Thread: Engine Miss Puzzler...

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Hi All,

I'm a marine mechanic dealing with a odd engine miss. I'm working on a 1978Mercruiser with a 305 SB stocker. I was asked to do a tune-up because the engine was running rough.

What I have found is the engine has a miss at cylinders #4 and #6 at idle. I checked the compression and all the cylinders are about 150 to 160psi. I changed the spark plugs and check the plug wires for proper firing order,( At plugs and distributor cap) and resistance, all were okay. The old plugs in those cylinders were black. I tried another set of new plugs in those cylinders with no change.

I sprayed around the manifold & carburetor with carb cleaner looking for vacuum leaks, no leaks were found.

It has a 2 bbl Rochester which I took apart and inspected and didnt find any noticeable problems. I flushed with carb cleaner and blew out the passages with compressed air. Everything seems to be working on the carb.

I then test ran the engine on our dyno and the engine is producing the correct power output and reaching maximum rpm.

So other than the miss at idle, the engine seems ok.

Any ideas?

Carlos G

i would replace the plug wires just to make sure.

Vac advance have a hole in the diaphram? Pinch off the hose, if miss goes away, you found it. Sniper

Plug wires would be my first move, too. Could be a valve guide, or even a loose seat insert if the engine has 'em. How many hours on it?
4 and 6 are side by side, and on the same half of the intake. Just thinking out loud here, but could coolant be seeping into them? Head gaskets can leak without losing compression.

Hi All,

Mercruiser marine engines of this vintage operate with copper wire core spark plug wires. If you check for resistance you should find zero. I checked the wires and they were both good. Wiggling and shaking the wires did not produce any change in continuity.

There were no burn thru marks on the insulation of either of the wires. I pulled the boots from the plugs and distributor and was getting about a 1/2" spark jump at both points.

Distributors on most nearly all marine I/O engine dont have vacuum advance and rely on the distributor centrifical advance.

This engine relies on fresh water cooling to keep the heat under control. Water usually enters into the oil and turns it milky should the head gasket leak.

As far as the number of hours this should be a fairly low time engine, > 100hrs I suspect.

I think a leaking cylinder head gasket would show up as a loss of power on the dyno but, stranger thing have been known to happen...

If the gasket is just seeping it won't show up in the oil. Difficult on a boat, maybe, but water will show up in the exhaust gases if you put a cold piece of glass near the outlet. Much easier with individual pipes, of course. Or antifreeze, it stinks. Hard to find a lakefull of the stuff, though!
Just the fact that they are side by side keeps pointing back to the head gasket.

If the valve train has an adjustment feature you may find #4 & #6 too tight.

Especially if you're getting a light popping at idle.

Originally Posted by R Pope

If the gasket is just seeping it won't show up in the oil. Difficult on a boat, maybe, but water will show up in the exhaust gases if you put a cold piece of glass near the outlet. Much easier with individual pipes, of course. Or antifreeze, it stinks. Hard to find a lakefull of the stuff, though!
Just the fact that they are side by side keeps pointing back to the head gasket.

Just have to put a set of " Ears " on the motor, and connect them to a garden hose. they will provide the nessasary water supply to the engine w/o having to be in the lake.

I just retired from this industry, and I am leaning toward what R Pope is saying. When you have two adjoining cylinders missing at only idle, it could be telling you that there is a little coolant getting into each cylinder. The reason you only notice it at idle is because at higher rpms the engine can burn off the water easier and the plugs won't be fouled so easily. I had a 3 liter Mercruiser that only fouled # 4 cylinder at idle, at more rpms it ran great. Turned out to be a cracked exhaust manifold.

I would suggest a leakdown test and also checking the exhaust manifolds and risers to see if they are bad. Sometimes a leakdown won't even show it as the water only gets sucked in under compression, like when the engine is running.

Also look for tell tale rusting on those two plugs. Could be something else, but I am betting a new head gasket or manifolds will be the fix. Here in Florida manifolds only last about 3 years typically due to the salt water, so we change these and head gaskets quite a bit. Let us know what you find out.

Don