40hp Johnson won’t plane

[Gantz6]

86 40hp Johnson won’t plane, replaced tank, fuel lines, fuel pump. Saw air bubbles and low fuel in clear filter but seems to be running fine on idle and low speed

 

[racerone]

Sounds like it is running on 1 cylinder.---Check compression and spark.

 

[TMW123]

You can try a cylinder drop test to see if its running on one cylinder. At idle, use well insulated pliers to pull each spark plug lead. If pulling the lead makes no difference in sound/rpm then that cylinder is not running. Air bubbles mean there is an air leak and you should tighten the clamps or check connection seals. Try cleaning the carbs? High speed circuit could be clogged up. Honestly, in your situation i would just clean the carbs to cross that off the list.

 

[Gantz6]

I did cylinder drop test and both had good spark and motor stumbled with both. I’ve cleaned carbs multiple times, taken them off and thoroughly cleaned them. Found loose connection at tank nipple threads, tightened but stll can’t get clear filter to fill. Can’t really test until on water

 

[Gantz6]

I did cylinder drop test and both had good spark and motor stumbled with both. I’ve cleaned carbs multiple times, taken them off and thoroughly cleaned them. Found loose connection at tank nipple threads, tightened but stll can’t get clear filter to fill. Can’t really test until on water

 

[racerone]

You did verify that the high speed jets screwed into the BOTTOM of the float bowls are clean ?-----And if you cleaned the carburetors multiple times maybe it is NOT a carburetor problem !!-----You have checked the flywheel key ??---Post the compression numbers.----Perhaps test run with another tank and hose.

 

[Gantz6]

Before tightening tank nipple, last night when it wouldn’t plane I removed carb cover and both carbs seemed to be getting plenty of fuel, also am using 75-1 mix because plugs seemed dark and wet. And still look that way today. A little history, this began whe wife and I took boat out 1st time this year and it hit plane then after slowing down to stop, hit plane again but very hard getting there, then wouldn’t get there again and sounds like it’s working too hard

 

[racerone]

Using 75:1 mix proves once again that lubrication in 2 strokes is not well understood.-----Water pump impeller looked at ?----Thermostat in place and working properly ?----Something is wrong and up to you to do the inspection / trouble shooting.

 

[juno pierrat]

if you have clear filter that's just a plastic cylinder with two nipples, have seen them on commercial lawn mowers and they seem to always have an air bubble in them or is it a clear spin on
is spark advance/timing advance moving freely , not sticking?

 

[racerone]

Is it an " air bubble " or is it fuel vapour.----No need to automatically assume it is air !!

 

[iggyw1]

I am NOT a motor mechanic by no means, but I had a few boats in the past that would not go on plane all of a sudden.I know what my problems were and I see nobody addressing them here, so I will mention these two things to check out.

Check the screen on your pickup tube inside the fuel tank to see if it is blocked with some crud. That could be restricting the gas flow at a higher RPM. Second, check the hull to see if it is full of water or water logged preventing the boat from going on plane. Both of these were problems for me in the past as far as my boat planing while motor was at WOT. Just a few things to check while trouble shooting.

 

[racerone]

Some cheap plastic tanks and hoses are nothing but junk.-----That is why I often state ----" test run with another tank and hose"

 

[Gantz6]

Replaced all fuel lines and tank because of old fuel( bought used) and rebuilt carbs. Then insulation under cowling was filling carbs with tiny black flecks, took awhile to realize that hence so many carb cleanings! Now have new tank, new lines, clean carbs today again, checked floats, put back and as usual runs great in driveway. Also tightened loose nipple on fuel tank, will know tomorrow if any of that will fix it. Also removed vro and installed fuel pump. Will rebuild vro and reinstall at some point

 

[TMW123]

Check the condition of your ignition coils. My 25 merc did the same thing.. got worse as it heated up. Check them for cracks and ohm them out to make sure they are in spec. You could be dropping a cylinder at speed but not idle/low rpm. Easy thing to check.

 

[Gantz6]

Compression test both cylinders 115, removed carbs AGAIN and sprayed all jets clear and checked floats. Got on water still not planing, then began running horrible wide open, stumbling and zero power. I’m lost at this point. Are there any adjustments on Sierra fuel pumps?

 

[Klink]

If the engine has an electric priming pump that is controlled by pushing in the start key, pump it when wide open to see if it speeds up. That'll inject fuel directly into the cylinder, bypassing the high speed jets. If it speeds up, you have a carb jet problem. The jets need to be cleaned by taking the carbs off and injecting the jets with Isopropyl alcohol with a syringe, so you can visually see the alcohol come out the other end as straight as it comes out of the needle. Dunking the carbs in carb cleaner does not really assure clean the jets.

Did you do a synch and link?

 

[juno pierrat]

maybe spun hub in prop?