Carb vacuum caps

[Incubus]

I just finished synching the carbs on Succubus, the beautiful and mysterious 79F. Got them all to 9.7-9.8 inches of vacuum at 1,000 rpm. I was pretty pleased. My question, however, concerns the black caps that seal off the vacuum fittings on the No. 1 and 4 intake manifolds. The caps I removed were not snug. In fact, they are so stiff that it seemed like they are metal lined. I would have thought the lines should be airtight. These were not.

I was fixin' to cut a couple of rubber plugs and drill them with a 3/16 bit to create new, snug airtight rubber caps. Before I do, however, I wanted to confirm that I want these tubes to be airtight and leakproof, and not breathers.

Who can tell me?

Now I have to colortune the air jets, which in my case means riding five miles in town, pulling the plugs and adjusting the air jets according to the color of the plugs. If I do this repeatedly I can get them all tuned in to a real pretty tan.

Thanks,

Patrick

[TopCatGr58]

Hey Patrick,

Yes, they are to be capped and SNUG. Don't go thru all that trouble, instead go to your local auto store and get some vacuum ports caps! Did you have the synch hoses connected to ALL 4 ports? If not, then you possibly could have some leaks which could mess up your synch!? Another Xsive posted about his trouble with throttle response, etc., turned out his caps were leaking. Once replaced, performance improved considerably!

The only things that are NOT to be capped are the carb bowl vents, the upper "T" fittings on the carbs are to be vented!
T.C.

[Incubus]

Hi, TC,

When I synched the carbs I used four vacuum gauges I picked up at Harbor Freight, so all four fittings were covered and I was looking at four gauges simultaneously. Additionally, because I am well aware of the quality of the stuff HF sells, and because these gauges have been used to synch carbs at least eight or nine times, I then switched out the gauges to see if I got the same readings with different gauges on different ports. I did, so I figure the synch is pretty tight.

It only made sense to me that the fittings had to be airtight - I've never seen vacuum lines meant to vent. But Yamaha engineers have always marched to their own drummers. From what I've seen Yamaha is usually a couple years ahead with innovations that work and that other manufacturers copy, but they also are ahead with those changes that are evolutionary deadends and disappear after a year or two. YICS, anyone?

Thanks for your response,

Patrick

[Incubus]

And thanks for the tip about the vacuum Autozone caps. Still, I already have some acrylic plugs left over from a powdercoating project and a 3/16 drill bit. I just might go ahead with the homemade - free - replacements and see how they work out.

Patrick