Johnson 120 bogging on hole shot

[aw1575]

Hi there, I have a 1985 Johnson 120 VRO outboard (V6 gearbox) on a 15 foot runabout that we use for waterskiing. Unfortunately the motor “bogs down” when starting off (ie trying to pull up a skier) which renders deep water starts on a single ski impossible and some larger people even struggle to get up on doubles. The motor doesn’t miss, splutter or die but just doesn’t pull strongly. It also starts and idles fine. Once up and running it runs cleanly to WOT. Similarly acceleration from half throttle to full throttle is brilliant. It is running a 14.25 x 19 OMC 3 blade alloy prop. I have tried an adjustable 4 blade prop at 16, 17 and 18 pitch settings but there was absolutely no improvement on hole shot although there was a huge loss of top speed. I have stripped down and cleaned the carbs and checked the float levels. The boat shop has vacuum tested the fuel line and replaced the main fuel hose, filter and primer bulb and has checked timing & synchronisation, plugs, compressions and fuel flows at WOT and all are fine. They have also drilled the prop. I have checked the plugs and all look identical with clean light brown colouring and no oiling. When running the boat just off idle I have noticed that the motor shakes a lot from side to side, almost as if it is not running evenly on all cylinders at this throttle opening. Unfortunately I can’t tell you at what revs this occurs as the tacho is not working and I suspect that the regulator / rectifier is at fault as the charging system is not working either. However my guess would be in the 2000-3000rpm range which strikes me as being the same rev range that would be being used for that initial bite when trying to pull up a skier. Could a coil be at fault? Can I test this by removing one plug lead at a time or will this wreck the electrics? Any direction you can give would be much appreciated.

 

[miloman]

Re: Johnson 120 bogging on hole shot

carbs or prop I lean to the prop though

 

[OBJ]

Re: Johnson 120 bogging on hole shot

Alan....First, ya' really need to get that tach working. This is probably the single most important gauge on the dash board. This is real time indication of how your engine is running. Have you had this problem all along or is it something that has just cropped up? If the rig is new for you, ask a knowledgable person to look at the set-up of engine and boat. (Is engine to low on transom or to high.) Are you trimming all the way down for hole shots? Check the boats sticker for hp rating. Maybe just me but 120hp seems a bit much for a 15' run about. (Course this is sight unseen.) You may not have enough prop for what you want to do. A higher pitch prop may be the key. If I were looking at just the engine, it would sound as if he sync&link is out of adjustment. It won't hurt to momentarily pull the spark plug wire to see if the low end idle is effected. Be careful. These systems can give ya' a bad jolt.<br /><br />OBJim

 

[aw1575]

Re: Johnson 120 bogging on hole shot

Thanks for your replies. You mention props and that was my thinking also so I tried higher pitches but the problem was still clearly evident. I hear what you say about the tacho and it is on the to-do list. Is the regulator / rectifier hard to replace? Where is it located? In terms of synchronization, I've had one boat shop set it up and as part of other work, a second shop checked it and said it was OK. To answer the other questions, yes the motor is trimmed right in. Other boats I've owned which have struggled to pull up skiers due to insufficient horsepower have all pointed the bow to the sky until they got enough momentum to plane out. This is different. There is minimal bow-high attitude but just no get up and go. It gives the classic symptoms of being over-propped but different props leave the symptoms in tact which suggest that it is mechanical. I'm just struggling to find the cause as it's fine starting, at idle and WOT. Could a poor carb gasket be letting in air at one rev range but not affecting others? Thanks for your help.

 

[johnc62]

Re: Johnson 120 bogging on hole shot

I had the same similar problem. I opened up my fuel pump and the orange rubber inside was disintergrated. Some sort of valves. I was told by my tech that what was happening was that the particles were getting into my high speed jets. This type of rubber was evidently common on some older Johnsons. I went ahead and had my Carbs rebuilt and a new fuel pump installed. OUCH!<br /><br />My 2 cents,<br /><br />John

 

[aw1575]

Re: Johnson 120 bogging on hole shot

Firstly, thanks for all your replies and information. The tacho is being fixed tomorrow so should have some readings then. Could the symptoms be related to the reed valves? Does anyone have experience with Boyesen reeds for this motor and are they worth the investment? Thanks again for your help.

 

[Dhadley]

Re: Johnson 120 bogging on hole shot

Are the thermostats still in? What spark plugs (be specific!)? What grade fuel? Is the airbox on or off?<br /><br />Good luck!

 

[aw1575]

Re: Johnson 120 bogging on hole shot

Hi Dhadley, thanks for your interest. To answer your questions. Spark plugs are Champion QL78YC. Airbox is on. Thermostats are in. Fuel is unleaded 96 octane. The boat is in New Zealand so this may be different to USA fuel ratings. It is the highest rating octane we have and the one I have been advised to use by the dealers here. Thanks.

 

[aw1575]

Re: Johnson 120 bogging on hole shot

I've been reading about Boyesen reeds and, if you can believe the marketing information, they seem to address some of the problems I am experiencing with bogging on holeshot. Any comments would be appreciated. Thanks.

