Carb problem after start, at idling when cold

[marcovandevelde]

Hi Guys,
I hope you can help me out here with what looks like a simple problem but failed to disappear till now.
For the couple of years that I've owned my ever changing '81 GSX 1100 S engine, it always has had the same small carb problem. It has the std 34mm CV's btw.

When I start it when cold, it immedeatly fires up. BUT it runs very badly, like it is on 3 cylinders, popping noises all around. When about one minute is passed, it settles and idles beautifully. Carburation is fine throughout the rev-range. Starting when hot gives no problems, but when it sits for half an hour the problem re-occurs. No biggie but it doesn't look very nice when I start my special-built bike and it seems to, err how shall I put it, expell air out of its bottom.

Now, in time I've changed the ignition system completely from pickups to sparkplugs, so that can't be it. It ran without a fuelpump before and with one now, I had Dynojet needles and jets in it, had individual filters and an airbox on it, all of this makes no difference for the problem. Currently I have the standard jets and needles in for the turbo system. I suspect the system for the airscrews to be clogged somewhere. But why is that only a problem when cold?

Also I'd like to hear what the std setting is for the airscrews as it's at a guesspoint now. Adjusting it doesn't make a difference to the problem btw.

ANY IDEAS? I'm taking them off this week to check them, maybe it's a simple thing?

Thanks in advance, Marco.

[KEITH KRAUSE]

Re: Carb problem after start, at idling when cold

When I start it when cold, it immedeatly fires up. BUT it runs very badly, like it is on 3 cylinders, popping noises all around. When about one minute is passed, it settles and idles beautifully. Carburation is fine throughout the rev-range. Starting when hot gives no problems, but when it sits for half an hour the problem re-occurs. No biggie but it doesn't look very nice when I start my special-built bike and it seems to, err how shall I put it, expell air out of its bottom.
I suspect the system for the airscrews to be clogged somewhere. But why is that only a problem when cold?

Also I'd like to hear what the std setting is for the airscrews as it's at a guesspoint now. Adjusting it doesn't make a difference to the problem btw.

There is no standard setting for the mixture screws, but they're generally about 2 turns out. If the pilot circuit is functioning correctly and has the right size jets, you should be able to set them using the "highest idle" method.
With the bike warmed up and on the centerstand, adjust a mixture screw to achieve the highest rpm's. After you fine tune that screw for the highest rpm, turn the idle down to about 1,100 rpm's by adjusting the idle adjuster knob. Go to the next screw and repeat until all four are set.
As for your problem description, I'm not sure what you mean by "popping" and "expelling air out the bottom".
"Popping", makes me think of a rich mixture where a plug is fouling at start up but keeps trying to fire (pop-pop.......pop..) until the engine warms up a little which will help it fire.
"Expelling air", makes me think of "spitting" out the filter(s), which is definitely a lean mixture.
Which best describes your problem?
The choke can be used in varying amounts to help you determine if it's a rich or lean condition. You shouldn't need any choke to start a motor that's only been sitting a 1/2 hour, but if a small amount helps the problem, then it's a lean condition. If a small amount of choking worsens the problem, then it may point to a rich condition, allthough a warm motor doesn't like to be choked anyway.
When starting stone cold, we all usually get to know how much our bike wants to be choked. If you find giving more choke than normal helps the problem, then it points to a lean condition, but if more choke worsens the popping, it points to a rich condition.

[marcovandevelde]

Hi Keith,
Thanks for your reply. Your explanation of ''popping'' is exactly what it does. The noises are the same as your bottom makes sometimes when you're on the toilet. I can't be any more specific as I can't use foul language here, but I think it is afterburn in the exhaust or firing at an unusual time.
Now that we're talking about it, I do remember I had to drill one of the airscrews out and ''temporary'' replaced it with one from a set of 32mm CV GS850 carbs who (the screw) looked very much the same. But the problem was already there before that.
Using the choke worsens things and that gave me also the impression it was idling too rich, adjusting it gave a hotter engine but no improvement for the startup problem.
I think I'll just clean them out thorougly (I separated the carbs to replace the floatbowl-vent O-rings)and then adjust them as you explained.

