82 Yamaha 650 Maxim fires after petcock set to prime

Hi Everyone, I am new to repairing my 82 Yamaha XJ650 Maxim. I recently had the carbs cleaned and now the bike won't really start unless I set the petcock valve to prime for 15-20 seconds then set it back to on. Then it'll fire off and ride great for a few minutes then it'll become rough and eventually stall. Then, if I set the petcock to prime again, it'll run great. Rinse and repeat. I think there maybe a vacuum leak or something with the fuel injection, I am not sure.

Things I have done:

Cleaned carbs several times, including a rebuild kit
New spark plugs
New battery
I've tried starting fluid but I don't think that matters.
Boots on the carbs look fine and not cracked but I could be wrong.

Any help would be appreciated!

 

It sounds like your petcock isn’t letting fuel through when it’s set to ON. When you set it to PRI, the petcock functions just like a non-vacuum unit. It lets fuel run to the carbs no matter what the engine is doing. Your carb bowls will then fill up, and when you switch back to ON, the bike will run just fine for a few minutes until the fuel in the carbs is used up.

Have you tried reserve to see if that works? If neither ON nor RES allow the bike to run, then you probably have a problem with the vacuum part of the petcock. Chacal sells a rebuild kit for those, and it’s fairly easy to take them apart.

I’ll also say, there’s nothing inherently wrong with riding on PRI. As long as the float needles in your carbs are working properly, your bowls won’t overflow no matter what the petcock is doing. Just remember to switch it back to ON or RES when you shut the bike off.

 

I will try with the reserve on. I've never actually ridden with the petcock set to RES. It's good to know that running set to PRI won't do much riding for now. I will look at rebuild kits.

 

As a quick first check, make sure there is still a vacuum line from the petcock to the vacuum synch nipple on one of your carb intake boots, typically #3 but it doesn't matter which. 3 nipples should have caps, one should have a line to your petcock.

If the vacuum line is present, then holler at Chacal for a petcock kit as that's the most likely culprit.

 

Thanks for the reply! I am ordering a petcock kit this week!