1986 Johnson GT 150 Low RPMs

[StevenG10]

I just bought a 1986 Winner Tournament bass boat with a 1986 Johnson GT 150. Took it out today for the first time and although it handles great and seems to run fine wide open I was only able to reach around 4400 RPM and 47mph. I know this motor is capable of much more but I dont know where to begin to look for power loss. The motor currently has a 4 blade shooter prop but Im unsure of the pitch as of now. After some reading I thought that maybe this prop is causing the low RPMs but Im not sure. Also this boat takes forever to get on plane. Just not sure where to begin checking this motor. Any help would be appreciated. Thanks

 

[Joe Reeves]

Re: 1986 Johnson GT 150 Low RPMs

Remove "all" of the spark plugs, then take a compression test. What are the psi reading of each individual cylinder?

With the spark plugs still removed, take a spark test. The spark should jump a 7/16" gap on all cylinders with a strong blue lightning like flame... a real SNAP! Does it?

Spark plugs should be Champion QL77JC4 plugs gaped at either .030 or .040 (original and revised gap)... your choice.

 

[emdsapmgr]

Re: 1986 Johnson GT 150 Low RPMs

Follow Joe's recommendation to determine if the engine is making full hp. Once you confirm the engine is running normally, then turn to your setup. The prop pitch will make a difference in the WOT speed. What pitch prop is running on the boat? I'd think a 22 pitch prop would be normal. Also possible the engine is mounted too low on the transom, causing excess water spray/drag. That GT engine will make about 165 hp at the prop, a solid performance engine. You'll want to prop the engine so that it runs at least 5500 at full throttle-assuming the engine is making full hp now.

 

[StevenG10]

Re: 1986 Johnson GT 150 Low RPMs

Sorry for the delayed response. I've checked my compression and found in the left bank: 81, 76, 76. Right bank: 86, 81, 86. Does this seem low? I have yet to check spark but the plugs that are currently in the engine are gapless NGK plugs (BUHX). I order a set of the correct champion plugs recommended which will arrive this week. I've also got a chance to look at the prop. It's currently running a shooter 4 blade SS 13 1/4 x 25P. I may have room to raise the motor some but I want to make sure I get the motor running to its full potential first. I'll check spark and measure where my prop shaft is shortly. Also would like to decarb soon.

 

[Joe Reeves]

Re: 1986 Johnson GT 150 Low RPMs

That 1986 150hp model hits its hp at 5000 rpm. The top rated rpm, not to be exceeded, is 5500 rpm.

Yes, that compression is low BUT that may simply be due to a gauge malfunction and/or the speed at which the engine is turning over. Best to check the compression and the spark with all of the spark plugs removed in order to obtain the highest cranking speed possible.

Normally on that model, you would be reading approximately 95 psi on the starboard bank and 90 psi on the port bank, the difference being due to the engine's design.

There is a 10 psi difference between the lowest and highest reading, not necessarily a real bad thing but not good either.

Using a spark tester whereas you can set a gap to 7/16", the spark should jump that gap with a strong blue lightning like flame... a real SNAP! Does it? (No tester... available at auto part stores, or build the following)

(Spark Tester - Home Made)
(J. Reeves)

You can use a medium size philips screwdriver (#2 I believe) inserted into the spark plug boot spring connector, then hold the screwdriver shank approximately 7/16" away from the block to check the spark or build the following:

A spark tester can be made with a piece of 1x4 or 1x6, drive a few finishing nails through it, then bend the pointed ends at a right angle. You can then adjust the gap by simply twisting the nail(s). Solder a spark plug wire to one which you can connect to the spark plug boots, and a ground wire of some kind to the other to connect to the powerhead somewhere. Use small alligator clips on the other end of the wires to connect to ground and to the spark plug connector that exists inside of the rubber plug boot.

