Ummm...how do you use a petcock with "Prime"

[cptarmy2002]

Ok...I feel like a total noob for asking this...but here goes.

Brought my 1980 GS550 back to life (118K miles) always been a great runner, but after sitting for so long the tank finally rusted out to the point of no return...

Rebuilt the carbs and purchased what I thought was a NOS 1980 GS550 gas tank. Turns out it was a 1979 GS550 gas tank with the old style "Prime", "On", "Reserve" petcock and vacuum tube.

Bike started and ran beautfully with a few shots of starter fluid...ran it about 20 miles and pulled it into the garage to grab something from the house. Went back out to start it up...and nothing...no start.

Petcock was in the on position.

Bike cooled down about 45 minutes...went out and figured it has something to do with the "Prime" so I put that on for like 20 seconds, pulled the choke and switched it to on and bang...2 cranks it fired...

Ok...so my long drawn out explanation leads to my question again...what's the starting procedure for a bike with the old type "Prime" petcock?

Thanks,
Frank

[82GS850]

Frank,

The ON and RESERVE positions use engine vacuum to allow flow. The PRIME position does require vacuum and is used to fill the carb bowls.

[avastyematey]

the prime position is a mechanically operated flow position on your vacuum based carb. on and res both use the vacuum that is pulled off of your carbs, while the PRIME (Sorry i Transposed my positions) position is a gravity fed mechanical position that bypasses the vacuum. Any of them should work.

Does your bike have a vacuum hose that runs off of the carbs?

If it ran for 20 minutes and then didn't it might not be the carbs. If it happens again check to see if you have spark, if not it sounds like my problem, which people seem to think is the ignitor....

[Dave8338]

PRIME DOES NOT USE VACUUM. That is why it is there. If the bike sits for a long time OR if the carbs have been off and need to be refilled...use the PRIme function.

Don't believe me, test it yourself. Heck, put it on PRIme and go inside for the night (do this outside please) and when you come out in the morning, if your float needles are not working perfectly...I suggest a change of oil before you attempt to start the bike...