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1995 120hp V4 johnson Gearbox Causing issues.. BIG KNOCK at full speed!

G

Guyph_01

Contributing Member
Hi Guys,

More issues to solve and I once again ask for help:)
I just purchased a boat and its fitted with a 1995 V4 120hp Johnson, J120TXARC, G 03593832. Before purchase we went on a quick run and everything was normal, the next weekend I took it out again with friends, at full speed and with 6ppl pushing a 19ft Caribbean Intruder hull it gave me at two times this big knock, both feeling it through the hull and hearing it from the engine like the boat/engine had hit a submerged object.

Brought the boat home, checked the oil and its blue and that as far as i got. From asking around I was told maybe cable adjustment or the dog clutch is failing.

Any though and advice? what can i do to pin point the issue and fix it?

Thankyou
 
Step[ # 1 is to remove the lower unit.----Then put the lower unit in neutral.----Measure the distance from mounting surface to the center of the pin hole on the shift rod.----It needs to be at a certain distance or shifting issues can occur.----Post that distance when you get it.
 
I assume that you did not encounter this "knock" (jumping out of and back into gear) on your test run with the previous owner? And if not.... was the rig held at full throttle long enough for it to take place? Possibly the previous ower was operating the rig and avoided holding the engine at full throttle for obvious reasons?

The "knock" as you call it is the sudden jar of the engine on the transom jumping back and forth as the clutch dog jumps out of the lobes of forward gear and back in.

(Jumping Out Of Gear - Manual Type)
(J. Reeves)

This pertains to lower units on all OMC manual shift outboard engines, or any OMC engine with lower units defined as a Shift Assist or a Hydro Electric Shift unit which incorporates a "Shifter Clutch Dog".

Within the lower unit, splined to the prop shaft is what is most often referred to as a clutch dog, hereafter simply called dog. The dog has at least two lobes protruding from it on both ends, facing both forward and reverse gear. The forward and reverse gears also have lobes built into them near their center area. When the engine is running, in neutral, the gears are spinning constantly via the driveshaft being connected directly to the powerhead crankshaft, but the propeller does not turn due to the fact that the dog is centered between the two gears, and the dog lobes are not touching either of the gear lobes.

When the unit is put into either gear, shift linkages force the dog (and its lobes of course) to engage the lobes of the gear. The lobes of the spinning gear grab the lobes of the dog, and since the dog is splined to the prop shaft, the propeller turns.

The lobes of the dog and gears are precisely machined, most with right angled edges that could be installed in either direction, and some with angles slightly varied that must be installed in one direction only (one end only must face the propeller). Dogs that can be installed in one direction only, if reversed, even if the dog and both gears were new.... would jump out of gear almost immediately. Keep in mind that the lobes are precisely machined with sharp angles!

Due to improper adjustment or worn shift linkages, but usually due to improper slow shifting, those precisely machined sharp edges of the lobes become slightly rounded. Now, with those lobes rounded, as the rpms increase, the pressure of the gear lobes upon the dog lobes increases to a point whereas they are forced apart (jumping out of gear), and due (usually) to the shift cable keeping tension on the engines shift linkages..... the unit is forced back into gear giving one the sensation that the engine has hit something, and the cycle continues.

Some boaters with manual shift engines have the mistaken belief that shifting slowly is taking it easy on all of the shifting components..... Wrong! Shifting slowly allows those precisely machined sharp edges of the dog and gears to click, clank, bang, slam against each other many times before they are finally forced into alignment with each other..... and this is what rounds those edges off! The proper way to shift is to snap the unit into gear as quickly as possible.
 
racerone said:
Step[ # 1 is to remove the lower unit.----Then put the lower unit in neutral.----Measure the distance from mounting surface to the center of the pin hole on the shift rod.----It needs to be at a certain distance or shifting issues can occur.----Post that distance when you get it.
Click to expand...

Havent had time to do that yet, will post once I do it.


joereeves said:
I assume that you did not encounter this "knock" (jumping out of and back into gear) on your test run with the previous owner? And if not.... was the rig held at full throttle long enough for it to take place? Possibly the previous ower was operating the rig and avoided holding the engine at full throttle for obvious reasons?
Click to expand...

HAha yes exactly! he drove it from start to finish as we had not paid yet and everything was normal! :(

Whats the best way to fix this? Could it be a cable adjustment or def the dog? is the part easily available? any exploded diagram available?

Thx
 
Racerone and Joereeves, as you may have noticed,or not, I always come back with a new motor and a new problem! ;) I love playing and fixing boats, cars, bike, garden equipment, basically anything with a motor! and find that forums are always very to find solutions to problems. I try to search and read first and post if I can't find anything about my problem. Anyway just wanted to say a big thank you to you both as you have helped me a lot and enabled me to learn a great deal too:)
 
Guyph_01 said:
HAha yes exactly! he drove it from start to finish as we had not paid yet and everything was normal! :( Whats the best way to fix this? Could it be a cable adjustment or def the dog? is the part easily available? any exploded diagram available?Thx
Click to expand...


What I would suggest is to follow "Racerone's" lead suggestion as thee previous owner may have had the lower unit off and accidentally changed the critical measurement of the vertical shift rod.

The measurement is to be taken in "Neutral" from the top surface of the gear-case housing to the very center of the shift hole. No, I don't know what that measurement is... you'll need to find that on your own... OR... most likely one of the other members will come up with it.

If the measurement is indeed wrong, correct it, re-install the lower unit and hope for the best. Also center the shift cable as follows.

(Centering Shift Cable)
(J. Reeves)

When all is as it should be, the proper method to adjust the shift cable is to disconnect the cable from the engine. Move the shift linkage on the engine to find the center of the play in neutral, and when found, leave it centered.

Now, grab the end of the shift cable sleeve, push and pull it to find the center of the play there, and center that play.

Adjust the trunion on the threaded portion of the shift cable so that the centered play of the cable lines up with the centered play of the engine's shift linkage. Install and lock the shift cable with the retaining clamp in that position. That's it.

Exploded diagrams?... Only ones I've ever come across are the brochures found at various dealerships that tend to emphasize what would be improvements, upgrades, whatever, in gear-cases, power-heads, etc. Also (sometimes) in service manuals. The parts manuals may be of some help to you as every part is more or less placed where it belongs but separated so as to number it.
 
racerone said:
Step[ # 1 is to remove the lower unit.----Then put the lower unit in neutral.----Measure the distance from mounting surface to the center of the pin hole on the shift rod.----It needs to be at a certain distance or shifting issues can occur.----Post that distance when you get it.
Click to expand...

So From the gearbox cast cast mounting face to the center of the pin hole on the shift rod i get 685mm.. haha we measure in metric here:)
What does this tell you?
Thx
 
Metric?... Personally, I don't have a clue! You'll possibly need to convert to inches.

For instance, the 1986 120/140 shift rod is measured in inches as follows:

1986 - 120, 140hp Loop Charged = (L) 21-15/16" plus/minus 1/32" - - (XL) 26-15/16" plus/minus 1/32". Have shift rod in neutral.

Keep in mind... the above is for a 1986 model NOT a 1995 model!
 
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