The usual non-starting post. '95 GS500e

[jrtaylorthird]

So after getting a whole boatload of problems with my last engine, I found someone selling a '95 engine for 500. Heard it start on the original bike, sounded great all the way through, installed it in my frame, but to no avail.

I've got a '95 engine with '92 carbs. 40 Pilot 140 Mains, idle-air screw 3 full turns out. The bike will turn over and over, sounding like it's running really lean and after about a minute it'll backfire. (Also, backfires are much louder than I expected.)

I've tried with a K&N Lunchbox, a regular stock airbox, and I just don't know. The carbs were fine when I last used them on the old engine, started that one right up. I've done a thorough cleaning, didn't misplace the little rubber o-ring under the black cap...I just don't know.

Any ideas?

[mr72]

I feel your pain. Did you check the plugs? Float levels? Intake boot o rings? Petcock?

Just things to check.

[jrtaylorthird]

Plugs are new, float levels even with gasket. I haven't check the intake boots, they don't look the happiest but the engine started on the other bike so I figured it would be something more towards my own carbs. I'll check the boots tomorrow. Any other ideas?

[Watcher]

As long as you get fuel, air, compression, and spark you'll start...  Usually jetting won't in it of itself result in a no start as long as the jets aren't clogged or something, so for now just put on whichever filter seems to be in better shape.

Can you clarify what's going on?  Title says no start but you mention it sounds like it's lean.  If you can get that far it must be starting.
Will it fire up and die shortly after?  Or does it just struggle to light?
Because this will change our diagnostic procedure.

Won't start at all I'd begin with verifying spark with each plug grounded on the head.  Just be sure you do this somewhere well ventilated, you don't want any fuel vapor getting ingnited!
If you're getting good spark I'd take starting fluid and try it in the throttle body.  If it starts this way then you aren't getting fuel somehow.
Still no start or very weak start I'd spray it around the intake boots.  If it fires this way then you have a cracked or torn boot.  If the engine is sucking air through the boot it won't be pulling air through the carbs and won't be picking up fuel.

On the other hand if it WILL start but idles weak and dies I'd take carb spray and try it around the boots and the carbs.  If there's a vacuum leak it'll cause the idle to race.  From here it's just identifying the source of the leak.

[jrtaylorthird]

Would 40/140 be good with a Lunchbox? Because I do have one.
The bike will not start. Just won't light. I'll check for proper spark asap, but if it does manage to backfire there must be some sort of spark happening, right?
Timing was perfect, never changed.

I have realized now that the exhaust gasket is completely gone. Idk what happened to it but there is just zero presence of a gasket whatsoever. I don't know if that would cause any problems with starting however.

[Watcher]

40/140 sounds appropriate for a lunchbox.  IIRC I used to run 40/130 with an aftermarket exhaust and stock intake...

No header gasket can cause problems like overheating and valve burning and such.  It won't cause a no start.

Backfiring is fuel igniting in the pipe where it gets a thinner fuel air mixture.  Could mean weak or intermittent spark, or could indicate a mixture that's too rich.
Or it could be just from cranking it's ass off and not starting.  If you keep cranking and cranking and sending fuel through an engine that won't ignite then it all ends up in the header. When it DOES catch it'll be a backfire.
The missing exhaust gasket isn't helping this.  Just a source for more air to mix with the unburnt fuel being thrown into the pipe.

While you have the plugs out make sure they're gapped correctly.  Also see if they're wet at all, and see what color they are.

[mr72]

While you have the plugs out make sure they're gapped correctly.  Also see if they're wet at all, and see what color they are.

Yeah this is what I meant by "check the plugs". Was typing on my phone so I was too brief.

I bet at least one plug is wet with fuel or maybe there's a bad connection making it get no spark at all.

[Watcher]

I'm guessing no spark as well.

If the engine was running in the donor motorcycle we know the compression is good and we can probably assume good signal generator as well.  And if the carbs recently came off a running motorcycle we can assume they aren't the problem either.  I'm also guessing that this engine is going into the frame that matches the carbs so those coils should be good.

But he said he DID change the plugs.

