Mine is exactly the same as yours Jimmy. Runs great in reserve, cuts out in on. And like Sandy, tore it all apart and found nothing wrong.
Things to check:
1st make sure there's no gas in the vacuum line, very important. If there is, replace or convert to manual as shown in the tech section.
2nd, replace the vac line. Darn thing cracks. And put on hose clamps, any kind.
3rd, remove petcock and completely tear it apart. Check the diaphram for holes, dead mice, etc.
and last but not least, although alot more involved, check your float valve and the screen in the seat. I've just discovered this little gem and have yet to do it. Crap in the screen could restrict flow just enough to need the extra pressure that reserve will give you.