Intake or exhaust valve stuck after winter storage

[HorizonblueDK]

I'm preparing a friends 20 footer for the season. It is still on land. The engine/drive is a mix of Mercruiser and Volvo Penta.

Engine:
4.3 V6
4 barrel Rochester
Thunderbolt 4 ignition
Crank driven raw waterpump
Fresh water cooling

Outdrive: Volvo Penta 290

This setup ran great last year.

When I startede the engine the first time a couple of weeks ago, a valve was stuck. I could hear that the engine skipped a cylinder, when I was turning the key. And when it startede, it definitely ran on 5 cylinders only. I have since then started the engine a number of times, for a limited time, since I don't have access to a garden hose, where it is now. The impeller is not installed yet, so no damage to the raw water pump.

The last couple of times I've started the engine, it fired up on all cylinders and ran great for a short time. Then it started to stumble, like it lost a cylinder again. When I give it a little gas, the stumble goes away, and the engine seems to run fine again.

At this time I have to stop the engine, cause I feel it's getting warm. So I don't know what will happen, if I let it run.

When I start the engine 1/2 hour later, the same again; starts and runs fine, but then begins to stumble. A little gas and it seems to run fine. Then I have to stop, again because of rising temp.

I would like to find out, if there is anything wrong, before I move it.

So my question is:
Can I rule out that there still is a valve problem, is it a carb issue then?

 

[Don S]

Re: Intake or exhaust valve stuck after winter storage

You are probably burning up the inside of the exhaust hoses that connect the risers to the Y pipe.
Have you done a compression test to determine a stuck valve and to locate which cylinder?
When was the last full tuneup ? Could be just old dead spark plugs, cap, rotor, plug wires, etc.

 

[HorizonblueDK]

Re: Intake or exhaust valve stuck after winter storage

You are probably burning up the inside of the exhaust hoses that connect the risers to the Y pipe.
Have you done a compression test to determine a stuck valve and to locate which cylinder?
When was the last full tuneup ? Could be just old dead spark plugs, cap, rotor, plug wires, etc.

I'll keep the exhaust-hose in mind. I haven't done a compression test. When I tried starting the first time (when the starter turned the engine, before it fired up), it was clear that a cylinder was "missing".

It sounded somthing like this
"Wor Wor We"
"Wor Wor We"
"Wor Wor We"

When I started it today, again, when the starter turned the engine, I could hear from the starter-sound, that all six seemed to have equal compression.

I'll take a compression test anyway, it is good to see the general condition of the engine.

I'm thinking about; is there a chance that the troublesom valve is stuck, then loose, stuck, loose, stuck, loose. etc, until the rust on the valve stem has been "grinded" away?

It has been some time since a tune-up. The ignition parts looks ok, I checked the spark plugs, they were black, but not worn/burnt, the cap looks fine also, no cracks. I actually have some new plugs laying around somewhere, think I'll replace them, don't know how old the old ones are.