Non-constant idle

[sraustx]

So I was riding a couple of weeks ago, and my intake boot (I think that's what it's called: between the carbs and the engine) popped off of carb 1. I had recently bought my 79 Special, and so I figure it was something the guy before me did. Anyway, I turned off the petcock and rode home (a mile or two) and then put the boot back onto the carb.

After that I went to NY for two weeks and when I came home, the thing wouldn't start. Whenever I would try to start it, it would smoke like hell, turn on for a second or two and then die. Also lots of thick brown oil/gas would flow out of the airbox.

I took my carbs to a mechanic and he helped me make sure it was clean (we also replaced the float needles--which were basically fine), I replaced the spark plugs, ripped out the octopus (just by what looked obvious, my housemate has a 79 Special without the octopus and we just followed his hose routing. Then I checked the petcocks (which seemed to work fine) and I changed the oil.

The oil itself was mixed with a lot of gas. Anyway, I put everything back together and for some reason it couldn't hold an idle anymore. It would just die. So we adjusted the master idle screw a bit and reset the idle screws on the carbs to 2 1/4 turns and then adjusted a slight amount from there.

Now it idles between 2-3k rpms. When I first start it, it idles at 1k rpms. Sometimes it'll be idling at 2.5k rpms in neutral, I'll kick it into first and then when I start to go, it kicks down to 1k rpms. There's also a bit of smoke coming out. Also the throttle now sticks a bit. Sticks might not be the right word. Sometimes I'll open it up a bit and it will there will be a delay then it might jump up 3000 rpms and stay there for a while after I've let off. This I don't understand either. Honestly, I have now idea what could be causing the non-constant idling. Maybe it's something with the mixture, but I'm not quite sure. Any ideas?

[Doo-Daa]

Sure hope you changed the oil!!!! Especially once you found it full of gas.

Sounds to me like you still have a vacuum leak around the intake boot or boots. Why did it come loose? And how did you manage to get just one carb loose? And you might check to see if that one boot isn't installed upside down on the head.

Doo-Daa

[sraustx]

Actually I was wrong. It was the boot between the carb and the airbox. But I just checked, and it looks like it's installed correctly. I'm thinking that the band around it just wasn't tightened properly, and the vibrations made it pop off. Could this happen?

[TopCatGr58]

Hey there Sraustx,

Well, after you had your mechanic work on the carbs, there's a good chance that they got out of balance! You'll want to resynch the carbs, check the tech tips on the breadstick tie bench method first, then you'll need to get/borrrow a set of gauges to get it properly synched! Also, check to make sure you connected the vacuum advance hose to the correct nipple, on the CARB , not on the Intake Boot!! Also, make sure the carb bowl vent tubes are not obstructed, free to breath, either where they attach to the air cleaner box, or just to some simple filters and suspended under the tank!
HTH, T.C.

[chevy45412001]

throtle cable

i've recently had a jumpy idle symptems myself. Turned out to be a sticky throtle cable. New cable works like a champ now.Just a thought if all else is unchanged.

[Dennyz]

This symptom can be caused by the idle mixture being lean (idle screws) and the idle speed turned up to achieve the 1100rpm idle speed. The increased air flow produced by turning the idle speed up can pull the slide up slightly on one or more carbs.

One way to check for this is to adjust the idle mixture screw on each carb for max RPM gain. If the idle RPM increases as you go, you may have found your problem. A carb synchronizer really helps see what is going on while making the adjustments. A short cut that works most of the time is to adjust one carb idle mixture for best RPM, record the number of turns, and set the other 3 to the same setting.

One should always begin their carb adjusting process by verifying the float bowls are set correctly and the carbs are synced. Either of these elements can have you running in circles if not correct.

[macnut xj]

Originally posted by Dennyz
One should always begin their carb adjusting process by verifying the float bowls are set correctly and the carbs are synced. Either of these elements can have you running in circles if not correct.

Very good advice! I would also highly recommend building Ken's stand for setting floats levels. You can see it in the tech tips section. It is also highly useful for doing the bench synch. It's well worth the time to build one of these!

[sraustx]

Thanks everyone for replying. Tomorrow (I have some time to work on it then) I'm going to put in a new throttle cable and then try to balance and synch the carbs. We'll see how much that improves it. I'm still fairly new at this, so I have one or two more questions...

Originally posted by Doo-Daa
Sounds to me like you still have a vacuum leak around the intake boot or boots.

How would I check this? The boot is installed correctly, but how could I test for a leak?

Also, TopCat, which hose is the Vacuum advance hose? (I don't have all the terminology down yet).

Thanks for all your help. I'll post more after I try these and tell you if that fixed it or not.

[chevy45412001]

checking for vac leak

have the motor running. Then take a spray can of wd-40, starting fluid or the like,If you have the straw to put in the can would be better. Then spray small amounts of Whatever your using at suspected leaking areas. Carb to motor boots ,vacum hoses etc. If you have a leak the motor will usually race up in idle. Like said above if the straw is on the can you can pinpoint a smaller area than just spraying a big cloud of the stuff all over.

[John]

The Vacuum advance hose is routed from the #2 CARB to the vacuum advance on the left side of the engine. Under the cover that says yamaha on it. The is a small nipple on #2 carb, not on the boot up next to the head. That is the vacuum source for the advance. The Nipples on the carb boots are for the petcocks/ octopus.

[ratbyk]

Carb Problems

If you are referring to the intake manifold as being the carb "boot" then to test for leaks; spray the area of the manifold with WD 40. Watch closely, if there is a vacuum leak then some of the WD 40 will be drawn in and the engine idle speed will change.

The vacuum advance hose fits on to a brass sleeve on the top of #2 carburetor/throttle portion just before where the carb would slide into the intake manifold. It does not fit into the brass sleeve sticking up from the top of the intake manifold next to the cylinder head.
The area of the carb where the brass sleeve goes down into the carb is right where the throttle butter fly closes in the throat of the carb. This shields/reduces how much vacuum is supplied to the vacuum advance at idle. As you "crack" the throttle the movement of the butterfly exposes the vacuum advance port allowing vacuum to be directed to the advance.
Clear as mud I bet. Hope this helps,
Ken/Sooke
78E Ratbyk
82 FT500 "lilRat"