Johnson 120 bogging on hole shot

[Chum Slick]

Re: Johnson 120 bogging on hole shot

Alan,<br /> Listen to what dpminc is telling you about the V6 and V4 gearcase ratio. Some of the 120 V4s came with a V6 gearcase which had a different ratio and larger prop spline than the conventional V4 gearcase. I had the exact same motor with a 1985 120, V6 gearcase/lowerend with the exact same problems. I tried every combination of size and pitch props for that unit. I finally changed the lower unit with one off a V4 that would take the smaller diameter prop and completely cured the problem. My 120 was on a 20' Mako CC. Difference was like night and day.

 

[aw1575]

Re: Johnson 120 bogging on hole shot

Dhadley. Thanks for your reply. The boat is a simple mod V runabout. Not sure what you mean by pad type but if that equates to have a plank shape on the bottom, no it doesn't. The hull is reasonably flat at the back but still has a mild V. Bit hard to describe! Yes, the prop is aluminium, not S/S and is a standard Johnson prop. The cavitation plate is 35mm (1.38 inches) above the bottom of the hull. Not I haven't tried raising it although it appears someone has tried raising it one notch.

 

[aw1575]

Re: Johnson 120 bogging on hole shot

Chum Slick - interesting comments and they seem to line up with what the service guys were saying. Can't argue with someone like yourself that has "been there done that". Now as it happens I know someone who has blown up a 140 Johnson but the bottom end is OK. From memory it is a 1978 motor. Is this likely to be able to bolt up to my motor? I will have to find out serial numbers and shaft lengths to compare.

 

[Chum Slick]

Re: Johnson 120 bogging on hole shot

If the 140 is a V6 your are going to have the same problem. I believe some of the older 140s were V4s. I don't know if the older 140s had the large gear case or the smaller one. You may also run into a minor problem with the upper end of the shift linkage. The V4 crossflow shift linkage is a little different than the V4 loopers. Make sure you measure the length of the shift linkage on your original lower unit so you will have a starting point when you change them. I had to heat mine and modify the bend angle to make it work.

 

[93bronco]

Re: Johnson 120 bogging on hole shot

the size of the case is not the issue, its the gear ratio that was intened for the 120/140. again you can have the larger v-6 case with the correct ratio and it will be fine.<br />chum slick- what cured your problem was that you got the rite gear ratio for your motor, not the change in housing.<br /><br />alan- the ratio you posted should be fine, if not dhadley would have suggested otherwise, hes the one that knows these motors.<br />dont really think its a good idea to be mixing and force matching something as important as a shift rod.

 

[Dhadley]

Re: Johnson 120 bogging on hole shot

Alan, the 1978 lower will bolt up and work by simply changing the shif shaft (it just screws in). The first thing to do is determine which lower you will use. The props will not interchange. Either case and ratio will work. Its simply a matter of fine tuning the set up. Most likely you will need to buy a prop anyway so just decide on which case you will use. Its a matter of X dimension.<br /><br />If you use the case you have most likely the holeshot will improve, as will top rpm, by raising the motor. The aluminum prop may not bite well. Perhaps thats what happened before. A ss prop will be the way to go most likely. The simplest, cheapest thing to do is raise the motor and try what you have. Try that and well see how the hull responds.<br /><br />Good luck!

 

[aw1575]

Re: Johnson 120 bogging on hole shot

Thanks for these last few replies. I plan to look at the 140 tomorrow and check shaft length, gear ratio and condition. It also comes with a S/S prop although I'm not sure of it's specs yet. If it is OK, it may actually be a cheap way of dealing with a number of issues including the prop. And it would be a "no improvement no purchase" deal. It has also been suggested that I change the mid range jets to make the motor run a bit richer in that lower rev range. Apparantly that was a mod that was done regularly here on these motors - may be related to our fuel. They also mentioned planing 50 thou off the heads although that sounds a bit drastic!

 

[93bronco]

Re: Johnson 120 bogging on hole shot

my thought is to keep the heavier duty v6 case that you have and work with it, it has the rite gear ratio. mine has the larger case on my 18ft w/a trophy and got me up on plan just fine. but yet a hole shot in no concern to me, just happy to be out the water. i am trying to up grade cooling to get rid of water control valve. thats another story.

 

[ICEMAN]

Re: Johnson 120 bogging on hole shot

I Been watching the posts on this issue and have a few ideas. Back when this motor design was new, which is about 1985, I'm recalling there was a modification to the exhaust system. Though I have not found the info in my records yet, I will see if I can find it. I would hate to tell you to do the modification, without checking the service bulletins, or letters, for the exact info. The modification had to do with relieving exhaust back pressure, which helped increase the hole shot. Also there is another modification that can be done to the prop to increase hole shot performance. Both require drilling a specified size hole in a certain location.

 

[aw1575]

Re: Johnson 120 bogging on hole shot

Hi Iceman. I have performed the prop modification by having that drilled. I have heard something about the exhaust mod but don't have details either. If you can find something, it would be much appreciated.

 

[Dhadley]

Re: Johnson 120 bogging on hole shot

The exhaust relief drilling was not written anywhere that I remember (I know, I know!) but we did drill into the exhaust. We went in the existing exhaust relief holes and started with a 3/8" bit. Drill thru both walls. If that helps we went slightly larger in steps up to 1/2".<br /><br />Again - at only 5600 rpm the holeshot may be sluggish. If you take care of improving the top rpm most likely the holeshot will improve also. <br /><br />Good luck!

 

[aw1575]

Re: Johnson 120 bogging on hole shot

Hi there. It's been a while since I've been here as I've been waiting for parts. I have fitted size 16 intermediate jets in place of the 20's to make the mixture a bit richer in the mid range. This was a big improvement and pretty much sorted out the bogging down problem. I had a 17" prop to try also but the boat shop had done the prop nut up so tight I couldn't shift it. My understanding was that they didn't have to be done up real tight - am I correct? So the results of the prop test will have to wait for another day. By the way, it was a pleasant 27 degrees celcius today (81 F I think). Just thought I'd mention that for those of you suffering through a US winter!

 

[93bronco]

Re: Johnson 120 bogging on hole shot

torque for prop nut:<br />V6 and V8 housing calls for 70 to 80 ft.lbs. (95 -105 Nm)<br />V4 120 in. lbs. (14 Nm)

 

[aw1575]

Re: Johnson 120 bogging on hole shot

Thanks for the torque settings. I've just tried the 19" x 17 pitch 3 blade aluminium OMC prop. Hole shot was definately better. Top speed dropped by 5mph and WOT rpm went up to 6200. I would say it would definitely improve the boat for waterskiing.

 

[aw1575]

Re: Johnson 120 bogging on hole shot

Seems I'm not out of the woods yet. We were skiing all day yesterday and while the flat spot was not evident the boat still took ages to get up an plane. It would struggle to pull up a single skier and at 15 feet with 120hp, it should fly. Makes it very frustrating to use and makes you think twice about going skiing. I'm still using the 19" prop as although the 17" gave slightly better holeshot, it still bogged off idle and i figured there's no point paying for a new prop if there are still other issues. In laymans terms, it's like you just can't get the prop spinning fast enough - again it's like starting a car in 2nd gear. You can literally hear it struggling until the revs pick up to a point where it can free it's throat and take off. I still have a nagging concern about that V6 gearbox. I appreiate that the ratio's etc are correct, but is it somehow strangling my motor and making it hard for my motor to develop revs quickly? We keep trying ....