JD H hand start no longer wants to start

I restored a 41 H a few years back. Entire engine was overhauled, bored .060 due to scoring. Always started easy, often on the first spin of flywheel. Secret formula for my H was no throttle, choke on, as soon as it fired immediately take choke off. The engine has truly never been broken in like it should be, unfortunately it sits most of the time in the barn. Early summer I put it in a parade and had it running for a couple hours. Back to the barn it went for a month. A friend came over and wanted to hear it run, literally started on the first spin. Sat for about three weeks and now all it will do is run for a few revolutions. Almost every time I spin the flywheel it will pop but won't stay running. Occasionally it will give me what seems like a mini backfire from the air cleaner. I have a nice blue spark from both spark plugs even from about a 1/4 inch away. Gas tank has fuel, if I open valve on bottom of carb bowl gasoline comes out. I can't understand how it can run so nice one day and not at all the next. I feel like it is not getting enough fuel but what can happen to a carb in a matter of weeks? The occasional mini back fire makes me think the timing is off but how can that be when I never touched it? The magneto bolts are good and tight, nothing has moved. My next step is to remove the carb and give it a good inspection. I had it running for about 30 seconds today by constantly turning the choke on then off, on then off, until eventually it quit. I can't understand how it can run so nice one day and not the next.
Any suggestions?

Float maybe sticking or you may have something in the fuel line. You need a good steady flow of gas out of the carb drain plug that will fill a pint jar in less then 3 minute 2 minutes is better. Had that happen to me this year with my Oliver S88. Found some junk in the sediment bowl assembly. Cleaned the sediment bowls assembly and problem went bye bye

ck the oil level mine just did the same thing. I finally cked oil and it had a lot of gas in it probley a gallon. I changed oil and filter cleaned plugs and started right up.

Its pretty easy and can happen for a speck of dirt etc to get lodged in the needle valve and seat area and/or gum and varnish to clog up and restrict gas flow (and needle valve full open and closing) in the same area. Its not all that hard to remove the line and the carbs bottom bowl to get to and inspect and clean the needle valve and seat. Or else you may be able to use spray gumout to clean the needle valve and seat area. If you open the carbs bottom drain cock and turn gas on you should get a continuous steady stream at least the size of lead pencil lead. Also insure any sediment bowl or other gas line filter are clean n free. I have also seen crud accumulate in the gas tank bottom in the outlet into sediment bowl area that restricts gas flow. Open it up and insure gas flows freely into and out the sediment bowl assembly, if not, check the gas flow out the tanks bottom. Is there a brass three way (gas/distillate) valve as used on all fuel tractors ?? If so they can get clogged up. Regardless of where any restriction is, you have to have a steady stream of gas flowing out the carb bowls bottom the size of a pencil lead.

Having to use choke sounds like inadequate gas flow so check all the above, carb needle n seat, sediment bowl, gas tank outlet area, any filters, brass three way valve. Is the tank properly vented ???

Those carbs also have that small circular screen/filter where the gas enters on the side which can get clogged up. If all else passes that may be the cause ??

This assumes it a gas flow problem NOT an ignition problem, I haven't addressed any of that.

John T

Thank you for all the suggestions. I will check each one out and let you know how it goes.