Vacuum Cap Keeps Popping Off

Hello everyone,

My buddy has an 82 XJ650. He has been having an issue where his #3 vacuum cap keeps popping off of the manifold. Whenever he pulls into work after a ride, it is missing.

Any ideas of what could cause that to happen? Very strange.

 

Btw: he has not checked his valve clearances yet. (SHAME SHAME!)

I will be assisting him soon with that project, asap as far as I'm concerned. Could tight valves cause back pressure through the manifold causing the cap/plug to pop off?

I cautioned him to not ride the bike anymore until we check those valves. He's gonna park it til' the weekend.

 

an intake valve not seating properly would certainly do that. When the picton came up on the compression stroke the compression would go out the intake.

Has he done a compression test on the motor?

I'd start there, then valves. It may even be running on 3 cylinders.

 

Honestly he hasn't done much with the bike. I am going to go through the whole process with him though. I've ridden it, and it doesn't feel like it has a loss of power...but we shall see.

 

A lean idle mixture on that cylinder will cause lean misfire. Thats when the fuel/air mixutre pre-ignites and pops back into the carb. That could cause the cap to come off. Of course he should have a clamp on it so it will not come off.

 

Well on the agenda is getting him shimmed to spec, checking his floats, and syncing his carbs. His PO paid a shop (has receipt) to rebuild carbs, but chances are they missed some things. Since I know have experience with all this stuff I'm gonna walk him through it.

 

several things are happening....

1. too much back pressure from the intake valve not closing all the way, is causing a

2. most likely stretched, hardened, and old vacuum cap that is now loose to pop off which

3. can lead to excessive air on an intake stroke causing a lean running condition in that cylinder which leads to

4. low compression as well as higher temps that can burn up the piston or cylinder or exhaust valves.

Dave F

 

Can also make bike run like SH@T!

 

Just a quick observation here-- Isnt the #3 vacuum port supposed to connect with the petc##k?

 

On mine, and all others I have seen the vacuum line runs to the #2. I don't really think it matters though.

 

For all the newbies, the carbs are numbered the same way we here in America read; left to right e.g. 1 2 3 4.

 

And no, it doesn't really matter where the vacuum come from, the factory decided that #2 should get the duty.

 

That, and it is directly underneath the petcock. For the factory, perhaps that translated into (marginally) lower cost of parts as that required less vacuum line. 2 inches X thousands of bikes = $

 

Actually, the vacuum line to the petcock was originally connected to the #3 intake manifold on the 650/750, and to #2 on the 550/600.

Don't forget that the YICS effectively pressurizes the intake tracts of any valves that are closed; it could just be a sync issue too.

 

Sorry, need to make a correction. As soon as Fitz mentioned YICS it hit me. He has a 1980 650, no YICS.

 

The aftermarket caps Chacal sells are tight enough you don't need clamps.

 

use clamps anyway. As the rubber ages and hardens, you will need the clamps to keep them tight.

Dave F

 

umm . . . care to elaborate??

 

HOld on........my understanding of the YICS passage is that it is basically a balancing tube passage that enhances fuel swirl. Since it goes between all of the carbs, then it would seem to me that on a YICS system if even only one intake valve doesn't seat tightly then ALL the rest of the carbs are compromised as an appropriately portioned bit of fuel will be drawn laterally through the YICS passage to the cylinder with the lowered compression. That leads to a case of a lower-compression cylinder trying to fire an over-rich mixture. The YICS passage isn't an actively sealing chamber. How would the YICS passage pressurize the closed valves? The pressure comes from the pistons squish when all valves are closed.

Now I probably confused myself even more trying to type that........


dave

 

I think the air flow thru the YICS system is TOWARDS the ONE cylinder that's on the intake stroke, stealing from the other 3.

The intake caps can pop off with a back-flash under a lean condition. And why?? Because the sparkplug fires on the exhaust stroke just about when the intake valve opens. That's the "waste-fire" igniting an unburnt charge AND the intake charge with quite a "POP".

 

A backfire can pop 'em out even if new... Jedd clamp it

 

Clamping them sounds like a good idea. But that is putting a bandaid on the real problem. He has the same style plugs as I do. They are new. I have never had a cap pop off.

I guess bottom line, we need to adjust his valve clearances and possible lean mixtures.

 

Agreed -- Any backfire will pop 'em -- the source or reason for the backfire is another issue

 

Time, I'm not questioning what you said. I agree with that....my question was regarding what Fitz wrote, too, not your reply.

For those talking about clamps......ALL my xj's have clamps that originally cam on the caps....I didn't add them. And yes,.......a good backfire surely can pop them off. I've had it happen enough times to know.

dave F