'80 SG driveability issues, want your thoughts...

[Bonz]

I posted this in the "bad timing advance" thread because that thread is the closest I've seen to describing my problems, but thought it would get more viewers as a thread unto itself...

Thanks in advance!

For my 1980 SG Special:

Since the day I bought it and rode it home, 2 1/2 years ago, when its warming up and otherwise cruising fine at light throttle, it will simply go flat and not accelerate, but not slow down either. Just a "blaaaaaaaaaaaaaah" sound no matter how much throttle I give it while in gear, if I roll out and roll back on,while in gear, it does not go away.

But, I get off the throttle, pull the clutch in and re-engage the clutch, it usually goes fine, occasionaly it will go flat again. Once warmed up (I mean, really up to operating temp after miles of riding) it is more or less fine. Then, when I get to half a tank, no matter what I do, it continually wants to go flat again if the petcock is "on" and I'm at low speed, light throttle. I can switch to reserve and it goes another 20 or so miles (still plenty of gas in the tank, just under half full now) before it begins going flat at low speeds continually. It will begin to feel like it's starving out at higher speeds as well by this point. So I just fill the tank and call it good. But I'd like to get more than 70 miles or so before having to do this ritual. I.E., I'd like to run a normal tank through it!

When I got the bike it would do a full tank (still had to low speed flatness issue, but would otherwise swallow a whole tank without having to go to reserve until it actually needed to), but has degraded to this point and stayed here for a year. Running on prime makes no difference, and I thoroughly took apart the carbs, cleaned them thoroughly, etc last fall. Because the flatness issue was there before and after the carb job, I tend to think I have a fuel supply issue (Octy) vs. a fueling issue from the carbs themselves.

As well, when it's really hot outside, and the engine is very hot as well (such as yesterday when going through Denver, 97 degrees outside air temp, slow speed through construction zone on I-25) it will continually need constant throttle and clutch tap dancing to get it to pull away cleanly from a stop or crawling speed. Otherwise it wants to be herky jerky, and sometimes if I don't do it just right, it'll just quit. Starts back up with a little work on the throttle, but this bike absolutely hates the hot air temps and low speed, no matter the fuel level in the tank. Cooler days (70's) and it runs like it never had a problem in it's life, and yesterday, or any day, at any speed above a crawl, it runs flawless despite the crazy heat. Make sense?

My mind has a lot of things going through it...

First, I have new fuel line and such to eliminate the Octy, and will do that tonight I hope. Start there and maybe that will be the end of my problems.

Second, I am going to pull off the fuel cap and makes sure it's venting. I can hear a hiss when I park it after riding, so that tells me something is venting, but is that also an indication the vent could be plugged/partially plugged and my hot day blues come from not enough air coming into the tank to replace the fuel that leaves?

Third, could my carb diaphragm slide springs be too tight/stiff and causing the issue of the flatness until it's warmed up? All it takes to set it right is to pull the clutch in and re-apply throttle to usually clear it up... Until I get to a half tank, then it wants to go flat all the time at light throttle regardless of how hot it is outside/how hot the engine is.

Fourth and last on my list, is an issue with the vacuum advance or pick up wire. YES the vac advance is hooked to the carb body, and the octy to the carb boot, so that is in order.

OK, that about covers it all, so have at it boys, love to hear if I'm thinking the right stuff...


Thanks much!

[CatatonicBug]

The fact that your cap is hissing when you stop is a red flag. Clean that vent in the cap, and I bet the problem will all but go away! The vacuum in the tank is starving the engine of fuel, but giving it just enough to allow it to run.

[b.walker5]

Also pull your petcocks and check the tank filters.

[Bonz]

I firmly believe I have both of those going on at the same time... Fuel screens need to be examined for the half tank issue, and the vent needs cleaning/fixing/whatever. Found some good simple posts elsewhere on this forum that speak to both of those.

Most of the battle is figuring out what system is causing the problem, or most likely causing the problem, then going after it!

Thanks for the help thus far, I feel pretty confident I am on the right track after the last two posts I read.

[Bonz]

Ok, pulled the tank and the petcocks. Screens were immaculate, barely a speck on them. Good deal there. As well, took the fuel cap off, took it apart, opened up the hole in the fiber disk so there was no doubt it was clean and clear, re-assembled the cap, put it back on the tank, put the fuel back in the tank that I'd drained out to do the above work, and it fired up fine (it should, had plenty of fuel still in the carbs). Let it idle for awhile, couldn't really tell anything other than "Yep, she runs". Hope to get out for a ride in the morning before work, although it won't get out of the 60's tomorrow here in Colorado. I'll know right away if the half tank issue is present, as there was just about 2 gallons in the bike when I drained and strained it, so there is just about 2 gallons back in it after re-filling.

[BigRed]

I hope what you did fixed your problem, but here's my guess:

From reading your symptoms, I'd bet it's a broken pick-up coil wire. These things can often act like carb issues because when you work the throttle, the vacuum advance wiggles the pick-up coil wires, and makes or breaks contact if the conductor inside the wire is broken. Take off your timing cover, and try to stretch the wires. If they stretch, the copper's broken, and you can solder in some multimeter lead wire (it's more flexible than regular wire). You can also try to jiggle/stretch the pick-up coil wires with the bike running to see if you can hear engine bog, but be careful not to get your hand or a sleeve caught on the spinning parts!

Good luck!

[Bonz]

BigRed and others,

Thanks for the ideas and thoughts, the pickup wire has been mentioned half the time in the suggestions I am getting, so there's at least a 50-50 chance it's part of a solution!

