poor spark? vacuum leak? bad plugs? bad battery?

[79-GS550-L]

Hi everyone...

I'm pulling out my hair trying to solve several problems which I suspect are all related... Sorry for the really long post but I'm trying to give as much info as possible.

1. The bike fouls spark plugs incredibly quickly (just puttering around the neighborhood a few times). They turn flat black and sooty (rich condition or weak spark). My carbs are set to baseline settings (fuel screws out 1 turn, air screws out 2 turns). After the plugs are fouled badly enough, it won't start.

2. I'm on my 4th set of plugs and it's getting expensive, so I sandblasted a few old sets and they seem to be working ok (although I suspect new ones would work much better). Is there permanent damage to the plug once it has been fouled?

3. I connected a spare plug to one spark plug wire and set it on the engine block and fired the bike up. The spark is purple and orange, at any RPM.

4. The bike usually does not make a spark (or maybe a verrry weak yellow one) when you use the starter, only when you use the kick start.

5. The battery voltage drops to 8v or so while cranking with the starter. It's a little over 12v when idle or when shut off. If I rev it to 4-5000 rpm, I get 14-15v. If I turn on the headlight, it drops down to 11-ish and the rpms slowly drop and then it stalls. If I keep the idle up with the throttle and try to drive with the headlight, I can ride it but it has a noticeable decrease in power (hard to go up a hill without stalling).

6. I've been trying to fix what I thought was a vacuum leak. When I rev up the engine, it hangs around 2100 rpm. To get it to go down I need to lower the idle with the idle screw a whole lot until it starts falling, then tighten it back in to catch it before it stalls. I can get it to hover around 1500, but as soon as I rev it up past 2000 rpm it stays there until I
loosen the idle screw a whole lot again. I just replaced the carb boot O-rings, put some silicone lube on the boots, redid the black rtv from around the vacuum sync ports. I am 99% sure there is NO vacuum leak between the carbs and the head.

7. Sometimes I get a pop while trying to start the engine (between kicks). I don't know if it's spitting back through the carbs, or backfiring, or what.

8. I can't adjust the idle air screws because I can't really get the bike to idle properly and at 2k rpm the screws don't really do much of anything.



Things I have done:

1. Set all valve clearances
2. Compression test (OK)
3. dipped carbs, replaced all O rings, bench sync
4. new petcock
5. tested both coils, resistances are all to-spec.
6. set points gap and static timing (I don't have a timing light so I haven't tried the 5k RPM timing but unless my timing advancer is bad it should be fine)
7. replaced carb intake boot O-rings, re-sealed vacuum sync ports, cleaned and
lubed all rubber boots from the carbs to the head and airbox
8. put a bit of weather stripping around the airbox lid
9. rinsed out the air filter - it was a green foam one, and the bottom 1/3 or so was soaked in red oily stuff that turned to pink froth when I rinsed it, I assume that's an excess of air filter oil

I have NOT done a vacuum sync yet because I wanted to solve this vacuum-leak-like condition first.

So I think the charging system is OK since the
voltage across the battery terminals is good at higher rpm. The battery itself is pretty small (smaller than the stock one, there are some rubber bits wedged in next to it to hold it in place). I think that's why it won't spark well while cranking. Could this be the cause of the weak sparking? I thought the charging system took care of all that. I've done everything I can think of besides throwing random new parts at the bike...

 

[Guest]

First things first. You need a new battery. Without a doubt.

2nd, you might need a carb synch. That should cure the idle problems.

 

[Guest]

I agree on the battery and clean the connections from the ground on the battery to the crankcase, the solenoid, starter....
Get crazy with the dielectric grease and fine grade sandpaper. \\/

 

[79-GS550-L]

Ok, battery it is then!

 

[SqDancerLynn1]

I would seriously consider replacing the coils too !!!

 

[Steve]

You have already agreed to get a new battery, spend a couple extra bucks and get a sealed unit.

One other thing that caught my attention:

I just replaced the carb boot O-rings, put some silicone lube on the boots, redid the black rtv from around the vacuum sync ports.