Known working components mashed together with a "foreign" part and no start?  I'd be looking at the "foreign" part.
Could be gapped wrong or just faulty plug(s), or poor connection.


But we have to see what happens.  It's just a process if elimination.  Start simple and work your way down.

[jrtaylorthird]

Alright, so a bit of an update.

When I checked the new spark plugs, I checked them both on the right side using the right coil. They both sparked, so I installed them and obviously figured "The spark plugs work, can't be the spark plugs."

But what I didn't think of was the left coil. Sure enough, when I was checking the plugs again today, the right one worked, the left one didn't. Switched the plug to the right (since they're new plugs) and it worked. Took a gander at the coil and plugs, nothing seemed wrong.

Then I remembered I had to replace the little knob holding the clutch plates down, and to get there, I had to remove the wiring on the right side of the engine that connect to the harness and actually give the spark plugs a spark. Replaced those two black bits and the wiring that runs back near the battery with an extra set I had laying around, and presto! The bike started like new.

Only now, another problem.

The bike will start fine but after idling will start...coughing? I'm not sure how to describe it. Almost like it was fuel starved.

On top of that, when I pull on the throttle...no increase in RPM. There's a sound coming from (I believe) the carbs that sounds like more air coming in, but no gas going in as well.

Some things I've checked:
1. Tank petcock on.
2. Frame petcock primed, gas is getting to carbs after full float bowl drain.
2.a. I did replace the frame petcock from a 89-00 with a newer '08 model. Could that make a difference?
3. Throttle cable properly attached.
4. Choke closes completely.
5. Choke cable properly attached.

Any ideas?

[Watcher]

You may want to leave the petcock in the prime position.  In the on position it's still actuated by vacuum, so any issues further down may cause a stoppage of fuel flow.
Prime bypasses this and just flows fuel relying on the carb floats to stop fuel flow.


Does throttle response change when you mess with the choke?  The choke has its own fuel circuit, if adding more fuel causes the issue to subside you might need to open up the carbs and clean them out.  Jets might be clogged.


Or you might have a bad vacuum leak, if the engine is drawing air from somewhere else it won't be pulling much fuel from the carbs.  Might be enough vacuum to draw off the idle circuit but not enough to actually draw from the jets.

Now that it starts and idles, take some carb cleaner and spray around the carbs/boots.  Idle change will be obvious, and spray location will direct you towards a leak.


Also wondering if maybe the carb slides aren't actuating...  Easy to visually check this by taking the filter off.  But a vacuum leak will interfere with the operation of these so first things first.

[jrtaylorthird]

Alrighty, here we go.

Now the bike won't idle at all. Just baBUMbaBUMbaBUM --dead--.

[Watcher]

...  How are you fuelling it?

The petcock might be vacuum actuated, but the fuel is still GRAVITY fed...

[mr72]

Also wondering if maybe the carb slides aren't actuating...  Easy to visually check this by taking the filter off.  But a vacuum leak will interfere with the operation of these so first things first.

I am guessing this is why the bike wouldn't rev off idle with opening the throttle, back when it was running. Big vacuum leak somewhere so when you open the throttle and increase vacuum instead of pulling the slides up it sucks in extra air through the vacuum leak.

Or, if the carbs have been shot through with carb cleaner there may be damage to the diaphragms on the slides due to contact with carb cleaner. Carb cleaner ruins the diaphragms if it touches them.

And by running it this way, some other issue has been exacerbated, which is why it will start and run only a minute right now. My guess here is float levels or float valves sticking.

If you have a piece of clear fuel hose you can connect to the float bowl drains, then connect it and hold it upright beside the carburetor. Then open the drain valve and let fuel flow into the clear hose. It should flow up and stop at the level where the floats are, this is how you can check the fuel level in the float bowls (and float height) with the carbs on the bike. The fuel should be level with the top of the bowl/gasket. Repeat with the other carb.

Also, after you run your bike for the 5 seconds it'll run, pull the plugs and check to make sure they aren't wet with fuel.

Plugs wet with fuel means you are fouling them while it's idling and that probably means your idle mixture is super rich for one of a couple of reasons, maybe it's just adjusted wrong, maybe you're leaking fuel past the idle mixture needle o-rings, maybe the float level is way too high or maybe the float needles are leaking and letting it flood at idle.

