Re: Can someone explain back drag air jet?
What the Back-Drag does is lean out the motor at higher speeds, for economy. The suction on the float chamber lowers atmospheric pressure inside, and this restricts fuel flow. The vent jet provides a controlled opening, i.e. a smaller jet would restrict air rushing in the float chamber and result in a leaner mix; a larger jet would restrict less air and do the opposite. Merc found out this wasn't such a good idea, leaning out an I-6 makes it prone to detonation and burnt pistons/rings. Especially with the alcohol-laden fuels we have today.
It's recommended to remove the vent jets if you can get them out. If not, it's not gonna hurt that much since the Back-Drag feature won't work if the little black hoses are plugged. Since the bottom vent jet is already loose, and it's the hardest carb to get at, the other (2) should be a piece of cake!
Take the back-drag hose off at the float cover, plug with a small BB or something similar and then you can plug the hose back into the float cover.
On your idling/dying in gear issue, it may be merely a case of idle mixture adjustment along with idle speed. The back-drag feature has no effect at idle, BTW.
What you'll want to do is get the motor up to operating temperature, then adjust each idle mixture screw, one carb at-a-time, first in Neutral then in gear.
In Neutral, slowly turn the first carb's idle mixture screw in (Clockwise) until the motor speeds up, then starts to die. If the motor just starts to die and never speeds up, you're way too lean already. Then, slowly turn the mixture screw CCW (rich) until the idle speed picks up; turn it a bit more and it'll go a bit "burbly" then slow down noticeably if you go any more CCW.
Note the range of adjustment you just had, from full lean to full rich, and leave that mixture screw on the rich side of the adjustment.
Do the same for the other (2) carbs. These adjustments will probably affect your idle speed, so you'll need to adjust the Idle Stop at the right side of the distributor stop bracket. Loosen the locknut and turn the idle stop CW to raise speed, CCW to lower. Be sure not to touch the Spark Advance or Throttle Stop screws, which are the 2 screws closest to the carbs.
Once you have a decent idle speed in Neutral, shift to Fwd gear and let it idle.
Repeat the idle mixture screw adjustments until you get a smooth idle. Leave all (3) mixture screws on the rich side of the adjustment.
Your 1500's carbs have no accelerator pump, as an automotive carb would, so the idle mix has to be a bit rich or it'll bog on acceleration. This rich setting doesn't necessarily make for the best idle, but it's a compromise and that's the best you can do.
After you've tweaked the carbs, you may need to readjust idle speed to get a stable idle in gear. Don't run the idle speed way up, just enough so it'll idle smoothly. If the idle speed is way too fast, it's gonna be real hard on the gearbox. When shifting to a gear, be sure to make a sharp motion with the control lever, and don't let the gears grind, as it'll damage them.
Check your acceleration and if you get a bog, adjust each carb approx. 1/8 turn CCW (rich), then check for bogging again. If it still bogs, try another adjustment. If no success after a couple more adjustments, you may have other issues we'll have to address.
But try the carb adjustment first, most times it's the problem.
HTH...........ed
