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Old 05-03-2019, 10:28 PM
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Default Backfired and dead

I've got the 212 hemi and I've done your normal mods, removed gov, ported the head, air filter, straight pipe exhaust. It was running great and I was actually happy with it. I'm going down the road at a pretty good speed, roughly 4500-5000 rpm, I go to slow down to turn around. As it slows down, it gets to a low rpm, not sure how much, it backfired and died. And that's it. Will not crack for nothing. It's getting spark, carb veins are clean, everything in the head is good. Would it be the flywheel key? Or what else could it possibly be?
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Old 05-03-2019, 10:33 PM
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Fairly good armchair diagnosis. Are you sure it was a backfire? Sometimes the flywheel key shearing in two is accompanied by a pop or knock.


To be a true word-meaning stickler: If it's out the exhaust, it's an afterfire.
Backwards or through the carburetor is a true backfire.
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Old 05-03-2019, 10:44 PM
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So check the flywheel key? It was a loud pop/knock. No I'm not sure it was backfire I just and assumed that's what it was, but you know what you get when you assume. Like I said, it was running good so it caught me off guard.
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Old 05-03-2019, 10:58 PM
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Wouldn't hurt. Check it.

Might be a good time to lap the flywheel to the crankshaft if you haven't already. Remember: the flywheel key only locates the flywheel in the correct position relative to the crankshaft and thus to the piston. A well mated taper on the crank and flywheel, plus the nut is what holds it on.

If the tapers match, free of obstruction, tool marks, etc, you can actually discard the key and set, retard or advance the timing just with the tight fit.
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Old 05-04-2019, 01:48 AM
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If everything under the valve cover looks hunky-dory, then I’d also suspect a sheared key. The two times I had an engine unexpectedly stop working it was A.) rocker arm came off the pushrod or B.) the flywheel key broke.

Took me a while to diagnose the flywheel key. I had compression, I had fuel, and I had spark. Then I realized the spark wouldn’t do any good if it happened at the wrong time. Lo and behold, sheared flywheel key.
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Old 05-04-2019, 06:47 PM
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Well checked flywheel key it's intact and fine. I'm gonna go back to the head and go back thru it and double check. If everything is fine there, where should go next???

Hellion, you said "lap the flywheel", not familiar with that. I've been inside the case and the head but this is first time in the flywheel side of the motor. And actually fairly new to the small engine. My experience with small engine are from boat motor.
Thanks for the help.
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Old 05-04-2019, 07:33 PM
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Engine's not seized right? It will turn over but not fire? Could be piston/connecting rod (heaven forbid) or an issue in the valve train (a pushrod, broken valve or valve spring?) leading to a loss of compression.
It is really hard to diagnose engine problems over the interwebz.

Lapping the flywheel to the crankshaft is old school. It's actually a preventive measure to keep from shearing flywheel keys. Lapping a boat motor flywheel (outboard motor) would be ideal to help prevent you being stranded out on the water.

Go to the 6:30 mark in the video to get right to the lapping process:



Our member KartFab did one too, at about the 1:19 mark:

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Old 05-05-2019, 12:56 AM
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Did you run out of gas? Lol
Step 1.) make sure there is fuel in the tank.

You can get an idea of compression by pulling the spark plug and putting your thumb over the spark plug hole while pulling the pull start. You should be able to feel the compression try to push your thumb off the hole.

Did you try to start it with starting fluid?
Take off the filter, open the throttle, squirt 1-2 seconds worth of starting fluid into the intake port, then attempt to start?
Report back with the results.
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Old 05-05-2019, 08:50 AM
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Frankly I'd check the ignition coil/wire/boot first..
those can overheat and crack.. and the bad thing is..
they could spark when cold or the wire is bent to make proper contact, but still fail when installed.

If you got a timing light, check for sparks with that,
if you got a spark indicator plug boot, try that..
best: if you got a spare running engine, swap the coil and test with the 'new' one

I assume you checked your flywheel magnet and realigned the coil when taking a look at the flywheel key, right?
with a bit of luck the coil just moved out of alignment (bump in the road or such)

And if that's checked and known to be working
my next guess would be carb manifold gasket (sideair causing ill air/fuel ratios)
or worse valves (which show as compression issues)
Anything really (valve seat, bent pushrod, misaligned rocker... sheared tooth off the cam...)

