compression question 1998 johnson 40hp

whats the normal compression on a 1998 johnson 40hp 2 cycle outboard? im trying to trouble idling problem.

engine starts and runs great in the morning with a full tank of gas.


we can cruise all day BUT
later in the day runs great past idle speed but does not want to idle.

things that have been done by previous owner in march of this year.

new fuel pump
new water pump
new plugs
carbs rebuilt and adjusted

things i have done
new gas tank (12 gallon portable) replaced the existing 12 portable
new fuel lines, primer bulb
sea foam ran thru 2 different tanks
on last trip out yesterday i brought an extra 5 gallon can of mixed gas and when tank was almost 1/2 full i topped off tank and still ran ok

i tested it several times by cruising at full throttle for awhile , going to neutral and idled fine
run at 1/2 throttle cruised several minutes, pulled back to neutral and idled fine

there is about a 5 minute no wake zone close to boat ramp where i cruised at about idle speed,, then we got back to boat ramp then it won't idle... AGAIN

this is a consistent problem.


temps and humidity are high... 90 degrees plus everytime we have taken it out this summer

is electrical ,, stator heating up and causing problems?

tank is vented and i have also adjusted screw cap tighter and looser with no effect. primer bulb appears to maintain pressure

im borrowing a compression gauge to rule out compression problem first and foremost

as always thank you for the help and advice

Get that compression checked and report back.

Boobie said:

Get that compression checked and report back.

Click to expand...

Thank you sir will do should have gauge later this week

If possible get us a cold test and hot test. Throttle, (carb butterflys), should be open, especially important on 4 strokes. Take all spark plugs out for test. Do a spark jump test too when cold and also hot. The 60 Evinrude I just finished will jump 1/2" with a crackling blue spark. When you do your spark test have the throttle in the same position as where you are experiencing your problem.

Finally had chance to do a cold start test. Top and bottom cylinders dead on 125 psi each. Will report back when i can do hot test