I've have a stock 76 shovel. Through the years it has performed flawlessly with this exeception .. the idle became steadly worse, to keep it running at rest I either had to set the idle speed real high (not acceptable) or constantly vary the throttle position.

I recently did a valve job and while I was at it, I did some minor cleanup of the intake ports and manifold, where I removed only a very little of the metal to give what I guessed would be slightly less restricted flow.

At reassembly, I replaced the original intake clamps and S&S clamps, thinking my idle problem might have been due to air leaks.

Now, I have decent idle and good low speed performance, but when I am running beyond maybe 1/3 throttle, I get the an intermittent cough, enough to be very bothersome. (It's very rideable in town, but not on the highway.)

The bike has the original Keihin carb, with a .100mm (.0394) slow jet and a .170 (.067) main jet.

Any thoughts?

 

Try increasing the main jet size Sounds like a lean condition

 

Thanks, I'll try that next.

Do you know of any sites where I could can get an overview of the symptoms caused by particular intake problems - such as idle jet too rich or too lean, main jet too rich or too lean, air leaks, float adjustment too high or too low, etc?

 

HD CV carbs are great or S&S B or E's are good

 

I've seen this caused by several things ..... timing is off, carb is jetted incorrectly, cam set up is wrong, too much overlap on the lobes (sometimes you can "time" this out). If we eliminate the cam possibility for now you need to make sure the bike is timed correctly, tehn look at jetting.
My $.02

 

I would say since you JUST pulled the intake off, you have an intake leak. I just went through this on my bike. My HD tech said it was my S&S carb having a worn butterfly shaft causing an intake leak at part throttle. I changed my pipes and the bike was nearly unrideable. I ended up pulling the entire thing apart, New intake gaskets all around, and went throught the carb. Replaced all orings, seals, gaskets, float needle, reset the float level, made sure it was SPOTLESS... bike runs like a million bucks now. Best 60 bucks I have spent in a while. I also picked up the timing sight glass, I am going to check/set timing on the bike tonight just to be sure.

 

Thanks to all of you for your help so far, I have some things to check, for sure. But you've got me thinking:

I had the valve work done 6 years ago, because I'd sucked a valve, and I've just recently got around to putting the bike back together.

Damn. I'd forgotten that when I had the heads done, I also gave the lower end to my guy to look over, and while he was at it I had him put in a "slightly hotter' cam.

Yeh yeh. My memory sucks.

Anyway, I have no idea what he put in, I just told him I wanted "a little more". Who knows what the hell he did.

So this is not really the engine I thought I was dealing with. Damn. I've been assuming everything should work as it was.

So .. "slightly hotter" cam .. does that beg a larger main?

 

May need to fatten up both the main and the slow, possibly adjust the needle position as well. Have you gone through the carb? You say the bike has been sitting, I would pull everything apart and make sure there is no varnish in the jets, passages, and that the slide boot has no tears in it and is soft. Make sure your needle valve is good, and check the float height as well. once you know the carb is clean and functioning 100%, then start bumping up the jet sizes. The nice thing about that carb is its cheap to work on.

http://www.keihin-us.com/tune.htm

 

My 75 shovel did the same thing and after new pistons, wrist pins, intake gaskets it didn't help. A friend came over & we went thru my bendix carb & found the problem-both the idle screw & main jet wobbled in their bosses so we put a little teflon tape on & screwed them back in...it now idles & runs again

 

The carb is thoroughly clean and looks good to me. It has all new rubber in it. The idle needle is not scoured, I think that's okay. I think I have the idle working right, it only seems to cough when the butterfly is open past the idle circuitry.

There's history I haven't mentioned, which doesn't pertain to my problem at this point, but here it is: When first got the bike together a few months back, the idle was real bad. Because I mainly ride through town to work I never really determined what was happening at highway speed.

I fought the idle problems by increasing the idle jet size, but I couldn't get a grip on the problem. But my plugs were oiling up badly, and I found that the machinist I'd used was a hack .. the valve guides were too loose, and he hadn't used stem seals, and I was sucking a LOT of oil. So this is my second valve job.

Of course, after putting the heads back on this time, the idle was again bad, because I still had a fat idle jet in the carb. The bike sputtered and hacked full time at low speed. When I put the idle jet back to stock (.100mm) the idle smoothed right out.

Now it runs fine at low speeds, but I'm getting the occasional, yet predictable cough as I come off of the idle circuits.

So, I'm going to try bumping up the main jet size. It's currently a 170, and I have 175, 180 and 185 jets in hand to work with.

Any further thinking on carb symptoms in general, I want to hear them.

With any luck, it won't be raining this weekend, I'll post the results.

Thanks to all of you.