So yeah, as suggested by hellion and bob, test as much and thoroughly as you can
for spark fuel and compression, and if everything is looking good,
you will need to take a more thorough look inside
starting with the valve cover turning the engine over slowly w/o a plug to see if there's something fishy going on with the valves or rockers..
and if that also looks okay we will all have to cross our fingers for your engine internals.

I have not seen a rod failing on just 5k, but since you ported the head and likely increased compression while being at it....
maybe you should've indeed gone for a billet rod already..
maybe yours' now slightly bent or you busted a bearing or something.
*shrugs*

I hope it's something as simple as a cracked spark plug wire.

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Old 05-05-2019, 11:57 AM
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Why would air be coming out carb?
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Old 05-05-2019, 12:39 PM
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It depends on where.
Where it meets the cylinder head, or at the air filter?
It there's air coming out of the carb, it's probably a vacuum leak, and that could keep the engine from running.
The "backfire" you heard could, just could have been an intake gasket blowing, but IDK.
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Old 05-05-2019, 03:11 PM
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When I'm pulling pull cord, it makes a sort of hissing sound. I'm at a total lose on this. Bout to the point of just going and getting new motor.
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Old 05-05-2019, 03:19 PM
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Hissing when you crank on it? Won't start?
Sounds like you've lost compression.
Check the head gasket, head bolts, valvetrain(especially valve faces and seats). Doesn't seem like that unfixable a problem.
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Old 05-05-2019, 05:11 PM
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Is there a video I can watch, so I can make sure I'm checking everything right. I've checked checked and checked. I've tore it down twice and went back over to check myself both times. It's fixing to go in the lake. Being frustrated. I'm just not getting something or missing something.

---------- Post added at 06:11 PM ---------- Previous post was at 06:10 PM ----------

To see if I'm missing a gasket or something wrong or whatever.
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Old 05-05-2019, 05:58 PM
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A hissing sound heard on every stroke of the piston (hiss-hiss-hiss-hiss) as you're pulling the recoil starter sounds like a vacuum leak. You coulda blown your head gasket. Otherwise everything being AOK, you'd likely hear air escaping just from the exhaust stroke (out of the pipe), more or less.

Also, when you're looking at the head gasket, it may not be an obvious, catastrophic "blow" with carbon/soot/discoloration everywhere. It could be a tiny break in the material of the gasket. You got to examine it closely, take it off and examine both surfaces.

Where you live (city and state)? Maybe someone nearby can do a house call.


Here's a last ditch option--take off all your speed parts and extras, return it to stock configuration and with receipt in hand, return it to Horror Fright for an exchange/refund.... assuming of course you're within 60 (or is it 90 days?) from date of purchase.

Not uncommon to have a faulty engine right out of the box, and while you did add "speed parts" and were probably stressing the motor slightly beyond its specified, boring industrial specs, it coulda been defective from the get-go.
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Old 05-05-2019, 06:09 PM
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Hardly within 60 days, I've had going on 3 years and never had this or anything close to this, hence why I'm confused and frustrated. Thank you all for the help, I'll check the head gasket once over real good and still have nothing it's water bound minus the good parts. On a good note I've got my money's worth. And will have another, been thinking about the 420cc. Thoughts on it?
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Old 05-05-2019, 06:17 PM
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Post some pics of your engine during tear-down, maybe someone will notice something amiss?

Make sure your pushrods are straight, not bent, and are still connected to the rocker arms.

420cc? No experience with it and not sure it would fit on my Mancos and other assorted yard karts which were configured for Tecumseh or Briggs flatheads of the era. . .

Going up so drastically in engine size and bulk, I personally would be looking into a motorcycle engine and gearbox or something. Maybe even 2-stroke.
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Old 05-05-2019, 06:26 PM
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uh if there's really coming air out the carb it's valve train of sorts.
engine surgery is due I'm afraid.

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Old 05-06-2019, 08:24 PM
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Finally. The exhaust valve and intake valve were backwards. The exhaust was opening when piston was descending, hence blowing air and puff of gas back out the carb. I just assumed when I lined dots on crank and cam shafts that the valve was in time too. Still construction how it happened but lesson learned, like always the hard way.
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Old 05-07-2019, 08:22 AM
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The moment when you realize you put valvetrain in backwards.
Oops.
Glad ya figured it out!
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