Talk to you later, greetings, Marco.

[KEITH KRAUSE]

Hi Keith,
Thanks for your reply. Your explanation of ''popping'' is exactly what it does. The noises are the same as your bottom makes sometimes when you're on the toilet. I think it is afterburn in the exhaust or firing at an unusual time.
Using the choke worsens things and that gave me also the impression it was idling too rich, adjusting it gave a hotter engine but no improvement for the startup problem.

How do you know what noises my bottom makes!?

OK, your popping at cold start ups and also after the bike has sat about a 1/2 hour sounds like a rich mixture problem.
You say the bike will do this at cold starts and after sitting a 1/2 hour. Sitting for a 1/2 hour, the bike will still be warmed up, so it should not be anything to do with engine temperature. Even though you say it will start up fine if hot or it will start up fine if turned off and immediately re-started, I don't think it's engine temperature related. Hot, or cooling for a short while, the engine is still at operating temperature.
I think it's fuel level related. It's the only thing that could significantly change after the bike has sat a short while. A fuel level problem makes even more sense after the bike has sat over night.
If the float valve(s) or float operation is allowing fuel to rise too high in the bowls, you'll start the bike (cold or warmed up) with a high fuel level that will cause the plug(s) to start fouling until that excess fuel level drops to the correct level (about a minute). After a minute, the bike will clear up and run fine. The bike will run fine as long as you're riding. If you turn it off for just a few minutes and re-start, it'll still be fine because the slowly leaking valve/float won't significantly raise the fuel level yet. But if you turn the bike off and go have lunch with your friends or park it for the night, that's plenty of time for the bowl level(s) to rise too high. It only takes an extra 1/20th of an inch, even less. Bikes parked on the side stand seem to leak more than bikes parked on the centerstand too.
The leak could be in one or more carbs. The high fuel level could be caused by dirt interfering with a valve tip, a worn valve, burr on the valve, weak valve spring, bad o-ring/gasket for the float valve seat, loose seat, incorrect float adjustment, tweaked/sticking/ruptured float.
A simple test to see if you have a leak:
Install a clear fuel line. Start the bike and let it run a couple of minutes or long enough that you know the bowls are completely full. KEEP THE BIKE STRAIGHT UP AS IT RUNS. Turn off the bike and park the bike as you would normally (sidestand?) and let it sit over night.
Look at the top of the clear fuel line and you'll see that the line is full or maybe a small air bubble in it. If the fuel level drops in the line by morning, you have a leak. Fuel level can be hard to see in an old clear line, so use a new line to be sure.
This test exposes a leak MORE OFTEN THAN NOT. For some reason, the fuel line level will not always drop every single time even though later tests/inspection proves a leaking valve or float is obvious. It depends on how worn the part(s) are.

[marcovandevelde]

Hi Keith,
Thanks again for your help. If I think it over your story makes full sense in relation to the symptoms and I'm 99% sure this will be the problem. As the carbs are off and apart yet, I'm not able to perform the leaktest now. So I'll replace the o-rings for the valveseats and thouroughly inspect the neelde-valves themselves. I had this kind of problem before on an XJ 650 Turbo where the engine would fill up with petrol(!) overnight because of a leaky valve. I'll also set the floatlevels again.

Talk to you later Keith!

Greetz Marco.

[KEITH KRAUSE]

OK. Let us know what happens.
I was also thinking a leaky petcock could cause the same symptoms. Of course, you would still have to have a leaking valve or float problem involved. I've found the valves and floats to be the problem more often though.
You can also test the petcock over night if you want by running a line into a bottle or just connect a line and fold the end over. ANY fuel collecting at all means a leak.
Another fuel level test you can do later if you need to, is before starting the bike in a situation where it likes to pop, catch and drain some fuel from each bowl drain first. Then put the petcock on prime and re-start and see if it pops.
When you put the carbs back on, be sure to wait until you're ready to start it before sending fuel to the bowls. If it starts and doesn't pop, then this backs up what we're thinking. The fuel levels would have to be good at that moment, as long as you adjusted the floats good and are sure all the parts are OK.
I would also clean off the plugs good before the re-start, just in case any of them are a little fouled from previous things.