1979 Johnson 70hp motor. starts fine idles fine but dies when put in gear

uiuwrestler

Recruit
Joined
Mar 4, 2015
Messages
1
Hi all,

I bought this boat rebuild in march. re did the inside flooring and boat looks great. Started the engine up on muffs and all sounded great. Took her to the river started it up and let it idle for 5 minutes just to make sure all cobwebs were out of it. went to put it in reverse to back off the trailer and she died. started her up again and let her idle for a few minutes and then put in in forward and she died again. Frustrated at this point as I am new to boats but have always been around them in the water but never had to fix anything before on them. I am somewhat handy but not a mechanics by any means. I have already replaced the spark plugs even though they were under a year old. the carbs were rebuild by previous owner before winter time and I have put in new gas/oil. I am confused on what the problem may be. the carbs all look flawless clean and the fuel pump looks to be clean as well. I have read on here that timing or the jets are clogged but was hoping for some more clarification on the two or maybe other suggestions you all may have for me to look into. This is the last stop before id have to spend big bucks for a marine mechanic. ( I do have the manual for the boat motor as well)
 

1983 ercoa 21'

Chief Petty Officer
Joined
Jan 12, 2014
Messages
632
Step #1 do a compression test

step #2 do a cylinder drop test

it don't matter if the carbs were rebuilt yesterday if something was not cleaned properly they still will not perform as they should. Complete the steps report results and we will go from there.
 

1983 ercoa 21'

Chief Petty Officer
Joined
Jan 12, 2014
Messages
632
Welcome to iboats
there is alot of good people here that are more than willing to help.
 

willamettejeff

Chief Petty Officer
Joined
Nov 15, 2004
Messages
550
If all else is fine with your engine then problem may be as simple as having the idle set too low. When on muffs there is far less load placed on the motor by the water pump and back pressure from the engine exhaust so will idle at higher revs then when in the water. Adding the additional load of turning the prop when put in gear would then lower the revs even more. Correct idle adjustment can only be made with the engine in water. My 1978 70hp Johnson worked well when set at 1100 rpms on muffs. Would drop to 900 when in water and a little lower after engine all warmed up after a good boat ride. It is my understanding and experience that 70hp J/E outboards just do not like to idle at low rpms.
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,047
Those 70 hp motors in good condition will idle / troll all day long !!
 

willamettejeff

Chief Petty Officer
Joined
Nov 15, 2004
Messages
550
Those 70 hp motors in good condition will idle / troll all day long !!

That is true if the idle is set correctly. When I said low rpms, I meant in the 500-700 range which has been my experience with a 1978 70hp Johnson that did exactly what the OP posted at first, but with proper adjustment will idle all day and has for years. My .02 anyway.
 

1983 ercoa 21'

Chief Petty Officer
Joined
Jan 12, 2014
Messages
632
I ran one on my pontoon for awhile that died every time I put it in gear drove me crazy running jugs .
then I dropped from a 15 pitch to a 13 pitch 4 blade prop and it stopped.
 

1983 ercoa 21'

Chief Petty Officer
Joined
Jan 12, 2014
Messages
632
Have you checked for service bulletins there is a jet change that didn't get corrected on alot of these motors. I remember reading about it when I was trying to figure out my problem.
when I changed the jets it helped but it didn't stop till I dropped pitch on the prop .
 
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