Which side of the vacuum sync ports? If you did the inside, your RTV has now gone through the engine. If you have hardened, cracking intake boots, replace them. Yeah, they aren't exactly cheap, but the bike will run so much better.


.

 

[79-GS550-L]

You have already agreed to get a new battery, spend a couple extra bucks and get a sealed unit.

Which side of the vacuum sync ports? If you did the inside, your RTV has now gone through the engine. If you have hardened, cracking intake boots, replace them. Yeah, they aren't exactly cheap, but the bike will run so much better.
.

On the outside. The bike had allen bolts in the ports, with a flat washer and a split washer, and some rtv around the flat washer that had hardened.
This is the same thing the PO had done around the gas tank, which leaked (split washers are obviously not watertight!). So I removed all that junk and scraped off his old rtv, sanded the boot smooth at that place (it is actually metal right there) and put a thin layer of new rtv. It shouldn't have even touched the threads, let alone gotten into the engine.
The boots themselves were still pretty soft and flexible, no cracking, so I left them as is.



What size battery should I get? The one that's in there right now says "9 amp hours @ 10 hr rate".

 

[79-GS550-L]

I would seriously consider replacing the coils too !!!

The resistances looked good on across both windings of both coils... could their performance still be diminished even though their resistances are still good? I haven't looked for stock coils yet but I searched for dyna coils a while back and was blown away by their price :shock:

 

[t3rmin]

RTV is NOT gas resistant. You can't use it to seal intakes or gas tanks. If it hasn't already been compromised, it will be soon.

The intake sync ports are usually sealed with an allen bolt and a single flat metal washer. No sealant required. If you've got split washers and RTV on there, I'm willing to bet that's your problem!

The petcock is secured to the tank with bolts and a flat washer as well. The flat washer is some kind of plastic on my GSs. You'd probably be able to find a nylon washer that'd substitute at a hardware store. No sealant needed, especially NO RTV!

Also make sure the vacuum hose to your petcock is on tight at the carbs and the petcock. That's often a source of big vacuum leaks.

 

[79-GS550-L]

RTV is NOT gas resistant. You can't use it to seal intakes or gas tanks. If it hasn't already been compromised, it will be soon.

Good to know! I'll remove it and use flat nylon washers instead. I just put it on there a day or two ago and haven't run it much, so I doubt it caused too much damage in that time. Vacuum hose is on there nice & tight.

I used jumper cables to hook up to a fairly new car battery and tried to run on that. It is a bit better, as I was able to start it using the starter, but I still managed to stall the engine by turning on the headlight. And when I looked at a spare spark plug while it was running it was still a bit orange, but it's pretty hard to tell if it's any better since it's sunny out

So before I go out and buy a new motorcycle battery I'd like to figure out why the headlight is still conking out my sparks... maybe my old battery is still ok after all...

 

[focus frenzy]

if you get good voltage with headlight off but not with it on you have a dead leg from your stator and it may be the one that runs all the way up through the light switch. I lost one leg once and the battery ran down, I unplugged the headlight bulb and jumped it off and without the headlight the charging system managed to just stay ahead of the demand and it built the battery back up.

clean, re gap or better yet replace your points and condensers first before replacing the coils. Suzuki coils are sensitive to voltage and don't fire worth a damn if they are not getting good voltage from the battery.

 

[79-GS550-L]

if you get good voltage with headlight off but not with it on you have a dead leg from your stator and it may be the one that runs all the way up through the light switch. I lost one leg once and the battery ran down, I unplugged the headlight bulb and jumped it off and without the headlight the charging system managed to just stay ahead of the demand and it built the battery back up.

clean, re gap or better yet replace your points and condensers first before replacing the coils. Suzuki coils are sensitive to voltage and don't fire worth a damn if they are not getting good voltage from the battery.

Points are brand new and gapped properly. I'll check on that stator leg, that sounds like it could be the culprit. Battery voltage does drop with the headlight on... it dropped about 1v with the motorcycle battery and about .2v-.5v with the car battery. Is that bad? It doesn't sound like much to me, but I would expect that with the extra stator leg I might actually see an increase in voltage with the light on, no? I've hear about excess voltage from the stator being sinked into the headlight. Maybe this only happens at higher RPM though...

