I'm wondering about my 77 Sportster having a unstable idle. I've replaced the stock carburator with a brand new S&S Super E and have done all the fine tuning as described in the installation paper. I can set the idle at 1000 or 1200 but after a short ride it seems to raise some. I don't have any manifold air leaks that I can find. The carburator butterfly is not binding at all, I've checked this very good. It has the dual throttle cable setup but I disconnected the closing one to make sure it wasn't hampering the butterfly but it made no difference. Do I have a possible ingition problem ? The bike runs great at all speeds off idle, better than it ever has with the old stock carburator. As I can remember many years ago it seemed to do the same thing with the old carburator installed too.
I'm constantly having to adjust the idle.
Any suggestions would be appreciated.

 

My '73 does the same thing. I just keep it running with the throttle until she is warm & after she is warm she idles fine. Just set the idle with it warm IMO

 

Same thing with my '73. My theory is, with 50 wt oil, the oil is very thick when the bike is cold. Once it warms up, the oil thins, which causes less restriction and the idle increases.
May not be correct but is sounds good, eh?

 

If it's below 60 degrees, it can take 15 minutes or so to warm up. It's imperative to fully warm the engine prior to adjusting idle. If the mixture screw is out more than 1 3/4 turns after adjusting idle, you may need the next larger intermediate jet. If it's less than 1 1/4 turns out, the next smaller jet. It could also be the advance weights not working properly, make sure they're not sticking. The springs can get gummed up.

 

Does the 1977 Sportster use points or electronic ignition or maybe even electronic ignition with mechanical advance?
What I am leading up to is that if the bike has a mechanical ignition advance, it could be the advance weights are hanging up and not going fully advanced until after the bike warms up which would cause the idle to increase. If your bike has mechanical advance and this is what is happening, all that would need to be done is get in there and clean/lube things up, install new advance springs (change 'em with every new set of points or every 15,000 miles [or so] because they are thin and the heat gets to 'em) and re-time the bike. Then set your carb.
If you ain't got mechanical advance, then excuse me and I'll sit down and shut the hell up.

 

+1 on the advance parts.

How have you tested for intake leaks? My experience is that the usual tests will identify only large leaks. The best test is to use propane. Aim the torch UNLIT!!! right at the joins between the carb and the intake manifold and the cylinders. Any change in RPM indicates a leak.

The other popular choice is WD-40 as it has very fine particles. Propane is the best of the two; any other choices are not as good. But even the propane will miss very fine leaks.

Best is to use T-bolt clamps, also referred to as aircraft style clamps, to seal up both the intake manifold and the exhaust headers.

 

If you ever do go with those t-bolt clamps, be aware that they are strong clamps and if you tighten them too much you might break the head spiggot or the manifold. They are outstanding clamps, the best I've seen. But they can be tightened too much; I expect, even on an ironhead.

 

I found a problem with the idle screw turning on it's own after the throttle is twisted and vibration occurs, so some locktite solved the problem.
Thanks again for all the very useful information.

 

Not sure if you mean the idle level screw or the idle mixture/pilot screw. Each has it's tension controlled by a spring. In either case a better solution than the locktite would be to replace the spring.

For the pilot screw, you may have to turn the carb upside down to remove the screw; and then use a pipe cleaner to remove the spring, washer and o-ring. Remember to re-insert these parts in the correct sequence.

 

You have an intake manifold leak. I've heard of guys pouring water from a hose over the intake to check for leaks. If it sputters, it's leaking. Haven't tried that myself.

Next time you have the carb and intake manifold off, hold the manifold up against the cylinders (without the O rings) and check to see that the manifold and cylinders mate up well. If they don't, you can loosen the bolts at the base of the cylinders and rotate them slightly until the manifold lines up correctly without any gaps.

I pretty sure that the idle speed problem has nothing to do with the ignition. Having said that, its always a good idea to check out those little springs and weights and make sure they are in good working order. Better yet, scrap out the ignition system and put an electronic ignition in it.