Carb won't stay on the bike!!! UPDATED 3/6 Video (they stay on)

[thousandlegs]

 
Ok, I don't know what's going on now.  I got a replacement head and got the bike running.  Then I went ahead and put the tank back on and hooked up all of the fuel lines and then when I tried to start it the carbs popped off. 

I searched around and found that it may be a vacuum problem and somehow related to the frame petcock.  I took the tank back off, plugged the vacuum line going into the left carb and put fuel in a funnel directly into the fuel line going to the carbs.  Just like before the bike turns over and then (maybe the compression) comes back through the carbs and pops them right off of the mount.  I tried it with the vacuum line not plugged...same thing. 

Does anyone know why the carbs may do this?  I'm going bald on this one...just about pulled out all of my hair.

[fred]

There is a hose clamp looking thing that holds the carbs on to the intake tubes.... Do you still have one on each carb? Are your clamps all old and worn out? Are they in the right place? I just took my carbs apart Sunday. They are held in by just shoving them into the intake tubes then tightening the clamp... Do you have the intake tubes? I guess if you had a new head, it might not have come with them and if you didn't transfer them off your old head, you wouldn't have them... I'm talking about parts 21 and 24 in this diagram:

[ohgood]

incorrect cam timing ?

just figured, since you have a new head and all. if the intake tubes were missing, there wouldn't be anything to hold the carbs to the head, right ? i mean, the intake holes and the output from the carbs is a good deal different in size - right ?

wonder wonder ...

[bill14224]

I've been a motorcycle owner since 1977 and this is the first time I've heard of someone having trouble getting the carbs to stay connected to the engine.  Clamps, anyone?  Those round things on the garage floor? 

But then I figure, no one can be that dumb, especially a GS 500 owner, so assuming he has tight clamps and the carbs are getting pushed off the intake tubes anyway, the clue lies in the fact he has new heads.  If the cams are degreed totally wrong, the intake valves may be open when the pistons are coming up, not down.  That would push them off, alright!  But then, it surely wouldn't run, and he says it runs.  Could it possibly be he was concentrating so hard on getting the heads together right he forgot to put the clamps back on?

[fred]

I've been a motorcycle owner since 1977 and this is the first time I've heard of someone having trouble getting the carbs to stay connected to the engine.  Clamps, anyone?  Those round things on the garage floor? 

Yeah, there isn't much to holding them on... Also, if your timing was crazy far off, wouldn't the back pressure to the carbs destroy something in the carbs before pushing them off? Seems like lack of clamps is the most likely cause, second only to not pushing them in the intake tubes far enough.. I just put new intake tubes on my bike and it took a bit of effort to get the carbs seated right...

[thousandlegs]

I've got clamps and intake tubes and they're together tight. 

I am suspect of the timing.  But I don't know how it would get out of time...especially so quickly.

[BeerGarage]

Ok, I don't know what's going on now.  I got a replacement head and got the bike running.  Then I went ahead and put the tank back on and hooked up all of the fuel lines and then when I tried to start it the carbs popped off. 

Assumptions: you installed the head, and the bike ran.  Engine running, making vroom noises, fuel going in through the carbs and exhaust coming out the exhaust.  Then you put the tank on, hit the start, and the carbs "pop" off.  "Pop" means they get pushed off by pressure.

This indicates that the timing did a 90deg switch from exhaust to intake on the upstroke.  Did you not adjust the cam tensioner?  That might allow the chain to skip that radically that quickly?  That's all I got.  Interesting puzzle!

[thousandlegs]

Assumptions: you installed the head, and the bike ran.  Engine running, making vroom noises, fuel going in through the carbs and exhaust coming out the exhaust.  Then you put the tank on, hit the start, and the carbs "pop" off.  "Pop" means they get pushed off by pressure.

This indicates that the timing did a 90deg switch from exhaust to intake on the upstroke.  Did you not adjust the cam tensioner?  That might allow the chain to skip that radically that quickly?  That's all I got.  Interesting puzzle!

I did not adjust the tensioner.  I merely uninstalled and reinstalled it.  Tomorrow when I check the timing I had intended to check that as well.

[thousandlegs]

Alright, I took the valve cover off and the timing was right.  Put it back together and it starts but idles very poorly and there is no throttle response.  It is sputtering at idle around 1100 or so but even at wot it stays the same...just seems to have less resistance with the throttle open and runs a _tiny bit more smoothly.

[GeeP]

I have the feeling you have timed the engine 180 degrees out, or your intake valves are WAY out of adjustment.   Get out your manual and go though timing from the top.  Verify TDC though the spark plug hole.  Be careful, the GS is an interference engine, it can bend the valves!

There should be no air pushing back though the intake side that would cause the carbs to pop off.

[fred]

Are you running so rich you're backfiring the carbs off the bike?

[thousandlegs]

That's kind of what I'm wondering.  Maybe it's running too rich...in fact, I believe it probably is.  How can I lean it up?  I know I can search and I will but any suggestions would be good too. 

[lawman]

Where are your mix screws set, incidentally?

What jets are you running?

[thousandlegs]

jets are 118 and mix screws have been at 2 1/4 and 1 3/4.  I've had the carbs apart and they're clean.  I'm 100% certain that I've installed the cams/chain/ect properly.

[fred]

Are your float heights good? Did you accidentally mess with them when you took the carbs apart?

[thousandlegs]

I didn't change anything about the float heights.  How would I go about adjusting the float height anyway?  I guess I need to check the compression too. 

[thousandlegs]

Ok, I just pulled off the float bowls and measured the float height.  15mm for both of them.  So:
New Main 118 (choose 118 because that's what was in it already..also have 128s that I can put in)
Pilot 40
Float Height 15mm
Mix Screws at 2 turns
Timing set
Stock Exhaust
K&N Filter

Turns over fine.  Has run since I've had it but I've not had it on the road.  Carbs still pop off often and no response from the throttle. 

[fred]

Did you check the float heights using clear tubing and the carb drains? This seems to be a more reliable method... I have also tried measuring them, but haven't always been able to get my measurements with the carbs apart to agree with the vinyl hose test, so I've deferred to the vinyl hose test as the more definitive one of the two...

[the mole]

Do a compression check. If the inlet valves are not closing properly (ie. less than no gap), then when it fires the carbs will blow off?? Might also explain lack of power and rough running.
We need more info too. Does it start easily? What sort of noises is it making? Is there a loud bang when the carbs come off? Do they always come off together, or sometimes just one?

You need to go through it in a logical sequence. Seeing as the engine has been apart:
1. Compression test.
2. Check valve clearances.
3. Double (triple!) check valve timing.
4. Ignition timing.
5. Carbs and fuel system.

[thousandlegs]

Yeah, I'm gonna try and lay me hands on a compression tool tomorow, as for the timing carbs etc... I'm fairly sure that everything is right.