Need some help- idle RPM's and choke

I've been doing some work on my bike, trying to get it ready to ride.

Tonight I did the coil power relay mod (or whatever that's called) just to be sure that wasn't a problem.

The main issue I seem to be having is actually two separate problems, but I'm sure they're probably related.

First off, the engine starts fine with the choke on and runs fine with the choke on, but you have to adjust it down a bit or it seems to want to idle around 3000-4000 RPM. But, no matter how long you run the engine, you can't seem to take the choke completely off or it just dies.

Second problem is once it's running- As I said, with full choke the RPM's will want to run around 3-4000 RPM. I usually back the choke off a bit so it will idle around 2000. Quite often though, if you ride it a short ways, sometimes the RPM's will start to climb, and sometimes if you stop and pull in the clutch the RPM's will shoot up to 4000 RPM for no reason and stay there, and you have to play with the choke again to get it to come back down to a more reasonable level.

Otherwise, the bike seems to run great.

So, I need some help here. I'm not very familiar with carbs. Also...when I bought the bike, I had a guy at the place I bought it from bring it out to me. When he brought it over I started it and it died, and he told me that on the way over he adjusted the idle because it seemed like it was idling kinda high. So I'm wondering if he screwed up the idle when he adjusted it.

Thanks

The pilot circuit needs to be cleaned thoroughly; probably on all the carburetors and the pilot jets replaced. Set the fuel level using the service fuel level method. Don't operate the bike for extended periods with the enrichener (aka choke) engaged.

Umm...What now?

Again, not very carb literate. I don't know what you mean by the pilot circuit needing cleaned.

Edit- OK, I bought a service manual on CD and was just looking at it, and I see the pilot circuit, but I'm still unsure on how you clean it.

Separate the carburetors and remove all rubber/plastic parts and dip them in carb dip overnight then blow them out with compressed air. Pay special attention to blowing through the holes associated with the pilot circuit. Alternatively (and a better solution) is to disassemble and run through an ultrasonic cleaner. Blow everything out thoroughly with compressed air when finished. When done, blow carb cleaner through the associated pilot air jet so that it comes out the hole in the venturi where gas is diffused and then blow carb cleaner through the pilot well and ensure it comes out through the diffusion hole in the venturi. This last step is done as vefication there are no internal obstructions.

Changing pilot jets means unscrewing the pilot jet and installing a new one in the pilot well.

What George said! The pilot jets are tiny on these bikes, and get clogged very easily. I would add - make sure the carbs are seated properly into the intake manifolds, and you have no vacuum leaks.

So, guessing that I'll have to pull the carbs to clean the pilot jets, right?

Are the carb gaskets still readily available off the shelf?

By the way, can you get bigger jets for these carbs? I'm thinking about pitching the mufflers on the bike for something a little lower restriction (and louder) and I don't want it to run too lean.

I have the gaskets on my shelf but you may not have them on yours. I suggest you leave the factory airbox and pipes in place. I am not certain you will find larger replacement main jets for the 78 BS38 and unless you rejet, you will just hurt performance by modifying the bike. Just my opinion. Lots of 750 owners will probably disagree. Last, I NEVER suggested you clean the pilot jets... I mentioned replacement. They are Mikuni BS30/96 type and if memory serves, size #42.5 but take one out and look at its badging on the side to verify.

I took the bike out for a ride tonight, put about 35 miles on it, and I may have at least partially solved both problems.

First, apparently the guy I bought the bike from turned the idle WAY down...I adjusted it up while I slowly backed off the choke until it would idle at 1500-ish RPM with the choke completely off, then adjusted it a little more as the bike warmed up.

So, I got it to where it idles pretty decent most of the time.

However, occasionally the idle will jump up to 3000 RPM for no apparent reason...sometimes if you blip the throttle it'll go back down to a normal idle, sometimes it's very stubborn and doesn't want to go back down. I'm not sure if something is sticking or what.

Other than that, it seems to be running great...I had a lot of fun riding it

A hanging idle can be a manifold air leak, or misrouted/dirty/kinked/old throttle cables.

To check for an air leak start the bike and spray some WD-40 or something similar on each of the rubber manifolds attaching the carbs to the engine. If the idle changes then you have an air leak on that manifold. The manifolds may be cracked, or just not sealing good to the head.

I have removed my manifolds and used a bit of hi-temp silicone to help seal them to the head. Other guys have sealed the cracks with silicone too. The manifolds are still available from places like Z1 Enterprises. The best thing to do would be replace them, if you see cracks in the rubber.

I still think your pilot circuit is dirty. Manually increasing the idle will tend to compensate, but doesn't solve the problem.
People here have used Seafoam with good results. Put some in your tank with fresh gas and run the bike. If it works, you'll see your idle increase, and you'll need to back it down manually.
WG's right that a good cleaning is what's best, but if your jets aren't too far gone, the Seafoam may help.
My .02.

Well I already bought carb rebuild kits for the bike...so I'll try what's been suggested here, if that doesn't work I'll rebuild the carbs and probably replace the manifolds too.