Backfire, then no start

[jspringator]

My 25 hp backfired when trying to start it, then nothing; not a hit, after several tries. Any ideas?

 

[HighTrim]

Re: Backfire, then no start

Have you recently had the flywheel off?

Backfiring out of the exhaust port is caused by having the ignition system fire a particular cylinder when the exhaust port is open ( ie piston down). This would sound like a shotgun going off..... is this the condition that you're experiencing?

This is usually due to a sheared flywheel key, a flywheel with melted (moved) center sensor magnets if they exist, timer base leads routed incorrectly to the powerpack, or having the leads from the powerpack to the coils reversed to mention a few.

 

[jspringator]

Re: Backfire, then no start

Yeah, I just had the points and pick up coils replaced before I left for Florida. If it is sheared, will that be expensive to fix? If not, I'm going to take it somewhere else. And to think I didn't want to take off the flywheel for fear of not getting it on tight enough. I'm in a campground on the Suwannee River, and this has caused me to miss the last 2 days of boating. A plastic retaining pin that connected the cam follower to the throttle advance failed on the first day. Luckily I could get home by removing the cover and slipping it on when it fell off. A parts guy gave me the piece free. I just bought this motor off e-bay and already have $900 in it. I had to replace the drive shaft and re-seal the lower unit. If I could have gotten a unit this light, I would have bought new.

 

[wbeaton]

Re: Backfire, then no start

What year is this motor? Flywheel key or plug wires reversed are the first things that come to mind. Possibly the timing cam is upside down.

If the woodruff key is sheared it could have damaged the flywheel, crankshaft or both.

 

[jbjennings]

Re: Backfire, then no start

I was just working on my "new" 25 yesterday and it did exactly what you described because the float was stuck and practically no gas was going in the carb. Just an idea, though. Does it run at all if you squirt a little gas in the throttle bore of the carb? If you squirt some in and it runs a second or two pretty smooth, you know you're not getting gas in the carb.
Just a thought,
JBJ

 

[jspringator]

Re: Backfire, then no start

I didn't smell any gas, and because of that thought of a fuel related issue, but didn't think of banging the side of the carb to free the float. I hope thats it. I need a carb rebuild anyway. BTW, its a 1975. It was running earlier, so don't think plug wires were reversed.

 

[Lakester]

Re: Backfire, then no start

If the woodruff key is sheared it could have damaged the flywheel, crankshaft or both.

hello,

yes, as it is possible to break any mechanical system given enough force. a back fire could possibly do that kicking the timing out of tune. one thing to be sure in any event is that the woodruff key is properly positioned and aligned in the crank's keyway for ur particular engine.

my '71 evinrude 85HP's crank end has a tapered fit to the flywheel. the oem service maual is quite specific about how to place the key in its slot and to check its alignment to the crank's taper. it is possible to place the key parallel to the taper, but that is not the correct position.

while my key was ok first time i popped the flywheel off, the key was not correctly aligned to the crank's taper.

let us know what u discover, please.

regards
lakester

 

[Lakester]

Re: Backfire, then no start

My 25 hp backfired when trying to start it, then nothing; not a hit, after several tries. Any ideas?

hello,

check your plug for firing, relative to piston position. if none or incorrect u have: answer

if plug fires, try some fuel in hole and see if it fires off. see if it runs or just pops and dies.

that is what i would do if it was mine... for: what to do next?

if i got loud back fires, i would stop this and pop flywheel.

good luck

regards
lakesgter

 

[jay_merrill]

Re: Backfire, then no start

If I remember correctly, backfiring is also a symptom of damaged reeds in the intake manifold.

 

[CATransplant]

Re: Backfire, then no start

If it backfired just once and then wouldn't fire at all, it's almost certainly a sheared key. If the OP takes it back to the place that did the work and explains what happened, they should pop the flywheel off and fix it at no charge. Sounds like someone didn't torque the flywheel nut properly. The fact that this occurred shortly after the flywheel was off is further evidence.

While it's possible that damage occured, it's unlikely. The key is softer metal than either the flywheel or the crankshaft. Sometimes you'll have some spalling of the key material onto the surfaces, but that's not too difficult to remove.

 

[jspringator]

Re: Backfire, then no start

Apparantly it was my fault. I redid the throttle stop screw so that there was a nut on each end of the thread through the throttle. Originally 2 nuts were jammed together on the head end of the screw. According to my mechanic this did something that caused the backfire, and then the no start. He mentioned this was the same procedure used to kill the motor? I still have a hard time envisioning exactly what I did, but he said it starts and runs fine. Oh well, I'll figure it out eventually. I think the throttle stops may be more trouble then they are worth.