missfire till warm

[Mac_55]

hi ive read alot of the threads and no one seems to have my type of problem so i hope someone can help

its a 1981 suzuki gs 400

runs nice when on the street , at first start up with choke out (on) it only fires on right side then when warm and choke is in ( off) left side starts to sputter and back fire and starts to fire and then runs fine ?

carb?
valves?
rings?

anyone else have this problem i'll be pulling the carbs tomorrow after work to clean again

thanks

[almarconi]

I would adjust your valves if you have not done it, clean the carbs and see if the problem goes away.

[hampshirehog]

Hi and welcome to the nuthouse.

It's not rings unless it's smoking so you're quite right to suspect either the valves of the carbs. If you've lurked in these corridors for a while you'll know how important it is to make sure that both of these are sorted.

[tkent02]

Check the choke circuit in the left carburetor, and the choke feed passage in the float bowl itself.

Something may be clogged in there.

[chuckycheese]

Check the choke circuit in the left carburetor, and the choke feed passage in the float bowl itself.

Something may be clogged in there.

Tkent....I understand where the "choke feed passage" is in the float bowl but which part is the "choke circuit" in the carb body? Thanks! (I've rebuilt my carbs a bunch of times but have never understood exactly how they work!)

[Steve]

I've rebuilt my carbs a bunch of times ...

Which only leads to the question ... WHY?

When done properly, one rebuild per set of carbs should last for more years than the typical ownership duration.



Now, as to how the "choke" works ...

When your "choke" knob is pulled, there are pistons on the top, front of the carbs that move. If you looked into that area when you rebuilt your carbs, you should have noticed that it opens a large-ish hole in the side of the chamber under the diaphragm. This chamber is vented to the carb throat on the intake side, it's that large oval opening at the top. Opening the "choke" piston allows air to flow through this hole, instead of the closed throttle butterflies. Also connected to this air passage is the tube that sticks down into the float bowl. When air is allowed through the passage, it sucks fuel up from the float bowl, providing its own rich mixture to the cylinders. The outlet is on the engine side of the throttle butterflies, so it relies on the high vacuum generated to pull the gas up from the bowl. If you open the throttle AT ALL, that vacuum is greatly reduced, along with the effectiveness of the "choke".

To make sure the fuel pickup tube is working, use a single strand of copper from a wire. Poke it up as far as you can, then put on your safety glasses and spray carb cleaner through the tube, then follow with some compressed air. Also check the well in the float bowl that the tube lives in. Be careful spraying in there, it will come right back at you from the other end of that passage.

I always put "choke" in quotes when talking about these carbs because there is nothing that phisically blocks the intake of the carb throat, "choking" it down to force it to draw extra gas, like a car's carburetor does. Instead, our carbs have what is known as an "enrichener" system, but everybody knows what you mean when you say "choke".

.

[piester]

What shape is the coil. Plug wires. Boots. And plugs themselves in? What about condensation under the plug cap?

[chuckycheese]

Which only leads to the question ... WHY?

When done properly, one rebuild per set of carbs should last for more years than the typical ownership duration.

.

OK... I'll take a couple of minutes to explain it. I've owned my 750 for about 7 1/2 years and my 1100 for a little over 2 years. The carbs on both bikes were in poor shape when I bought them and one was filled with the wrong size jets. I rebuilt both as soon as I bought them.

I travel a lot on trips of long duration and don't take my bikes with me; the airlines won't allow it and I woudn't even if I could. Consequently, they sit, idle, often for weeks at a time. I've found that they seem to run best when the carbs are rebuilt about every 2 years. Every time I've done it, I've found the carbs to be filled with crud, so I don't think I was wasting my time. I also have an extra set for both bikes and, you guessed it, I rebuilt both when I bought them. Now.....there's more.

When I mentioned that I've "rebuilt them a bunch of times", I was also referring to other bikes, as well. I own 2 now but I've owned more than a dozen others in the past (I think 16, counting my dirt bikes)...and I've rebuilt the carbs on the majority of them. Each time I've done it, they've seemed to run much better. So....that's how I came up with my "bunch". Based upon the many posts I've read through the years, I doubt my rebuild schedules are probably all that unusual. Maybe I'm filling up at the wrong gas stations!!!!!

[almarconi]

I've resurrected three bikes that have sat for long periods of time. I've found that no matter how well I have soaked and cleaned the carbs, I usually have to clean them more than once to get them thoroughly clean. I've also made the mistake of replacing the petcock after cleaning the carbs, only to find my nice clean carbs filled up with rust and other debris.

When done properly, one rebuild per set of carbs should last for more years than the typical ownership duration.

In Steve's case, I think he's just lucky. He also gets "free" motorcycles, how often does that happen to the rest of us?

[Steve]

OK, sounds somewhat reasonable. The way it appeared when I first read your statement, it sounded like you had done each set several times. Now I see that you have also done several other sets, too.

It's just that I rebuilt the carbs in my wife's bike almost 5 years ago and they have not been apart since then. Since we live in oHIo, we have several gasoline-powered engines that only see seasonal use. Besides the bikes, this includes mowers, snowblowers, weed whackers, etc., so you can see that several of these engines are idle for months at a time. By using either Stabil or Seafoam in the fuel system before parking them, I have never had any problems with "dirty carbs" when it's time to use that device again. Chances are, if you use some sort of fuel stabilizer before you leave for your month or so, things should be just fine when you return.

.

[Steve]

In Steve's case, I think he's just lucky. He also gets "free" motorcycles, how often does that happen to the rest of us?

Yeah, I'm always "lucky", but sometimes it's bad luck.

As far as getting a "free" bike, sure, there was no money that changed hands at the time, but so far, the tab is at about $675 to get "Freebie" back on the road (not counting registration and insurance).

I am going to be changing clutch springs on him tomorrow, anyone want to tag along?
Sunny central Florida (at least until it starts raining in the afternoon), temps are finally back up where they belong, should be a decent day.

.

[1948man]

I'd love to watch and help out what little I can I but can't make it Saturday.

[Johno]

Electronic ignition and battery

Does your bike have electronic ignition?

You should have a good fat blue spark on both plugs.

Make sure your battery is fully charged or you may get a weak spark on one or both cylinders.

Don't ask me how I know! It took me nearly two years to get mine right and the difference is amazing. I hardly need any choke now and warm up is a breeze in comparison to what I was putting up with.

[SqDancerLynn1]

I would check the ignitor. If yours can be taken? apart do so and check the solder connections on the circuit board...
make sure you have 12 volts at the coil

[Mac_55]

ok carbs pulled, still looks as good as 2 years ago when i rebuilt them,

would the valves (I/Ex) give me this effect, weak coil or ....

also does anyone know how to get the air box out with out bending every mounting tab on the frame?