Using the above, one could easily build a spark tester whereas they could connect 2, 4, 6, or 8 cylinders all at one time. The ground nail being straight up, the others being bent, aimed at the ground nail. A typical 4 cylinder tester follows:


..........X1..........X2

.................X..(grd)

..........X3..........X4

 

[James R]

Re: 1986 Johnson GT 150 Low RPMs

Thanks once more Joe for some valuable info, this time on the 150GT. I recently bought a 1984 model and checked the comps. All are 90 or 90 plus.
I thought that this was a little low. However on removing the heads I found that the bores were within .001' of standard and still showed xhatching and so assumed the comps were good. Good to have that confirmed. Further inspection shows that the motor has been used very little. I will however replace the VRO with the 1983 twin pump set up.
Thanks again.

 

[emdsapmgr]

Re: 1986 Johnson GT 150 Low RPMs

I think you are asking a lot of that engine to turn a 25 pitch stainless. Have you got another prop you can try of lower pitch?

 

[StevenG10]

Re: 1986 Johnson GT 150 Low RPMs

I think you are asking a lot of that engine to turn a 25 pitch stainless. Have you got another prop you can try of lower pitch?

I was thinking that also but wasn't quite sure. Unfortunately the only other props I have are 13 spline and will not fit. I'm going to keep asking around to try to find one to test.

 

[StevenG10]

Re: 1986 Johnson GT 150 Low RPMs

Would the diameter of the prop make much difference? I see all the recommended props on a parts diagram are between 14 1/4 and 15 whereas my current prop is 13 1/4.

 

[emdsapmgr]

Re: 1986 Johnson GT 150 Low RPMs

Prop manufacturers make props with all different diameters. That diameter number is not as important as is the second number, the pitch. I'd worry more about the pitch-as in the case of your 25 pitch on your current prop. The pitch number measures how far the boat will theoretically move through the water when the prop spins 1 rpm. Usually you need a lot of hp to turn a 25 pitch prop. About 15% less to turn a 22. A manufacturer will vary the diameter of the prop slightly in order to hold a specific pitch. In the case of your 4 blade, it's a different type of prop technology. The horn is significantly smaller than the diameter of the gearcase (takes a different forward thrust hub.) The advantage is that 1. it's easier for the torque of the engine to spin a smaller diameter prop than a larger one. (better holeshot.) 2. The smaller prop horn provides for a certain amount of exhaust bleed-which will aerate the prop blades, providing some limited slip. The slip lets the engine rev up somewhat more during holeshot-then typically quits "slipping" as the engine reaches planning speeds.

 

[StevenG10]

Re: 1986 Johnson GT 150 Low RPMs

So I checked the spark and found that cylinder 4 has no spark and cylinder 6 has weak spark. As a side note the plug wires on cylinders 1&3 were once replaced as they are not the original yellow wires. Not sure what to check next. Thanks in advance.

 

[Joe Reeves]

Re: 1986 Johnson GT 150 Low RPMs

Doing a spark test, have all of the spark plugs removed.

Switch the leads from the powerpack to the weak firing coils with wires that are firing other coils properly. This will tell you whether the problem is a coil or not.... if the bad coil fires properly with the other wire attached, then obviously the coil is okay.

However, do a coil continuity check also, as follows.


(Magneto Capacitance Discharge Coils - Continuity Test))
(J. Reeves)

Check the continuity of the ignition coils. Remove the primary orange wire from whatever it's connected to. It may be connected to a powerpack screw type terminal, a rubber plug connector, or it may simply plug onto a small boss terminal of the coil itself.

Connect the black lead of a ohm meter to the spark plug boot terminal, then with the red ohm meter lead, touch the ground of the coil or the powerhead itself if the coil is still installed.

Then touch (still with the red lead) the orange wire if it's attached to the coil, or if it's not attached, touch the primary stud of the coil. You should get a reading on both touches (contacts). If not, check the spring terminal inside the rubber boots of the spark plug wire. Poor or no continuity of a coil is one reason for s/plug fouling.

Also, keep in mind that the engine (with all spark plugs removed) must crank over at least 300 rpm in order to have the stator energize the powerpack properly........ slow cranking = weak erratic ignition.