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post #1 of 7 Old 08-08-2011 Thread Starter
72 C&C Corvette
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Atomic 4 slippin

Hey all you atomic 4 owners!!! I have an 1972 C&C Corvette and have noticed this season that when under power I have lost a knot or so, crusing at about 4to 4.5 knots. When more throttle is given the A4 will slip out of gear, reduce throttle and it engages again. Any thoughts???
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post #2 of 7 Old 08-08-2011
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I had the exact same problem and followed Don Moyer's tip in the link below to tighten the forward adjusting collar on the reversing gear (transmission):
Moyer Marine Atomic 4 Engine Rebuilding and Parts
It took care of the problem.

Easy to do.

O'Day 30 - Stuart, FL
Oceanis 411 - South of France
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post #3 of 7 Old 08-08-2011
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Found this Doc on the web.....

1) Place the cockpit shifting lever in neutral.
2) Recheck to be sure the reversing gear is in neutral by turning the prop shaft. The
neutral position is at the point where the prop shaft turns most freely.
NOTE: If the forward clutch assembly is not in a good neutral position prior to
adjustment, it will be very difficult to rotate the notched adjusting collar in step 6. It's
frequently necessary to move the shifting lever slightly in the reverse direction for the
reversing gear to free completely.
3) Remove the access plate on top of the reversing gear assembly.
4) Rotate the gear case cluster until the retaining pin of the adjusting collar is facing
5) Loosen the retaining pin until the staked collar can be turned on its threads. It is not
necessary to completely remove the retaining pin from its threads to turn the adjusting
6) Turning the adjusting collar clockwise (as you would be facing the engine from the
rear) will tighten the clutch disks when in forward. As a frame of reference, one notch on
the adjusting collar make a large difference and is usually sufficient to prevent slippage.
7) Retighten the retaining pin.
CAUTION: It is very important that the end of the retaining pin extends into one of the
notches on the adjusting collar before final tightening. If the end of the pin presses on the
collar itself (between notches); or if the pin is simply over-tightened, it is extremely easy
to break the cast iron pressure plate.
8) Place the cockpit lever in and out of the forward detent several times to insure a
proper "feel". A solid detent should be felt while going in and out of forward, but the
adjustment should not be so tight as to cause any concern that the ships cable and levers
may be over stressed.
NOTE: Moving the forward adjusting collar one notch makes a rather profound
difference in the force required to get the clutch assembly into and out of the forward
detent. In some cases (particularly in pedestal mounted shifting levers) one setting can
result in more force than might be desired, while the very next notch looser results in
some slippage of the clutch assembly at high power settings. In the very latest engines
(circa 1979 - 1981), Universal installed forward clutch adjustment collars with notches
closer together to provide more control when adjusting the forward clutch assembly. You
can check the difference in the two collars in our online catalog at,
product number: OREV_05_306.
9) If, after readjusting the forward clutch assembly, the neutral position of the shifting
lever in the cockpit is in an awkward location, you can adjust the cable shackle at the
engine, or cockpit shifting lever, until the cockpit lever is in a more natural neutral
1) When the forward mode adjustment is correct, recheck the reverse mode for proper
adjustment. There should be a well defined neutral range when coming out of the
forward detent, and reverse mode should be felt comfortably before the shifting lever in
the cockpit reaches the limits of its rearward travel.
NOTE: There is no detent in the reverse mode.
2) If the shifting lever in the cockpit reaches the limits of its travel before reverse mode
is securely established, turn the 3/4" hex-headed nut of the reversing brake band
3) If the reverse mode is reached too soon, and/or the neutral zone is so small that it is
difficult to find a spot where the prop is not turning (one way or the other), turn the
adjusting nut counterclockwise.
NOTE: It is not necessary to remove the retaining spring in order to turn the nut on the
reversing band adjusting bolt.
By way of background, pedestal mounted shifting systems typically have somewhat less
cable travel than those which are mounted on the side of the cockpit. This makes them
very prone to having problems associated with being able to reach both forward and
reverse, and still have a reasonable neutral zone.
It's very important that the cable assembly is adjusted so that you're able to engage the
forward detent near the end of the travel in the forward direction. This adjustment is
necessary so that you will have sufficient travel in the rearward direction to accommodate
reverse, and still have a reasonable neutral zone between forward and reverse.
In the past, we have seen several pedestal mounted systems where the range of cable
travel had shifted so far in the forward direction, that there was barely sufficient travel
remaining to reach the forward detent before encountering stops within the cable system.
In this configuration, whenever the forward adjusting collar is set to provide a "stiffer"
adjustment, the additional force required to get the reversing gear into the forward detent,
results in the cable system reaching the limits of its travel before the detent is reached.
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post #4 of 7 Old 08-08-2011
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If you don't have Moyer's A4 manual, that is a good resource.
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post #5 of 7 Old 08-08-2011
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You might find that after following the instructions that it is harder to get it into/out of gear. That's probably normal. I adjusted my '72 one notch at a time and it took two notches so that it would not slip. However, it's MUCH harder to get in and out of gear. I think that is normal.
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post #6 of 7 Old 08-08-2011
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You might want to check your prop for growth. If it cavitates, it will feel like the tranny has slipped out of gear, and dropping the rpms will let the prop grab again, making if feel like it's back in gear.

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post #7 of 7 Old 08-09-2011 Thread Starter
72 C&C Corvette
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Thanks all for the tips, they are very helpfull. I think I'm going to get a little oily this weekend, Its time for a little surgery on the ole girl. I will let you know how I made out
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atomic 4 slippin

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