Tune-up help

I've got a 76 900 that I need some help tuning. I bought it not running, but it has had some engine work done to it. It is said to have Webb cams in it, but not sure. It does have a set of K&N air pods, a 4 into 1 header, open can. Dyna electronic ignition, aftermarket coils, aftermarket wires. Oil pressure gauge mounted on engine. Aftermarket clutch cover, different brakes, different size sprockets.

Anyways, I had the carbs off of it, and checked the jets. It has the factory jets in it. It also has NGK plugs in it.
Could you guys give me some input on what the best plugs would be and their gap? Also, should I change jets?:S
It runs good when your really on it, but seems to have a miss at idle and part throttle. But when you really crank on it, the miss clears up. Seems to be smoking a little black, like it's rich.

Thanks,
Mike

Let's assume you have the stock 76 VM26 carbs. These have an ID mark Z12 00. You can see the mark on the flange. Let's also assume the ignition is timed correctly and the valve lash set properly... OK...

The 76 VM26 used #115 main jets, #17.5 pilots and 5DL31-3 jet needles. First thing you need to do is get some info about that engine. Check the compression on all four cylinders. Do this when the engine is warm... first remove plugs and crank throttle wide open. Turn it over a few times till max compression is realized. You will likely have about 125-140 psi if you have a stock engine; more if you have high compression pistons. Second step is to look at the cams. See if they have adjustable (slotted) cam gears. If so, you do likely have aftermarket cams....

The MAIN problem with the 76 VM26 is that is can only run so much gas through it. The petcock, fuel line and fuel pipes are very likely going to act as your main jet if your engine is pretty modified. You can still make them run as good as you can. First thing is to swap the main jets for #122.5, the pilots may be OK and raise the jet needles a notch by reclipping to the 4th slot (#1 is top). The problem, as I see it, is that your symptoms don't sound like lean carburetion... they sound more like a weak spark.

You might want to look at the coil-powering mod in the Filebase. It may be marked on the right of the Filebase page as "most popular download" or something like that. That is why plugs soot up and you are likely getting some backfiring.

Go ahead and use the jetting I noted, do the coil-powering mod and save your money for some carbs that can keep up with your built engine.