Trouble starting engine

[tealetm]

After taking apart/cleaning the carbs, adjusting valve clearances, and reassembling it all- we still can't get the 93 gs500e to start. 

To make a long story short- we grabbed the bike from a friend who claimed it would only run for about 20 min before both plugs fouled up and it shut down.  I had it running for a few minuted, then decided to clean it and do valve shims. 

Now that its all back together (mixture screws at 1.5 turns), it won't start.  And, it backfires out of the Exhaust (very loud- my ears can still attest to it).  Turned it over and over and over again- same thing -there is fuel getting into the bowls.  I have good spark, however the plugs seem to become covered in carbon (due to...?)  I increased the mixture screws to 2.5 turns- still nothing.

Any clues?  I didn't mess with the timing, but that doesn't mean it could have been off previously.  Why is it backfiring?

[mbbikes2005]

Backfiring is caused by either over fuelling or if the valves are still open when its on ignition stroke. it cant be the ignition timing because its pre-set so i would check carb float height, valve timing or other possibility is shims to thick holding valves open or valve sticking in guides.

[tealetm]

When I first opened the valve cover, I had to replace all the shims with smaller sizes as there was no clearance between any of the 4.  I put new ones in, checked it then buttoned it up.  I can check again and see if something has changed. 

Valve timing- havne't don't that before.  I'll look into it though

Float height- I'll check that too.

[LimaXray]

hmmm... it seems strange to me all the clearances were off... I'm sorry to ask, but just to make sure you did measure the clearances when the cam lobes were pointing away from the buckets right?  If not, the clearances may now be too loose.  Otherwise, it is possible the clearances were so tight the pistons hit some valves.  I would check your compression just to rule that out

[tealetm]

Yep, there was no clearance between any of the shims and the "non-contact" portion of the lobes (where you are supposed to measure).  I seriously doubt valves made contact with the pistons- I'm ruling that out.

The only reasoning for this besides normal wear on the valves is... aftermarket cam with different clearances?  I'm just throwing that out there- no clue at all.

[tealetm]

Well I thought that the valve timing was off- so I pulled the vavle cover again.  To my dismay, the timing was dead on.  I'm truely baffled by this.

I have a strong spark on both plugs.  When I turn the engine over it doesn't even attempt to light off, until about 15 seconds of starting attempts and then a very loud backfire from the exhaust (and a small back fire from the left carb as well).  After this, I pull the plugs only to see that they are a little sooty- but not wet and not too fouled with carbon for ignition. 

I am going to pull the carbs apart again- to check the float height.  There is definately fuel in the bowls. 

A couple of possibilities:
1)  Fuel is not getting into the cylinders.  I want to discount this because of the fact that it is indeed backfiring.  Fuel is getting into the intake manifold (moist when carbs removed).  This could only mean that the vavles aren't functioning corrrectly.  Hmmm

2)  Valves are screwed up.  I don't know how this oculd be as I did have it running... and i never screwed the timing up.

3) Exhaust is plugged.  I put a "new" oem assembly on- but it doesn't seem plugged when I blow into it (let alone when it backfires)

4) Poor compression.  I maybe have to break down and buy a $10 gauge- but... again it backfires?

I'm at a loss here...

[LimaXray]

check your float height first.  use this method.  it's easier, you don't have to take the carbs off, and it's more accurate.

if that proves not to be the problem, get a compression tester.  if the valves are sticking open for whatever reason, you would get a backfire and you wouldn't have great compression.

[Kerry]

I didn't catch it before, but this is starting to sound familiar - and electrical.  I had a similar problem a couple years ago.  It seemed like I had a strong spark on both sides, but my symptoms were similar to yours.  In my case it turned out to be a corroded connector in the "pulse generator coil" circuit for one of the cylinders.

This may not be your problem at all ... but you can perform some simple, non-invasive tests with an ohmmeter to eliminate a few possibilities.  (Start at this old post and work your way through the rest of that page.)

Just curious - has the bike ever been down on the right side?

[tealetm]

I picked up the bike from a buddy- he hadn't  dropped it but wasn't sure about the previous owner (I'm sure it has been at one point in its ugly little life)

Everything looked good when I peeked at the pulse  generators and everything did look good.  I had a definate, strong spark.  Although as I have little other ideas, I will investigate further.  Thanks

[Mandres]

Are you positive the carbs were put back together properly?  Did you double check the little o-rings on the top vacuum ports?  Are the pilot jets completely clear of obstructions?  

Try taking th exhaust pipe off and putting a piece of paper over the ports.  Pull the plug cables and crank it over.  Is the paper getting wet from the fuel/air mixture?  

-M

[tealetm]

Excellent suggestion Mandres- I will check that fuel/air is getting into the cylinder via exhaust ports.

But still... how can it backfire without fuel?  It can't...

I have the carbs sitting on my kitchen counter- going to pull them apart again tonight.

[Mandres]

Since you've got the carbs off try this little check too:  remove the valve cover and rotate the engine manually so that both intake valves are closed.  Pull the plugs and shine a small light into the plug holes.  Look through the intake ports in the head and look for light leaking around the intake valve seats.  Repeat for the exhaust valves.  If you find an obvious leak the head will have to be removed for service.

-M

[tealetm]

I went at it again last night.  Took the carbs apart- all looked good.  Put the carbs back on, took the exhaust off and turned it over without the plugs in it.  Fuel/air mixture was coming out of the exhaust ports in pleanty.  It's getting fuel.

So at this point I was completely dumbfounded.  So I took a ride on my 95 Triumph Speed Triple to put the smile back on.  While out- I figured geez, maybe the battery is strong enough to give a visual spark- but just weak enough to screw up the starter turning fast enough or something.

