78 35hp backfires and doesn't start

lotsofish

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May 25, 2009
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Had the boat on the lake and the motor was running fine at first. Started it a half dozen times and ran around, and it was working fine. Then all of a sudden it just wouldn't start. Ran the battery dead trying to start it, pulled the rope for a half hour, and it didn't even get going.

I have tested for spark, rebuilt the carb (needed it anyway), and I don't believe fuel is an issue. If I crank it for 10-15 seconds, it will backfire out the exhaust.

From what I have found on this site, it looks like maybe the flywheel key has sheared or become loose? The problem came on all of a sudden, so I think this makes sense.

So question is, can I somehow check this without pulling the flywheel off? As far as I know, the flywheel has never been pulled on this motor.
 

JB

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Re: 78 35hp backfires and doesn't start

You have very likely sheared the flywheel key, lotsofish.

The only other thing that can cause that kind of backfire is switching the spark plug wires. Doesn't sound like you did that. :)
 

lotsofish

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Re: 78 35hp backfires and doesn't start

You have very likely sheared the flywheel key.

Yep, that's what I am thinking.

Can anyone tell me the size of the nut on the flywheel? I see I can rent a puller from Autozone, so I will do that.

I don't really know what the key looks like, or what the other parts under the flywheel look like. What should I be looking for. Anyone have pictures of a good key and a sheared one?

Where do I buy a replacement part if this is indeed the problem?

Thanks!
 

bktheking

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Re: 78 35hp backfires and doesn't start

Key

flywheel_key.JPG


sheared key


key.jpg



You'll know
 

torbjorn

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Re: 78 35hp backfires and doesn't start

Yep, that's what I am thinking.

Can anyone tell me the size of the nut on the flywheel? I see I can rent a puller from Autozone, so I will do that.

I don't really know what the key looks like, or what the other parts under the flywheel look like. What should I be looking for. Anyone have pictures of a good key and a sheared one?

Where do I buy a replacement part if this is indeed the problem?

Thanks!

A steering wheel puller will work but you'll need longer 1/4" bolts, preferably
hardened steel, if available, and some thick, strong 1/4" washers. 27mm or 1 1/16". Evinrude dealer for part.
 

lotsofish

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Re: 78 35hp backfires and doesn't start

Thanks guys. I will update when I get the flywheel off.
 

lotsofish

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Re: 78 35hp backfires and doesn't start

I rented a puller kit from the parts store. It's got a couple different types of puller and comes with a large array of bolts, all of which are too big!

Can anyone tell me exactly what size bolts I need to get when I go to the hardware store tomorrow?

Edit: 1/4"x20
 

lotsofish

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Re: 78 35hp backfires and doesn't start

I got the flywheel off. That was a chore...

Here's a picture of the key. Looks fine to me.
Sorry it's blurry, I guess my phone can't focus that close.



The white sensor thing was covered with some grease and metallic shavings. I cleaned everything off. Then I used the ohm meter to test the two sensors. The black and green one was normally ~500 ohms. As the magnets came by, it dipped to ~200, then open (or at least > 10k.) Then it would do the reverse when the next set of magnets went by. The white sensor read a steady 40 ohms.

After I put it all back together, I tried to start it. The first several attempts had lots of backfires. After that, it didn't backfire anymore, but it wouldn't run on it's own. There is exhaust coming out of the motor while cranking it.

So, what's next?
 

bktheking

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Re: 78 35hp backfires and doesn't start

Have you got spark on both plugs?
 

Brokeback Johnson

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Re: 78 35hp backfires and doesn't start

Check the low speed jet. I have seen where the rubber needle valve retainer does not provide enough friction and the needle turns out of adjustment during operation. I had one quit on me for this reason.....needle in fact turned itself to the seated (closed) position. Retainer is part number 0315232
 

lotsofish

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Re: 78 35hp backfires and doesn't start

@Brokeback johnson - you are referring to the externally adjustable knob for the carb, normally set to 1-1/4 turns out? Yes, this is set correctly.

It does have spark on both plugs. I am going to pick up a new set of plugs though and give that a try. They were new last year though.. The coils are 2 or 3 years old. If that doesn't work, I'm going to try to get my hands on a compression tester.
 

bktheking

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Re: 78 35hp backfires and doesn't start

Before you go and get something else, have you tried spraying 50:1 directly into the carb throat? Also, is the choke solenoid working when you try and start? Could be low compression or a fuel issue.
 

lotsofish

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Re: 78 35hp backfires and doesn't start

I was able to borrow a compression tester. 35 top, 30 bottom. That's gotta be it. I'm thinking head gasket because both cylinders have "gone out" at the same time. Does that make sense?

I'm going to need to find a repair manual before I start pulling the internals out...
 

bktheking

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Re: 78 35hp backfires and doesn't start

Probably the head gasket. You shouldn't need a manual to pull the head, once you determine if it's bad I can supply you with the bolt pattern and torque to reassemble.
 

lotsofish

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Re: 78 35hp backfires and doesn't start

Alright, I will give it a shot. The head gasket is original, so it's probably going to need to be replaced once I pull it off, but hopefully I will be able to tell if it had some problems.

When I was inspecting it around the gaskets, I found something strange. I don't know that this is causing my problems, but it's something I should probably replace. This is an aluminum button that is right between the two cylinders. Can I find a part number for this piece?

 

bktheking

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Re: 78 35hp backfires and doesn't start

That's just a disc with the model and SN of the motor, no need to replace it, it doesn't seal anything either.
 

lotsofish

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Re: 78 35hp backfires and doesn't start

I just got back around to working on the boat. Overtime at work and other house projects (landscaping, putting in new kitchen floor) took precedence.

The head gasket between the two cylinders was completely gone. I put in the new head gasket (I found somewhere that the bolts should be torqued to about 15 foot lbs). My battery was dead so it was hard to get an accurate reading on the compression test, but they were both at least 80 (cold) and the motor started!

I also found that the thermostat was looking pretty bad and I have one on order, so once that comes in, it's look out fish!

Thanks everyone for your help.
 
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