1985 Johnson GT 150 STILL Surging!

[jskise3]

Here's the rundown... after first trip out this summer I decided to dump some money in this ole boat and try to fix it. I ordered service manual, got all gauges back working, rewired all the switches to working order, replaced the stator, glued a couple magnets back under flywheel, replaced regulator/rectifier, 2 new power packs, all 6 ignition coils new, 2 new thermostats, new plugs and wires, rebuilt fuel pump (fuel side only, oil is plugged and I premix), doel fin, new fuel lines and primer bulb, new inline filter/water separator, linc n sync, timing set to 24* (Joe Reeves method so its really at 28*). I took it out today and I must say it ran A LOT better... BUT.. still a few things:

1: Idling only abut 500 rpms, would like to know how to get that closer to 7-800 rpms.
2: Up to 4000 rpms this thing runs excellent, after 400 rpms it starts surging. Loses 1000-1200 rpms, then it picks it back, loses it, then picks it back up all without me touching the throttle. Pumping primer bulb and choking doesn't affect it, still does the same thing. What could this be?
3: At WOT I'm runnin about 55-57 mph, BUT Im only at 4500 rpms. This was my first trip with a working tach.
4: I noticed after runnin up the lake and stopping to fish, I leave n oil trail for about 5 minutes?

PS: The prop is 14 1/2 by 24 3 blade stainless. Its not horrible... but its not great either. The blades have ate their fair share of rocks. None are bent but the edges of them are beat up pretty bad. What would be a good size to go with? Could the prop possibly be causing this situation?

Thanks in advance!

 

[jskise3]

Re: 1985 Johnson GT 150 STILL Surging!

Also rebuilt carbs and seafoamed

 

[Bosunsmate]

Re: 1985 Johnson GT 150 STILL Surging!

You could try increasing your minimum timing setting to get the idle up, i like it as low as i can get it as it uses less fuel and is much nicer on the gearbox when you shift into gear.

Is your primer bulb holding firm with engine off?
Im wondering if you are getting some leaking past the floats. needle valves in a carb/s at higher rpm which can cause flooding and that trail of oil behind the boat.

I was just out today in my 82 60, no sign of any oil from mine

 

[jskise3]

Re: 1985 Johnson GT 150 STILL Surging!

How do u increase minimum timing?
Usually after the engine is off for a while the bulb is super hard but it's not really soft either.
I may take the carbs back apart and check the floats

Also spark test was good and compression was:
#1 86
#2 92
#3 76
#4 92
#5 86
#6 92

 

[jskise3]

Re: 1985 Johnson GT 150 STILL Surging!

I want the idle up because at idle and with the throttle barely in forward the boat wants to die some which makes gettin it in the trailer a *****

 

[71Windsor]

Re: 1985 Johnson GT 150 STILL Surging!

Sorry I didn't feel like going out in the shop to get my OMC manual I know Clymers sucks but it was closer to me. Any way says tie the boat at the dock and bring it up to operating temp then adjust for idle.

 

[racerone]

Re: 1985 Johnson GT 150 STILL Surging!

Those compression values do not look good.--Take the bypass cover off #3 and have a look at piston and rings in there.

 

[emdsapmgr]

Re: 1985 Johnson GT 150 STILL Surging!

The #3 cyl is low on compression-below the normal 10% acceptable variance. The others look "normal". You can pull the side covers, as mentioned, also I'd pull the head to check for a damaged top piston ring. If it's ok, I'd run a can of Bombardier Engine Tuner through the powerhead, then see if the compression comes up a little. Surging at higher rpm's may be a sign that the VRO fuel pump is weak. If it won't pump enough fuel volume, the carbs will consume more than the pump can supply, and you will get that rpm periodic "pulsing" problem that you describe-at higher rpm's only. When you observe this rpm variance, have someone go the back of the boat and constantly prime the fuel hose bulb. If the engine runs fine at higher rpm's, as long as someone constantly primes that bulb-you've got a fuel delivery problem. Which could be a bad fuel pump, or fuel restriction, bad anti-siphon valve, etc. I would not jump right into replacing the fuel pump (an expensive proposition) until you sort out the compression variance in the powerhead. If the powerhead is damaged, the fuel supply issue becomes secondary.

 

[71Windsor]

Re: 1985 Johnson GT 150 STILL Surging!

The part name is escaping me wt the moment but the line from the crank case to the fuel pump that supplies the pulse theres a valve in the middle of that hose. I'm having a brain cart this morning. Some one will know what its called

 

[jskise3]

Re: 1985 Johnson GT 150 STILL Surging!

Plan on rings and stuff this winter... How do I get the rpms up to the 5500 range? Is prop selection the only thing to change that?

 

[emdsapmgr]

Re: 1985 Johnson GT 150 STILL Surging!

Your 85 model has no pulse limiter. That came out later, on the 87 models. Maybe yours is not an 85.

 

[racerone]

Re: 1985 Johnson GT 150 STILL Surging!

Determine why you have compression issue on #3.--------If you keep running it with a bad piston and rings you risk more serious engine damage.-----Motor may come to a " dynamic halt " and that will likely be a $$$$$$ issue for you.