1985 120hp Johnson missing after starting

[Kiwi482]

Hi all - have purchased a 5.2m(17ft) boat with 120hp Johnson. Its in really good nick for age as its not run alot and fresh water only. However after starting and running for 2-3mins to get out of harbour then accelerate it starts missing badly and really jerking engine until pull back to 1000rpm. Does this for about 5mins then runs beautifully for next 3-4hrs use!!
Have given de- carb,fuel filter clean etc to no avail.Seems like its lack of fuel but only early on. Have looked through posts for info. Any ideas welcome. cheers

 

[oldboat1]

Re: 1985 120hp Johnson missing after starting

Hi Kiwi. need to supply a model number. What is meant by missing -- a cough at a carb, for example, or maybe at the exhaust?

 

[Bob_VT]

Re: 1985 120hp Johnson missing after starting

Welcome to iboats.

First I would suggest a real original shop manual and here is a free electronics troubleshooting guide to check out the ignition Technical Support | CDI Electronics

 

[Kiwi482]

Re: 1985 120hp Johnson missing after starting

Thanks for replies guys - its J120TLCO so 120hp v4 1985 20" shaft. What happens is you start ok, head out idling or about 1500rpm, then when you go full throttle it will either a) start missing badly and wont reach high rpm or b) gets up to say 4000rpm then after a minute starts to cut out almost completely and really 'cough',miss etc - you bring back to idle or low rpm then go again. It might do this for 2 or 3 times then -bingo - runs smooth as! Just runs perfectly for rest of the day!! Quite frustrating I have to say. When she runs for skiing etc its great. Seems like it needs to warm up yet its got quite warm coming out. Or its fuel related...
cheers

 

[boobie]

Re: 1985 120hp Johnson missing after starting

Sounds like an ignition problem. Do a cylinder drop test on it when it's acting up and see what cylinders are acting up. Or else use a timing light.

 

[stratos150xp]

Re: 1985 120hp Johnson missing after starting

Just throwin this out there, but i had a similar problem, i would start the boat throw it in gear, and it would jerk like crazy, after several good jerks, it would go.........i could idle back down, put it in neutral, back in gear and it was great.....but everytime i would shut the motor off and start it again it would do the same thing, jerk real bad several times, tgen it was fine till the next time i shut the motor off....my problem was it wasnt fully going into gear. But was weird cause tge only time i experienced the problem was after i had just started tge boat.....as long as i kept it running, i could go from forward to neutral to reverse, and vice versa all day long.....

 

[Kiwi482]

Re: 1985 120hp Johnson missing after starting

Hi all - just an update. Thanks to Bob for the CDI info - that was great. I checked the plug leads and coils. Had no spark on #3 cyl or it was intermittent. Swapped over coils, cleaned all connections and ran engine. Got a spark on all 4 cyl & ran sweet on the trailer. Just have to do an on water test now and Im hoping its cured it !
cheers