Johnson GT 150...Need help diagnosing!

mickjenks

Recruit
Joined
Oct 24, 2013
Messages
3
I have a 1985 Johnson GT 150. It will run and idle roughly on hose muffs for a few minutes but will eventually cut out. It will start but not keep running in tank for more than a few seconds. I put new plugs. I have done as others have suggested in this forum and tested spark on all cylinders with a 7/16 gap spark tester and spark is good, however some cylinders will spark simply while turning key in ignition while others will not spark until engine actually starts to turn over. I don't know if this is normal or abnormal. I have tested compression and all cylinders are around 90 psi. I have loosened the gas cap that leads to fuel tank to test if vent line is possibly closed and got no difference in the engine performance. I have had someone keep squeezing the fuel bulb once engine starts to see if fuel pump might be issue but that doesn't keep engine running. I took carbs off and they looked very clean but I cleaned with carb cleaner and air compressor and put back on with new gaskets. The engine has new power packs, new stator, new spark plugs and wires. What should I do next to diagnose the problem? Would a bad impeller or water pump keep the engine from running? How can I determine if timing is ok if engine will not keep running? Any advice on how to proceed is greatly appreciated.
 

levi_tsk

Ensign
Joined
Dec 26, 2010
Messages
907
Re: Johnson GT 150...Need help diagnosing!

power pack could be on the fritz... they have 2-3 diodes for each cylinder and if one of them burns out the other one(s) can and sometimes will cut in and out make sure your battery is fully charged aswell because while the engine doesn't use the battery to fire the plugs it does need to make full cranking rpm to fire the plug
also check the grounds one each of the coils stator and the powerpack(s) I had two BRAND NEW powerpacks burn up because they didn't have a good ground take some 80 grit sand paper to the terminal and the spot where it grounds on the engine this can cause erratic firing too if its not grounded and everthing starts heating up and expanding
the water pump shouldn't be the issue unless the block is getting too hot too hold your hand there for a couple seconds - shuld be uncomfortable after about 5 seconds of holding you hand on the cylinder head if its too hot to touch al all the engine's running too hot and this could be the problem
use joe reeves' WOT timing procedure to check the full advance on the timing using a timing light the procedure is in the "top secret" file at the top of the main J/E repair forum
 
Last edited:

emdsapmgr

Supreme Mariner
Joined
Dec 9, 2005
Messages
11,551
Re: Johnson GT 150...Need help diagnosing!

levi's right. The engine needs to crank about 250 rpms in order to fire the ignition. If it cranks slower than that, it won't fire the plugs. So, make sure the battery is well-charged. When you run engine, do you know the plugs are getting fuel? Did you pull some out to look at them? Do they appear dry or wet? Do all of them look the same? That will tell you something. It's unusual not to have spark on all cyls when cranking. That would point me in the direction of an ignition problem. Remember, weak ignition components tend to fail once the engine heats up to normal operating temps. May run fine when cold, then crap out once the engine warms up. Can you identify which plugs don't fire properly? Are they all on one head? When the engine finally quits running, did you immediately check all 6 plugwires for spark? Sounds like your engine has dual power packs. They can be swapped-allowing you to test for changes in plug firing when cranking. The one ignition component you've not changed/tested is the timer base. You can qualify it by checking for voltage output with a special "peak reading" voltmeter. See this website for an ignition troubleshooting guide: cdielectronics.com When you had the carbs apart, did you pull out all the jets (high, mids, idles) for a visual examination?-in order to make sure they have no debris in them. That engine should warm to 145-150 degrees at idle. You can test this with a laser temp gun. Most auto parts stores rent them by the day.
 

James R

Commander
Joined
Feb 1, 2007
Messages
2,660
Re: Johnson GT 150...Need help diagnosing!

Checking the timer base is a good idea. If you get on the CDI trouble shooting you can find the resistance tests which you can do with a regular multimeter.
 

mickjenks

Recruit
Joined
Oct 24, 2013
Messages
3
Re: Johnson GT 150...Need help diagnosing!

Thanks for the suggestions. All I've had a chance to do is recheck the compression. All cylinders were right around 90 psi +- 5 psi. What I noticed is that all the plugs have alot of oil on them. I've always suspected a fuel issue because the engine will start from the initial primer bulb fuel, but will bog down and shut off shortly after it starts running. It seems that once the VRO pump takes over that the pump is over pumping oil and not enough fuel. Does that seem to be a reasonable explanation for why there is so much oil on the plugs?
 

James R

Commander
Joined
Feb 1, 2007
Messages
2,660
Re: Johnson GT 150...Need help diagnosing!

More likely that the oil is correct but not enough fuel flowing so the mix is oil heavy. I have recently bought a 1984 150 GT and the mix seems ok but as I don't trust VROs it is out of there and I am replacing it with the 1983 twin pump set up and 50:1. Of course I am going right through the motor and lower and doing the whole thing.
 

emdsapmgr

Supreme Mariner
Joined
Dec 9, 2005
Messages
11,551
Re: Johnson GT 150...Need help diagnosing!

It's pretty normal for the plugs to be wet if running at idle. These crossflows do overfuel at low rpm's. Having said that, it is possible you are not getting the right oil/fuel mix into the carbs. You can try starting and simultaneously/constantly prime the fuel hose bulb. That will force fuel around the pump, into the carbs. See if that makes any difference. If it continues to run "normally" if you continue to prime the fuel hose bulb, you may have a weak fuel pump.
 

mickjenks

Recruit
Joined
Oct 24, 2013
Messages
3
Re: Johnson GT 150...Need help diagnosing!

I tried pumping the fuel bulb and it still cuts out within 5-10 seconds. When I do the initial prime of fuel, the bulb gets hard and the clear inline fuel filter is full of fuel, but I noticed that after it cuts out, there is very little fuel in the clear inline fuel filter as if maybe fuel is not being pulled through. Would it make sense to block off the oil side of the vro and run premix from an exterior fuel can to see if the fuel pump side of the vro is the issue?
 

pn

Petty Officer 1st Class
Joined
Apr 20, 2013
Messages
374
Re: Johnson GT 150...Need help diagnosing!

try checking for an air leak in your fuel lines, or any drippy fuel, cracks, etc, take a look at your priming system too. also do a search on disconnecting vro fuel pump, i think the secret tips at the top of the page will give good info. good luck and investigate the obvious...
 

emdsapmgr

Supreme Mariner
Joined
Dec 9, 2005
Messages
11,551
Re: Johnson GT 150...Need help diagnosing!

If the hose bulb does not collapse when running, the fuel system sounds normal. Have you checked for spark after it quits?
 
Top