Hi Guys,
I just want to say that I check this forum almost everyday and it's been very helpful. Piniongear, I know you know more about the early sportsters, so I relieve you of any responsibility that you may have to answer this post. However, I'm willing to listen to any suggestions that you may have! Xxxflhrci, you have the same year bike that I have, so I'm wondering if you may have had the same problem. Here we go!

It's a 1977 xlh sportster 1000. I know the carb is stock and by looking at pictures I determined that the carb is a Keihin. The problem is the carb coughs or backfires (sometimes) between35 and 50 mph especially when I open her up. She is running a little rich (the plugs are a dry black color). I've adjusted the carb and just about ended up at the same place she was adjusted at when I started. I haven't checked the points yet, but plan on doing that today. Shehas the get up and go and she runs great except for the carb coughing. Any ideas? Should I get a carb rebuild kit?



Thanks guys! you don't know how much help you've been in the past.

Chuck

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Chuck...
Sooty black plugs mean you have a problem with the mixture of air and fuel with things being heavy on the fuel side of the equation. I really do not think adjusting the carb is going to solve this.
I know zip about Keihin carbs, but I had a similar problem with mine running rich and it took me a l-o-o-ong time to find what the problem was. I run a Bendix and the mainjet holder has an O ring around the neck that screws into the carb body. This seals the raw fuel in the float chamber from dumping into the venturi. All fuel must travel through the main jet to reach the venturi. Well. my O ring had badly degraded and was allowing raw fuel to dump right into the venturi, along with the fuel going through the mainjet.
You may want to see if the Keihin has a similar design, with a similar way of sealing off the fuel.
On a more serious note......
Several years back my Sporty developed the carb cough. It was ridable, but a pain in the butt to put up with the cough. I discovered that the rear cylinder intake cam surface was badly galled. I replaced the cam with another used intake cam (used but smooth) and that fixed the problem.
It is a pain to remove the push rods and then the lifters to check the cam surfaces, but that is the quickest way I know of doing it. If it turns out that you have a bad cam, at least you have already done a third of the work required to replace a cam.
I would look at the carb first. I have attached a pic of my Bendix bad O ring.......pg


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Piniongear,

I received my carb rebuild kit Friday. I've been working on the carb for the last two days putting it on the bike and testing it. She started right up and ran good. When I would open her up in third or fourth gear, she would hesitate like it was flooding out, recover (all in less than a second), and take off like a bat out of hell. When I parked her, she started puking gas out of the carb overflow hose. I've adjusted the float and it pukes less gas, so I guess I'm heading in the right direction. Am I heading in the right direction and what causes the hesitation when I open the throttle?

I did not find a o-ring or a place for one on the main jet. I guess the Keihin are different from the Bendix carbs. Thanks for your help! I really appreciate it!

Chuck

 

Scarletthehog.........
First let me give you this because I am not familar with the Keihin (www.gadgetjq.com/keihin)

First the overflow issue:
The float is still allowing the fuel to rise too high in the float chamber. The float is not closing the needle in it's seat before the fuel level reaches the overflow tube and leaks out. Since the needle has not seated, fuel continues to run out of the fuel tank into the float chamber and then out onto the ground.
You are going in the correct direction.
Bend the tab so the the float is a little lower. This will cause the fuel to raise the float up and the tab will reach the needle base and push it into it's seat before the fuel level reaches the overflow tube.

For the carb hesitation:
At wide open throttle you are on the main jet. There is a tapered needle that has a number of notches on the top end of the needle. This sits in a recess in the throttle slide where a clip is inserted in 1 of the notches to hold the elevation in place. The bottom end of this needle is tapered and this goes into the main jet.
When the throttle is off and the bike is idling, the tapered needle sits tightly in the main jet, thus sealing it off. The bike idles fine because it is running on the primary jet side of things (low speed jet).
As you increase the throttle the slide rises up and starts to bring the tapered needle out of the main jet. Just a little at first, but the main jet is letting a small amount of fuel into the venturi. The slide is also letting a little more air into the venturi as it raises up. (Gotta keep the air/fuel mixture in balance at all times you know)
As you continue to open the throttle even more, this needle now is quite a ways out of the main jet hole. The tip is tapered, so there is still some fuel restriction of the main jet, but not much. Open the throttle wide and the needle is clear of the main jet and the fuel is pouring through the jet and the full volumn of air is flowing into the venturi throat and you are now flying down the road.
That is the way the thing works basically.

So, when you open the throttle you are getting a dead spot (like it's flooding) and then things go wild as it lights up.
Try this:
Take note of which notch the needle clip sits in. Pull the clip and raise it 1 notch higher. This drops the needle down into the main jet a little bit with the result that it has a leaner mixture now.
Go for a test ride. Is it better? Is it worse?
If it is worse, then take the clip out of the needle notch and lower it 1 notch lower than it originally was (this is 2 notches lower than it now sits) This raises the needle out of the main jet, thus enriching the mixture.
Take it for another test ride.
Let us know the result. It sounds like you are really close to being where you want to be with it...........pg

 

You have "traditional"(not CV) butterfly Keihin (with AMF stamps ?) what size jets do you have ?
Do you have original airbox ? without cutted backplate ? paper of foam filtter ? original exhaust ? dragpipes with or without bafflers ? Hesitation in that speed indicates lean mixture or bad ignition timing.... of course if you are running with points ( or electronic Dualfire) you can't totally eliminate this.

 

Make sure the air bleed for the inter. to main cross over is clear. (or you will be on the main very early) that would also cause you to be running rich at lower speeds. also check that the accl. pump is working and that it is spraying to the center of the bore. check the intake manifold O-rings for a good seal. Just a thought, I have run into some problems with float needles in the aftermarket not sealing.