Hi, I have a '85 R100rs. Lately there has been an issue with the idle being lumpy and faltering particularly when cold. I noticed there was a vacuum cap missing just behind the back of the fairing. Putting my finger over it smoothed out the idle. This seems to work better when the engine is warm. So i got some caps from Auto-One and went for a ride (120km) bike went well, as i got close to home it started to surge and  feel like it was missing. When i got off the bike the cap was missing and the idle was "lumpy". Putting my finger over it smooths it out again. Today I ride the bike after putting another cap on. As I accelerate I feel something hit the inside of my knee, the bike starts to surge again. I pull over and the vacuum cap was again missing, Im pretty sure thats what hit my knee. What would cause the vacuum cap to be blown off.?  The others remain in place. Thanks.

shintaro wrote: What would cause the vacuum cap to be blown off.? 

A fit that is too loose would contribute to that.

Using the wrong sort of glue?

Here in France we would tend not to use those little black plastic things that fit over the holes, but after 2,000 years of wine production we would probably develop some very small wine corks that fit nice and tight inside the tubes.

I have seen caps made from different materials...some are a vinyl type material and others are rubber-like. The rubber type material would hold on better than the vinyl type.

The fit should be fairly tight...but this is something that you now already know!

brickrider2 wrote:"Using the wrong sort of glue?"

Only vacuum glue can be used.

Inge K. wrote:

brickrider2 wrote:"Using the wrong sort of glue?"

Only vacuum glue can be used.

I suppose that any sucker will know where to get some.

Yes, I tried both Rubber and a Plastic type one also. 4mm size as mentioned in a thread here. My thought was to put a drop of silicone on  and leave overnight to make it stay. I thought maybe there was something else at play here as Ive had the bike for nearly 2 yrs and this has not happened before. Thanks.

About the only thing that would make them come off is a backfire but you would hear that. I would say that they probably were a marginal fit in the first place. maybe silicone as suggested or rubber caps which will grip better as against plastic which would tend to slip off.

Once they start to rot, then they'll come off really easily. Fall off, in fact. I made a set from 3mm id silicon tube with the ends blocked off.






Okay, if you really want to know, I used 8mm long no.10 stainless steel self-tappers to block the ends off. Go on - you can laugh now