I jumped the battery leads from the GS to the Triumph- bingo we had action.

Barely.  She putted and spat, did run though.  Misfires all over the place- backfiring out of the exhaust and both carbs (back and forth one to the other).  I wouldn't start if you gave it any throttle at all, and I could only get it to rev up to about 3k rpm.

My thoughts now are... carb sync.?  Or... valves need to be lapped/replaced.  I'm thinking the latter because the clearances from the shims was 0- valves probably had a lot of blowby and now aren't seating smooth and uniformly.

I'm going to dig into the valves tonight- anyone taken the top end apart to lap the valves?  Is it doable with the engine in frame?
I'll try the flashlight into the spark plug hole trick while maknig sure that the valves are "closed" when they are supposed to be. 

I'm throwing in a picture, I was just filling the fuel line into the carbs directly (oz. by oz.)  There are no other vacuums which need to be attached correct... the hose on top of the vavle cover is a breather hose to the airbox (which no loger exists)?  That breather hose was origianlly connected to the top T on the carbs (which is another breather to the orig airbox correct?)

[LimaXray]

did you ever do a compression test?  i wouldn't go taking things apart unless you were sure there was a problem there.  if there is a problem with the valves, you're gonna have a loss in compression.

was it running while connected to the triumph? was the triumph running too?  if it runs when connected to the triumph, but won't run on its own, that sounds a lot more like an electrical problem then anything else.  first off, you can't properly evaluate spark by the way it looks.  it may look great, but it may be weak, or it may not be firing when it is supposed to.  there may be a bad contact or something somewhere and that extra bit of current can make it go from not running to running like crap. 

i doubt it has anything to do with the stater not turning fast enough, if the stater doesn't go 'click click click' when you try to start it, there is plenty of current to start it.  also note, when you jump start it, you increase the current supplied to the starter, thus increasing the amount of torque it produces; but the voltage remains the same, and it's the voltage that determines how fast a motor turns.

start off by checking the various wire harnesses for damage or corrosion, namely the aux ground wire harness coming from the battery.  next i would try swapping the signal generators and ignition coils to see if that makes any difference.  if you have the shop manual, it gives you basic methods to test these things, so you should also try that.

edit: if you go to an autoparts store you can pick up a small generic coolant overflow tank for a couple bucks that will make a good testing fuel tank.  there are threads about this somewhere if you do a search.  also, the only line i think you need to worry about is the vacuum line that goes from the carbs to the petcock, as it should be plugged otherwise that one carb will run a little lean.

[Mandres]

There are no other vacuums which need to be attached correct...

Well, other than the missing diaphragm cap on the right carb you're correct.  Be advised that without those caps the engine will run like crap.  There is really only 1 vacuum hose in the system: from the left carb to the frame petcock.  It sounds like badly-sealing valves to me too; the backfiring is a dead giveaway.  Like LimaXray said, a compression check should really be your next step. 

Yes, the head can be removed with the engine in the frame.  You'll need 4 of the head/cylinder o-rings, new head gasket (or you can clean and reuse the old one with a light coat of gasket sealant) and a can of gasket remover (for cleaning off old gaskets and decarbonizing the combustion chamber).  When pulling the head try your best to keep from disturbing the gasket between the cylinders and block.  A local machine shop could probably disassemble and clean up the head for you relatively cheaply.  If you want to do it yourself you'll need a valve spring compressor (check the FAQ section for Dgyver's homemade one).  If you have any questions drop me an e-mail or PM.

-M

[tealetm]

Ha, I would hope you didn't think I tried to run the bike with the right carb diaphram cap missing! 

I'm also leaning towards valves.  I'll do a compression test, then the old oil for rings test, then pull the head if needed.  I've lapped valves before, but if the case is that the valves are really bad- how much are a new set? Can't be too much for only 4...

I should take a sound clip and post it- I laughed yesterday when it "started".  Cough, pop, spit, cough pop, etc. etc.

[Mandres]

Intake valves are about $20 ea.  Exhaust are about $30 iirc.  Hopefully you won't have to replace 'em; a good scrub and lap-in can do wonders.  If you do it yourself take a real close look at the seats and guides.  I had a cracked guide that I almost overlooked. 

-M

[tealetm]

maybe I'll just grind lap the hell out of it them for that price! 

[Kerry]

Well I thought that the valve timing was off- so I pulled the vavle cover again.  To my dismay, the timing was dead on.  I'm truely baffled by this.

[...]

A couple of possibilities:

[...]

2)  Valves are screwed up.  I don't know how this oculd be as I did have it running... and i never screwed the timing up.

I respectfully submit that it COULD be a timing issue - just not a valve timing issue.  I still think there is something wrong with the signal generator circuit, which affects the timing of the spark.

You said the pulse generators look OK.  When my bike had this problem, I never even looked at them ... but only because I found the corroded connector upstream first.

As you aptly pointed out, the bike was running before.  With clean carbs and adjusted valve clearances, it should run even better now ... something else has changed to cause the backfires, and the timing of the spark is the only thing that makes sense to me.  Do the resistance readings from my previous post check out?

BTW, your photo shows some odd hose connections.  Standard practice when removing the airbox is to add a PCV(?) valve on the airbox end of the hose from the valve cover.  (The hose that is attached to upper T between the carbs in your photo.)  The vent hose that is normally attached to the upper T is left open on the other end, as always:

[tealetm]

Kerry,

I agree with you something has changed.  I just can't find it.  I also agree with you observations re: hose connections.  When I first got the bike the breather hose from the valve cover was hooked right up to the upper T on the carbs.  Wrong- I know but thats how it was given to me...  What does the PCV valve do when added?  Does it act the same way a automotive PCV valve does (drawing vapors off the engine)?