Topic: Please Help Diagnose: Piston not firing

[drdreas]

Started my bike up for the first time this winter since it was a warm day, and the exhaust sounded odd. I rode it around, filled up the gas, and when I got back, checked to see if all 4 headers were hot, and one was not. I pulled the plug while the motor was running, and nothing changed, so I got confirmation that this cylinder was not firing at all. Pulled the plug out, left it connected to the plug wire, and held it against the block, plug sparked fine.

Next I unscrewed the carb bowl drain screw, no fuel drained. I unscrewed the drain screw on the carb on the other side of the bike and fuel poured out. So it looks like Im not even getting fuel to that one carb bowl. Any ideas? What do I do from here to fix it?

Just before I winterized the bike, a car backed into it and knocked it over, so I thought maybe the cause of the problem is a stuck float. I took a hammer and tapped the side of the bowl a bit, but still no fuel came out with the drain screw loosened. Any ideas?

Thanks

[bender01]

Stuck real good

[Gordon]

I was going to suggest tapping the side of the bowl with a screwdriver handle, but it looks like you already tried that. 

If it's not getting any fuel, then it's either a stuck float or stuck float valve.  If tapping it doesn't do the trick, then you need to get in there with your hands and see what's stuck.  If the floats move freely, then the valve may just be glued shut with gas varnish. 

[TwoTired]

Float or float valve stuck in the carb for the non-firing cylinder.
If tapping on the carb doesn't free it, you'll have to go in and make it work again.

A me too, for Gordon's faster typing.

Cheers,

[drdreas]

I've never gone inside the carb before. Can I leave the carb in and just take off the carb bowl to check whats going on in there? Does it just unscrew and drop down?

[mattcb350f]

If it's the outside bowl, it's not too hard to drop with the carb rack on the bike; depending which bike it is, but i've done this on the 350F and 750F.

Drop'er down and see what's going on. Like everyone else says it's probably a stuck float or needle...or both. It could also be something lodged in the fuel inlet, but that's more rare.

 Matt.

[Gordon]

It could also be something lodged in the fuel inlet, but that's more rare.

 Matt.

I didn't even consider that, but in your case this may be a possibility.  Considering your bike was knocked over recently, if the fuel filter isn't in good shape, some debris or rust could have worked its way down the fuel line to that carb. 

I don't know how your float bowls are held on (don't know what bike you're working on, and I make no assumptions based on what's in someones signature or avatar), but the bowls can be dropped with the carbs on the bike with varying levels of difficulty depending on what bike and which carb. 

With the right tools and amount of patience you can even remove the floats and float valve with the carbs still installed, but when it comes to the inner two carbs, many times it ends up being much easier and much less frustration just pulling the whole bank.  Getting the float bowl back on with the o-ring in its correct place can be the hardest part.

[drdreas]

Its a 77 CB750k, and from what I can tell has stock carbs. Its an outside cylinder, so I imagine access won't be a problem. Should I drop the bowl while leaving the carb on. I have the owner manual, but it doesnt really have good diagrams showing how this would be done.

[bender01]

Since your in Boston you might as well take them off and clean them. Its a couple months til youll want to ride all the time. Search Carb cleaning here and its not that hard. Taking them off less likens stripping the float bowl screws. I have found. Then its more work and $$. The bike might really wake up too which would be another positive. I like Simple green.as it doesnt melt the rubber parts.
 

[rshelbert]

Not that cleaning them is a bad thing but It may be a big overkill.  My number 3 carb does this now and then.  The float was bent badly at one time and it hangs up now and then, esp if I run it out of gas or when I go to reserve.  Mine is a 72 so it has the single large springy thing holding the bowl on so its very easy to take the bowl off.   It takes about 20 seconds to turn off the gas, drop the bowl, manually operate the float a time or 2, reinstall bowl, turn on gas and go.   

I would turn off the gas
drain the bowl with the drain screw
hold the float in the closed position all the way up (dont put too much pressure on it just seat it lightly or you will bend it)
place a rag folded up under the carb or a container of some sort to catch the gas
slowly lower the float (you should see a steady stream of gas coming out around the needle)


If the above works, replace the bowl and see if it fills by again trying to drain the bowl with the screw.  If it does not fill your floats are hitting the bowl somewhere due to bent or being out of adjustment. 

If when you lower the float no gas comes out your passage or fuel line is blocked somewhere inline or your needle is stuck in place.  In this case you might can drop just the float and clean that area, but it would probably be better to just drop the carbs and clean the whole system out.






With that said I have a 550 that has screws holding the bowl on and it would be much harder to do. 

I would try this before I opened the carbs up for a cleaning.  If it made it all the way to the gas station, back and you only noticed it was running unusual then I would think the other 3 are running pretty well.

[drdreas]

This was exactly my thought. The carb was rebuilt/cleaned last spring, and the bike runs PERFECTLY. Even with one piston out it ran smooth, pulled hard and I rode around for several miles with no problem. I am almost afraid that a cleaning would upset the condition that allows it to run so perfectly!

Do you know if my 77 has the same setup as yours?

thanks

[moham]

This was exactly my thought. The carb was rebuilt/cleaned last spring, and the bike runs PERFECTLY. Even with one piston out it ran smooth, pulled hard and I rode around for several miles with no problem. I am almost afraid that a cleaning would upset the condition that allows it to run so perfectly!

Do you know if my 77 has the same setup as yours?

thanks

72 & 77 carbs are different. Since the offending carb is on the outside, you should be able to remove the bowl with everything in place. I have a couple 90-degree screwdrivers and small stubby screwdrivers that work for this (as long as the screws are not stuck fast but if your carbs were re-built lately then they should be ok to free up, still take care to not strip the screws, though). Like the other folks have said, check the float and see if it moves as it should. You might also be able to pop the float pin out and remove the float and float valve from there (when removing the float don't drop the valve if it's loose) but re-assembly might be a bit tougher from that angle. See if gas flows or not. If not, then on to plan B. I've attached a blow-up of the internals...

[rshelbert]

If there are screws that will be the only difference, on my 550 i use a removable scewdriver bit extension and a adjustable wrench to apply upward pressure.  Just make sure you use the right size screwdriver.

As far as checking the carb out the procedures are the same. 

I wouldnt remove the float pin unless I had to (and you may) as it takes 3 hands and a fine balancing act to get it in witht the carbs in the bike, but it can be done.  if all is installed correctly nothing should fall out when you take the bowl off.   

[moham]

If there are screws that will be the only difference

77/78 carbs (both 750 and 550) differed from pre-77 in bowl shape, presence of accelerator pump, jet size and more...

[rshelbert]

When you say different thats not helping him much. 

Does that mean it cannot be done or just harder.  I truely dont know as mine is pre 77.   I dont want to steer him in a bad direction if ii cannot be done.  From the diagram above it looks like the carbs work like every other bike carb I have ever worked on.

[Hush]

Its' an outside carb so no problems, just drop the bowl (screws or retaining clip) and gently move the float up and down a few times.
If (with the gas tap on) no petrol arrives then you are blocked further up the line.
You should be able to see if the float valve (the wee bit that the floats move up and down) is moving, I'd probably give that a wiggle if it didn't move at that point of time as there can only be some old gas/varnish holding it up.

[moham]

Does that mean it cannot be done or just harder.

It means they are different. Not harder, just different. I was trying to be specific. The question of whether or not they are the same carbs had come up in an earlier post. I was attempting to answer the question with, well, an answer...

[drdreas]

Thanks, I'll give it a shot this weekend and post to let you guys know if there is any trouble.

Thanks again