Run away throttle - '80SG

[Under Dog]

I think I'm having a carb issue, but need some input. Here's what I've got...

On a cold bike, I pull out the choke all the way. One kick and it fires right up. A little warming and I drop it down to half choke. Then a little later i can put the choke all the way in. The bike is now idling fine just sitting in the driveway.

If i blip the throttle now, she revs up and then comes right back down.

As the bike warms up more, either sitting in the driveway or out on a ride, the revs no longer come back down after i close the throttle. I'm finding I either need to 1) ride the gear down to force the revs to drop or 2) hit the kill switch until the engine stops racing then turn it back on before the bike dies. Unfortunately, option 2 always results in a pretty substantial backfire.

From what I've read I'm probably looking at an air leak or lean carb jetting. I tried spraying propane around the intake manifold, around the carb itself, etc... The only time the idle change is when I spray propane into the air filter itself. So... I'm ruling out an air leak for now.

The bike has stock air filters, and the exhaust is a home-built 2 into 1 with a fairly loud and free flowing muffler. I don't recall what jets are inside at the moment. If I can find where I wrote them down, I'll post them. Otherwise it'll have to wait until the carbs come off again. The air screws are currently at 3.5 turns out.

At first I was leaning toward dirty carbs, but I've cleaned them 4-5 times. I'm pretty sure they are ok. Oh, and I do have a fuel filter on the gas line too.

Last time out, the idle started out fine, but within 5-10 minutes of riding the revs were no longer dropping back down. At one stop sign I did the kill switch trick and got a nasty backfire. After that the bike seemed to be riding much better, but what happened was it blew the cap off one of the vacuum barbs. For some reason the bike's revs would drop normally with that vacuum cap off, but the bike would no longer idle without me coaxing the throttle at stops.

So... I think that covers everything I know at this point. I am open to any and all suggestions at this point...

 

[Bizzy13]

That is usually due to a lean condition. But it seems to me opening up your mix and jetting without freeing up air would make you rich, not lean.

 

[Bizzy13]

I've actually notice mine do this in the past to

 

[willem]

yea you're probably running lean. Check your plugs to make sure.

If you have white plugs, start looking for bigger main jets. It sounds like your idle is doing fine (at least when you put that vacuum barb back on) so don't worry about that one yet

 

[Scorpio1963]

The last time out riding my bike has had this malfunction.It raced up to 5k`s about half way home home.I killed it and caisted the rest of the way home.This also ties into the problem Im having with the gear shifting also.

 

[Under Dog]

I thought of this late last night. If the racing idle is due to a lean condition, would running with the choke half out correct it? In other words, could I confirm it IS a lean condition problem by doing this?

 

[Bizzy13]

^it should

 

[Under Dog]

Ok, I spent a few minutes out in the garage again. It turns out pulling out the choke makes absolutely NO difference. The revs will run away regardless of the position of the choke. Does this mean I'm not fighting a lean/rich issue but something else?

Also, I noticed a fairly persistent backfire through the left carb upon closing the throttle. What might this indicate?

 

[Bizzy13]

Backfires through the carb also normally indicate lean mixture.Check your plug on that side. Also try spraying some ws40 on your intake boots while idling. Check for leaks there.

 

[Under Dog]

Pretty sure there are no intake leaks as I was spraying propane all around them last night without any change to the idle...

I was thinking too lean, but pulling the choke out doesn't seem to make any difference. The idle still revs way too high even with the choke pulled out... ?????

 

[Scorpio1963]

Have checked the throttle cable while the bike is not running to make sure the throttle is snapping back like its supposed to?Your problem could be as simple as doing some maintance on the cable by taking it off and squirting some type of lubricant in it and working it back and forth.You`ll be surprised of what can come out of a 30 something year old cable.You also may have to adjust it where it connects up on the handle bars.

 

[Under Dog]

Here are pics of the plugs... The two pics on the left are the left side (backfire in carb). The two on the right are the right side.

I don't know how well you can tell from the pics, but I'd call them both fairly blackened on the outsides with the tips being tan in color... ??? Wouldn't that make the carbs too rich???

So confused at this point....

 

[Bizzy13]

Tan is what you want, black is rich. If you're rich in one cylinder turn in that mix screw a 1/4 turn. Keep playing that game till its right.

 

[Under Dog]

How can I have black/tan plugs and yet still have the run away idle (btw... throttle cable snaps back fine...)

Seems like I'm having rich and lean symptoms at the same time?

 

[Bizzy13]

Looks to me like your left side is rich & right looks about like it should

 

[willem]

right is a bit lean if you ask me. That's maybe what's happening, left running rich & right running lean. Check and play with your idle mixture screws first, and if that doesn't help have a look at your jets. Also a carb sync would be in order.

 

[Bizzy13]

Ideally you want your plugs to be a tan color. Right one looks pretty ran to me. But yea, if your carbs haven't been cleaned in awhile so that first. Strip them down completely and soak the bodies for 12-24 hours then get a toothbrush and scrub them inside & out, run small wires through all your jets and holes carefully and blow everything out w/ air or a can of throttle body cleaner. Don't just soak them and reassemble, that's not enough. Do at least a bench/wire sync, and put them back on. Then play w/ you're mixture. Id suggest writing down where they are away now and start back at that point and just lean out your left one of it still rich after cleaning. Cleaning may ungunk your right jets and richen it, so just play w/ your mix.

 

[blibzool]

I am having a problem very similar to Underdog's

I have an '81 xs400 with the BS 34 carbs. I got it last year and it ran pretty good but this year I thought it wouldn't hurt to clean/inspect/adjust the carbs. I only had to replace one float needle and adjust the floats - the float height was much lower than the specifications. Everything else looked good to me, and all jets are stock.

Now the bike is acting really weird:
1. When I start it cold, pulling the choke out makes it rev up to 4k rpms.
2. Once it warms up hitting the throttle causes the rpms to go up and stay up - it just revs at 3 or 4k until I cut it off or slowly pull out the choke until it starts to cut out and then the rpms go back down.

My mixture screws are at 3.5 turns, I sprayed starting fluid around the carbs and boots and detected no vacuum leak, and the high revs are definitely not my throttle cable.

I shot some video to post but have no idea how to do that.

 

[willem]

high revs with the choke on is normal, but it should come down to a normal idle when you push the choke back.

How are your plugs looking? It can be just the same thing, running lean except when you've got the choke on..

Any modifications to your exhaust or air filter systems?

 

[Bizzy13]

Man this problem is way to common seems. Mine stop doing that when I richened the mix. I was gonna ask bout your intake and exhaust to. Modify those w/ stock jets you'll be lean