84 Johnson GT 150 Knocking???

[bighead]

I just purchased a used boat with a "bad" engine. It's a 84 GT 150 that was described by the previous owner as having a knock. I have not had time to tow to the lake and start the engine to hear it myself yet but I've been reading past threads here to try to figure out whats wrong. One thread mentioned bringing the piston past TDC and pushing on it to see if there is any slack present. I did this on each cylinder a couple of times and do not notice any movement. Each cylinder seemed to have at least some compression checked with just a finger over the spark plug hole. I've got a proper gauge but don't know if this should be done with the engine warmed, oiled, or ? to get an accurate reading. With all the plugs out and rotated by hand there are no hard spots but there is a squeaking sound from somewhere in the engine. If anyone can offer some advice on digging deeper into this problem I would be greatful. bighead

 

[Dhadley]

Re: 84 Johnson GT 150 Knocking???

Welcome!!<br /><br />The squeaking noise is probably the water pump impeller. No biggie. The first thing is to get a compression reading. It really doesnt matter how you do it as long as you do it the same each time. And with the same gauge. We arent looking for a specific number, rather a difference of no more than 15 lbs low to high.<br /><br />Maybe you got lucky and its just piston slap.

 

[bighead]

Re: 84 Johnson GT 150 Knocking???

Can you tell me if the plug wires need to be grounded when running a compression test? Seem like I heard you can damage the ignition somehow if you don't.

 

[TxShorty]

Re: 84 Johnson GT 150 Knocking???

If you jump across the starter solenoid to spin over for compression check, ignition doesn't even come into play....

 

[bighead]

Re: 84 Johnson GT 150 Knocking???

Found my compression gauge and went to work tonight. Shot a little bit of wd40 into each cylinder since the engine had not been run in almost a year. Spun the engine as suggested by TXShorty. Facing the rear of the engine the left bank measured from top to bottom: 95, 100, 98. Right side: 110, 110, 110. This was better than I expected. Any ideas on the knock that I might be able to look into until I can actually get it started? Thanks again, bighead

 

[Dhadley]

Re: 84 Johnson GT 150 Knocking???

OK, good numbers. Next lets drain the lower unit oil. There will be a magnet on the drain screw. It should have some metal "fuzz" but there should be no chunks on the magnet or in the oil. If the oil looks good then you can refill it. Dont forget to pick up some new seals or O rings (whichever it has) for the screws.<br /><br />Actually if the oil looks good, you might as well get a water pump kit and 3 carb kits while youre at the dealer.

 

[bighead]

Re: 84 Johnson GT 150 Knocking???

Got back to the boat today. Drained the lower unit grease and noted that it was milky with some fuzz but no chunks. Had a couple extra quarts that had been laying around a few years so pumped them through to flush out as much water as possible. Refilled with a fresh bottle. Pulled the boat down to the gas station and put in about 10 gallons of regular to try to put some life in the 7-8 gallons that were in there for the last year. Went back home and hooked up the ear muffs hoping to at least start for a moment. After a pumping up the VRO and gas bulbs, a little choke and she fired up. Kind of sputtered for a couple minutes then straightened up and sounded good. Could not hear any knock but the exhaust was pretty loud. Idle seemed a bit high so searched the forum and someone said that they do idle high out of the water. Decide to make a run to the local lake and fire it up in the water. Didn't notice any unusual noises when fired on the trailer so thought I'd run it a bit and see what happened. The idle was still high and banged into gear. Broke the plastic linkage to the shifter so no reverse. Ran a ways just fast enought to get on plane and only noticed what seemed to be a little detenation. Didn't really surprise me considering the fuel and that the motor was kind of lugging. The noise would clear up if the speed was changed a little or trim raised. Ran at low speed for 30 minutes or so never to far from the ramp incase of a big pow. Got a little braver and trimmed it up and floored it. Boat (18' Skeeter) lurched forward and topped out around 60ish with smooth, hard acceleration. Too much wind to hear engine noises but ran good. It was pretty cold out so headed to the ramp feeling better about the whole deal. Dropped back to idle and then could hear the noise I'd been told about. It was a low tapping sound that would get quiter and faster if the idle was slightly increased. Could this be piston slap? What causes piston slap and is there anything that can be done about it or is it rebuild time? Seemed to run better than a motor that was about to go south but you know what they say about running the best just before they blow! Thanks for any insight or advice you can offer. bighead

 

[Solittle]

Re: 84 Johnson GT 150 Knocking???

Your compression readings tell you that the internals are basically healthy. You might check the timing, do a sync & link and adjust the idle. I think your rpms at WOT should be around 55-5800. Other than that & the normal stuff in bringing a sitting engine back in service I would run it.<br /><br />I have twin 1987 V4s & they make all kinds of noises but run just fine.

 

[G DANE]

Re: 84 Johnson GT 150 Knocking???

Take my advise, NEVER run on the old fuel, and ALWAYS add oil to fuel untill oil consumption is confirmed, by putting a piece of gaffa on tank at oil surface and observice oil level to drop accordingly to gas consumption. Especially if motor sat and lost oil prime, but on all that is new to me. Could have been the last ride.

 

[gatorred]

Re: 84 Johnson GT 150 Knocking???

try cheep gas, plus or better no knock try lead add to age of motor

 

[gatorred]

Re: 84 Johnson GT 150 Knocking???

89 oct or higher

 

[Dhadley]

Re: 84 Johnson GT 150 Knocking???

As long as the motor is stock, it's made to run on 87 octane. You can probably get away with 89 for quite some time but dont run it on 93 unless you rejet the carbs. Especially if youre set up at less than the optimum 5800 rpm. Or if you run something other than Champion plugs.