1/2 Throttle Only

[Get The Net]

Hi all,I was out with a mate the other day when his motor just drop revs and started to sound like you were trying to start the motor,but if you 1/2 throttle the motor would sit on 2500 rpm with no problem.The motor is a johno 70hp 92 model.Any idea

Steve

[a boat]

Hi Steve, if it was a sudden RPM reduction it sounds like you dropped a cylinder due to lack of spark. A bit strange though that it will run fine at upto 2500 RPM. What your mate wants to do is the basics first-make sure it has three good even compressions, it has three good sparks (able to jump about 1/2 gap) and the fuel is good and no water in it. If the carbies have never been serviced, being that it is 15 year old engine might be time to give them a once over.

Do the basic checks first and let us know what the results are.

Cheers,

Huey.

[tafc010]

Steve,

Had the exact same thing happen last week, Johnson 70 HP 1986 model. Ran fine all day on two of it's three cylinders. The one that was a dud was number one (the top one)

Got back to the shop and started playing. All cylinders had spark, so we swapped plugs (maybe a dud plug) but all were OK, swapped coils, same story, number one did not fire but had spark so that tells us no fuel.

Pulled air filter thingie off to get to carb drain bowl screws. start at the bottom, number 3, bowl full, number two bowl full, number one dry as a (yeah you know).

So now lets look further, the fuel bulb (black thing you can pump up) would not go hard and would not push fuel through vaccum the fuel pump fitted to motor. however if we pressurised the tank fuel would run out through the filter no problems.

So the end result was the bulb had passed on and there was not enough fuel pressure to fill the top carb, replaced bulb and rock n roll, it all works again.

CYA

Geoff

Edited October 24, 2007 by johnno

[pelican]

Steve,

Had the exact same thing happen last week, Johnson 70 HP 1986 model. Ran fine all day on two of it's three cylinders. The one that was a dud was number one (the top one)

Got back to the shop and started playing. All cylinders had spark, so we swapped plugs (maybe a dud plug) but all were OK, swapped coils, same story, number one did not fire but had spark so that tells us no fuel.

Pulled air filter thingie off to get to carb drain bowl screws. start at the bottom, number 3, bowl full, number two bowl full, number one dry as a (yeah you know).

So now lets look further, the fuel bulb (black thing you can pump up) would not go hard and would not push fuel through vaccum the fuel pump fitted to motor. however if we pressurised the tank fuel would run out through the filter no problems.

So the end result was the bulb had passed on and there was not enough fuel pressure to fill the top carb, replaced bulb and rock n roll, it all works again.

CYA

Geoff

Geoff you might want to check the diaphram in your fuel pump if the issue you encountered was as described. Fuel bulb may not have been your only issue or you may have had a sticking or dud float in the carby bowl. Have had ones where there is gunk in the bowl and even with the drain screw removed nothing comes out until you poke a toothpick in. You really don't want one cylinder running lean. You can always clean the plugs run it and check their colour if you are not sure if all pots are resonably tuned. Good to see you were able to diagnose the issue and as with all 2 strokes 4 big options - fuel , spark , compression and a battery to turn it over.

Edited October 25, 2007 by pelican

[Get The Net]

Thank you,I have spoke to my mate he had no problem when he flushed the motor out only when is on a load it played up,we feel that it's electrical but not 100% sure.

Steve

[IWANNABROCK]

If playing up under load check coils and plugs, still wouldnt hurt to check fuel in carbies though as well.

[tafc010]

Hey all, just got back from the shop after jacking around with our Johnson 70 which was not a happy camper.

Between us and the good folk at Fishraider we fixed it.

1) The one way valve in the bulb had collapsed (actually broken)

2) A small peice of junk had lodged under the needle & seat of Carb #1 from the one way valve

3) Half an hour later and it runs like a dream.

4) Hopefully out this weekend after some jew.

Thanks to all for their suggestions.

CYA

Geoff

[pelican]

Good to see you up and running again and isn't it a great feeling when it is a cheap simple fix.

About the fuel bulb, make sure it is a decent quality one. Many of the cheaies are crap and will deteriorate quickly and let you down. Some suffer really quickly from ethanol or fuel additives. Also make sure it is one of the larger ones as the smaller ones don't have enough flow rate for older carby motors.

Pel

PS Often after seeing bulbs replaced I see air / fuel leaks. Make sure to trim the old pipe back an inch or so which will enable the barbs to get an airtight seal. Also use quality stainless clamps of the right sze and not so tight that they cut the fuel hose or have sharp bits protruding. Hose that is old or has deteriorated or cheap will often kink and may restrict fuel flow. If very old the inner lining can errode and clog carbys and fuel bulb valves

Edited October 30, 2007 by pelican