93 Johnson 25hp Backfire-ing multiple times at lower RPM'S

[cchildr84]

Re: 93 Johnson 25hp Backfire-ing multiple times at lower RPM'S

I couldn't see any spark, but if you say the sparks are coming fromt he fuel pump side of the motor, I had this same issue and it just turned out I didn't have the fuel pump fastened tight enough to the motor. So air a fuel were escaping where the fuel pump connects to the intake port.

 

[Triston Elston]

Re: 93 Johnson 25hp Backfire-ing multiple times at lower RPM'S

at 34-38 seconsds is when you cans see the sparks. C-oming from behind the fuel pump side but more towards the tiller.

 

[racerone]

Re: 93 Johnson 25hp Backfire-ing multiple times at lower RPM'S

There certainly is an air leak-------Can be coming from a number of places.----Any evidence that this motor was apart ?----------Bad bypass cover gasket ?

 

[Triston Elston]

Re: 93 Johnson 25hp Backfire-ing multiple times at lower RPM'S

I never took it completely apart i just do the normal maintenance to it, water pump, impeller, fuel pump, carb cleaning, lower unit oil etc. but where I see the spark is on the fuel pump side behind the fuel pump and towards the carb. I have no idea what that part is called.

 

[racerone]

Re: 93 Johnson 25hp Backfire-ing multiple times at lower RPM'S

Is it a round part with yellow wires going to it ??----That would be the rectifier and it is part of the charging system ?----Any chafed wires ??

 

[cchildr84]

Re: 93 Johnson 25hp Backfire-ing multiple times at lower RPM'S

Is it a round part with yellow wires going to it ??----That would be the rectifier and it is part of the charging system ?----Any chafed wires ??

My motor doesn't have a charging system but as raceone mentions the rectifier is attached to the starboard bracket. See part #33 in the attached diagram for the starboard bracket. Keep in mind this diagram is on an 81 25HP, but the bracket is in the same location on your motor.

 

[cchildr84]

Re: 93 Johnson 25hp Backfire-ing multiple times at lower RPM'S

[QUOTE.

Where is the base gasket located?[/QUOTE]

See item #8 in this parts diagram. I took a stab at your model number, but for this gasket the moded number is irrelevant. It's the same part from the early 80's to the late 90's. To change this gasket, obviously the powerhead must be removed, which can seem like a daunting task, but when you get down to it, isn't all that diffecult, just a little time consuming.

 

[Triston Elston]

Re: 93 Johnson 25hp Backfire-ing multiple times at lower RPM'S

I finally got around to messing with the motor again. As one of y'all said earlier in the thread to tilt the motor up and squeeze the bulb to see if gas comes out the carb. and I did and I did see gas slowly coming out the carb. when I squeezed the bulb. My question now is if this is normal or not b/c I would think that if you put too much pressure on the gas it will eventually find a way out the carb. but again I'm not too experienced with working on outboards. I want to thank y'all again for being such a help to a new iboats member.

 

[Triston Elston]

Re: 93 Johnson 25hp Backfire-ing multiple times at lower RPM'S

also would this be causing the back-fireing problem or not?

 

[cchildr84]

Re: 93 Johnson 25hp Backfire-ing multiple times at lower RPM'S

I can't speak with certainty whether or not this would be causing a back fire. Where on the carb is the fuel coming out? Is it somewhere on the carb itself or in between the carb and the intake manifold. I can say that no fuel should be coming anywhere out of the carb. At the very least I think a carb re-build is likely in order if it is leaking internally. That is to say leaking from the body of the carb.

 

[oldboat1]

Re: 93 Johnson 25hp Backfire-ing multiple times at lower RPM'S

Hey Triston -- I can't get a sense of what you would be comfortable with, so you probably have to decide that early on, if you haven't already. It's neat to have an action shot of the motor -- looks to me like it is leaking (blowing out air/fuel) on the side, but hard to tell. If you have a manual or a parts blow up (like the online one supplied), you can try to get at the gasket that might be a problem. Obvious point is that nothing should be misting out. If you feel like giving it a shot, you might first try to tighten up everything on the right side (side shown in the clip) -- fuel pump, exhaust cover bolts -- whatever is on that side. Do it by hand with a box wrench, and don't force bolts that don't want to move. Retest the motor after that exercise. If still leaking out, see if you can pinpoint where it is, even if you don't know what you are looking at.

On gaskets, there is one behind the carb, and another one behind the intake manifold behind the carb. Then the leaf plate and leaves behind that (which may be broken, by the way), and another gasket behind that. This is before you get to the cylinder/crankcase assembly and really the guts of the engine. You can get at these gaskets (and one or two I might have missed) without taking off the powerhead. My thought is that you might be able to get at offending gaskets without getting too far into disassembly and reassembly. This is pretty generic, but trying to give you a sense of what you might do without getting into something you would rather not deal with.

The exhaust cover should also be on that side at the rear of the engine (bolts you should have checked for tightness). There is a plate and gasket underneath, and you can get at that if the leak appears to be there.

So try tightening things up first, and if that doesn't cure the problem, maybe pull enough to get at the gaskets I've been describing.

There are many guys on this site who can do this blindfolded, and a number who have done it in professional shops for many years. I'm not one of those and not a trained mechanic, but I would take on the fixes I just described, so it can be done. And the second time around it would be a lot easier, but it just depends on what you are comfortable with.....

whew. kind of long-winded, huh?

 

[boobie]

Re: 93 Johnson 25hp Backfire-ing multiple times at lower RPM'S

Agree+1.......:laugh:

 

[oldboat1]

Re: 93 Johnson 25hp Backfire-ing multiple times at lower RPM'S

think I'll try a book. Oh wait.....just did that!

 

[Bosunsmate]

Re: 93 Johnson 25hp Backfire-ing multiple times at lower RPM'S

I finally got around to messing with the motor again. As one of y'all said earlier in the thread to tilt the motor up and squeeze the bulb to see if gas comes out the carb. and I did and I did see gas slowly coming out the carb. when I squeezed the bulb. My question now is if this is normal or not b/c I would think that if you put too much pressure on the gas it will eventually find a way out the carb. but again I'm not too experienced with working on outboards. I want to thank y'all again for being such a help to a new iboats member.

That was me and that is not normal. Your primer bulb should hold firm and no fuel should be leaking out and out the carbs. So long as you havent tilted it up much then the more you press on the bulb shouldnt really matter. The more fuel that goes in just makes the float go higher and makes it even harder for more to get in. Often if you do squeeze very hard (normally some muscle bound guy who likes using his guns) something will break on the hose lines etc not in the carb bowl.

If the prime bulb is not holding firm for a while with engine off and with that fuel running out the carb you need to adjust the float at the very least. But you may need to buy a new valve and seat and perhaps a float. Its not expensive and isnt hard to do so long as you dont drop parts in between the floorboards.

If you are getting too much fuel in there it will run rough and it will also backfire when that extra fuel in the exhaust housing decides to ignite now and again.

Pinpoint where the fumes are coming out before going further on that issue

 

[kodibass]

Re: 93 Johnson 25hp Backfire-ing multiple times at lower RPM'S

You have a gasket/leak problem, run the engine in the dark, when you see the flame mark the area with a paint pen, then you will know the area of the leak, this will home you in on which cover/part has the blown gasket/loose/corroded/warped/cover etc. Also remove your spark plug wires have someone pull the motor over and listen in the area of the paint pen mark you will most likely hear a hiss, for a visual place talc powder in the area and pull engine over look for a puff.