Stumble?

[jjz28]

Sometimes my bike will kinda act like it hangs or just didnt fire when idling. Its hard to explain but it act like it shuts down for a millisecond and fires right back up, just like it was idling fine and kinda hangs up for a second and then is fine. Its a 79 special all stock. anyone know what this is?

[DGXSER]

First and most likely source would be the pick up coil wires.

[James England]

It does sound like a tiny electrical blip which may indeed be the pickup wires. Often though, these only produce problems when the vacuum advance starts pulling them but, you never know and DGXSER may indeed be correct; there may be a fracture in there when the wires are 'at rest'..

Failing that, I would check obvious electrical things like plugs, HT caps and wiring block connectors etc.

Also, it could possibly be carb tuning. Getting a totally perfectly timed tickover beat requires everything to be just right and is often not achieved. The tickover can be a little 'lumpy' and sound like a tiny misfire, in my experience. I reduced tickover on my bike to about 700 rpm and was able to get the beat just about exact. Then, I increased to a more usual 900-1000 rpm and it sounded perfect. But that was after carb kits, diaphragms, coils, caps etc. I had the tickover so that you could count the beats, one after the other, with no variation once the engine was warmed up. Most bikes do seem to have an uneven tickover beat which may, to you, sound like a tiny blip or misfire maybe?

If the bike sounds like it does for a millisecond, and only on tickover (with no misfires at higher revs), I'd say it could be carb tuning. I'd synchonise the carbs with vacuum gauges for a start, then get the mixture screws right and see what's what.

[motoman]

+1 what James stated. Annoying to say the least, and imagine up to 4K which is in the idle circuit. Alot of variables can cause this, fuel bowls actual running levels which changes mix ratios, a tad lean on the mix screw settings. Normally, I would lean toward pri. or sec. ignition glitches, but getting anal with associated carb settings seems to resolve this.

[jjz28]

The bike revs great, though i have not ridden it on the road except up the hill and back (about 200 yards) and it runs really good to me but I know for a fact that the carbs are not tuned right. They have only been eyeball bench sycnched with the hole at the top, i halved the little hole like i did with my xs750 carbs (mk1's). Also, it seems i have to rev it just a little to take off good. It used to be really bad until i found out my vacuum advance wasnt hooked up (i deleted the octi and forgot to hook it up). basically it runs really good probably in the mains but i think maybe the primarys may be where my problem is. are the primarys the ones that are in the carb "bell"? They say something like 180 on them. Thanks guys, wish I could get this thing right.

[natemoen]

Ok since you mentioned the octy and the vac advance together I want to make sure about something. When you hooked the vac advance up you hooked it to the nipple of the left side of the #2 carb body and NOT to one of the carb intake boots correct?

[DGXSER]

First, at least read through the Carb Cleaning 101 thread, it will familiarize you with the names of the jets and their locations. THe ones in the inlet bells are the pilot air jets. They control how much air goes to the pilot (idle) circuit. The little ones inside the tower inside the bowl are the correpsonding pilot fuel jets, controlling of course how much fuel goes to that circuit.

If you have not yet tuned or synched the carbs, that could easily be your entire problem.

[jjz28]

Thanks guys, i really think I need synched. I rode it yesterday for about 60 miles and love it. It makes my xs750 feel like a scooter, lol. This is a great site with some great people!