 

[Dhadley]

Re: Johnson 120 bogging on hole shot

Good, specific answers. Now we know where we are. You may think about trying the lower octane. Over here it would be 87, thats what the motors are calibrated to run on. I would go strictly by the factory manual. <br /><br />Boyesen reeds are great. I have them in all my motors but I dont think they will "cure" a problem. <br /><br />Lets go back to Jims question - is this a new problem or has it done this all along? Have you tried bumping the primer as it bogs? If so did it get worse?<br /><br />Let us know!

 

[aw1575]

Re: Johnson 120 bogging on hole shot

Hi Dhadley. Amazing how well these sites work considering we are communicating across the world! I have only owned the boat for a few months and the problem has always been there in my ownership. Unfortunately I can't contact the previous owner. No I haven't tried squeezing the primer bulb so can't say what effect that may have.

 

[Dhadley]

Re: Johnson 120 bogging on hole shot

Pumping the bulb is OK to check also. I was refering to the primer (choke). When it starts to "bog" tap the primer (push key in quickly). How the motor responds will give us a clue to what it's doing. <br /><br />If, when you tap the primer it picks up and runs better, its lean. If it makes the condition worse its rich. Rejetting will fix either. Dont push the key in and hold it. You will flod the motor. We are looking for what happens the instant you activate the primer.<br /><br />Also look at the side plate on each carb. Sometimes those leak and will suck in air. That causes all sorts of wierd problems.<br /><br />Good luck!

 

[aw1575]

Re: Johnson 120 bogging on hole shot

Dhadley. Thanks for this. The boat is going into the shop today to get the charging system fixed which will give me back a tacho. I will then be able to be more definite on the rev point at which the problem is happening. When I get it back I will take it for a run and do the choke test and reply then.

 

[dpminc]

Re: Johnson 120 bogging on hole shot

Your first post said V6 gearcase. NO NO NO It is the same size as a V6 prop wise, but the gear ratio is very different. Out of the water, put the motor in foward gear, unplug and isolate spark plugs, turn the flywheel by hand CLOCKWISE. The Flywheel should turn 2 1/4 times for every 1 time the prop turns. The early looper V4 large gearcase is 2.25:1 ratio. The V6 is a 1.86:1 ratio. This is a very big ratio change, and will cause the symptoms you describe. If yours checks out, Check some of the other ideas in the thread.

 

[aw1575]

Re: Johnson 120 bogging on hole shot

Hi all. The boat's in the shop as we speak. The regulator / rectifier was the cause of the non charging and no tacho. Stripping down the carbs revealed some poor condition rubber gaskets under the side plates that may be letting in air. They will be replaced along with other carb and air box gaskets. Apparently the original seal material was badly affected by our fuel quality and seals are now viton to counteract this. Not sure if that's a global change or not. <br />DPMINC - I will check the gear ratio when the boat returns. Thanks.

 

[93bronco]

Re: Johnson 120 bogging on hole shot

just my thought, you can have the larger v6 gearcase <as i do> with the 120, but the gear ratio is different. just because you have the larger case doesnt mean its wrong. the above check will determine. also go to the thread " new member evinrude 225 questions " there is alternative ways listed to check ratio. good luck

 

[93bronco]

Re: Johnson 120 bogging on hole shot

after looking in a 1990 manual there are 2 ratios listed for the 120/140. not sure what it is for 1985 but will give you these as a reference.<br /> 20 in shaft 2.00 to 1 <13:26>small v4 case<br /> 25 in shaft 2.25 to 1 <12:27>large v6 case

 

[dpmpat]

Re: Johnson 120 bogging on hole shot

In 90 the 20' model is the "small' gearcase. This runs the 2:1 ratio because the largest diameter prop to run is 14" The 25" can run up to a 15 and somting prop and requires the lower 2.25:1 ratio. to help spin the larger wheel!

 

[aw1575]

Re: Johnson 120 bogging on hole shot

Hi everyone. I now have the boat back from the shop so can respond to you all.<br /><br />2K BOAT, dpmpat, dmpinc: The gear ratio is 2.25:1 based on dpminc's test of rotating the flywheel 2.25 times for one prop revolution.<br /><br />Dhadley: 4 carb kits were fitted and this has made a huge difference. Although the transition from idle through intermediate to main jets is still more pronounced than I would prefer, the motor no longer shakes just off idle, there is less black residue on the back of the boat and it planes quicker. As I said, the transition to planing is still not totally linear but there is less bogging. The motor idles at 900rpm and pulls 5600 at WOT which equates to 50mph. You have to make sure you use FULL throttle though as the pull is not strong until it hits around 3000rpm. That last inch of throttle makes a big difference! I have been told that a lazy takeoff is characteristic of this model so perhaps I just have to live with what I have.<br /><br />Thanks everyone for your help. You have been great.

 

[Dhadley]

Re: Johnson 120 bogging on hole shot

I dont think the takeoff should be "lazy" but at only 5600 rpm on top it may be so. Lets now talk about your set up as it seems the motor is running well. What type of hull? Is it a pad bottom type? When you said the prop is alloy does that mean aluminum? Where is the anti ventalation plate in relation to the bottm of the hull? Have you tried to raise the motor (increase the X dimension)? The larger gearcase creates some extra drag but you can use a larger diameter prop so you should be able to run a fairly high X dimension.<br /><br />Good luck!