Those hoses......yeah those...........

[gibvel]

The ones that hook to the vacuum ports on the carb boots at cylinders 2 and 3 and go up above the carbs. Where do they go and what are they for?

[V7Goose]

Assuming you are asking about a 2nd Gen, the vacuum hoses are actually on cylinders 2 and 4 according to the shop manual (but in reality I have seen them on cylinders 2 and 3 as you stated), and they go to the Air Induction System (AIS) valve. These vacuum lines are what control the AIS and cause the valve to close when you chop the throttle, thus preventing after-fire pops through the exhaust.

 

Any vacuum leaks in the intake tracts will cause after-fires by either causing an excessively lean fuel mixture, allowing additional oxygen the be pulled through the combustion chamber into the exhaust during high vacuum, or preventing the AIS valve from properly shutting during closed throttle operation. Depending on where the vacuum leak is, it could be any one of the above, or a combination of all three.

 

Just as an editorial note - I know many owners have disabled or removed the AIS to eliminate after-fires; however, in my experience, this is completely unnecessary. As long as there are no vacuum leaks or exhaust leaks, and the carbs are reasonably close to sync, these engines do NOT pop on deceleration.

Goose

 

BTW- here is another extraneous piece of information about these vacuum hoses: I once had an intermittent sound of something hitting the cooling fan when it came on. It turned out to be one of these hoses! They are routed forward and down very close to the fan, and it just moved a little bit to allow it to rub a corner of the fan. Very difficult to find because you can't see it, and it did not rub when just turning the fan with my fingers.

[gibvel]

Thanks Goose!! I, kind of, thought that they may have been part of the AIS but I wasn't sure.

[gibvel]

Okay, then what's the best way to track down a vacuum system leak..... assuming it's not at the boots (I'll check them as well). Reason being, ever since I got this bike, it's had a problem on deceleration. Can't really call it a backfire cause it doesn't sound like gun fire. It's more like a gurgle or loud rumble or a series of small pops, while I'm down shifting (don't know if that makes much sense). I plugged the AIS once and that kind of took the edge off but didn't get rid of it. I synced the carbs and that doesn't make much of a difference. It does have some fairly loud after market pipes on it and K&Ns.

 

I'm wondering if it could be a leak somewhere either in the hoses or somewhere else in the system.

 

I did take your advice and check the port plugs and both of them, if you pull on the end (where it looks like two pieces were put together) look as though they come apart slightly. I don't think there is a hole or that they are all the way through though. I replaced one and tried to fix the other. Didn't seem to help much.

[V7Goose]

The after-market pipes might be the entire cause of the "burbling" - might just be normal engine running as accentuated by those pipes. Certainly hard to tell without actually hearing it.

 

As for those rubber caps on the manifold nipples, in my experience, if you see any cracks at all in the rubber, they are leaking. I didn't think mine were all the way through either, but they WERE the problem. An easy replacement is to just buy a foot of thin-wall rubber gas line the right size for the nipples, cut about 1 1/2 inch and double it over. Slide this on the nipple and then put the original clamp or a small zip tie over the whole thing.

 

The first thing I would do to trouble shoot it is to remove the vacuum lines and the existing rubber caps on the nipples and cover all four with a piece of gas line (or even vinyl tubing for the test) like I described above. For this test, you will also need to temporarily plug the AIS lines that go to the four exhaust ports too. Pull them off the AIS valve and put a screw wrapped in electrical tape in them. If you just pull the vacuum lines without plugging the lines to the AIS injection ports, you will CREATE after-fires.

 

If that still does not change the pops, or if you just want to skip that step, then the next way to look for vacuum leaks is to get a can of carb cleaner or WD-40 and carefully start spraying it around the intake manifolds, carb connections, vacuum lines, rubber caps, etc., while the bike is idling. If at any point the engine speed or sound changes even the slightest, you just found a vacuum leak.

 

Finally, if you have eliminated all possibilities of a vacuum leak, make sure that there are no leaks in the exhaust pipes, especially around the Y joints, as these will absolutely cause after-fires in the right conditions. Assuming no leaks in either the intake or exhaust, then the last possibility I can think of is that those after market pipes just do not have enough back pressure to run with the stock carb settings - it may be running too lean. You might be able to fix it by just adjusting the air screw settings, or it might take re-jetting. Either way, I wouldn't fiddle with the carbs myself; I'd take it to someone with a dyno and exhaust gas analyzer to get it set correctly (or just go back to stock pipes).

Goose

[gibvel]

Already had the carbs adjusted. If anything they are adjusted on the rich side. Was concerned about going down in altitude and didn't want them to lean out too much if I did.

 

Thanks for the info Goose. I'll give all that a try!!

[V7Goose]

Hey man, one more thought just came to me - read your plugs. Small effort to remove the front dog-bones to pull them, and the info could be valuable. First, no matter what color they are, all four should be IDENTICAL. If they are not, then it indicates something is off in the mixture of one or more cylinders. In my case, even though all four plugs looked excellent, there was just a bit of black soot showing on the upper 1/2 of the ceramic insulators (down deep in the plug as you look at it) on the two cylinders that had the cracked rubber caps on the intake nipples. This was the evidence of the vacuum leak causing a slight amount of misfires at idle. These misfires were not detectable in the way the engine ran, but it allowed enough unburned fuel to pass into the exhaust system to cause the after-fires.

 

Assuming that all four plugs look identical, then the next question is the color. They should be uniform light tan. There are a number of places on the internet to find pictures to compare. If there is any doubt at all about the plugs being in perfect condition, then I'd put new ones in for the duration of the testing and read them again after a good run.

 

Of course, for any of your testing to be valid, you will need to make sure the engine is fully warmed up. The only thing you can validly do at idle is the spray test. After any other changes, you will need to take the bike for a run through some varying conditions that cause you to do a fair amount of acceleration and quick throttle closes. In this case, keep the RPMs down; screaming acceleration is not what you want. I have found that if there IS a vacuum leak, the effects are most evident when you chop the throttle after a prolonged load on the engine (such as climbing a steep grade at medium RPM).

Goose