious noise in his motor so his diagnostic procedure was to pull MAX revs at MAX load ....

geeze... have some mechanical sympathy . you are DEAD right so far... you caught it early and have probably avoided major damage... don't screw up now. pull the vc's and look.

whats the downside ? if you are wrong and the springs are good ... thats great... youve wasted some time and a few gaskets... but if you are RIGHT then your conservative and intellegent approach has saved you a world of grief and a lot of money...

i don't remember reading ANYWHERE that if you suspect something bad mechanical is wrong, that you should continue running and turning the motor until it breaks for a confirmation that your intuitions were correct.

TwinTurboStroker

Ah, the "Trial by Fire" method of tuning and diagnoses. This is about as effective as pouring fuel on a stack of $100 bills and igniting them.