1974 85 HP Johnson outboard issue

flintman · Jul 24, 2012

OK I have been working on this outboard all summer long. It is mounted on a 1974 16SS star craft and I believe I should get more out of it. Full throttle I'm turning 4000 RPM and 29 MPH. The compression is good and I checked the timing and it is right on. Here is what I have noticed. The other day as I was pulling a tube with two adults on it we were moving at about 25 MPH and turning 3800 RPMS. All of a sudden the motor took off turning 5000 RPM and reaching speeds of over 35 MPH. This lasted for about a minute and then I dumped the tuber. I restarted and started to go again and realized I'm out of fuel. I haven't refuelled yet because it was getting late and I went home the next day. I will be going back up in a few weeks but I wanted to see what everyone thought of this. I have the factory service manual and re synced everything from the carbs to the timing. From what I can tell they are fixed jets. The way I was running out of fuel is telling me that maybe it's running really rich but I don't smell a real strong odor of gas out the exhaust. Thoughts anyone?

Pre thanks

BonairII · Jul 24, 2012

Re: 1974 85 HP Johnson outboard issue

Probably missing on a cylinder....and then occasionally the bad cyl begins firing again. 4000 rpm is def low.

flintman · Jul 24, 2012

Re: 1974 85 HP Johnson outboard issue

I installed all new plugs, Compression is good on all cylinders. After running the plugs all look the same. Only went above 4000 RPM when it was running low. It has run at 4000 RPM all season. I never had a tach on until this year so I don't know how it was last year. But from the engine sound about the same as last year.

crb478 · Jul 24, 2012

Re: 1974 85 HP Johnson outboard issue

What size prop are you running? You are 1000-1500 rpm to low. You do have fixed jets, but could have had a lean condition when running out of fuel. Are all your coils in good shape? A coil can work and then stop after it warms up, and then work again as it cools.

flintman · Jul 24, 2012

Re: 1974 85 HP Johnson outboard issue

I'm not sure on the coils. They look good. Now that you said prop it made me think. The guy i bough it off of put an adjustable prop to get the RPMS up to 4000. I put the stock prop on earlier this year and was only able to turn 3000 RPM at 32 MPH. Is there a good test for the coils? With what i described should I look at fuel side or ignition side. I could get all new coils and a new Power pak and see what happens.

BonairII · Jul 24, 2012

Re: 1974 85 HP Johnson outboard issue

Don't start throwing parts at your motor in hopes that it fixes the problem. Test spark on all cyls with an adjustable spark tester. Spark should jump 7/16" air gap.

Your prop is not your problem. Find out what cyl is dropping out....that is what's causing the low rpms.

flintman · Jul 24, 2012

Re: 1974 85 HP Johnson outboard issue

Thanks. I won't see my boat till aug 4th. Ill bring up a spark tester and test every cyl.
Thanks all for the help

BonairII · Jul 24, 2012

Re: 1974 85 HP Johnson outboard issue

If you have a timing light, bring that too. If you're dropping a cylinder while running on the water(not idling at the dock), you'll be able to use the timing light to figure out which cyl is not firing.

4000rpm is way too low....you're definitely 'lugging' the motor.

If your coils are older, it's probably a good time to replace them anyway....so probably not a bad idea to bring new coils to the boat also.

flintman · Aug 5, 2012

Re: 1974 85 HP Johnson outboard issue

So I tested out the old coils. Two of them could only jump a 1/4 gap and the other 2 jumped the 7/16. I replaced all 4 coils and also new spark plugs the one the manual calls for. Took the boat in the water and what a difference. My top end is now about 4500 RPM and when I Max throttle I use to hear a bog. No more bog. It's got alot more power now. 4 adults and two on a tube towed it around at around 30 MPH. Never could do that before. I know the top end book says around 4500 to 5500 RPM. Do you think i can get more out of it. I was thinking i should recheck the timing on my way home today what do you think?

Thanks again for the help

boobie · Aug 5, 2012

Re: 1974 85 HP Johnson outboard issue

If you have verified that it is running on all the cyl's with a cyl drop test, you may want to do some prop swapping.

emdsapmgr · Aug 5, 2012

Re: 1974 85 HP Johnson outboard issue

You want to settle on a prop which will let the engine run at the top end of the "normal" rpm range, or 5500. You are lugging it at 4500.

BonairII · Aug 5, 2012

Re: 1974 85 HP Johnson outboard issue

What pitch prop are you running? How heavily loaded is the boat(and could it be waterlogged)?

Lastly, when was fuel pump last rebuilt?

flintman · Aug 6, 2012

Re: 1974 85 HP Johnson outboard issue

It's an adjustable prop. I tied a I believe 17 and was only turning 3600 rpm but the speeds were better at lower rpm. The timing stop screw looks like it has been moved. I'm going to check the timing again. The fuel pump to my knowledge hasn't been replaced and I've had the boat for a year.

flintman · Aug 14, 2012

Re: 1974 85 HP Johnson outboard issue

So today I did a bunch of stuff. I pulled all the lineage off and reset everything to the factory setting. Time is perfect cables synced. I'm Turing 3000 rpm with the 13.25 17 pitch. Fuel pressur to the carbs is 3 psi. Retested compression 115 + or - 1. I have come to the conclusion its my carbs. There a port is clogged or my floats are t opening all the way. Does anyone know what to check. I have a signal to update and see if I'm on the right track. Thanks again for the help

Daviet · Aug 14, 2012

Re: 1974 85 HP Johnson outboard issue

You might try removing the high speed orifices and see if one of them is plugged or restricted.

flintman · Aug 15, 2012

Re: 1974 85 HP Johnson outboard issue

They can be removed but don't you need a special screwdriver to remove them? Or can you do it with a small flat head if I'm careful?

emdsapmgr · Aug 15, 2012

Re: 1974 85 HP Johnson outboard issue

Bombardier does sell a special jet removal/installation tool. 317002. It's not too expensive and really does the job.

flintman · Aug 24, 2012

Re: 1974 85 HP Johnson outboard issue

Ok so i pulled my high speed orifices. They had some junk in them. Cleaned them out. Got out on the water and before was pulling 3000 now pulling 4300. A big difference in power and speed. Still not giving me 100% but i was happier with it. After about a day of floating around and putting around i went to go tubing. Started off great then all of a sudden max RPM 2800. Pulled back into the docks pulled the orifices again clogged up with black crap. cleaned up and pulling 4500 RPM now. Needless to say i will be ripping apart the carbs this winter and doing a full cleaning.

Thanks all for the help. Ill update this next year if i get my +5000 RPM once ive finished cleaning out the carbs.

emdsapmgr · Aug 24, 2012

Re: 1974 85 HP Johnson outboard issue

Couple of comments. The only fuel "filter" on that engine is the screen inside the cap on the fuel pump. Most folks purchase a Bombardier white/clear filter and install it somewhere in the fuel tank hose. The original fuel lines and fuel pump bladders on that engine were not made to handle today's ethanol fuels. The ethanol will break down the fuel hose and you may find small black bits of hose residue inside the jets and carb passages. Best to replace all the fuel lines on the engine (and fuel tank) if they have not already been replaced. All replacement hoses will be 2012 fuel compliant.

1974 85 HP Johnson outboard issue

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