Port idle conking out

Where did the 3 quarts of oil go? If they're not in the bilge, you've got a serious problem with that recently rebuilt engine....... Either a dead/bad cylinder with low compression causing the oil to burn and the motor to not have power to pull a load at idle and dieing.......I'd start by doing a compression check.....Is there water in the oil?

What caused it to need to be rebuilt? How old are the exhaust manifolds and risers?

I'm guessing you might have been lax in checking your oil...until it needed 3 quarts. As for power and stall problem, check your alternator. Sounds to me like you have an electrical problem somewhere.

I think the engine is idling fine but issue is just in gear?

Easy things first.

Did you verify nothing is wrapped around the prop that could be causing the extra load to stall it? I would think you would have felt a vibration at cruise but who knows. Were you using a synchronizer at the 3200 cruise? If so, that could be almost full throttle with the extra load and you don't even know it.

While under the boat, spin the prop and make sure it moves normally for your boat.

Just came to my mind from out-there but something to check.

You may have already oil fouled a spark plug. Cylinder not igniting so no blue smoke. But you're running a V-7 or V-6 now.

With more solid motor mounts, sometimes it's hard to feel a mis fire in a boat.

>>>While under the boat, spin the prop and make sure it moves normally for your boat. <<<

Not familiar with this boat, but couldn't you just grab the coupling at the transmission and see if you can spin the shaft? If you are already under the boat it should be pretty obvious visually if something is wrapped around the prop or shaft.

Bob

It could also be ignition timing......

Hi,

Do you know if the plug that was replaced actually sparks?

The lead or distributor cap might be the culprit.

Before whipping the valve covers off run a compression test on the cylinder in question.

If you can change the sparkplug, you can get the flexible rubber compression tester hose screwed into the spark plug hole which is easier than threading the spark plug in there. That doesn't sound right. You can also plug the spark plug hole with your finger and should feel if there's enough compression for the cylinder to fire or very little......make sure the coil is unplugged when you do that.

750 cfm on a 350 SBC?
Rebuilt Chevys? Bent push rod or broken valve spring.
I don't understand his reasoning for not pursuing a few more easy checks. Looking forward to the results.

Hi,

Where do you get the idea that test one if its ok the rest must be too?

If I had even suggested such a thing to my foreman when doing my apprenticeship I would have been immediately transferred to the lawnmower shop.

you will have to expose all that gear if you do lift the head so a compression test is no major deviation, if it tests low and a bit of oil down the spark plug hole improves it you can fairly safely say rings are at fault, if no diff valves are likely.

That is still an oversized carb for a SBC on regular gas. Interesting.

You have a cylinder that is not firing. Supposedly it's not a plug. He can change a plug but not run a compression check on the suspect lung.

Get another mechanic before he takes any more of your time and money.

Basically five things are necessary to run an engine:
fuel
air
compression
spark (not on a diesel )
timing (spark and engine internal)

Also:
fuel air ratio
mechanical integrity of the engine


As I've understood the problem:
It's a gas engine with carburetor and distributor, a single cylinder misfire, the affected cylinder has spark.
A rebuild has been mentioned, the engine did work after it.

The usual suspects are:
no spark, already ruled out
not enough fuel, ruled out because carburetor instead of injectors, would effect more than one cylinder in this case
timing problems (spark OR engine internal)
Spark timing is unlikely, it should affect more than one cylinder.
Engine internal timing, say a skipped timing belt or chain, is unlikely as it should affect more than one cylinder.

low compression
Still unknown, compression testing an unaffected cylinder does not really help this case.

Also possible: Engine internal mechanical failure. Say a stuck valve as it would affect only one cylinder.


The question in diagnosis is always: What is the next easiest step?
I think compression testing the affected cylinder. If you can change a spark plug you can screw in a compression test hose.

Alternatively, using specialist equipment and expert knowledge, using an in-cylinder pressure transducer and graphing the compression curve of the affected cylinder and probably one other. This can give a lot of insights without opening the engine. (Mostly timing and compression but also much more obscure things like vale seat wear.)
Needs similar access as the normal compression test. This is certainly not a go / no go test, interpretation of the curves is key. Thus the need for someone with actual experience in this field.
(Probably way too much here, throwing it in as option for other cases.)


Also, there was an oil loss. Did it persist? Did the spark plug of the affected cylinder show signs of oil contamination while the others did not? Maybe even serve contamination?
That would indicate a mechanical problem, say piston rings.
So one could go for broke and and pull the cover off - if it's necessary in order to remove the engine to get at the pistons from below.

But even in that case, the compression test should be quick and only reinforce the decision to pull the engine.


The rebuild was carburetor only?
If it was also an engine rebuild don't forget to take a look at warranties that may have been included.