75 25HP won't start - backfires

[whywhyzed]

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[F_R]

Re: 75 25HP won't start - backfires

You have to remove the flywheel to see the key.
Coil wires mixed up?

 

[whywhyzed]

Re: 75 25HP won't start - backfires

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[HighTrim]

Re: 75 25HP won't start - backfires

What plugs are you running? What is their condition?

 

[JB]

Re: 75 25HP won't start - backfires

Is the backfire a "POOF", sneeze or a loud bang from the exhaust?

You might try the troubleshooting in Engine FAQs. Though it seems you have done some of it, you haven't done all of it.

 

[Ranger 330V]

Re: 75 25HP won't start - backfires

check the wiring to coils over and over..lol. I checked mine 4 times and still missed it. Someone had swaped the wires going into the plug from the powerpack. I'd also check timing. I know when I was getting my 140 going I had to make for sure the timing was at least 2 degrees at idle..if any lower it would get rough and I'd get a backfire once it retarded much under 2 degrees. Runs like a top now. Replace the coil and do a link and sync. Like someone said earlier is it a sneeze/cough or a loud pop/bang out of the exhaust?? I had both..lol. Just got done rebuilding carbs and took care of the sneeze. One of the small round gaskets on the top carbs jet plug was leaking air and I would get a sneeze...well more like a hard cough...lol. Good luck!

 

[whywhyzed]

Re: 75 25HP won't start - backfires

Is the backfire a "POOF", sneeze or a loud bang from the exhaust?

You might try the troubleshooting in Engine FAQs. Though it seems you have done some of it, you haven't done all of it.

It's a loud bang out the exhaust.

I went through all of the "Engine Won't start" in the FAQ thread. going back to review. What I know I didn't do was fog intake with gas as I didn't have any kind of sprayer. and I haven't checked the flywheel key yet.

 

[HighTrim]

Re: 75 25HP won't start - backfires

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

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

 

[whywhyzed]

Re: 75 25HP won't start - backfires

well flipping the ignition leads is certainly easiest to try. then I'll check the flywheel key. and I'll throw new plugs in. Thanks all. I will be back with results.

 

[F_R]

Re: 75 25HP won't start - backfires

That white stuff looks more like a bit of corrosion from here. You sure it's sealant? Looks like blue wire #1 is going to top coil, which is correct. I'd check the key 1st. And breaker points while you are in there.

 

[HighTrim]

Re: 75 25HP won't start - backfires

FR, who says ur old, u got eyes like a hawk lol jk

 

[whywhyzed]

Re: 75 25HP won't start - backfires

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[HighTrim]

Re: 75 25HP won't start - backfires

Are you using grade 8 hardened bolts on the puller? She should pop off. Thread them in 7/16 then give her heck. No oil or heating is needed or reccomended.

 

[Lakester]

Re: 75 25HP won't start - backfires

hello,

many times when using a puller, and parts stuck. u can load the puller up tight with its nut/shaft... good and tight!... almost to point of bending something... use some oil on threads, too... then let it sit super loaded for up to 1 hour. then come back with a BFH [big ball peen hammer] and smack it HARD with a cracking smack to the center shaft of the puller. often times this will upset the bond enuff for it to let go.

hope it helps.

regards
lakester

 

[F_R]

Re: 75 25HP won't start - backfires

A 25hp flywheel isn't hard to get off, compared to some others. Use your puller with grade-8 bolts as already said and tighten that bugger till it pops off. If it still won't come give the center bolt a whack or two with a hammer. But the big hammer is overkill, (note kill). A 12oz is ideal. It's the shock that does it, not the bash.

Don't be heating that poor thing.

 

[JB]

Re: 75 25HP won't start - backfires

As above, but use a mallet rather than a hammer or put a wooden buffer over it before smacking it with the hammer.

 

[G DANE]

Re: 75 25HP won't start - backfires

Those Cranckshafts have endplay. Have a helper lift the flywheel - only possible a fraction of an inch, when you whack the puller center bolt. Off it goes !!

That Motor looks brand new, btw, the cleanest I ever saw, take good care of it. 130 PSI is stock compression, new. If your buddy holds the throttle linckage open manually, it will only backfire, not start. Do a sparkcheck, put an appropiate nail in the sparkplug boot, let your buddy hold it with an insulated plier, with the nail 5/16 from ground. If both produce bright blue spark til ground with a snap, leave ignition out for now. It has points ignition and it will not produce a good spark, with key shared, unless flywheel is exactly 180 deg turned. Test and let us know. JB tinkered with a ton with that ignition, i bet, he helped me out with my first.

 

[whywhyzed]

Re: 75 25HP won't start - backfires

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[itstippy]

Re: 75 25HP won't start - backfires

What is the model number of your 1975 20HP Johnson? Just curious. I thought they quit the 20HP in 1973 but in the last couple days I've heard reference to both a 1975 and 1976 20HP.

 

[jspringator]

Re: 75 25HP won't start - backfires

Those 22 CID 25 HP OMC's are very valuable because of their light weight. Probably weighs less than a 4 stroke 10.