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1988 Johnson GT 175 issues

J

jtecker

Member
Just bought an old Mako for restoration that has a 1988 Johnson GT 175 on it. Engine has severe idling issues and the exhaust clearly has unburned fuel vapor in it. Once over about 1500 RPM's runs fine and run fines up on plane. But after the engine is really warmed up and you go back to idle speed it really sputters and the second you take it out of gear it stalls out, and sometimes will not restart. Once the engine is cold, push the key in to prime and it starts right up. I've taken the carbs apart, cleaned every little orifice of any crystal or gunk and put them back together. Seemed to help for a little while then not. I did not purchase OEM rebuild kits for the carbs and I don't have a tech manual to follow for proper rebuild instructions. Seems like too much fuel is dumping into the motor and that it's a carb issue. But before I spend the money on the kits and try to find a service manual I wanted to get some input from the old salts here. I'm experienced with old Merc's but have never had a Johnson. Thanks in advance.
 
Did this motor / boat come out of a backyard or field like so many today ?-----Eager buyers for long abandoned units.-----Not for you to worry about this compression.-----Just post your numbers using a good gauge.-----Then you get advise on what to do next !
 
I bought it from a gentlemen who was starting the rehab then had to sell it when his wife unfortunately had a stroke. All I really wanted was the boat and the relatively new dual axel trailer it came on. I restored and sold another old Mako and decided to jump in again. I was expecting the engine to be a throw away but it runs so well up on plane I figure it's worth a bit of effort to see if the low rpm issue is fixable. And in the current market there isn't a used 150-200HP outboard to be found for a remotely reasonable price. I'll test compression and spark over the weekend and repost.
 
Update. Compression between 92-95 all six cylinders. Spark on all six plugs. Took another look at the carbs. All three plastic bowls were warped and leaking. Wet sanded them flat. Bought and installed OEM rebuild kits. Floats properly set. All jets/orifices cleaned thoroughly. No more carb leak. All dry as a bone after running. Brand new fuel filters. Brand new properly gapped plugs. Starts right up and idles great on the muffs.

But as soon as we put her in the water and put her in gear she starts idling very rough and occasionally stalls. And using while docking or rafting up. In addition, she sometimes bogs down at WOT and jerks for a while before kicking back in. I'm at a loss on what to check next. Any advice would be very welcome.
 
Update and still looking for some advice. Rebuilt the fuel pump. One of the little rubber diaphragms was partially pulled out of its hole when I opened it up. With the fuel pump rebuilt I seem to have solved the idle problem. Starts pretty well, idles fine, and is fine underway until you try to get up on plane. As soon as I push the throttle the engines bogs down and shuts off. And it's a real challenge to get it re-started. I'm starting to think this is not a fuel problem but an electrical problem. Seems like the engine is getting flooded and the spark may not be advancing to burn it. Any thoughts out there on what could be causing this bogging/stalling problem as soon as you try to punch it?
 
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