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Carb choke circuit - how does it work?

  • Thread starter Thread starter Anonymous
  • Start date Start date
A

Anonymous

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In good weather, using the choke does what I'd expect - it richens the fuel mixture, makes the bike easier to start and increases the idle speed. In cold and/or damp weather though, using the choke seems to make things worse rather than better when I'm trying to start the bike.

From what I've read here and elsewhere this strongly suggests that the carbs in general need cleaning and the choke circuit in particular. What I don't understand is why the choke circuit doesn't act like it's blocked all the time - why just in the cold and damp? And why does the choke act as normal once I've got the bike started? Is it blocked or isn't it?

Roger
 
If you are saying you use the choke less in cold damp weather, I wouldn't have a clue. The choke circuit richens the mixture by letting more fuel in, unlike many other types of choke systems that richen the mixture by letting less air in.
 
Once the bike is running.

Once the bike is running.

Once your bike is running, and the choke is off you won't notice any trouble from it. My #1 choke is blocked. I can tell becouse the #1 pipe doesn't warm up thill the choke is off. I figured out today the choke circuit goes down to the intake bore, and also around the outside of the carb to the tube that sticks down into the float bowl. Once I get all these passages clean hopefuly it will be fixed.
 
here's a picture
site1130.jpg
 
rphillips said:
If you are saying you use the choke less in cold damp weather, I wouldn't have a clue. The choke circuit richens the mixture by letting more fuel in, unlike many other types of choke systems that richen the mixture by letting less air in.
Click to expand...

What I'm saying is that, in cold/damp weather, the bike won't start if I use the choke. What I have to do is squirt 'cold start' into the air filter box and get the bike started without any choke. As soon as it starts I have to quickly open the choke in order to keep it going. Once the bike is started and I put the choke on, the revs increase just as you'd expect.

I thought I might have an idea about this until I re-read your post and saw that the choke adds more fuel as opposed to letting in less air.

Thanks for the diagram Daveo.

Roger
 
Your choke circuit may be partially blocked and not working like it should.
Cold weather, especially damp cold weather, does not favor good combustion. There are other things that make the motor harder to start under these conditions. Ignition performance, compression, oil, etc, all can contribute to this. It may be something as simple as replacing/cleaning the plugs and re-gapping them and checking your coil/plug leads/caps for good/clean connections.
 
i have a choke circuit that isn't working properly either. it raises the choke when i hit the lever, but one cylinder doesnt fire until i come off the choke.

i had a little hesitation around 4500-5000 RPM last week, i hit the carbs with a little carb cleaner while the bike was running and it smoothed out. maybe i should do the same with the choke on.

~Adam
 
You guys may want to try just using a little choke, like 1/2, mine never likes full choke, too rich, I give her 1/2 to start and then back it off to none
 
if i had the engine running within an hour or two i'll use less choke...but normally mine likes full choke to turn over quick. it usually fires on half a crank, even with 3 cylinders. :)
 
AOD said:
i have a choke circuit that isn't working properly either. it raises the choke when i hit the lever, but one cylinder doesnt fire until i come off the choke.

i had a little hesitation around 4500-5000 RPM last week, i hit the carbs with a little carb cleaner while the bike was running and it smoothed out. maybe i should do the same with the choke on.

~Adam
Click to expand...

You know, your problem might not be the chokes fault.
If you have a cylinder that is running too rich, that plug will be dirty.
When you introduce a rich choke mixture to a dirty plug, you will temporarily foul the plug. The wet/dirty plug will allow the spark to track down the plug to ground, instead of jumping the plug gap. Electricity always takes the path of least resistance. The result is poor/no combustion.
The colder the engine/outside temperature, the more difficult it is for good combustion. With higher outside temps, or a little internal heat, combustion improves quickly and the rich running cylinder will be less noticable, if noticable at all. Just the friction of starting up will help the plug to begin firing. Heat vaporizes the mixture, so combustion improves.
 
so an easy way to find out which is not firing, would be to start the engine on full choke when its cold, and use a laser temperature gun to check the temps on the pipes as they warm up.

i did this when the engine was warm (didn't think about it when cold) and found one of my cylinders running 50 degrees hotter then the others (Celsius, so its a lot).

I will investigate.

~Adam
 
I don't know about having to open the choke ALL THE WAY. We all use different amounts of choke depending on several things.
The easiest way to tell which cylinder is not firing as soon as the others is to just touch each pipe quickly. You can feel which one is colder than the others. Also, take a plug read and compare it to the others. Take the plug read after your last ride, not during a cold re-start using the choke. If you find the "cold" cylinder is also giving you a darker plug read, then my info above may apply.
A 1/4 turn in or more (clockwise), on your pilot screw underneath, may stop the temporary fouling and make that cylinder run better. Of course, this assumes your pilot circuit is clean. You also have to set the side air screw correctly.
 
i'm not sure it is an air screw problem...pilot, maybe. all the plugs look pretty good compared to each other. there is no indication one is running particuilarly rich. i tweaked the pilots last time i had the carbs off (1/8th turn to richen all) and it still had this problem before then. i really dont consider it much of a problem, since it starts and warms up fine. these carbs came off the 78, now on the 79, and it did it on both bikes.

i will try hitting the carbs with a little cleaner on full choke. going out to start it up now.

~Adam
 
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