Throttle Mystery, I don’t know what is happening.

[MacTank]

When I throttle up the bike there is a chance it will bog out and die before the bike is warmed up. One the bike is warmed up it will sometimes go up normally but the revs will go down slowly. Or it struggles to go up high at all. I have my suspicions that the carbs need to be vacuum synced but I am unsure. Can anyone help? I do have a video but apparently it is too big...

[flyboy]

I had the same issue once and immediately suspected carbs. Three carb shops and 600 bucks later, as the issue continued showing up at random times, I decided to learn how to fix my own shit and found a bike shop to volunteer as a mechanic's assistant (and for the better, cause my transmission got stuck at the beginning of the riding season a few years later, but that's another story)

In any case, my issue was caused by a plugged vacuum line on the right side of the carb AND an aging petcock.

Cleaned the vacuum line, cleaned the petcock, and the bike immediately started running fine.

That said, it came back once a month later, in the scorching heat, in the middle of nowhere. I ordered a new petcock the next day, and once I replaced it, I never had any more issues of that sort (that was in 2014 I think).

Apparently on old petcock diaphragm can get sticky when hot, and it tends to show up at random times, counterintuitively even in the middle of the ride. Like, if a vacuum pressure dips for a sec while riding (such as when flicking the throttle), it can allow the spring to overcome the vacuum and allow the diaphragm to contact and get stuck (either fully or partially closed). This will start starving your engine of fuel. This may not be immediately apparent as there is a bit of fuel in the lines and the carb bowls left, but if you open the throttle, the air can't get into the line to replace the fuel fast enough, and it starts to show as engine bogging down.

One way to test if this is the culprit is to simply switch to prime the moment you notice this. Prime position mechanically overcomes the diaphragm spring in the petcock allowing the fuel to flow to the engine - you don't even need a functioning vacuum for this to work. It may take a minute to get the engine running well again once you switch, but just keep it there and see if the issue comes back. Just make sure to take it out of prime the moment you kill the engine - you don't want to rely on your carb float valves to keep the fuel from seeping into your ignition chamber (I have a "fun" story about that, but that's another post).

If the engine continues running fine on Prime, you either have a vacuum issue, or a bad petcock, or both.

Now, I'd definitely yank the carbs out, clean them well (if you're not mechanically inclined, let me know), and make sure the vacuum side is clear. I'd check the vacuum hose too.

The old me would also say to clean the petcock, then hook up the vacuum hose to it, suck on the other end, and see if that makes the fuel flow out of the tank. The old me would say that's an appropriate test of the petcock functionality. But the old me would also say that 4 Non Blondes rock.

The new me remembers how I did just that after cleaning up my petcock and it worked like a charm... in the shop... until it didn't, once I was on the bike in the middle of said nowhere.

Apparently I can suck really well... uh, on the hose... get your mind out of the gutter - I'm saying I can suck on that vacuum line better than the carb vacuum can, so it only seems like the petcock works when I'm in the shop. Carb vacuum can be very subtle. Myself, I'm not known for that quality.

Anyway, the new me says order a new petcock, rather than try to clean the old one. The new me also says Tom Waits' Nighthawks at the Diner is the best album there ever was. The new me also has more money than the old me, so if you're strapped for cash, go ahead and clean the petcock rather than getting a new one. But make sure to ride the snot out of it, and do it in hot weather, before you attempt any long trips. For some reason, the heat seems to exacerbate the stickiness of that petcock. So if it passes that test, I guess it's fine to ride to hell and back.

TLDR: switch to prime while riding, and if that makes the issue go away, clean the carbs (pay special attention to the vacuum) and change the petcock. And listen to Tom Waits. Or not...