Visit our Popular Forums

hobopacket · 15-05-2017, 15:19

Hey all,
Posting in other forums too but like the wealth of knowledge here too

My gm 350ci 260hp mcm 1983 vintage is running rough after it warms up. Runs great all through throttle range until 10ish mins and is like it's missing a cylinder very few revs. The Rpms surge.

Any ideas? I just did the cap, rotor, plugs and plug wires as I thought they might be the culprit. But, alas. Does this engine have a ignition module? If so. Anyone have any ideas on how to test?

Thanks!

**Pete7** · 15-05-2017, 15:40

Best guess, carb jets gummed up perhaps, or a problem with a valve seating?

Pete

**a64pilot** · 15-05-2017, 15:41

I'm sure it has a module, and I too suspect ignition, perhaps coil as they usually work fine until hot.

hobopacket · 15-05-2017, 16:10

I had an old is Zouzou with us in line four. When the ignition module got a hot after running for the while it would run rough how do I replace the module. Possibly the same thing here? Anyone know how to test coils and ignition modules?

dave948 · 15-05-2017, 17:36

Pull plugs 7&8 (back plug on each side) and see if one is really clean. Very common on these engines for the eyebrow shaped port at the back of the head to rot threw. T-stat opens water sprays in cylinder and you get a miss fire. Plug gets steam cleaned.

jeepbluetj · 15-05-2017, 17:49

I'd guess the '83 vintage it's got points, condenser and a ballast resistor or wire. I've got a friend with a late 80's skipjack with the 260 and it has points. OTOH, if you just changed the rotor, it should have been obvious if it had points. Also, even if the engine came with points it may have been changed to an aftermarket electronic ignition somewhere along the line. (GM HEI could be a pretty easy retrofit, but that would have been obvious during the cap/rotor change too)

Coils often go bad when they get hot. Coils go bad very quickly if they were supposed to
have a ballast and somebody rewired around it. A completely failed coil is testable with an ohmmeter, but for something that's failing dynamically that won't work. Luckily, coils are cheap.

An ignition scope is very very very useful to diagnose this type of issue. Unfortunately, most of us don't have one.

If it's points, a dwell meter is useful. If it's a module, it may just be replace parts till it's fixed.

You can tell if a cylinder isn't firing by pulling the plug wire and listening for the miss. No change, that one isn't firing. (I use a glove, some folks are braver than I)

On a mercruiser, another idea is to temporary bypass the shift cutout switch. As that dang thing causes all kinds of issues. If it's going off due to vibration it'll cause the engine to stumble.

hobopacket · 15-05-2017, 18:10

Quote:

Originally Posted by dave948

Pull plugs 7&8 (back plug on each side) and see if one is really clean. Very common on these engines for the eyebrow shaped port at the back of the head to rot threw. T-stat opens water sprays in cylinder and you get a miss fire. Plug gets steam cleaned.

Interesting. Didn't think of that, didn't realize that was an issue. Happened on all the gm 350 blocks around this time? First I have heard of it. Of course this engine was brand new on a shopping crate when I was born

hobopacket · 15-05-2017, 18:27

Quote:

Originally Posted by jeepbluetj

I'd guess the '83 vintage it's got points, condenser and a ballast resistor or wire. I've got a friend with a late 80's skipjack with the 260 and it has points. OTOH, if you just changed the rotor, it should have been obvious if it had points. Also, even if the engine came with points it may have been changed to an aftermarket electronic ignition somewhere along the line. (GM HEI could be a pretty easy retrofit, but that would have been obvious during the cap/rotor change too)

Coils often go bad when they get hot. Coils go bad very quickly if they were supposed to
have a ballast and somebody rewired around it. A completely failed coil is testable with an ohmmeter, but for something that's failing dynamically that won't work. Luckily, coils are cheap.

An ignition scope is very very very useful to diagnose this type of issue. Unfortunately, most of us don't have one.

If it's points, a dwell meter is useful. If it's a module, it may just be replace parts till it's fixed.

You can tell if a cylinder isn't firing by pulling the plug wire and listening for the miss. No change, that one isn't firing. (I use a glove, some folks are braver than I)

On a mercruiser, another idea is to temporary bypass the shift cutout switch. As that dang thing causes all kinds of issues. If it's going off due to vibration it'll cause the engine to stumble.

Pretty darn sure its points and condenser. I haven't seen the condenser but it has the points on the rotor.

I am going to inline test all 8 plugs to make sure I am getting spark to each and a steady spark. I will report back. I will snap some photos.

hobopacket · 16-05-2017, 04:48

Do some other folks have mentioned a fuel issue, as a clogged filter or clogged vent line. Hmmmm. More digging to do.

**a64pilot** · 16-05-2017, 05:09

Quote:

Originally Posted by jeepbluetj

On a mercruiser, another idea is to temporary bypass the shift cutout switch. As that dang thing causes all kinds of issues. If it's going off due to vibration it'll cause the engine to stumble.

I had a 1984 Yahama FJ 1100 in 84. After having it about a year it would "bust up" at high RPM, took me forever to figure it out.
The FJ 1100 has a side stand safety switch, the engine would not run with it down, anyway at high RPM the vibration was causing this switch to close intermittently, killing ignition for milliseconds and making it miss badly of course, ran perfectly at lower RPM.

If it is water ingestion, you need to address that ASAP, pull those plugs right away and check

hobopacket · 16-05-2017, 06:06

For water ingestion. After I pull plugs, what do I look for? I will check the neutral switch too

Mistress Sirena · 16-05-2017, 08:33

Quote:

Originally Posted by hobopacket

Hey all,
Posting in other forums too but like the wealth of knowledge here too

My gm 350ci 260hp mcm 1983 vintage is running rough after it warms up. Runs great all through throttle range until 10ish mins and is like it's missing a cylinder very few revs. The Rpms surge.

Any ideas? I just did the cap, rotor, plugs and plug wires as I thought they might be the culprit. But, alas. Does this engine have a ignition module? If so. Anyone have any ideas on how to test?

Thanks!

Most likely ethanol poisoning pull the carb and rebuild.

eddiej82 · 16-05-2017, 08:42

I have one and it did the same thing you are describing. Turned out to be the carb. Bought a rebuilt marine carb on ebay and it runs perfectly now. You could check the filter at the front of the carb or the water separator filter ( mines attached to the front and looks like an oil filter) and another area that gave me fits that is not documented anywhere is the tube that screws into your fuel tank that picks up the gas has a very small filter that looks like a pipe filter inside it that can clog. Mine did. Just push it out and replace if you want or leave it out. There are enough redundant filters to catch any debris. If your ignition sys will crank the boat it is probably not the culprit. They usually work or don't on these year models. Good luck.