To date, I have not pulled the cover and taken a look at the pick up wire despite knowing of the potential problem that exists with that since about the time I bought the bike... I'll give it a look tonight. Is this pretty much a 100% "failure" on the XS? I don't see many posts or threads saying "my pick up coil wire has always been fine"...

With that said, what's the concensus on this... My thought is the pick up wire would have nothing to do with the consistent half tank issue that I have developed, nor in my mind would the heat put out by the engine on really hot days have any effect on whether the pick up wire becomes "more intermittent" when I come to a stop and the bike is hard to make a clean launch w/o revving it a bit and slipping the clutch. On cooler days it does not have that problem. That's why I've been going down the path of checking out fueling stuff.

It was wet this morning (rainy mist, foggy) so I forewent taking the old girl out for a ride.

It'll be a mid-80's day tomorrow again, I'll be able to ride it to work and see how it is doing with the stuff I did last night.

[CatatonicBug]

I don't see many posts or threads saying "my pick up coil wire has always been fine"...

Well, I'll be the first then! My pickup coil wire has always been fine.

Of course, mine is an '81, and there is much less "wiggling" going on down there to break the wires on mine than with the earlier years.

[Bonz]

Ok, now that the petcocks and screens are verified as clean and good, and the fuel cap vent has a clean bill of health, I rode it to work this morning.

As well, I replaced all the fuel lines because the bike had original lines on it still and I tested the Octy diaphragm by sucking on the vacuum port after I'd removed the whole assembly. I could feel the instant build up and maintaining of vacuum and hear the diaphragm move inside the assembly as it pulled shut with the application of my scientific vacuum device. Also ran a new vacuum line from the boot to the octy. So, the octy as a whole seems to be in order.

Anyway, started my ride with just a bit less than half a tank. The bike still hits the flat spot that comes and goes at light throttle until warmed up, so that issue remains. As well, it still was not running like it does with a full tank, i.e. it acts like it's starving for fuel with the light fuel load, but it comes and goes. Stopped and filled the tank, ran well the rest of the way in to work, except for hitting the flat spot once when cruising at light throttle in traffic. Yep, time to check the pick up coil wires.

What I DO notice with this bike is at light throttle, it is either on or off. Meaning, if I keep rolling out slowly at light throttle, I hit that threshold where it feels like it goes from having some throttle applied to no throttle applied. Like it's a switch instead of a rheostat. Make sense? It's at that threshold where the flat spot hits, and I have to clutch it and re-engage to get it back to feeling fine. Until I hit that threshold again while slowing down or going slow in traffic, then I do the "wash and repeat" again with the clutch.

[natemoen]

Well, I'll be the first then! My pickup coil wire has always been fine.

Of course, mine is an '81, and there is much less "wiggling" going on down there to break the wires on mine than with the earlier years.

Congras on having quality wires BUT...

81 still had the vacuum advance did it not? Wasnt it the centrifugal advance that it lost? So because it still has the vacuum advance that would be just as much movement to affect the wires.

[cywelchjr]

Congras on having quality wires BUT...

81 still had the vacuum advance did it not? Wasnt it the centrifugal advance that it lost? So because it still has the vacuum advance that would be just as much movement to affect the wires.

That they do. That said, my 80 also has AFAICT the original wires and with no repairs done to them I can see, and they are working fine. I'm prepared to deal with it when the problem happens, but so far I have been really really lucky.

[DGXSER]

Yes, the 81 models have the vacuum advance but no mechanical advance, it was built into the programming in the TCI.

The vacuum does move the timing, but no where near as much as the mechanical advance and the vacuum combined. The 81 model has always displayed less succeptibility to the wires breaking. Not to say it won't or can't happen, just not nearly as likely is all.

[cywelchjr]

Yes, the 81 models have the vacuum advance but no mechanical advance, it was built into the programming in the TCI.

The vacuum does move the timing, but no where near as much as the mechanical advance and the vacuum combined. The 81 model has always displayed less succeptibility to the wires breaking. Not to say it won't or can't happen, just not nearly as likely is all.

You know, everyone keeps saying this, but the mechanical advance moves the rotating assembly, not the plate the pickups are on. This doesn't flex the wires at all. The only thing that flexes the wires is the movement of vacuum advance, which is also the violent and jerky movement as well, as the mechanical advance moves fairly smoothly as rpm's increase, where vacuum advance is jiggling most of the time its operating.

[DGXSER]

To be honest Cy, I have not worked on the earlier models in the timing area yet. So I will take your word on that.

I know that alot of the long time member gurus always have stated that the 81 model does not experience the issue as much as the earlier models due to no mech advance. And I do KNOW that I have worked on three 81 models that have never needed the pickup wires repaired.

[TomRodgers]

Hey guys....not to hijack a thread..but does the following sound like pickup wires as well??? or carb issue???

Quick question.....

After going to/from work yesterday bike ran fine. Let it sit about 3 hours...then went to get gas for today.... pulled the choke out, hit the starter and it fired right up....let it sit about 15 seconds (due to being about 65-70 degrees out), snapped the choke back in and it idled just at 1000 rpms....usually idles about 1200....took off to gas station.

i noticed the choke went in rather hard, and now the bike is weak on the low end...after 3000 on the tach, lurches like it was shot out of a cannon.... then on decel, starts running ragged again below 2500 rpms....and will barely idle at 1000.....in neutral, rev it, and it pops on decel like its starving for go-juice....

Any thoughts????