What makes a Johnson GT?

[Ginder75]

I recently acquired a 1988 Johnson GT 150? Anything special about this motor? It runs great and will make a fine addition to my boat, but the GT threw me for a loop.

Is it basically a 150 in nice metallic grey paint?

I have a new water pump and spark plugs on the way. Anything else I should consider?

Thanks,
Robert

 

[1730V]

Re: What makes a Johnson GT?

The GT's hasd some special "go fast" stuff. They were at the high end of their legal horsepower band. As an example, a 150 GT may really be a slightly detuned 175. BIA (Boating Industry Association) standards stated an outboard had to be within 10% +/- of its rated HP. You get the picture.

The go fast stuff was mainly carburetion, porting and fine tuning, possibly some lower unit gear ratios.

 

[Chinewalker]

Re: What makes a Johnson GT?

Didn't the GTs have fiberglass reeds originally? I remember seeing an asterisked note stating that they should not be run wide open for sustained periods as the reeds would disintegrate. Maybe that was just the 2.6GT/XP models...
Gear ratios were the same for all the V6s of that era 14:26. The hoods had louvres in them for added breathing. The '84 brochure also says they have "hard head" cylinders and custom pistons. Not sure what that means...

- Scott

 

[Dhadley]

Re: What makes a Johnson GT?

The scalloped pistons were in the 2.6's. The 150's had standard pistons like all 149 cid motors. The XP/GT heads were very good ones. Some of the 150 XP/GT's had port timing real close to the 200 crossflow. All standard 150's & 175's have the same port timing.

The XP / Gt's had harder mounts too. At one time the mounts were cheaper than the standard ones. Odd.

 

[emdsapmgr]

Re: What makes a Johnson GT?

You will find these models have the high output 35 amp alternator. You can tell by the louvers in the flywheel. Since the engine is new to you, I'd probably do a compression check just to baseline the engine for future reference. It's inexpensive to replace the thermostats and check the pressure relief valves for proper function when you have them apart.

 

[Ginder75]

Re: What makes a Johnson GT?

Thanks for all the information.

I got to know my motor a little more tonight. The motor has been pulled and is on a stand waiting for some parts. The thermostats were shot, and the plastic relief valve was cracked. Looks like I will be ordering a rebuild kit or two.

Everything else seems in order. But the compression.

I have 95 lbs on each cylinder, does this seem low? It was even across the motor, but I thought it should be higher?

I can't wait to get this one back together and let my little boat fly. This motor will fit nicely on my 17' Basscat Caracal. It is an older boat, but it is solid and fishes wonderfully.

If you have any more information on these fine motors, I would love to learn about my little GT.

 

[emdsapmgr]

Re: What makes a Johnson GT?

These engines did not run high compression numbers. Some of them ran high timing as a result. You should be OK with 95. What with your thermosat findings, your engine may have been running cool. As a result, it is possible that you may have some carbon on the rings, affecting your compression numbers. A can of Bombardier Engine Tuner run through the engine annually is a good idea, what with today's poor fuel. It will loosen up any carbon in the rings and piston skirt area and blow it out the exhaust. Once you do this, recheck your compression numbers. Your engine should be in the 165 hp range. Prop it to run in the upper end of the normal operating range. 5500-5700.