No vacumn on #4 on 78E

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Took the carbs off to reset the needles on the slides back to the middle slot, the bike has always run to rich and only got 28 mpg.
In going into this I noticed that on 1,2 & 3 all had a slide with a smooth white plastic ring that holds the diaphragm. In # 4 it has a circular groove running around the ring. Also the diaphragm rubber is a smaller diameter and much harder to seat into the carb top when putting back together. The main nozzle on #4 seems to sit a little higher in the throttle bore.

I put the diaphragm rubber side down on a photographic lightbox and also tried a small led penlight. I can see no pinholes nor any scoring on the slide piston.

I remember years ago having a bad diaphragm and having someone else do the work. I am wondering if the mechanic used a slide from a different year model and also changed the main nozzle? The needles are the same on all four. This was 40,000 miles ago with no problems from using the wrong slide.


Anybody got any ideas. By the way I have had these carb slides apart a few times for cleaning in the past and never had any trouble getting vacuum after putting back together.

I have access to another rack of carbs so I don't think I'll have any trouble correcting this problem. Just not today.

[John]

No vacuum?

Is there any chance you may have plugged the intake port to keep the undesirables out, and forgot to remove it when reinstalling the carbs? Are the throttle plates closing completely? How about something blocking the vacuum nipple? The diaphrams, I can't help you with. Maybe someone here has a diaphram you could pick up cheap.

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Same Problem with different slide

John did not plug the intake port at any time.

Went to Hippie Dave's Garage/XS11 shop and got a new (to me) slide and main nozzle. Installed and exactly the same problem. Gonna take the carbs back off and clean everything to see what happens.

Any other ideas would be held in sthe strickest confidence if not correct, but aplauded and bowed to if ever we meet.

[Ken Talbot]

What you've got in #4 is a diaphragm from a later model (80 or 81) carb. Besides the groove in the plastic ring that sandwiches the rubber, the later one three tabs that hold the top and bottom rings together versus the four that hold the older rings together:


Somebody has managed to pry the rings apart on a newer model slide to get a replacement diaphragm for your older model slide. The newer slides are about 1mm bigger diameter than the older so there's no way they just put in a whole different unit.

The groove diameter that the diaphragm sits in is the same on both model carbs so that would rule out the possibility of the replacement diaphragm being a different size from the start. I think what happens is that the rubber just tends to shrink a bit over time. I've noticed that some diaphragms seem to need a bit more of a stretch to get them seated than others.

[John]

How does the bike run? You never mentioned that. If it had no vacuum, then it would run like s#!t. It wouldn't pull the fuel/air into the cylinder if there was no vacuum. Can you run it with the airbox off and place your hand over the carb inlet and choke it down? How about when you pulled the carbs, the intake boot cracked? (Grasping at straws, now) Hell crank it over with the carbs off and place your hand over the intake boot. That will definitley tell you if there is any vacuum there.

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Done

John it was running OK after the restore just figured I'd change the needle hight to try to get a little better response and better milage.

I blew out the carbs with air then redid the bread tie sink. Put them back on and had a hard time sinking with a mercury stick but it is done now. Must have been some kind of trash in #4 to cause this condition but I never saw it.

The #4 slide and needle was as shown on the pic on the left but with the ring and the diaphragm from the pic on the right. The diaphragm is a little smaller but like I said before it did work for 40,000 miles. I don't know how he did it but at least we know it can be done.

I just got done so haven't had a chance to ride it yet. I'll ride it tomorrow.

Hopefully it'll jerk my arms out of the socket like it used to before I fixed it.

Thanks for the help.

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Rode it tonight, damn my shoulders are sore.