why am i blowing off vacuum barb caps?

[bloodthirstysystem]

78' with pamco ignition, valves set correctly, timing is set..

i have capped off my vacuum barbs but they keep blowing off.. i carry some extras but used them on my ride to work this morning. i had a hose connecting both but i switched it out for caps the other day.

anyone know why i am backfiring through the carbs so often? seems like timing but i'm right on with that, it advances perfectly also

i'm just going to cover a wood screw with super glue and permanently fill those fuckers...

 

[gggGary]

Normal. They need clamps to keep them on.

 

[YamiMami]

Where do you get those caps?

 

[retiredgentleman]

bloodthirstysystem:

Your carbs are not adjusted correctly....................pilot jets/mixture screws may need attention.

Even if they do stay on, those rubber caps don't last very long. They crack and give air leakage. Go back to using a black fuel line to join the 2 barbs.

 

[fredintoon]

78' with pamco ignition, valves set correctly, timing is set..
i have capped off my vacuum barbs but they keep blowing off.. i carry some extras but used them on my ride to work this morning. i had a hose connecting both but i switched it out for caps the other day.
anyone know why i am backfiring through the carbs so often? seems like timing but i'm right on with that, it advances perfectly also
i'm just going to cover a wood screw with super glue and permanently fill those fuckers...

Hi thirsty,
don't seal the barbs, you need them to balance the carbs properly.
So drag out the homebuilt manometer and do that, it may help with the backfiring.
Then go back to the ol' "link the barbs with fuel hose" trick.
If you are buying barb caps by the fistful you are most likely buying the caps that the local car parts store sells which are meant for air conditioning systems and won't stand up if used for a fuel system.

 

[grizld1]

Yep, vac plugs off the rack from a big box auto parts store fail pretty quickly. Mikuni blind plugs are heavier and use high quality material; they'll hold up for many years. An option is to block a short piece of fuel line with a 6 mm. screw (push the line off to remove it; if you try to pull it off it will tighten up, and you'll pull the barb out with it).

 

[Max Midnight]

Assuming the barbs are a similar size (can measure if required) then these will stay on - part # 5 & 6

 

[scabber]

Try down shifting before twisting the throttle to stop backfire.

 

[mrtwowheel]

I had this problem on our Honda CB750Cs when using the crappy plugs for the one vacuum port for the auto petcock. I used some of that TYGON fuel line, aviation stuff, and plugged them with the cut off shank of a stainless bolt, painted black with an enamel paint marker, no more missing plugs. I still check them every time I wash these bikes.

Scott

 

[5twins]

I've always just used auto parts store ones and never had a problem. I do put a nice clamp on them and maybe that helps preserve them. I think the proper size is important too. Force one on that's too small and it probably won't last long .....

 

[DogBunny]

http://www.oreillyauto.com/site/c/d....oap?ck=Search_N1167_-1_-1&pt=N1167&ppt=N1171
I have been using these nylon caps for a while. I used to do the ol' "link the barbs with a hose trick," but no longer. These are indestructable. They come as an assortment, Dorman "HELP!" part # 47397. Advance has them too, and probably most of the other auto parts chains.
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 5 of each color, a total of 20 caps for $2.69.
I DO zip-tie them when I put the blues on vacuum barbs! With one of those tiny 4-5" zip ties.

 

[5twins]

Yes, you have to secure them. Even in a perfect state of tune, these bikes still pop and fart occasionally.

 

[mrtwowheel]

I've had a lot of experience with these, this brand and others. I used the right size and they split and sometimes just disappeared. I'm sure that the ones that went AOL deteriorated and split before being absent. I will never trust them again. Leaking vacuum will burn valves, hole pistons and cook oil to destruction, just not worth the risk.

Scott

 

[5twins]

This is a 30+year old bike. You have to check them and everything else on it all the time. If this bothers you, shell out 10K to 12K and buy a new bike. Then your dealer can change the $2 rubber cap and charge you $40, lol. Hey, you gotta pay to play. But here, we'd rather turn our own wrenches for the most part. I know that's not possible for some, they don't have the knowledge or a wrench for that matter. What they can gather here is the know-how to use that wrench if they ever decide to buy it. I highly recommend they buy those wrenches.