Calling All Motorheads!

[Fastjeff57]

This SOB of a problem has got me stumped.

Background: Mark 78 converted to late model electronic ignition. Rebuilt the motor over the winter and fired it up. Motor started instantly, but it was running rough, smoking (had lots of oil in the cylinders) and idling at 3K with the throttle set to idle.

What the hell?

Distributor’s gotta be off a tooth, I predicted, but it wasn’t. I then backed the idle stop screw off a TON and fired her up again. No difference!

What the hell?

The carb butterflies gotta be open somehow. Checked and readjusted, fired it up and—no difference; still running rough and screaming at 3K.

What the hell!

Having nary a clue as to what was going on I started messing with the idle adjustments—I had to try something. Still no difference.

While I trying to figure what on earth to do, it suddenly smoothed right out and dropped down to a nice 1K idle.

What the hell!!!

From my 60 years of messing with motors, I've never seen a screw up of some kind (fuel or ignition) cause a motor to rev HIGHER; lower is normal.

The next day I took it to the river. Started up fine, then did the high, rough idle thing again. Rapped the throttle a few times (and good!) and the little b*st*rd suddenly dropped to a nice, 6 cylinder idle at around 1K. Snicked it into gear and she took the gas immediately without a hint of a bog. Ran the boat then and she ran normally for the rest of the day.

So what on earth is causing this crap! I'm at a loss. Thought it might be the switch box (somehow) so I changed it (and the coil late). It was running normal by then so I THOUGHT that had cured it, but the problem returned when I took the boat to the river.

Any suggestions will be gratefully accepted.

Jeff

[racnbns]

Try 24 volts! That's all we ever used on the 6 mercs.

Bruce

[Fastjeff57]

I meant serious ideas.

Jeff

[Master Oil Racing Team]

Have no clue myself, but just some input regarding a whole lost year of racing in 1970. OMC had just come out with the 50,000 volt ignition system with polar gap plugs. That was just a centralized electrode surrounded by the grounded plug. Because my Dad was an OMC dealer he decided that we had to have that system. An OMC employee that raced built a whole kit that included mounting plates,...etc. We were out front many races when it would cut out, then cut back in, but mostly the tiniest bit of spray would shut the engine off. Harry Pasturczak designed the most waterproof intake I ever saw. I think Bill Kurps had one. It didn't work for us, although others seemed to not have a problem. We did everything we could think of to make the motors race. They would be fine, then cut out. It was advertised as 50,000 volts, but I think that if the battery drops below 10 or 11 volts while racing, the spark is weakend and maybe misfires. What Bruce says I think would probably have cured our problems back then, but I don't think that is what is happening to you because of the revs. I'm not a mechanic so what I say is just merely looking at other angles like we had to do when things were totally out of whack trying to figure out the problem. This is just my suggestion as one point to look at. You say you have a late model electronic ignition. I don't know how late, or if it was late model for that motor, or something adapted to make the motor work. If you are able to install the ignition system the same or similar to what the motor came with, that's what I would try first to make sure it was not ignition and something else. The revving makes me think it was some newer system with maybe sensors and stuff. Just my idea, but I really am like you on it not making any sense.

[R Austin]

As has been said, need to know what generation ignition your trying to use. I assume not the ADI system because of the incompatibility of the top main cap and trigger system and you have a timing belt. That leaves either the first CDI with the replaceable trigger and 2 piece shaft or the single piece rotor shaft with the single piece potted trigger in the housing. With all that, need to know what you did with the carb pickup hardware mounted on the distributor for opening the carbs.

The carbs are closed until the distributor is advanced to a specific timing advance position, then opens as the timing is fully advanced and spring loaded linkage allows for complete carb opening after full distributor advance.

Wild idle RPM sounds like carbs are out of their synced position or the carb vertical connections are binding the butterfly shafts.

Alky loopers had to have 24 volts to run clean. Standard system has to maintain 12 volts. Know this sounds lame but check firing order and plug wire position in cap. check timing by placing number 1 piston in the advance side of top dead center firing position. Turn on ignition and with spark board or plug in number 1 wire, and distributor in full advanced position, turn the distributor pulley till number 1 fires place timing belt in this position. If a 2 piece shaft be sure that it is not binding vertically and sticking in different positions. Used this type shaft back in the days and had to weld a nugget in the notch to stop timing changes with rapid rpm changes as prop jumped in and out of the water. Shifting gear case I assume not direct reversing.

[Fastjeff57]

..."I assume not the ADI system because of the incompatibility of the top main cap and trigger system and you have a timing belt."

You obviously know your stuff! That is exactly why I could not go ADI, which I would have preferred to do.

The setup is a mid-70s up in-line fours and sixes clamp on cap distributor. Other than a problem with carb clearance (it's tight!) the system was easy to convert to and eliminates those clunky car coils, ballast resistors, points, etc. It ran super from day one, and NOW is running great again--took her for a spin yesterday and it ran perfectly.

So why the high rough idle that goes away when it feels like (and hasn't returned--for now)? Dammed if I know! Been messing with engine for 60 years and this one has me stumped.

Jeff

PS: FOrgot to add: I converted a Mark 55 type tower and FGS to the power head so I don't "wreck any docks".

[John Schubert T*A*R*T]

..."I assume not the ADI system because of the incompatibility of the top main cap and trigger system and you have a timing belt."

You obviously know your stuff! That is exactly why I could not go ADI, which I would have preferred to do.

The setup is a mid-70s up in-line fours and sixes clamp on cap distributor. Other than a problem with carb clearance (it's tight!) the system was easy to convert to and eliminates those clunky car coils, ballast resistors, points, etc. It ran super from day one, and NOW is running great again--took her for a spin yesterday and it ran perfectly.

So why the high rough idle that goes away when it feels like (and hasn't returned--for now)? Dammed if I know! Been messing with engine for 60 years and this one has me stumped.

Jeff

PS: FOrgot to add: I converted a Mark 55 type tower and FGS to the power head so I don't "wreck any docks".
Boy, the timing belt sure looks loose. I tend to agree with Dick Austin with regards to the 2 piece shaft but more so the sticking of the throttle advance as well. I experienced this myself, but not with CDI conversions.

[Fastjeff57]

..."Boy, the timing belt sure looks loose."

They usually are without the driven pulley!

...."...sticking of the throttle advance as well"

I hand my hand on the distributor body itself as the fool motor was reving like mad at the lower than normal idle position, so that theory can be eliminated.

Not sure what Dick meant by "two piece shaft": there's one solid piece from the pulley to the rotor.

This one's a dilly!

Jeff

[R Austin]

The first CDI came out in 1967. It was a separate unit just like the breaker point section. If you had used one of those all you had to do is replace the shaft in the upper housing. It had a half cut end as did the CDI unit, 2 piece shaft as in the direct reverse breaker unit. All other linkages would have remained the same.
Still think your problem is in the carb connections/linkage/syncro. The ignition will not wander in timing unless the trigger chopper wheel is not fitted properly.

[Powerabout]

Timing light would have been the first tool I would have picked up to check each cylinder after marking the flywheel

[Fastjeff57]

Just learned about that little trick (with the light) or I would have used it.

Motor is now running fine again, on its own. Still suspect the mystery problem will re-occur, but....

Jeff