If the plugs are dry then it probably means the opposite, you're running out of fuel. Probably not lean since you are starting it on choke (right?!?) so more likely there's a clog in the pilot jet or pilot fuel circuit (you have to clean this by running a .013" or so wire through it) or your float needles are sticking closed so you run the carb bowls dry before it can refill then gravity/prime lets it get a little bit of fuel in the bowls between starting attempts.

The other thing is maybe you have zero compression. Did you check the compression? Valve adjustment? Maybe super tight valves prevent good compression?

If it was my bike (and I just did all this to mine in the past few weeks for the same exact reason) I would #1 check the compression on both cylinders to rule that out, #2 check the valve clearances to rule that out, and then once both of those checks worked out, I'd #3 pull the carbs and tear them down for a thorough cleaning, replace the pilot jets since they are very difficult to clean properly (unless they are already brand new). Verify the slide diaphragms have no tears/leaks and the springs are installed correctly so the slides can move. When the carbs are assembled again make sure the slides move up and down as expected. I'd replace all of the o-rings in the carbs (including the pilot mixture needle o-rings) as well as the float needles and just for good measure replace the intake boot o-rings so you don't have a vacuum leak there. Then carefully put it all together to ensure there is no chance of a vacuum leak. I know this sounds like a lot but really it's like $30 in parts and 2-3 hours in labor to fully eliminate [most] failures that can't be adjusted out.

[Watcher]

mr72 I think you're overcomplicating this.

The bike was running after he sorted out the issue with spark, so compression isn't an issue he needs to worry about.
I also wouldn't be worrying about valves right now either.  But no, valves that are too tight won't cause loss of compression...


Right now our goal is to get it idling again.  We have spark and have compression, and from the looks of things it's obvious we are getting air.
Let's focus on fuel for the moment.
Sticky floats or a stuck needle might be a cause, but I don't think improper float height would cause a no start unless they're, you know, buried way low in the bowl and not letting ANY fuel in but I don't think thats possible. 
Jets don't really do anything at idle, and neither do the slides.  The pilot jet flows at low throttle and the mains come in above half throttle, and the slides only move with high vacuum when the throttle is open.

The main question I have right now is where are the carbs getting their fuel from?
No tank was installed, and he wasn't holding up a bottle or something.  I'm thinking he unknowingly wasn't supplying fuel.

[mr72]

mr72 I think you're overcomplicating this.

You're probably right. I guess I'm in a funk because bad compression caused every kind of doesn't-run-right problem with my own GS500. It's in my head.

[Watcher]

Any update?

I really want to know if the bike has complications or if this was a giant facepalm event!

[jrtaylorthird]

Yes, an update.

I've got the bike running a bit better now. I cleaned the carbs and checked float heights everything was fine, but saw there was a slight leak from my fuel line from frame petcock to carbs on the petcock side, so I replaced that tubing and the bike idles like a dream.

But now I've got another problem, of course.

In neutral, it'll rev like there's no tomorrow. Idles around 1400-1500, redlines and come back down without a hitch. Change into 1st, slight hesitation, but still a decent 1300-1400 idle. The second I give it gas though, it sounds like I'm running on one cylinder, almost. Boggy acceleration, bike won't rev to anything past 6k rpm and still takes too long to finally get there, and will backfire from exhaust a few times on the way up. This is with clutch pulled in.

Clutch let out, same thing, but more backfires and some dark smoke on decel from the top of 6k rpm. Any ideas?

TL;DR Initial problem was fuel line. Now idles fine, revs well in neutral, but bogs in gear.

[jrtaylorthird]

Here's a video of all my problems.

[Watcher]

That really hesitates on the way down.  Usually that's a sign of it being leaned out by something.

I'd spray some carb cleaner around and see if it's drawing air somewhere.

As to why it lugs when in gear, that's something I'm not experienced with.  I'll see what I can think up.

[jrtaylorthird]

The hesitation on decel is probably the throttle cable, I need to find a better way to route the sucker.

But yeah. Being in gear is what stops me. Should I start another thread?