struggling with '75 15hp manual start

The saga continues. Thought I was in the end-game of this rebuild but still struggling getting the motor to stay running. Motor will turn over and run for about 15 -20 seconds and then will just die. Have already replaced fuel pump, and rebuilt carb. Slow speed needle has been set in accordance with the info on the threads on this site. How can I tell if I am too lean or too rich? Can't seem to get the right combo of throttle, choke and carb settings. Also, the slow speed knob on the stbd side of the motor is gone. I can see there is a cam which appears to rotate the armature plate. Will this position impact the ability of the motor to idle without dying? Can't find anything about it in my service manual.

Thanks

When you overhauled your carb did you remove the core plug on top of the carb and clean the low speed passages?

Yes, cleaned and blew out carb, put in new carb kit and replaced slow speed needle which was worn on one side. When I first got the motor I checked the compression and it was low (around ~35psi). Since it was about the same on both cylinders and the fact that it is a pull start, I did not think it was an issue. Would low compression cause the motor to run this way.

3 things a engine needs to run, fuel ,spark, compression, the 35 lbs of compression seems pretty low the low idle adjustment nob could be turned in just a little to much or is your gas can looseing prime try starting engine then grab the bulb when she starts to die try pumping it up see if that helps, did you check your fuel pump it might have a pin hole in diaphram, try giveing her a little more throttle at start up to see if she will stay running

some of these motors can be pretty tempermental i have a 1955 7.5 a 1954 7.5 and a 1958 10hp and they all have there own little sweet spots where they like to run and not one of them have left me stranded

Bulb stays hard so think it is getting gas. I have been starting at almost full throttle and then backing off throttle and choke to try to get it to run. Seems to want to run at higher throttle settings, but dies when I throttle back. I think I am going to recheck compression with different gauge and continue to mess with slow speed needle setting.

I'd pull the head & take a look. That compression is too low to operate effectively. If everything looks good (cylinder walls, head, etc.) try a decarb on the engine.

Can you pull the cylinder head and check out the gasket without taking the whole power head off?

On most engines, yes. You need to use a "light touch" removing the bolts. If any are seized, you need to heat the threaded are of the head with a propane torch or you will break the bolts off, and have a world of trouble. (Light touch= no breaker bars, 3/8 drive ratchet, hold it half-way up the handle only, and don't force anything.) Also recommend 6pt socket so you don't round off the bolts.

Very sage advice from martino1. If the cylinders/pistons are bad you'll have to decide whether the rebuild is worth it on that '75 model.

Thanks all. Definitely going into uncharted waters here. Still looking to find out if I have to remove the whole power head just to check the head gasket. Manual walks through the power head removal before tearing the engine down, but also covers an entire engine breakdown- not just the head gasket (ie rings, piston, etc).