1985 Johnson 120hp poor idle/starting

bigd450

Cadet
Joined
Aug 11, 2024
Messages
6
Been searching this and other forums for months now. A lot of good info but still unable to resolve the issue.

First time out this year started up smooth and ran excellent for about 20min then it was just like the key was turned off. Started back up ram fine for awhile then quite and wouldn’t start. Went through the fuel system and it eventually started ran fine the rest of the day. Next day ran good for a bit then died. Started but would only idle any time you try to give gas it would die.

Months later after checking everything I could. The only thing that stood out was I was only getting 4v to the coils. But I was getting spark. Three power packs later and some help with cdi electronics. Ended up reversing the connection of the Dva tester so red to the block and black to coil it actually was reading 170v. Not sure there’s a problem with the Dva tester of what but left that and started on the carbs again.

Cleaned the carbs for probably the third time used oem carb kit with new floats. New fuel lines. Did find one fuel line that was in rough shape and leaking a bit of fuel into the housing. Checked the reeds and they look good as fas as I can tell from the carb side. Replaced the little check valve on the cylinder drain lines or what ever they are.

Start and runs quite good on the muffs. As soon as it gets into the water though it’s quite hard to start and still won’t idle and misses and jerks. Until it get me over 2000rpm. Ran for 30 min from 2-5000 excellent nice and smooth. Get below 2000 and it starts to miss. Shut it off and same thing hard to get running and into gear again.

Pulled the port side spark plugs looked good and the cylinder was hot to the touch. Pulled the starboard plugs and they were wet and the cylinders luke warm.

When I start it does start the really only way I can get it going is the quickly put it in gear push in the key give it kinda a kick in rpm and then I can give it throttle.

Gonna tear the carbs apart again. And look more into the idle or slow speed circuit. Can anyone confirm how these plastic carbs work? And confirm which jets should be were. I pretty confident I didn’t screw that up but who know. Also, what are the two bass tubes do? Vent or overflow? I know the one I could blow through but the other one didn’t really seem to move any air on all the carbs.

Any help, insight or where to look for info would be greatly appreciated!
 

saltchuckmatt

Commander
Joined
Jul 19, 2019
Messages
2,638
Spark jump test? Or switch coils and see if it follows.

At the very least do a cylinder drop test and see if it keeps running.
 

bigd450

Cadet
Joined
Aug 11, 2024
Messages
6
Yes done both those. Spark doesn’t seem to jump that far with all four cylinders. But always have good spark with the spark plugs. Champions spark plugs. When it’s idling I have unplugged every spark plug and everyone makes a bit of difference but the port side made the most.
 

saltchuckmatt

Commander
Joined
Jul 19, 2019
Messages
2,638
Spark jump test is good or not. Sounds to me it could be ignition coils or power pack. Mostly sounds electrical but you never quite know.
 

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,902
Compression? VRO still in service? Spark must be able to jump a minimum 3/8 open air gap with bright blue snap. If not DVA check stator output. These early small bore loopers are not the best idlers
 

bigd450

Cadet
Joined
Aug 11, 2024
Messages
6
Compression? VRO still in service? Spark must be able to jump a minimum 3/8 open air gap with bright blue snap. If not DVA check stator output. These early small bore loopers are not the best idlers

Compression 120psi each cylinder. Vro deleted older fuel pump installed. Checked fuel pressure 3-5psi at idle. Dva tested everything. All with in cdi electronics spec. 295v stator connected 170v at coil. Dva tester needed to be red to ground black to coil. Not sure if that’s normal cdi electronics didn’t think polarity mattered. Ended up with three power packs because of that.

I’ll have to double check the spark air gap again.
 

iggyw1

Ensign
Joined
Oct 24, 2011
Messages
954
Just a note to let you know about what I did when my motor acted that way. My motor is much smaller (35 H.P.) Mine shut off just like someone shut the key off and did that several times as I limped into the dock with my electric trolling motor. After a few minutes, my 35 H.P. motor would start up, run only about 2-3 minutes than shut off again, and again. I changed my power pack because it costed me only $48 on Amazon for a new one, and that did the trick. No longer had a problem with motor shutting off like the key was shut off! Your power pack may be reasonable to change too. Used ones are cheap, but may be no good.
 

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,902
Nedd to check DVA output when it dies. Could be a bad connection in stator and when it gets hot, epoxy expands and breaks connection.
 

bigd450

Cadet
Joined
Aug 11, 2024
Messages
6
Since the initial couple days it hasn’t completely die again. So can’t test it when it does. After completely going through all the electrical and fuel system it runs fine over 2000rpm. Anything under it misses and stalls. Have tried two other powerpacks same result.

Would like to find out how the idle circuit in these carb works and where the two brass tubes go.
 

cyclops222

Lieutenant
Joined
Mar 21, 2024
Messages
1,289
C D I Oh well.
You have adjusted Idle mixture and idle speed screws correctly ?
You seem to have 2 separate problems ? Low speed ? Hot engine stoppage ?
 

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,902
The small bores where briged port engines and bad about scuffing exhaust side of piston. Tis resulted in low crankcase compression to drive fuel pump and pull a smaller F/A mix. Also your carbs are fixed jet so no adjustments
 

bigd450

Cadet
Joined
Aug 11, 2024
Messages
6
The small bores where briged port engines and bad about scuffing exhaust side of piston. Tis resulted in low crankcase compression to drive fuel pump and pull a smaller F/A mix. Also your carbs are fixed jet so no adjustments
Kinda wondered about something to do with the fuel pump. But if I use the primer and the bowls will be full and that makes no difference.

Have thought about pulling the heads off. Should just order some head gaskets and check.
 
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