24 HP Intek Twin Backfiring through Carb

dcpropilotMay 13, 2014

Hi,
Please help!! I tried a bunch of troubleshooting things but still can't find the problem. When throttle is advanced, the engine 'bogs down' and backfires. So far I am stumped.
I have a new carb, new fuel lines, valves are good, new fuel. Taking the spark plugs off one side at a time has no effect, leading me to think the problem is common to both cylinders. My thought was that it could be a sheered flywheel key...nope. Next I tested the diodes. I tested both lines that lead to the armatures and they both are at 5.91V. Don't know if this is acceptable for determining if the diodes are good or need to be replaced. Any suggestions?

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jimmy56_gw

I'm not sure about this but did you check the governor.

    Bookmark   May 13, 2014 at 9:00AM
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mownie(7)

You reported that you are reading 5.91 V on the circuit to the magnetos. Are you seeing this voltage with the engine running or with the engine not running?
Have you tried running the engine with the kill wires disconnected from the magnetos?
Disconnecting the kill wires from the magnetos would certainly isolate an ignition system issue from any other reason for the behavior.
CAUTION!!! If you do disconnect the kill wires from the magnetos, you might not be able to kill the engine by turning off the key. You might have to operate the choke to kill it or smother the carb with a towel.
Caution!!!!, if you run the engine with the flywheel housing removed, you will overheat the engine due to lack of air flow across the cylinders. Keep any test runs with the cover removed to only a few seconds of time, with long rests between runs. A well positioned box fan or squirrel cage fan blowing across the engine will help it cool between runs.
Does the engine kill easily when the key is turned off now?
Or does the engine almost sound better for a second or two just after turning the key off?

    Bookmark   May 13, 2014 at 10:12AM
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dcpropilot

Voltage reading is being done via the service manual method, using a voltmeter using the diode test function, so the engine is off and there is a lead at the 'polarized connector' and a lead at the end of the wire that attaches to the armature/magneto.
No issues at all regarding the key being turned off. Good point about leaving the squirrel cage off. Hopefully the engine isn't damaged from any possible overheating. I'll disconnect the kill switch and report back. May be a couple of days though..waiting for replacement rocker cover gaskets..

This post was edited by dcpropilot on Tue, May 13, 14 at 15:17

    Bookmark   May 13, 2014 at 2:37PM
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dcpropilot

Ok...I found what might be the issue. The camshaft and crankshaft timing marks are off by about 6 gear teeth. Now I have to figure out how to pull either the camshaft or crankshaft up enough to re-align the timing marks. What would be the best way to do this? I have no idea how it got out of timing in first place.

    Bookmark   May 20, 2014 at 10:17AM
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mownie(7)

***" The camshaft and crankshaft timing marks are off by about 6 gear teeth."***
SAY WHAT?? !! ??
I don't think we are getting a complete story on the recent history of this engine.
There are only a couple of ways the camshaft gear and the crankshaft gear can "get out of time". One way is for them to have been installed "out of time", or two, the engine was treated carelessly while the sump cover was removed for some kind of repair.
It is possible for a camshaft to slip out of position if the engine is tipped the wrong way while the sump cover is off of the crankcase. It is also possible for the cam gear and the crank gear to "jump one another" if the engine is rotated while the sump cover is off of the crankcase.
But both scenarios REQUIRE that the sump cover be removed from the crankcase.
The camshaft and crankshaft gears DID NOT become "untimed" while the engine was operating.
And, how is it that you are able to report that you have learned the camshaft and crankshaft are off by about 6 teeth, without some sort of preface to the news such as " I took the sump cover off and I see that the camshaft and crankshaft are not timed correctly."??
You would not keep secrets from us, would you? :^)
To get the camshaft into the correct index, it is best if you remove the push rods (so there is no downward pressure on the camshaft) and then slip the camshaft out of its bearing bore in the crankcase just enough to turn the cam gear and crankshaft to where the timing marks on each align correctly.
Do not put the push rods back in place until the sump cover has been installed and properly torqued.

    Bookmark   May 20, 2014 at 11:22AM
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dcpropilot

It may have happened when I changed the crankcase gasket a few months ago. It's my own ignorance for not knowing that might happen. At this point, everything is back together after spending most the day on this. Tomorrow I'll fire it up. Hopefully light at the end of the tunnel!

    Bookmark   May 20, 2014 at 10:22PM
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dcpropilot

Started it up this morning....seems like it's back to normal!!

    Bookmark   May 21, 2014 at 8:40AM
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mownie(7)

OK, good to hear that. Lesson learned? :^)

    Bookmark   May 21, 2014 at 10:27AM
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dcpropilot

Definitely!

    Bookmark   May 23, 2014 at 8:56AM
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