Run away throttle - '80SG

Thanks for your input.

Everything on my bike is stock - original pipes and new oem air filters. That's why I'm confused - it definitely shouldn't be running lean with the mixture screws at 3.5 turns.

Also, my choke never seemed to make it idle so high. I used to leave the choke on for a minute or two until it warmed up and i think it would idle around 1600 - 1800.

I'm thinking i should set the float height back to what it was. Maybe i screwed something up by messing with it.
 
I think I found the issue...:thumbsup:

Since I couldn't induce a change in idle speed by spraying propane ALL over the carbs, intakes, etc... - I ruled out an air leak.

The pictures don't show it well, but the spark plugs looked more black than tan - I ruled out a lean mixture.

I've cleaned, scrubbed, sprayed, and blown compressed air through the carbs more times than I care to recount, and I've put an inline filter on the fuel line - that ruled out dirty carbs.

So, the problem had to be somewhere other than the carbs.:wtf:

Turns out it was the timing!!!

Darn bike has a TCI (no points). I initially figured since the TCI wasn't meant to be messed with, the timing was fine. BAD ASSUMPTION! I don't recall which way I ended up adjusting things, but I think the timing was too far advanced. Anyway, runs much better now. I took it out for 25 miles or so tonight and the idle didn't race away from me once. Also, the backfire through the carb appears to have disappeared too...

So simple... yet so elusive...:doh:
 
Not too bad. The tamper proof cover over the pickups was taken off by some previous owner, so all I had to do was remove the 2 bolts holding in on. Once that cover is off, it's a simple matter of loosening two screws and turning the plate so that the light is firing on the proper timing marks... Easier than points since you don't have to time each side individually...
 
Under Dog said:
I think I found the issue...:thumbsup:

So simple... yet so elusive...:doh:
Click to expand...

No, you found the symptom. And just like your doctor does, you treated the symptom. What you should have done was ask yourself "why has this fixed setting changed?"

You need to find the actual cause of the problem. Was it the previous owner messing with the settings? Or did this just occur recently? How are your cam chain and sliders?
 
Cam chain, sliders... all else seems OK.

I'm guessing the PO did something with it. When I bought the bike a lot of it was in pieces. He said he was trying to set up the bike for racing but realized he was in over his head and out of money. I've just now got it back to running condition, so I think I'm finding all his "racing mods". :shrug:

I'm going to give everything a good once over this weekend. Nothing to be assumed. If I can check it, I will.

#Biz, the light I was using was just a cheap automotive light gun. + and - hook ups to the battery and a clamp that goes over the spark plug wire. I took off the engine timing cover on the left side and shot the light on the timing marks as the bike was running.

Hopefully I can start logging some miles on this thing now...
 
Now I never had this problem till recently but even when the weather is very warm My bike would start without choke as soon as I hit the starter.I might take my carbs off later and adjust the needles up 1 clcik
 
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If the float level wasnt set high enough Im pretty sure this would cause a lean condition but what about the problem with the run away throttle?:shrug:
 
Run away throttle, or high idle and hesitation in decelerating rev can all be lean conditions. I solved all of these problems by cleaning my carbs and putting on the right pod filters.
 
I've been having this same problem as well. I cleaned the carbs, no problem there. That leaves 2 more solutions which I wanted to touch on.

Mixture:

Is the mix screw the same as the pilot screw, the one under the tamperproof seal? Given that it's still sealed, should I be messing with it?

Float height:

(if someone could explain how the float height affects the fuel/air mix I'd appreciate it, I see how it opens the needle valve but don't get how that affects the mixture into the engine)

They were initially at 20mm, which had the neutral-rev issue after the bike warmed up. I found that adjusting them to 25mm keeps the needle closed at all times, and 17 dumps gas out the air filter. Do I just keep trying every millimeter from 18 to 24, reassembling the carb 7 more times?

The manual says that floats should be at 32mm, but that seems implausible -- the floats would be back to a right angle and would touch the very bottom of the float bowl.

The manual does say, though, that the floats are made of brass, while mine are black plastic like the photo below.

$(KGrHqJ,!g4E-+VE(gi,BPzUp-tOQQ~~60_58.JPG


My current line of thinking, is that these are not the stock floats so the numbers in the manual are invalid and I'd have to just figure it out myself. Though maybe someone else has the black plastic floats and knows the correct height offhand?
 
If you messed w/ air such as pods or exhaust then yes you have to adjust the mixture screws. Drill out those plugs to get to them. The floats determine how much fuel goes into your float bowl. If its too high you'll get grip much and flood your carbs & foul plugs or be too rich. If its too low it won't fill up w/ enough gas to keep up. Id say go back to stock settings. As far as aftermarket floats, I don't know if that effects anything. Try to research them, find out what they are.
 
I kept on fussing with the float height last night, and finally got the bike to run again. Man, those things are tedious: 1mm too low and the valve is always shut, 1mm too high and it dumps gas out the air filter. Is it supposed to be that way, or is that a side effect of my using floats other than the brass ones mentioned in the manual?

Having gotten the bike running again, it still has the high rev problem and some backfire, with some hesitations. Again with lean mix.

I'm reluctant to mess with the floats again, as any deviation from where it is now means the bike won't run. So, how else to adjust the mix?

Aside from the mixture screw (and taking a drill to my carb?!) the manual mentions that the piston-needle attached to the diaphragm can have its height adjusted. Any thoughts on this method of enriching the mix? Less effective, more tedious, ...?
 

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