Another Carb question and still stumped

[seabeelt]

Been working on the 48, Its had starting issues since last year. Had the mag rebuilt and rebuilt the carb myself, but still no joy in getting the old girl running. Seems to draw fuel and flood the carb with the choke on but the plugs look to be dry. It will crank and backfire in the carb only occasionally but not start. So I thought I might have rebuilt the carb wrong (not my first carb rebuild btw). Went all through it again. Same results. Pulled an old carb off the shelf ( the fuel inlet threads are stripped and the pick up tube is broken off), but I noted one significant difference. In the previously running carb #63349-C91 the venturi in the carb is 5/8" while the carb from the shelf #251234R92 the venturi is 1/2" What gives????? Both have the same throat size going to the manifold (15/16")

Just as an aside, I have pulled the carb from the running 53 and get the same results as the other carb on the 48. Mag is fine, timing is fine with the impule kicking over right on the timing pointer. Firing order is correct and I have blue spark at each plug. Compression is 100+ across the board. Cant think of anything else unless the manifold gasket has failed or the intake manifold is somehow plugged. Going to pull it as soon as I get a new gasket just to be sure.

Any thoughts from the great brain trust here?????

r/
Michael

[Jim Becker]

I'd recheck the timing (from scratch).

[Eugene]

ignition timing problem.

Magneto is firing to early. Point gap? Rotor timing?

[seabeelt]

Just redid the timing from scratch. I know these are pretty simple motors so it's either spark or fuel.

My thought of ruling out spark is that a.) I redid the timing from scratch. b.)Point gap is correct, new cap, rotor and wires, new coil in the mag and c.) had it checked by one of the folks here that rebuilds them. d.)Firing order is correct and in the correct direction. e.) Blue spark at each of the plugs. f.) Impulse clicks right on que with number 1 cylinder at the timing pointer.

To me that rules out the spark part of the equation unless you folks think I have missed something here.

That leaves fuel. Like I mentioned, even with he carb flooded the plugs look dry. Only 1 and 4 look even remotely like they might be damp. With the carb rebuild and even a swap from a known runner I'm leaning to the fuel not getting to the cylinders.
Assuming that the carb is good from the runner, if it's not getting fuel from carb to engine that leaves carb to manifold gasket ( this is a new gasket) , the manifold gasket - looks to be original and the manifold itself - possibly cracked - haven't seen anything to indicate that either has failed from an extrior visual inspection.

I did notice that when cranking the engine the exhaust pressure from the muffler was higher than the vacuum pressure from the intake when placing my hand over the openings

Is it possible to partially plug the intak manifold???

Tractor is always in the barn when not used and the air breather and hoses are also in good condition so there doesn't seem to be any evidence of something getting in there through the the oil bath fllter and carb to plug it up.

I'm truely stumped

[Eugene]

Still ignition timing problem. Backfire out carburetor, ignition is firing early. And you swapped out with a know good carburetor.

Does your pulley have two timing marks?

Timing light. Have someone crank over the engine by hand. Check to see where the magneto fires in relationship to the TDC mark on the pulley and the pointer. Check the ignition timing on both cylinder #1 and #4.

Magneto impulse trips at the appropriate point. Great. The magneto fires when the points open, which is a separate issue from the location of the impulse coupling tripping.

Triple check spark plug wire routing.

[seabeelt]

Only one mark on the pulley.

[Bill V in Md]

Are you checking the spark with an in-line tester. The in-line tester insures that your plugs are sparking during compression. Also, check the point gap at both high points on the cam to eliminate misalignment. The surface of the points should also be aligned when closed, and make sure they are clean.

[seabeelt]

Yup, using an in line spark tester. Points are new and clean

[Rick Spivey]

Double check that the pinion gear and rotor are timed together, using small cast marks (line on rotor, dot, line, or triangle on pinion. It sounds like ignition timing to me. Possibly 180 out, and everything would look good except backfire instead of good fire.

[muleboss]

Valves stuck open???

[tmays]

Governor timed to the engine?

[seabeelt]

Thanks for all the suggestions. No stuck valves - good compression (100+) on all cylinders. I'll double check the mag setup tomorrow, but I'm pretty confident it is correct

[seabeelt]

Governor is also timed to the engine unless something broke internally.

[tmays]

Like the other guys, I lean toward a timing issue. You're thinking fuel related. Obstruction in breather? While you had carb off, did you place something in intake to keep critters out? Maybe it's still in there? Or dirt dauber has plugged intake up?

[Papa's Cub]

Seabeelt, It you're sure the timing is right and you're not getting gas into the cylinder to wet the plugs, then I agree with Thomas there must be a restriction/blockage somewhere between the carb and cylinders. Pull the carb off one more time and place the palm of your hand over the intake inlet while spinning the engine. You should feel significant suction. If not, you have a blockage. Just thinking, Tom.