 

[focus frenzy]

Points are brand new and gapped properly. I'll check on that stator leg, that sounds like it could be the culprit. Battery voltage does drop with the headlight on... it dropped about 1v with the motorcycle battery and about .2v-.5v with the car battery. Is that bad? It doesn't sound like much to me, but I would expect that with the extra stator leg I might actually see an increase in voltage with the light on, no? I've hear about excess voltage from the stator being sinked into the headlight. Maybe this only happens at higher RPM though...

actually Suzuki wired the system to kill one leg when the headlight was turned off to keep the regulator from overheating and failing. the voltage is controlled by shunting excess power to ground in the regulator.

if the contacts for the leg controlled by the light switch are corroded or burned out ( likely, and reason people re-wire the third leg direct and keep the headlight on) you will have a bike that charges as long as the light is off but draws down the battery with the light on.

straighten out the charging issue and the spark and plug fouling problem will likely greatly decrease or go away all together.

also check the voltage drop at the coils, with it running check the voltage on the hot wire at both coils and see what they are getting.

the fuse box uses brass clips to hold the fuses and the tarnish that forms on them is a very poor conductor and needs to be cleaned off.
the resistance from the tarnish also make the clips get very hot and it can (and does) cause the solder that holds the filament inside the fuse to mel and you will have a fuse that is blown but looks perfectly good.

 

[79-GS550-L]

that makes more sense now (about the leg killed when the light is off). I looked at the contacts, they were pretty nasty, but with a little cleanup I got .3 ohms going through the headlight switch (my meter reads .2 ohms when you short out the probes). The stator appears good, I get the right resistances between all 3 legs. I guess I've got a little electrical cleanup to do now. I was planning on tearing it all apart and completely redoing the wiring this winter with nice expensive connectors and nylon wire looms, so for now i might just remove all the bullet connectors and solder all the old wires together


Just a few more questions:
1. Once I get the wiring looking good, if I still have a poor spark from a fouled-then-cleaned sparkplug, could the plug be permanently damaged? Or should I still see a nice blue spark? I'd go out and buy a new set of plugs but I always get a blue spark from those, so I wouldn't know whether I actually fixed anything.

2. Could my "hanging idle" be caused by anything other than a vacuum leak? i.e. poor spark at low rpm, carbs unsynched, mixture screws set wrong? I wanted to fix any air leak before tuning the carbs but I've been searching and really can't find any sort of air leak at all. Everything is on there nice and tight. I will try spraying carb cleaner around the boots when I get all the electricals back together.

3. If it IS an air leak, that would be between the carbs and the head, right? I.E. it cannot be the boots between the carbs and the airbox?

 

[Guest]

With the VM carbs the trouble more than likely would come from the intake boots. Have you replace the o-ring behind them? Exuse me if this has already been answered. :-D

 

[RMC]

Just as an FYI......I sealed the threads of my petcock bolts with a thread sealant from Loctite. It's said on the package it was perfect for use with gasoline and oil. I pick it up at my local Advance Auto Parts. It's a low strength seal because I figured I may need to take these out one day!

 

[SqDancerLynn1]

You should have a 12-14 ah battery. I had problems with my 79 850 re weak spark. New coils fixed the problem even tho they checked good

 

[79-GS550-L]

ah yeah then my battery is definitely too small. Did you have the same problem with both of your coils sqdancer? Can anyone recommend good coils that won't break the bank like dyna coils? Preferably ones with replaceable wires.

 

[79-GS550-L]

Got a 14 Ah battery... it fits, but I need to modify the side covers to get them back on. No big deal, they were already cracked and needing fiberglassing

So if I were to bite the bullet and get dynatek coils... which ones do I want? And what do the numbers on the side mean? There are some on ebay that say "3 ohm 6/96". Is that the manufacturing date? What secondary resistance do I want?