I have a dinosaur johnson 15hp outboard motor that has never let me down. The problem im having is that as im trolling along it will sometimes almost want to stall so i gotta rev it like hell to keep it going and then its fine for another 20 mins or so. The carb is fine and i replaced the small fuel pump so they aint the problem. What i was told though that the connectors that connect to the gas tank and motor have some kind of seals in them that wear out after time and then air gets in and buggers up the fuel flow. I feel this is the problem as the bulb on the gas hose that you squeeze doesnt stay hard and i have to keep squeezing it every so often to build the pressure up again so i tend to think that the air is leaking cause the seals are shot. Hopefully thats all the problem is. Any suggestions?

Gary

I'm an old engine freak.

Do you have a single hose or a twin hose???  The twin hose actually pressurizes the tank.  Twin hoses are one piece but have a figure  eight cross-section.  Some times the Johnson has a button on the tank to pressurize the tank on twin hose models.  Twin hoses were the standard till about 1960.

I assume that you have a single hose with a single bulb about 3 inch long that looks like a lime????

  That Bulb has two check valves, on on the inlet and one on the outlet.  They are used to get the engine started with fuel.  There at no valves in the tank, only an air inlet valve, it should be open to run.  And that air inlet valve, usually on the cap never fails closed.  So it should not be the problem.

Most older engines had scavenge fuel pumps.  they use the negative/positive pressure below the piston to pump thru a double diaphragm pump.  the only moving parts are the diaphragms, and it sounds like that is what you replaced.  ???????  It is not necessary to have the bulb remain hard after it runs for 5 minutes or so.

Normally I'd have said to rebuild the pump diaphragms, that is pretty easy job.  If you did it that cause would be eliminated.

Some thoughts: 

Old hoses usually start to deteriorate, they can put out a small particle of rubber, they clog fuel filters and the needle valves in the carb.  I's change the hose.  It is cheap and you will get a new bulb.

The hoses sometimes leak air at the connectors, they stress crack there.  check them by bending them hard.  Air leaks in the hose reduce the fuel that gets to the engine.  Some people test this by smearing heavy grease on the hose at the connectors.  It results in a temporary seal and the engine doesn't stall.  You could cut back the hose about 5 inches on either end and put new hose clamps on it. 

Most engines have a thermostat in the engine head.  Often they are by-pass valves that let the water from the pump flow past the engine when the engine is cold.   some act just like the ones in your car.  Sand and grit get in and block them.  again they are usually cheap.  If you have to take the head off to change add a new gasket for sure.

Good luck.

BEAR

Hi Bear

Its the single hose line. The fuel pump is fairly new and was only used a few times towards the end of the fishing weather last year. How much cost am i looking at if i have to replace the thermostat? If i have to remove the head, the pull start wire coil wont spring all over the place will it. Ive only ever took the carb off to clean the needle valve cause it was sticking so ive never even though about the head?

Gary

When the needle valve gums or get clogged, it usually stays constantly bad.  The vacuum leak by a bad hose is something that can change with speed.

The Sherlock Holmes in me is interested in "The fuel pump is fairly new and was only used a few times towards the end of the fishing weather last year."  If that was not properly rebuilt, and one of the check vaves (thin rubber flap is hung up) it would work just like you say; not having enough suction to pull the gas. 

quick test:  When it starts to stall, if you pump the primer bulb quickly to make it hard, does it behave good.  If so I'd bet on the a hose replacement (about $20 at West or cheaper on e-bay) OR The new rebuild is not working correct!

BEAR 

When im at trolling speed and the engine starts to cough and sputter and wants to stall i start reving the throttle like crazy to get it purring again and its usually good for a little while then acts up again. Yes sometimes i prime the bulb and it does make a better difference as well. When i remove the spark plugs one is usually more soaked in gas than the other. I also have a 40 hp evinrude that im having probs with but ill ask you about that once we can figure this out.

Gary

Hey Bear,

Heres the problem i have with my Evinrude 40hp. When i go at full throttle it just flys beautifully along the lake. The problem im having is when i try it at trolling speed It jerks sharply like cachunk cachunk cachunk almost as if you try to drive off in your truck in 5th gear. Its probably needs an adjustment if anything but id like to hear what you think first. Its the same boat ill be doing the fiberglass work on for my charter business.

Gary

Gary,

Tough to get it all right on the net.

But, if it clears up on low speed when you pump the bulb and it clears up when you rev the engine; then it is surely a lack of fuel.  The pump/filter/line is not working right.  A new fuel line with connectors is a fast first step.  Also check the fuel filter, also a cheap replacement item.  What seems to be happening is that on high speed the fuel pump is pulling enough gas out of the tank, as the vaccum is sucking very hard.  On slow speed the engine is reving less and the scavenging pump doesn't have enough vaccum/suction to pull the gas up to the cylinders as the engine needs after the card bowl gets low.  Either the fuel system has a leak (that hose again) or the diaphram pump is running poorly.  Since you mentioned a recent rebuuilt on the pump, I'd consider that immediately.

The wet plug is something else.  Often a rich mixture.  Or a poor ignition on that plug, like a HV wire shorting out.

The plug and the pump could be related.  It the ring on one cylinder is shot (cracked,etc) then on low speed that same scavening diaphram pump does get the lower crankcase pressure it needs.  I'd make sure I had a full charges battery, and crank the engine fast with a pressure gage in that "wet" plug cylinder.  Then compare it with the same test in the other cylinder.  A bad ring will be very low pressure compared to the non-wet cylinder.

Boy, aren't these old engines fun.

Hope some of this is helpful.

BEAR

Well i went looking for some stuff for my outboard problem. I got a new needle valve, gas hose with bulb and i had to order spark plugs. They had a helluva time getting plugs even with cross reference as my motor is so old (1968 Johnson workhorse 15hp) that the original plugs it used werent even listed in the manuals. Tomorrow ill replace the fuel lines in the motor itself then we'll see what happens.

Gary

A '68, that's a new one.  The majority of the ones I worked on were '57 & '58 models.  OMC switched from dual hose to single hose systems in '62.  On the 15 I'd change the plugs if the fuel system checks out. 

On the 40 it's a low speed carb adjustment or ignition (advance) issue.  If you can get it to run well at low speed but then it won't run at high speed, meaning it will do either high or low but not both it's likely an issue with the crankcase seals, reed valve (if equipped) or the advance is sticking. Only other thing I've seen that has caused that is a repaired crankcase, (rod punched a big hole and it was welded up.)never could get it to run right after it was put back together.   

Hi Murf

I took my old Johnson in to get repaired and they had to replace the coils on that one because at idle it would misfire so you may have a point as to my 40 hp as well with the coil problem.

Gary 

Well guys, i got my Johnson outboard back today and there was alot more wrong with it than just fuel. The coils were cracked, the carb was plugged and the pistons were misfiring due to them overheating. The motor would run great then cough and die after 5 minutes in his tank. If he leaned it out more one way then the other piston would misfire so he couldnt balance it out. He ended up having to drill a hole and putting a plug in it. So thanx to you guys for the advice you gave me but it turned out to be a bigger job than i thought

Gary.