Vacuum Cap on Carb

[vlad_the_impaler]

I was riding yesterday morning and the bike was idling a little high randomly and started backfiring a little bit. Then I felt a blast of air on the inside of my thigh. Look down and see the vacuum cap missing on the part mounting cap of the carb on the LH side.

I can only assume that it blew off because I later felt a jet of air hitting my leg?

What could cause this? I couldn't really find much on it. I attached the best picture I could find of the part that blew off because my bike is at home and I'm at work. The part is circled in red.

Thanks in advance!

 

[5twins]

Many times a minor backfire through the carbs will cause that. It's pretty common actually. The original rubber cap is about the best. It's big and should have a spring clip on it to keep it in place. Both are still available from Yamaha. In the mean time, you can fit a short length of rubber hose with a screw in the end. You want parts 4 and 5 in this diagram. I'd order extras to have on hand in case it happens again .....

https://www.partzilla.com/catalog/yamaha/motorcycle/1983/xs650sk/intake

 

[vlad_the_impaler]

Thanks 5twins! That was a big help, I tried searching and couldn't find anyone saying that the same thing happened to them.

The bike now idles at 2-3k warm and has small, but fairy frequent backfire at idle. I'm assuming that could be caused by the giant air leak it has now?

 

[5twins]

Yes it is, lol.

 

[mrtwowheel]

I've tried all of the brands of vacuum caps available at auto stores. They all crack and disappear. A short piece of Tygon hose with a cut off stainless bolt for the plug has worked well for me. Black felt marker or paint for color.

Scott

 

[vlad_the_impaler]

Yes it is, lol.

I've tried all of the brands of vacuum caps available at auto stores. They all crack and disappear. A short piece of Tygon hose with a cut off stainless bolt has worked well for me.

Scott

I appreciate the help, hopefully it fixes the issue. I just ordered the plugs and springs from partszilla so I'll see how those hold up!

 

[5twins]

The originals work well and last a long time. Like I said, they're about the best.

 

[vlad_the_impaler]

Hopefully it fixes the high idle. I put my finger on the port to create vacuum once I saw it and the idle stayed at around 2.5k, but hopefully that was just a fluke haha.

 

[gggGary]

Spitting vacuum caps off is common on lots of old bikes. A ball bearing pushed in tygon fuel line works good or better than a bolt or screw. I've spit out a bolt or two out of fuel line also. You should (have to) clamp what ever you use.

 

[vlad_the_impaler]

I'll definitely try that if the caps with clips from partzilla don't work since I already ordered them. But I feel like this explains the high idle, idle backfires, lack of wanting to idle... etc lol. It all started right after I realized the cap was missing.

 

[DogBunny]

https://shop.advanceautoparts.com/p...-color-vacuum-cap-assortment-47397/89015274-P
I have been using these vinyl caps for a loooong time. They are heat resistant and indestructible. They come as an assortment, Dorman "HELP!" part # 47397. O'Reilly's keeps them on the shelf.
The blue is perfect for the vacuum port barb.
The red is perfect for blocking one of the fuel inlets on your BS38s when you go to a single petcock.
The red also fits the petcock nipple for when you remove a tank with gas in it but the petcock leaks.
The white slips over 1/4" Tygon fuel hose.
Just an incredibly useful assortment of caps that don't disintegrate, and you get 4 of each color, a total of 20 caps for $2.99.
I DO zip-tie them when I put the blues on vacuum barbs! With one of those tiny 4-5" zip ties. Never had the slightest bit of trouble.

 

[vlad_the_impaler]

View attachment 127769
https://shop.advanceautoparts.com/p...-color-vacuum-cap-assortment-47397/89015274-P
I have been using these vinyl caps for a loooong time. They are heat resistant and indestructible. They come as an assortment, Dorman "HELP!" part # 47397. O'Reilly's keeps them on the shelf.
The blue is perfect for the vacuum port barb.
The red is perfect for blocking one of the fuel inlets on your BS38s when you go to a single petcock.
The red also fits the petcock nipple for when you remove a tank with gas in it but the petcock leaks.
The white slips over 1/4" Tygon fuel hose.
Just an incredibly useful assortment of caps that don't disintegrate, and you get 4 of each color, a total of 20 caps for $2.99.
I DO zip-tie them when I put the blues on vacuum barbs! With one of those tiny 4-5" zip ties. Never had the slightest bit of trouble.

Sweet, thanks! The ones I ordered come in the mail today so I'll see how they hold up. If not these will be the second option since they're so cheap!

 

[fredintoon]

I was riding yesterday morning and the bike was idling a little high randomly and started backfiring a little bit. Then I felt a blast of air on the inside of my thigh. Look down and see the vacuum cap missing on the part mounting cap of the carb on the LH side.
I can only assume that it blew off because I later felt a jet of air hitting my leg?
What could cause this? I couldn't really find much on it. I attached the best picture I could find of the part that blew off because my bike is at home and I'm at work. The part is circled in red.
Thanks in advance!

Hi vlad,
hopefully DogBunny's rainbow assortment will work for you.
Or you could use that vacuum connector to operate a ScottOiler instead to keep your drive chain lubricated.
BTW, your photo shows pleated tapered air intake filters.
List consensus is that your carbs will be happier when they are sucking through UniPods.

 

[mrtwowheel]

Myself, I wouldn't trust those vinyl caps any more than electrical tape held on with zip ties.

Scott

 

[vlad_the_impaler]

Hi vlad,
hopefully DogBunny's rainbow assortment will work for you.
Or you could use that vacuum connector to operate a ScottOiler instead to keep your drive chain lubricated.
BTW, your photo shows pleated tapered air intake filters.
List consensus is that your carbs will be happier when they are sucking through UniPods.

They're the tapered filters from MikesXS. I'm not opposed to trying UniPods though. I'm assuming I'll have to retune the carbs as well if I install Unis? Or should it be okay to just install and keep it moving? I'm not familiar with these carbs yet unfortunately, but so far they seem a little finicky.

Myself, I wouldn't trust those vinyl caps any more than electrical tape held on with zip ties.

Scott

Hopefully the OE caps and clamps I got hold up well!