Weird problem after trying to fix vaccuum line

Admiral Crunch · July 06, 2006, 09:14:12 AM

Well, I've just given my bike its first valve adjustment (since I've owned it anyway) and got everything back in spec. I also did a little work on the fuel hoses. The previous owner told me the fuel hoses weren't right, and it only runs in PRIME. It goes dead in a minute when in ON or RES. So I decided to see what was up while I had the tank off for the valve work.

Everything else was hooked up properly except the vaccuum line. It was attached correctly to the petcock on one end, but the other end was not attached to anything, and it had a scew screwed into the end of the hose to seal it off. The port where the vacuum hose should attach to the left carb was capped off by a plastic cap and held on by a spring clip. (At least I think it's the correct port. It's on the right side of the left carb toward the front.)

I removed the screw and the cap, and I plugged the vaccuum hose into the port on the left carb and used a spring clip I had to secure it. I fired up the bike, and after a few minutes, it died in ON, like it used to. I switched to PRIME, and it fired up again, but it's running rough at anything above an idle. It runs, and it moves down the road, but it kind of stumbles, and the power is coming unevenly. It idles fine, but the problem starts at about 3500 rpms.

Could a poor vaccuum cause these symptoms, even with the PRIME setting? I guess I need to undo what I did to the vaccuum line and see if the problem goes away. I don't think the valve adjustment would be responsible, but it's one way to find out for sure.

The Buddha · July 06, 2006, 09:17:56 AM

Vacuum attaches to the left carb 1/2 way down on the front ... and its facing the right carb ... that needs to be capped off if not used for vaccum. the top is OK ... it will give you vacuum ... but cap the lower one off if you use the top.
Cool.
Srinath.

Egaeus · July 06, 2006, 09:19:31 AM

Sounds like you could either have a leaky hose, or a busted petcock diaphragm. It sounds like a vacuum leak.

Admiral Crunch · July 06, 2006, 09:32:32 AM

Quote from: seshadri_srinath on July 06, 2006, 09:17:56 AM
Vacuum attaches to the left carb 1/2 way down on the front ... and its facing the right carb ... that needs to be capped off if not used for vaccum. the top is OK ... it will give you vacuum ... but cap the lower one off if you use the top.
Cool.
Srinath.

I'm using the one on the side, pointing toward the right carb. The one on the top is just supposed to be used for balancing the carbs, right?

Admiral Crunch · July 06, 2006, 09:33:52 AM

Quote from: Egaeus on July 06, 2006, 09:19:31 AM
Sounds like you could either have a leaky hose, or a busted petcock diaphragm. It sounds like a vacuum leak.

Thanks. I'll take the hose off and see if I can find any leaks in it using water. Is there a good way to test for leaks where the hose attaches?

aaronstj · July 06, 2006, 10:22:12 AM

Suck on it.

At least, that's what I'd do. *shrug*

The Buddha · July 06, 2006, 10:25:49 AM

OK runs in prime ... and dies in on or res ... OK yes big freaking leak from that vacuum line ... Sorry didn't read it quite right. Forget testing it, replace it ...
Cool.
Srinath.

Admiral Crunch · July 06, 2006, 01:03:29 PM

Quote from: seshadri_srinath on July 06, 2006, 10:25:49 AM
OK runs in prime ... and dies in on or res ... OK yes big freaking leak from that vacuum line ... Sorry didn't read it quite right. Forget testing it, replace it ...
Cool.
Srinath.

It does run in prime, but it runs like crap. Does that still sound like a vaccuum issue?

Thanks all for your help!

The Buddha · July 06, 2006, 01:12:29 PM

Yes, vacuum leak will make 1 cyl very lean ... or if leaking in both ... it will make both lean ... not what you'd call ideal ... so it will run like sheite ...
Cool.
Srinath.

Admiral Crunch · July 06, 2006, 01:20:40 PM

That makes sense. There's a leak, so the carb is sucking in more air than it's meant to, thus lean mixture.

Thanks!

Wrecent_Wryder · July 07, 2006, 04:04:33 AM

j7

Admiral Crunch · July 07, 2006, 07:13:20 AM

Quote from: Wrecent_Wryder on July 07, 2006, 04:04:33 AM
Which carb the vacuum line runs to depends on what year it is, too....

It's a '91. The service manual says it hooks to the left carb, but the diagram doesn't actually show it attached, because the angle of the picture is from the front left of the carbs, so the hook-up is behind them.

LimaXray · July 07, 2006, 08:25:03 AM

As Egaeus said you could also have a problem with the petcock/petcock diaphragm. Try replacing the hose, and if you still have a problem, it may be sucking fuel through that vacuum line making it run rich and run like crap. My bet would be the hose, but it's just a thought if not.

Admiral Crunch · July 08, 2006, 11:33:59 PM

Update: I replaced the hose with a new one, and the good news is the bike now runs in ON. The bad news is the rough running above 3500 rpms is unchanged, and it happens in ON and in PRIME. I'm starting to suspect that something might be screwy with the choke, as I had to move the cable a lot to get the valve cover on and off. If the choke is still partially on, that might explain the roughness. Just a guess though.

If I can't see anything wrong with the choke, I think I'm going to try re-doing the vaccuum line like it was before I started working on it, just to see if that changes how the bike runs. I just hope my valve adjustment didn't have anything to do with it. I don't see how though. I triple-checked all the measurements, and it's in spec on all four valves, which wasn't the case when I started.

Admiral Crunch · July 18, 2006, 05:35:51 AM

Update 7/18/06:

I've restored my vaccuum line to the way it was when I bought the bike. The line is now only attached to the petcock and capped off on the other end. The hose attachment on the left carb is also capped off. The bike only runs in PRIME, but it runs normally.

So if I run a brand new vaccuum line from the petcock to the carb, I get stumbling above 3000 rpms, but it will run in the ON position. If I cap off the vaccuum line all together, the engine runs normally (only in PRIME, of course). Any thoughts on what could be the issue here? I don't think there's much chance of a vaccuum leak with the new hose. It's in perfect condition, and it attaches very tightly at both ends. Is it possible the petcock's diaphragm is ruptured? Wouldn't that prevent the bike from running in the ON position, even with the good vaccuum line attached? Other than that, I'm out of ideas at this point.

MarkusN · July 18, 2006, 05:57:03 AM

Quote from: Admiral Crunch on July 18, 2006, 05:35:51 AMIs it possible the petcock's diaphragm is ruptured? Wouldn't that prevent the bike from running in the ON position, even with the good vaccuum line attached?

Not necessarily. If there's only a small leak the petcock may still open, but you would be sucking fuel into the left carb, making it rich.

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Weird problem after trying to fix vaccuum line