Briggs misses on full fuel flow

[38racing]

288707 flathead with walbro lmt carb on an old craftsman that I pulled out of storage. My recollection is that it was running when stored. So carb was totally sludged. I replaced it with one that had been on an engine that I had bench tested. Engine was used for a repower but it got the pump carb of the original. So it starts and runs but develops a puff like miss. I discovered that if I close the shutoff it smooths out in about 5 seconds. Reopening it miss comes back in 10 seconds. Very repeatable. I figured it was flooding. I decided to clean a different carb which I bench test and no flooding. Installed it and exact same performance. I bench tested first carb and it wasn't flooding. Jus for heck I installed it again. Same exact performance. Something I noticed once was thst it ran fine until I finished installing air cleaner assembly. Then missing started. Subsequent testing it would miss even with assembly off. Only other thought is that muffler heat is affecting gas in bowl. There's no heat shield as I've seen on some setups. Anything else I should look at?
One other observation. Even using a tach adjusting low speed mixture seems to have no effect.

[bgsengine]

Try a higher octane gas , just for kicks...

[KE4AVB]

To me it sounds like either you are pump feeding a Gravity flow carb or you do have needle seat problem under pump pressure. Pressurize to 7-10 psi and see if it holds that for 30 minutes while the carb is inverted.

Currently I got a Robin engine that if the fuel bowl is drained and then refill it leaks. But if I jar it, it stops. New needle doesn't fix the problem either. Nor recutting the seat helps. Now I got to order a carburetor and rework it so the current choke setup is transferred to the new carb. Current carb is NLA.

[38racing]

Try a higher octane gas , just for kicks...

Already on 91 ethanol free.

[38racing]

To me it sounds like either you are pump feeding a Gravity flow carb or you do have needle seat problem under pump pressure. Pressurize to 7-10 psi and see if it holds that for 30 minutes while the carb is inverted.

Currently I got a Robin engine that if the fuel bowl is drained and then refill it leaks. But if I jar it, it stops. New needle doesn't fix the problem either. Nor recutting the seat helps. Now I got to order a carburetor and rework it so the current choke setup is transferred to the new carb. Current carb is NLA.

Gravity feed. One of them ran fine on another motor (31 series) as gravity feed. I have maybe another 28 .Going to check shape of the carb on it. I will pressure test them all. Forgot about doing that.

[KE4AVB]

If running 31 series carbs on a 28 series that might the problem as they are jetted differently.

[38racing]

If running 31 series carbs on a 28 series that might the problem as they are jetted differently.

I thought of that but one is from a 28. It's 12.5 HP and I do have another 12.5 , just hoping its carb is cleanable.

[38racing]

If running 31 series carbs on a 28 series that might the problem as they are jetted differently.

you are right it appears , maybe. At least one carb that acts up has the jet screwed into the side of the tower. I don't know if the one currently installed is the same. I have 2 others from the flathead 28s and there are just has large holes on both sides of the tower, no jet. One was generally clean but the throttle plate was seized open. Of course I couldn't be patient and snapped the rod. so still in the process of making up another carb to try.

[KE4AVB]

Those removable nozzles are main the jets. If you look at the IPL for the engine you are working on you'll see depending which LMT it is there is specified main jet/nozzle.

In order to remove these nozzles you will need a straight sided hollow ground flat tip screwdriver bit. Most your 1/4 inch hex bits will be hollow ground but you will need to file off the corners of hex to fix inside the tower threads. Hollow grind help prevent cam out of the bit which destroys the nozzle otherwise.

[38racing]

well getting the broken throttle shaft out wasn't easy. I soaked that end of carb in MMO/acetone mix overnight. Still not moving. Only 1/4 of shaft to vice grip. Resorted to some heat. Guessing that made the difference but also discovered the heat went right to other end and melted the choke shaft lol. So I replaced the stuff. I did the qtip valve compound on the seat. Float level looked good. Inverted I could pump to 10 psi and then it opened. If I stopped before it opened it went down to 3-4. I tested the other 2 carbs and one acted the same and the other would not pump up to any psi. Anyway I installed it and gas poured out. I benched tested and it did flood. So I pulled needle and float from the other carb where float was slightly above level and it held pressure if I lightly held float. It bench tested fine. Installed on engine and it runs good. I was able to adjust rpm adjusting the low speed mixture. Was going to get deck back on today but then found the brake needed servicing so maybe deck tomorrow.