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Discussion Starter #1
I have a 1977 stock sportster that I purchased brand new. When I bought it the stock exhaust was taken off and staggered dual pipes installed. Later it was recommended that I increase the jet size because those pipes could make the engine run lean so I changed the main jet. Also at this time the bike was at sea level and it always ran good. Over the years a head came off for an oil leak and I always had trouble sealing the intake manifold. But I could always get the bike to run good and it idled nicely and had plenty of power. Fast forward to today. Now I live at 4000' of elevation and I had an issue that required pulling the carb and intake manifold. In my old age I finally figured out how to install that manifold and O-rings without sealant and actually have the manifold truly seal now. With the carb off I can look into the manifold and see it truly installed perfectly. Well now I have an issue of an over rich mixture at cruise throttle. Apparently the manifold leaked just enough vacuum over the years that I had the best mixture at cruise. The bike idles well and for the first time I am able to lean out the idle mixture screw and get a good adjustment. But at cruise throttle, about 2100 RPM the engine bucks like a rich mixture and I have to gear down to speed up the engine. I'm thinking the main jet is too big but where could I find these old jets to down size and try? Any idea's on a jet size for a bike that lives at 4000' now?

I certainly appreciate your help and advice with this. The bike is still as fun and as fast as it always was but this choppy cruise is truly anoying now at this stage of my life. ;)
 

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You can find individual jets and jet kits online, even some Harley dealers have them. Try J&P or Dennis Kirk, they both sell individual jets, needles and rebuild kits. There's also CVP and I'm sure other websites.

It would be your main jet if it's to rich at cruising AND high rpm. If it's just the mid-range, if you have an adjustable needle, drop it one notch leaner (move the clip up). If it's not adjustable, try a leaner needle.

If your top end is where it should be, you don't want a smaller main to fix the mid because then you'll lean out the top.
 

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Discussion Starter #3
You can find individual jets and jet kits online, even some Harley dealers have them. Try J&P or Dennis Kirk, they both sell individual jets, needles and rebuild kits. There's also CVP and I'm sure other websites.

It would be your main jet if it's to rich at cruising AND high rpm. If it's just the mid-range, if you have an adjustable needle, drop it one notch leaner (move the clip up). If it's not adjustable, try a leaner needle.

If your top end is where it should be, you don't want a smaller main to fix the mid because then you'll lean out the top.
Thank you "sardonicus87" for taking time to reply. The bike really runs hard through all the RPM ranges, it's just this part throttle cruise issue that I've always had but it's worse now. Here are some pictures of the carburetor out of the old Harley manual I have for the bike. I wish it had the adjustable needle that you're talking about but as you can see in the pictures not this carb.
Also thank you for the information of where I could find the main jet. That is what I modified 42 years ago and I'm truly thinking that is where I should be playing now.
Thanks again for your help.
Lisa
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Mid range part throttle issue, just rolling on the throttle ? That sounds like an accelerator pump symptom.
Those earlier Keihin butterfly carbs are easy to tune. Make sure the float level is correct and the needle tip isn't dented.
 

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Discussion Starter #5
Mid range part throttle issue, just rolling on the throttle ? That sounds like an accelerator pump symptom.
Those earlier Keihin butterfly carbs are easy to tune. Make sure the float level is correct and the needle tip isn't dented.
Thanks "Schmidty" for your response. I just rebuilt the carb with new parts and the accelerator pump is doing it's job. I also payed very close attention to the float level adjustment so I'm good there. The bike has no hesitation on acceleration at all. It's just the part throtttle cruise where the engine is just not running smoothly. Roll hard into the throttle and the bike will just plan take off and run. Now, digging through all my records I came across a note I made on "drilling out" the low speed jet. I believe the original low speed jet was a #76 or .760mm orifice which turns out to be .0292". In my notes I drilled it out to .032" of an inch and that was when the bike lived at sea level. Now I live at 4,000' elevation and I truly need to go back to a #76 or smaller jet. I'm sure this would cure my issue. But the real problem I'm having is finding original jets for that carb. The carb on this bike is a 38mm Keihin "butterfly" carb and not the later CV carb that all the jets are advertised for. So that is my issue today is finding the right jets. If anyone has an idea where to find these I would appreciate that advice. I have beat down the sources that "sardonicus" recommended and come up empty handed. So that is where I am now with this issue.

Thanks again for your time, reply and help.

Lisa
 
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