A Mystery to Me--Loong Post

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Bill E Bob
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A Mystery to Me--Loong Post

Postby Bill E Bob » Fri Jun 24, 2016 3:45 pm

'56 LoBoy, fresh engine overhaul. New .030 aluminum pistons, new crankshaft, new std bearings for crankshaft, new valve train/valve job, different crankshaft gear. This block was previously sleeved (all 4 cyl) all were bored to .030. Ring end gaps were checked and in spec. All bearings checked and in spec using plastigauge. 12v conversion using rebuilt alternator.
Oil pump primed before start up.

Problem: Immediate start up with good oil pressure and runs fine for a while, no undue noises etc. Engine will start to labor and shut down, would suspect seizing, but immediate restart with no drag on starter, runs awhile longer, labors, shuts down. Checked fan hub and oiled, checked alternator not seized. Pulled head, checked cyl walls, no scoring, scratches etc. Pulled rod caps, all bearings OK. Reassembled and same problem: Runs fine for awhile, labors, shuts down, immediate restart--repeat. Hydraulics not installed. converted to 12v coil. Plenty of fuel.

Am at a total loss with this one. :surrender: Any and all suggestions/opinions appreciated.

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Re: A Mystery to Me--Loong Post

Postby Don McCombs » Fri Jun 24, 2016 3:53 pm

You say plenty of fuel, but have you checked for good fuel flow from the tank through the carburetor?
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Re: A Mystery to Me--Loong Post

Postby Eugene » Fri Jun 24, 2016 4:16 pm

Sounds like a low fuel flow problem.

If it were an ignition problem, engine wouldn't immediately restart. Probably take 15 to 20 minutes before restarting.

If the engine were seized from heat, wouldn't immediately or within a few minutes restart.

Edit. Oil pressure about 20 to 30 seconds after engine start up?
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Re: A Mystery to Me--Loong Post

Postby Boss Hog » Fri Jun 24, 2016 8:10 pm

when it starts to cut off, pull out the choke and see what happens
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Re: A Mystery to Me--Loong Post

Postby slowpoke » Fri Jun 24, 2016 8:41 pm

A long shot maybe but the other fellow mentioned fuel flow.Friend had a similar problem with his boat turns out the vent for the fuel tank was plugged so that once you drew enough fuel out of the tank you created enough vacuum that the pump could not overcome .If you fuel cap vent is plugged the same thing would occur.Try running without the cap and see what happens.

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Re: A Mystery to Me--Loong Post

Postby Bill E Bob » Sat Jun 25, 2016 5:48 am

Currently using pony tank with in-line fuel filter. Will remove filter and see what happens. Will post results. Thanks for suggestions.

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Re: A Mystery to Me--Loong Post

Postby John *.?-!.* cub owner » Sat Jun 25, 2016 9:58 am

Bill E Bob wrote:Currently using pony tank with in-line fuel filter. Will remove filter and see what happens. Will post results. Thanks for suggestions.
Might try loosening cap on pony tank in case it's vent is plugged, also make sure filter is for a gravity flow system. If it is for fuel pump system it will restrict flow.
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Re: A Mystery to Me--Loong Post

Postby housebg » Sat Jun 25, 2016 10:46 am

I had a similar problem it did not die but it would stall down the power wire had broken and was just touching the starter and it did all kinds of stuff . Try running with a jumper wire from battery to coil and see cheep fix and may help Billy :)): :)):

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Re: A Mystery to Me--Loong Post

Postby Crimson Tim » Sat Jun 25, 2016 2:18 pm

Had a similar problem. There was rust in the tank. Cleaned out the fuel lines and it ran great. For a while.

So I rigged up a pony tank, keeping the inline fuel filter that the tractor had used before. It ran great. For a while. There was a little rust sediment in the bottom of the filter, but it looked ok. I removed the filter, and just ran the line about 8" directly from the pony tank to the carb with nothing else that could in any way cause a problem.
It has been running fine ever since, and have mowed 3 acres a couple of times with it now.

It did die on me once, but a quick shot of carb cleaner into the carb inlet screen, and it cleared right up.

From my admittedly limited experience with Cubs so far, in spite of the fact that Cubs are considered to be very fuel efficient, when they need fuel, they need fuel, and are VERY sensitive to any restriction or diminishment in flow!

Another quick test to determine if fuel is your problem: when it starts to stumble and die, unplug the hose from the air cleaner and give it a shot of starting fluid into the air inlet side of the carb. If it picks up and runs fine on that for a few seconds, then you have air, spark and compression, and you have just provided it a temporary source of fuel. Therefore, fuel is the missing element to run properly on its own.

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Re: A Mystery to Me--Loong Post

Postby Boss Hog » Sat Jun 25, 2016 5:27 pm

Bill E Bob wrote:Currently using pony tank with in-line fuel filter. Will remove filter and see what happens. Will post results. Thanks for suggestions.


Pull the choke out when it dies, if it helps you have fuel problems
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Re: A Mystery to Me--Loong Post

Postby Bill E Bob » Mon Jun 27, 2016 12:05 pm

Update:
Checked and changed carbs (prev one was flooding). Started and ran fine, oil pressure at high idle 40# low idle 20#
Plenty of fuel, timing good, runs fine till it gets warmed up, lugs down, then stops, immediate startup--repeat. Engine will low idle for longer period of time than it will at high idle or load. I give up! Will completely disassemble engine, check all clearances/areas of possible overheating/seizing and report back. This WILL NOT beat me!!
Thanks to all who contributed, you deserve an answer, may be awhile, but I will let all know what the problem was.

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Re: A Mystery to Me--Loong Post

Postby Eugene » Mon Jun 27, 2016 2:00 pm

Bill E Bob wrote:I give up! Will completely disassemble engine, check all clearances/areas of possible overheating/seizing and report back. This WILL NOT beat me!!
Probably be faster and a whole lot cheaper to solve the fuel flow problem.
I have an excuse. CRS.

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Re: A Mystery to Me--Loong Post

Postby slowpoke » Mon Jun 27, 2016 2:12 pm

Absolutely right about the fuel flow.Don't take that engine apart.

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Re: A Mystery to Me--Loong Post

Postby Clark Thompson » Mon Jun 27, 2016 2:22 pm

Assuming fuel and ignition are in working order I would suspect improper piston and cylinder clearances. I also had that problem with a engine that had been machined to book specifications. These days it is required to have the pistons in hand so the machine shop can properly fit them.cast iron pistons require .003 and aluminum pistons require .004. I had this problem a while back. test run a cub engine . When it got to operating temp the engine would labor and stop.Cooled down, would fire right up. Had to have machine shop refit the pistons.Solved the problem.
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Re: A Mystery to Me--Loong Post

Postby Bill E Bob » Mon Jun 27, 2016 4:21 pm

Thanks Clark, exactly what I'm thinking. Fuel flow is good, rebuilt carb, timing spot on, fan hub good, alternator good there is nothing left to explain the problem. This ain't my first rodeo and is my 4th engine rebuild. As mentioned, will report back, maybe a fault in my original measurements.


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